Papers about pb-free
by Dr. Ning-Cheng Lee , Iris Artaki, James Slattery, John R. Sovinsky, Paul T. Vianco
Environmental and toxicity concerns related to the use of lead have initiated the search for acceptable, alternate joining materials for electronics assembly. This paper describes a novel lead-free solder designed as a "drop in" replacement for common tin/lead eutectic solder. The physical and mechanical properties of this solder are discussed in detail with comparison to tin/lead eutectic solder. The performance of this solder when used for electronics assembly is discussed and compared to other common solders. Fatigue testing results are reported for thermal cycling electronics assemblies soldered with this lead-free composition. The paper concludes with a discussion on indium metal availability, supply and price.
pb-free, surface mount, SMT, solder paste, reflow, electronic, lead-free, soldering, solder
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee , Paul A. Jaeger
As one of the major approaches to address the CFC issue, no-clean solder paste has received rapidly increasing attention. Although currently the industry seems to accept full residue paste as a temporary solution, the low residue no-clean paste technology using inert or reactive atmosphere advances immensely to meet the challenge. Presently consensus has not been established yet regarding how low a residue level could be achieved and how inert the atmospheres needs to be. In this study, a semi-empirical model is proposed to predict the soldering performance of low residue solder pastes under various levels of inert reflow atmosphere. The model predicts that the soldering performance would improve rapidly then gradually level off with decreasing oxygen content. The soldering performance vs oxygen content curves are superimposable, with the lower residue one leveling off at lower oxygen level. In general, the experimental data match this model fairly well. However, the data also indicate that, although inert atmosphere improves soldering performance, the optimum condition for bond strength performance seems to demand the presence of some oxygen. This unexpected behavior suggests that a very tight low oxygen level control may not be required. The mechanism responsible for this phenomenon can be attributed to oxidation-induced resin crosslinking. This slows down the flux drying rate as well as hinders the permeation of oxygen through the flux layer.
solder, solder paste, no-clean, low-residue, soldering, reflow, flux, nitrogen, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Yan Liu, William Manning, Dr. Benlih Huang, Dr. Ning-Cheng Lee
Voiding is attributed to the flux outgassing within the solder joints when the solder is at molten state. The effect of reflow profile on voiding at microvia for lead-free soldering is strongly dependent on the flux chemistry. In general, wetting is more important than
melting outgasing behavior, and can be enhanced by employing a higher melting energy, including both higher peak temperature and longer dwell time. Use of a high soaking energy can help drying out volatiles hence reduce the melting outgasing and result in low
voiding, but may also increase oxidation for pastes with poor oxidation resistance and cause a high voiding. Testing oxidation resistance of solder paste beforehand will promise a more accurate selection of soaking energy.
reflow, profile, microvia, lead-free, solder, SMT, voiding, void, CSP, BGA, soldering, pb-free
[Permanent Link to this Paper ]
Posted on 2 Mar 2010
by Dr. Ning-Cheng Lee , Dr. Richard Ludwig, Steven Rocco Marongelli, Sergio Porcari, Sunil Chhabra
In order to achieve ultra-fine dot solder paste dispensing, both solder material and dispensing equipment have to be optimized. Dispensability of solder paste was evaluated in terms of “dispensing rate”, consistency of dispensing rate, and the stability of dispensing rate with time. Within the given conditions, threshold values for dispensability seem to exist for viscosity, powder size, and metal content. Small nozzle inner diameter is definitely needed to deliver a small dot size. Archimedes Metering Valve shows a greater flexibility in metering the volume than Positive Displacement Pump, primarily due to a greater sensitivity in dispensing volume to variation in pressure, and nozzle ID, besides being very sensitive to variation in encoder count. For success in high speed ultra-fine dot dispensing process, solder pastes with a low viscosity, small powder size, low metal content, and a high thixotropy are desired to deliver a high dispensing throughput. Controlwise, a high pressure and high encoder count may be promising. The consistency improves with increasing metal content, thixotropy, pressure, nozzle size, and encoder count. Viscosity, powder size, and delay time appear to have negligible effect on consistency. The stability increases with increasing flux activation temperature, and is expected to be poor for low thixotropy and low viscosity. Large powder size may cause immediate clogging, while small powder size may cold weld under repeated pressure cycling using pneumatic pump systems. In general, a very careful design and tight control of parameters discussed in this work has to be implemented in order to succeed in ultra-fine dot solder paste dispensing.
dispensing, dispensability, solder paste, flux, surface mount, SMT, pump, fine dot, Archimedes, piston, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Mario Scalzo
Chinese version of Addressing the Challenge of Head-in-Pillow Defects in Electronics Assembly.
CHINESE LANGUAGE, halogen-free, head-in-pillow, pb-free, solder defects, solder paste, solder reliability, lead-free
[Permanent Link to this Paper ]
Posted on 11 Mar 2010
by Mario Scalzo
White Paper Video
Addressing the Challenge of Head-in-Pillow Defects in Electronics Assembly
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The head-in-pillow defect has become a relatively common failure mode in the industry since the
implementation of Pb-free technologies, generating much concern. A head-in-pillow defect is the
incomplete wetting of the entire solder joint of a Ball-Grid Array (BGA), Chip-Scale Package (CSP), or
even a Package-On-Package (PoP) and is characterized as a process anomaly, where the solder paste and
BGA ball both reflow but do not coalesce. When looking at a cross-section, it actually looks like a head
has pressed into a soft pillow. There are two main sources of head-in-pillow defects: poor wetting and
PWB or package warpage. Poor wetting can result from a variety of sources, such as solder ball
oxidation, an inappropriate thermal reflow profile or poor fluxing action. This paper addresses the three
sources or contributing issues (supply, process & material) of the head-in-pillow defects. It will
thoroughly review these three issues and how they relate to result in head-in-pillow defects. In addition,
a head-in-pillow elimination plan will be presented with real life examples will be to illustrate these
head-in-pillow solutions.
halogen-free, head-in-pillow, pb-free, solder defects, solder paste, solder reliability, lead-free
[Permanent Link to this Paper ]
Posted on 11 May 2009
by Dr. Ronald C. Lasky , Daryl Santos PhD, Joseph R. Cloyd
It is now widely accepted that using designed experiments is the most effective way to optimize surface mount technology (SMT) processes. This situation begs the question “what is an effective strategy in implementing this powerful tool?” This paper will present such a strategy that incorporates Taguchi’s approach for screening, full factorial analysis for optimization and central composite design for precise modeling. We will present these techniques using MINITABTM Release 13 statistical software and printed circuit board industry applications.
pb-free, lead-free, design of experiments, DOE, line optimization, continuous improvement, process modeling, process improvement
[Permanent Link to this Paper ]
Posted on 31 Mar 2010
by Dr. Ronald C. Lasky , Timothy Jensen
The clock is ticking, on July 1, 2006 the WEEE Initiative will take effect. Thereafter, all electronic assemblers that sell products in Europe must be ready to convert their assembly processes to Pb- free. The nearness of this date raises the question of what can be done to get ready. In response to this need, we will review a pioneering effort in establishing a Pb-free process.
RoHS, WEEE, lead-free, pb-free
[Permanent Link to this Paper ]
Posted on 4 Mar 2010
by Dr. Ronald C. Lasky
Miniaturised electronics and the advent of lead-free soldering have added new challenges to the SMT electronic assembly process, most notably in the arenas of stencil printing and reflow. Recent work on improving these assembly processes and advances in solder paste technology can help to minimise these process challenges.
halogen-free, solder paste, solder, solder reliability, flux, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 15 Mar 2009
by Timothy Jensen , Dr. Ronald C. Lasky
The electronics industry continues to strive to provide more environmentally friendly products. This movement is partly due to legislation from various countries, partly due to public outcry from well publicized 3rd world recycling practices, and partly due to non-government organizations (NGOs) testing and publishing information on electronic devices regarding their content of various toxic materials. One set of materials targeted for reduction and eventual elimination are halogenated compounds. Halogens are found in plastics for cables and housings, board laminate materials, components, and soldering fluxes. Replacing these halogenated compounds can have a dramatic affect on the PCB
assembly process. In this paper those challenges will be discussed as well as techniques and practices that will help ensure high end of line yields and continued reliability.
halogens, halogen-free, halide-free, head-in-pillow, graping, reflow, pb-free, activator, oxidation barrier
[Permanent Link to this Paper ]
Posted on 4 Mar 2010
by Dr. Ronald C. Lasky , Timothy Jensen
Chinese version of Challenges of Implementing a Halogen-Free PCB Assembly Process.
CHINESE LANGUAGE, halogens, halogen-free, halide-free, head-in-pillow, graping, reflow, pb-free, activator, oxidation barrier
[Permanent Link to this Paper ]
Posted on 22 Mar 2010
by Vahid Goudarzi, David Day, Richard Brooks
This paper will outline the issues relating to the implementation of a Pb-free solder paste into a
standard Sn/Pb manufacturing facility and product. The Pb-free study includes the compatibility and impact on the various manufacturing processes that include, printing, component placement, reflow process, and solder joint quality.
These parameters must be fully characterized to ensure that the lead-free solder paste meets the manufacturing and product quality requirements. In addition, a lower super heat temperature is critical to reduce the thermal stress on components, since the lead-free alloy composition (Sn/Ag/Cu) liquefies at about 34 C higher than the current leaded material (63Sn/37Pb). Several implementation issues were discovered during the pilot phases and resolved prior to full-scale production manufacturing. Some of the problems revealed were BGA voiding, chip component tombstoning, and component integrity as it relates to termination plating and moisture sensitivity.
pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 4 Mar 2010
by Dr. Ning-Cheng Lee , Dr. Benlih Huang
Tombstoning of SnAgCu is affected by the solder composition. At vapor phase soldering, both wetting force and wetting time at a temperature well above the melting point have no correlation with the tombstoning behavior. Since tombstoning is caused by unbalanced wetting force, the results suggest that the tombstoning maybe dictated by the wetting at the onset of paste melting stage. A maximal tombstoning rate is observed at 95.5Sn3.5Ag1Cu. The tombstoning rate decreases with increasing deviation in Ag content from this composition. DSC study indicates that this is mainly due to the increasing presence of pasty phase in the solders, which is expected to result in a slower wetting speed at the onset of solder paste melting stage. Surface tension plays a minor role, with lower surface tension correlates with a higher tombstoning rate. SnAgCu composition with a Ag content lower than 3.5%, such as 2.5Ag, is more favorable in terms of reducing tombstoning rate with minimal risk of forming AgSn intermetallic platelet.
surface mount, lead-free, flux, solder paste, soldering, solder, tombstoning, pb-free
[Permanent Link to this Paper ]
Posted on 4 Mar 2010
by Dr. Ning-Cheng Lee
Effect of solder alloy composition and properties on tombstoning of SnAgCu has been investigated. Both wetting force and wetting time at a temperature will above the melting point have no correlation with the tombstoning behavior observed at vapor phase soldering. Since tombstoning is caused by unbalanced wetting force, this unbalanced wetting force may occur at the onset of melting DSC study indicates that the tombstoning rate decreases with increasing pasty temperature range and increasing mass fraction of solid in solder at onset of melting. This slower wetting in turn results in a more balanced wetting force and accordingly reduces the tombstoning. The mass fraction of solid may be the more essential factor. Surface tension also plays a role, with lower surface tension correlates with a higher tombstoning rate. Tombstoning of SnAgCu can be regulated by the solder composition. A maximal tombstoning rate is observed a 95.5Sn3.5Ag1Cu. The tombstoning rate decreases with increasing deviation in Ag content from this composition, particularly toward the end of lower Ag content. SnAgCu composition with a Ag content lower than 3.5%, such as 2.5Ag, is more favorable in terms of reducing tombstoning rate with minimal risk of forming Ag3SN intermetallic platelet.
tombstoning, solder, soldering, solder paste, flux, lead-free, surface mount, pb-free
[Permanent Link to this Paper ]
Posted on 4 Mar 2010
by Dr. Ning-Cheng Lee , Dr. Benlih Huang, Dr. Arnab Dasgupta
For SnAgCu solder, the voiding rate at microvia was studied with the use of simulated microvia, and was the lowest with 95.5Sn3.8Ag0.7Cu and 95.5Sn3.5Ag1Cu, and increased with further decrease in Ag content. Results indicated that voiding at microvia was governed by via filling and exclusion of fluxes. The voiding rate decreased with decreasing surface tension and increasing wetting force which in turn was dictated by the solder wetting or spreading. Both low surface tension and high solder wetting prevented the flux from being entrapped within microvia. A fast wetting speed might also facilitate reducing voiding. However, this factor was considered not as important as the final solder coverage area.
solder, soldering, lead-free, void, voiding, microvia, surface mount, reflow, pb-free
[Permanent Link to this Paper ]
Posted on 4 Mar 2010
by Dr. Mikolaj E. Jozefowicz, Dr. Ning-Cheng Lee
The IPC-SF-818 Surface Insulation Resistance (SIR) test data taken with the use of a variety of halide-free no clean fluxes are analyzed against Bellcore TR-NWT-000078 Electromigration (EM) test data. Neither test results show correlation with bulk flux resistivity, flux water extract resistivity, flux residue moisture pickup, and flux corrosivity without bias. However, in the case of rosin fluxes, the insulation resistance behavior in both SIR and EM tests is a function of pH value of fluxes. This phenomenon is more profound in SIR test. In the case of low residue no clean fluxes, only SIR test displays such a pH dependent relationship. Data suggest that the 50 volts bias voltage used in SIR test may be responsible for this, and can be explained with a high-bias-voltage-induced electrolysis mechanism which is further promoted by a high pH environment. This failure mechanism is absent in EM test which utilizes 10 volts bias voltage, and probably will not occur at normal 5 volts application condition. Overall, the SIR test seems to be more stringent while the EM test appears to be more realistic.
SIR, electromigration, solder, soldering, flux, EM, surface insulation resistance, no-clean, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Karl Pfluke , Richard H. Short
The advent of Lead-Free Soldering presents many manufacturers with the need to Wave Solder using Lead-Free Alloys. These alloys melt and are soldered at temperatures well above conventional SNPB processing temperatures. This creates several well-documented problems. This article offers a proven and practical alternative to the Lead-Free Wave Soldering Process.
lead-free, wave solder flux, pb-free, Rework, solder preforms, solder paste, solder reliability
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Karl Pfluke , Richard H. Short
German version of Eliminate Lead-free Wave Soldering.
lead-free, wave solder flux, German language, pb-free, Rework, solder preforms, solder paste, solder reliability
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Xiaohua Bao, Dr. Ning-Cheng Lee , Rajkumar B. Raj, K. P. Rangan, Dr. Anu Maria
Rheology of a solder paste has a significant effect on its stencil printing, tack, and slump performance. This paper describes a series of tests designed to investigate the rheological properties of a suite of solder pastes and fluxes, and the correlation with the solder paste performance prior to reflow. Data indicate that 1) print defect is proportional to the compliance (J1 and J2) and inversely proportional to the elastic properties (G’/G’’ and Recovery) and meta-rigidity (Yield Stress); 2) slump resistance is proportional to elastic properties (Recovery), solid characteristics (Stress [G’=G’’]), and rigidity ( êG* ê); 3) high elastic properties (Recovery), low compliance (J1 and J2), and low solid characteristics (Stress [G’=G’’]) are required in order to achieve high tack value. Good correlation between fluxes and solder pastes are observed for Yield Stress and Recovery only, suggesting those two properties are primarily dictated by fluxes.
solder, soldering, solder paste, rheology, print, slump, tack, flux, viscosity, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee
This paper reviews the status of lead-free solder developmental works. Some of the solder systems, Bi-Sn, Bi-Sn-Fe, In-Sn, Sn, Sn-Ag, Sn-Ag-Zn, Sn-Ag-Zn-Cu, Sn-Bi-Ag, Sn-Cu, Sn-Cu-Ag, Sn-In-Ag, Sn-Sb, Sn-Zn and Sn-Zn-In are discussed in more details, while the others are briefly commented on. In general, compared with eutectic Sn-Pb solder, all the lead-free solder alternatives investigated more or less exhibit some shortcomings, such as price, physical, metallurgical, or mechanical properties. Relatively, Sn-In-containing systems are more promising in terms of solder mechanical properties and soldering performance, although the price of In may be a concern. Eutectic Sn-Ag solder doped with Zn, Cu, or Sb exhibits good mechanical strength and creep resistance, due to refined microstructure. The Bi-Sn systems doped with other elements may have a niche in the low temperature soldering field. Eutectic Sn-Cu has a good potential due to its good fatigue resistance. Eutectic Sn-Zn system modified with In and/or Ag may be promising in mechanical properties. Finding a lead-free alternative for high temperature solders presents the biggest challenge to the industry.
solder, soldering, lead-free, electronic, tin, lead, pb-free
[Permanent Link to this Paper ]
Posted on 4 Mar 2010
by Dr. Ronald C. Lasky , Timothy Jensen
pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 31 Mar 2010
by Dr. Ning-Cheng Lee
In this article, the interconnect infrastructure for SMT, BGA, and flip chip are reviewed, with particular emphasis on the bonding technology. Interconnection technologies are the vital part of electronic packaging. Obviously, interconnections of SMT industry, from components to boards to board-level assembly methods, are the most mature and well established technology. BGA, on the other hand, intelligently utilizes the knowledge of SMT interconnections and re-engineers the design through combining the strength of various interconnect technologies and successfully comes up with a great family of versatile packages. Flip chip interconnects, while also trying to incorporate existing technology, place a good deal of emphasis on the polymeric systems, and very much develop a new arena of interconnect concepts and processes. The impact of flip chip interconnect progress is expected to ripple through the rest of electronic industries in the near future.
SMT, BGA, Flip Chip, CSP, Interconnection, surface mount, ball grid array, Packaging, assembly, soldering, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee
Lead-free soldering, originally started as an environmental issue, is evolving rapidly into a business survival tool for the worldwide electronic industry. Promising lead-free solder alternatives for surface mount assembly applications include eutectic Sn/Ag, eutectic Sn/Cu, Sn95/Sb5, eutectic Sn/Bi, Sn/Ag/Cu, Sn/Ag/Cu/X, Sn/Bi/Ag/X, Sn/Zn/X, and Sn/In/Ag/(X). However, for wafer level area array solder bump interconnects, most of those options fall short in terms of fatigue resistance. Sn/In/Ag/(X) appears to be superior when compared with Sn63/Pb37, as demonstrated by Sn/In/Ag/Cu. For applications involving high lead solders, no solder alternatives have been developed yet. While the industry is advancing toward being finer, smaller, lighter, and faster, wafer level packages using area array solder interconnects is suffering from the soft error due to alpha emission from the lead in the solders. Although lead-free solder alternatives for eutectic Sn/Pb are virtually free from alpha emission, the continuous dependence on the use of high-lead solders for C4 applications indicates that the challenge of alpha emission from lead-containing solders will persist regardless of the lead-free move of the industry. This challenge is getting tougher with the rapid advancement of IC design toward further miniaturization. Low alpha lead can be obtained from cold lead ore, old lead, and laser isotope separation process, with the latter having potential as a long term solution. The price of those low alpha lead is very expensive when compared with the regular lead. Due to the increase in I/O density, requirement on alpha emission level may soon move from LC2 to LC3 level. The supply of low alpha lead for wafer level interconnects does not seem to be an issue.
lead-free, solder, soldering, wafer level interconnect, Flip Chip, CSP, BGA, alpha emission, low alpha solders, soft error, indium, pb-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee
A host of lead-free solder replacements are coming to the fore, each presenting the user with certain tradeoffs. Many of the replacement solder systems are based on adding a small quantity of a third or fourth element to binary alloy systems to lower the solder’s melting point, which increases
wetting and reliability.
pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 8 Mar 2010
by Dr. Ning-Cheng Lee
Lead-free soldering for electronic industry is a segment of global trend toward lead-free environment. Although initiated in U.S. in early 1990's, it advanced much more rapidly in Japan and Europe. This differentiation in Pb-free progress triggered great concerns of users of Pb-containing solders about maintaining business opportunity, therefore further expedites the advancement of Pb-free soldering programs. The favored Pb-free solder alternatives vary from region to region. However, in general, high tin alloys are preferred, including Sn/Ag, Sn/Cu, Sn/Ag/Cu, Sn/Ag/Bi, and various versions of those alloys with small amount of additions of other elements, such as Sb. Sn/Ag/Bi systems are used in some Japanese products already. However, Sn/Ag/Cu systems are more tolerant toward Pb contamination than Bi-containing systems, therefore are more compatible with existing infrastructure for the transition stage. Pb-free surface finishes for PCBs include OSP, immersion Ag, immersion Au/electroless Ni, HASL Sn/Cu, Sn/Bi, electroless Pd/electroless Ni, electroless Pd/Cu, and Sn. The challenge for components is greater than for solder materials or PCBs. Although some Pb-free surface finishes for components exist, such as Sn, Pd/Ni, Au, Ag, Ni/Pd, Ni/Au, Ag/Pt, Ag/Pd, Pt/Pd/Ag, Ni/Au/Cu, Pd, and Ni, the performance remains to be verified. In addition, options for higher melting temperature solder is still not available for high temperature applications, including first level interconnect within the components. Thermal damage can be a concern for both PCBs and components.
lead, lead-free, solder, Pb, sn, soldering, alloy, pb-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee , Dr. Benlih Huang
Tombstoning has plagued the surface mount assembly industry for decades. While the problem seemed under control, it has begun creeping in again due to the miniaturization of discretes such as 0402S and 0201S. This article studies tombstoning behavior on a series of SN AG CU Lead-Free Solders and attempts to find a way to control the problem.
reflow, soldering, pasty range, solder alloy, SMT, solder paste, solder, tombstoning, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Aniket A. Bhave, Daryl Santos PhD, Dr. Ronald C. Lasky
Numerous studies have shown that greater than 60%i of end of line defects in SMT assembly can be traced to solder paste and the printing process. Reflowing adds another 15% or so. In light of this fact, it is surprising that no simplified procedure for solder paste evaluation has been documented. This paper is about such a procedure.
pb-free, lead-free, stencil printing, solder paste, solder paste evaluation
[Permanent Link to this Paper ]
Posted on 31 Mar 2010
by Dr. Ning-Cheng Lee
No-clean soldering process is the cheapest available process alternatives in the post-CFC era. In order to enjoy the benefit of no-clean process, care should be taken to assure the cleanliness of products before and after assembly. In addition, the no- clean soldering materials have to be properly formulated in order to deliver the high reliability and adequate flux residue appearance. Due to the elimination of cleaning process, issues such as solder beading, solder balling, probe testability, wire bondability, compatibility with polymeric coatings or wave soldering fluxes have to be addressed. No-clean fluxes typically utilize hydrophobic chemicals and often are in line with RMA flux chemistries. Nitrogen is required if a low residue level is desired for reflow process. Some conventional testing methods may not be adequate for evaluating no-clean soldering materials. Concurrent trends of shifting toward finer pitch, higher reliability, lower residue, and air reflow processes pose a great challenge for no-clean soldering process.
no-clean, solder, soldering, flux, paste, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Amanda Gronau
The move to lead-free electronics has had a slow start and many detractors, for a variety of reasons that range from the rational - added costs, new process parameters, extensive testing and approval, the need for consensus within the industry, to the irrational - fear of the unknown. The transition will not be easy, but given the market demand for environmentally friendly electronics, it seems inevitable.
pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 8 Mar 2010
by Dr. Ning-Cheng Lee
The reflow profile is engineered to optimize the soldering performance based on defect mechanisms analysis. In general, a slow ramp-up rate is desired in order to minimize hot slump, bridging, tombstoning, skewing, wicking, opens, solder beading, solder balling, and components cracking. A minimized soaking zone reduces voiding, poor wetting, solder balling, and opens. Use of low peak temperature lessens charring, delamination, intermetallics, leaching, dewetting, and voiding. A rapid cooling rate helps reducing intermetallics, charring, leaching, dewetting, and grain size. However, a slow cooling rate reduces solder or pad detachment. The optimized profile favors that the temperature ramps up slowly until reaching about 180°C. The temperature is then gradually raised further up to 186°C within about 30 seconds, then raised rapidly until reaching about 220°C. After that, the temperature is brought down with a rapid cooling rate. The conventional profile was developed due to the limitation of past reflow technologies. Implementation of the optimized profile requires the support of a heating-efficient reflow technology with a controllable heating rate. Vapor phase reflow can provide a rapid heating, but has difficulty to control the heating rate. Infrared reflow can regulate the heating rate, but is sensitive to variation in parts features. Emergence of the forced air convection reflow provides controllable heating rate. In addition, it is not sensitive to variation in parts
features, thus allows the realization of the optimized profile.
reflow, profile, defect, flux, solder paste, soldering, convection, infrared, vapor phase, SMT, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee , Dr. Chingchen S. Chiu
The solder bumping process for BGA is investigated by using solder paste alone, solder spheres with solder paste, and solder spheres with fluxes. Also explored is the use of integrated preform together with either flux or solder paste. For bumping process involving Sn62 or Sn63 spheres, use of paste for sphere attachment produces excellent alignment results. In the case of using fluxes for Sn62 or Sn63 sphere attachment, the defect rate increases with decreasing flux viscosity, decreasing solvent volatility, decreasing pitch dimension, increasing flux deposition thickness, increasing flux activity, and increasing pad diameter. For overall better yield, a solder paste with long stencil life, good printability, and good solder ball performance should be the most promising eutectic sphere attachment material. For systems using pastes for Sn10 sphere attachment, no missing is observed, and the alignment improves with decreasing paste deposition thickness, decreasing solvent volatility, increasing sphere solderability, increasing flux activity, increasing pad dimension, increasing metal load, increasing pad solderability. Paste viscosity, pitch, and reflow profile has negligible effect on the Sn10 bumping yield using Sn63 solder paste. An easily releasable solder paste is crucial for area-array BGA if a regular print-release process is desired for bumping with solder paste alone. Bumping with integrated preform is promising. Reducing the thickness and width of the solder link is considered essential for improving the bumping success rate. Other potential bumping processes may include (1) dispense paste/reflow, (2) print paste/reflow/release, (3) apply solder mask/print paste/release /reflow/strip solder mask, (4) solder jet/reflow, and (5) sphere welding, and are briefly introduced and commented on.
solder, balling, bump, Bumping, BGA, solder sphere, solder paste, integrated preform, flux, defect rate, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Manchao Xiao, Paul A. Jaeger, Dr. Ning-Cheng Lee
The probe-testability of no-clean solder paste flux residue at in-circuit-test is determined mainly by the residue amount, residue location, and residue hardness. The testability increases with decreasing amount of residue, decreasing amount of top-side flux spread, and increasing amount of bottom-side flux spread. The residue amount, top-side flux spread, and bottom-side flux spread affect primarily pad probing, pad probing, and pin-tip probing, respectively. Inert reflow atmosphere helps probe penetration. Higher metal load effectively reduces the flux spreading. Among all, the soft residue approach appears to be most promising in providing successful probe contact.
probe, testability, solder paste, flux residue, no-clean, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Steve Buerki, Amanda Hartnett
Paper Interview
Process and Reliability Advantages of AuSn Eutectic Die-Attach Solder
This video requires Adobe Flash Player to play correctly. Download the player for free .
Amanda Hartnett provided a tech paper she presented at iMAPS 2009 : 42nd International Symposium on Microelectronics (November 1-5, San José, CA) on the advantages of gold-tin die-attach solder. RF mounting and MEMS packages both benefit from the solder alloy's properties.
wafer scale packaging, pb-free, eutectic die-attach, automated pick-and-place, solder preforms, die bonder, AuSn solder
[Permanent Link to this Paper ]
Posted on 5 Nov 2009
by Dr. Benlih Huang, Dr. Ning-Cheng Lee
The prospects of 10 major lead-free solder alloys for being widely used for reflow soldering are studied in this work. Compatibility of those alloys with a variety of representative flux chemistries is considered essential, and is determined for performance in handling- ability, including shelf life and tack time, and soldering capability, including solder balling, wetting, and solder joint appearance. Results indicate that the control 63Sn37Pb is still the most compatible alloy, rated 27.1 in compatibility out of a full scale 30 when using warm profile. The primary factor which distinguishes 63Sn37Pb from the rest alloys is the soldering performance, particularly the wetting and solder appearance. As to the solder balling, although 63Sn37Pb is also the best, it is fairly close to the best lead-free systems. Among the lead-free options, both SnAgBi alloys studied here, 91.7Sn3.5Ag4.8Bi and 90.5Sn7.5Bi2Ag, turn out to be on the top of lead-free systems, rated 22.9 and 22.8, respectively. This is mainly attributed to the better wetting and solder balling performance. Shelf life and tack time of the SnAgBi systems are also fairly good, while the solder appearance is at best considered average. The six alloys, 99.3Sn0.7Cu, 95.5Sn3.8Ag0.7Cu, 93.6Sn4.7Ag1.7Cu, 96.2Sn2.5Ag0.8Cu0.5Sb, 58Bi42Sn, and 95Sn5Sb, show fairly comparable performance to each other, with compatibility ranging from 19.3 to 20.3. In general, the whole group displays a quite noticeably poorer wetting than SnAgBi systems. 58Bi42Sn exhibits a fairly poor solder balling performance, but an outstanding solder appearance among lead-free systems. 96.2Sn2.5Ag0.8Cu0.5Sb shows a relatively poor performance in both wetting and solder appearance among these six alloys. 96.5Sn3.5Ag, rated 17.1 in compatibility, is ranked below the other alloys described above, mainly due to poor performance in solder balling, and particularly the poor wetting. 89Sn8Zn3Bi, rated only 2.2 in compatibility, falls far short in every category when compared with all other alloy systems. Obviously, this is attributable to the very reactive nature of zinc, which results in excessive oxidation of metal and excessive reaction with fluxes, and consequently a definitely unacceptable performance for solder paste applications. High-tin-content lead-free alloys seem to display a thicker IMC layer than eutectic SnPb when reflowed.
lead-free, solder, soldering, reflow, paste, flux, wetting, solder balling, solder appearance, shelf life, tack time, pb-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee , Kevin J. Lawless, Dr. Manchao Xiao
The 12 mil pitch processing is achievable with solder paste. It may also be the limit of solder paste printing
technology, mainly due to the scooping problem associated with thin stencils. With decreasing pitch size, both smear and insufficiency rate increase. Tapering of stencil aperture helps thick stencil prints, but hurts on thin stencil printing. Aperture with orientation parallel to squeegee movement results in a higher print defect rate.
Overall, use of fine powders is the most effective means to meet most challenges. It helps on achieving high
performance in printability, tack, and non-slump, with acceptable trade-off in rheology and tack time. Solder
balling may be the primary hurdle. The problem may be resolved by using inert reflow atmosphere or via flux
chemistry improvements. A metal load of 90.5 to 91% seems to be the optimum for most properties.
solder paste, Fine Pitch, print, slump, solder balling, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 9 Mar 2010
by Dr. Ning-Cheng Lee
Reflow soldering of solder paste is the primary interconnection method used in SMT assembly process. The major issues which plague the reflow soldering performance include, but not limited to, bottom-side-component-holding, bridging, dewetting, low-residue, opening, solder balling, solder beading, solder-fillet-lifting, tombstoning, defective balling for BGA, and voiding. The mechanisms, causes, and cures for each issue are briefly discussed in this article.
reflow, soldering, solder paste, SMT, bridging, dewetting, opening, solder balling, solder beading, solder-fillet-lifting, tombstoning, balling for BGA, voiding, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Jim Hisert , Sigurd R. Wathne PE
The best way to test a flux is to conduct the test in the production line under actual working conditions. This can be impractical if too many materials are included in the evaluation process. There are, however, ways to understand the capabilities of a wide range of flux materials without scrapping a large amount of production parts and time. This article will outline a test procedure that can be used to initially compare fluxes with minimal time, capital expense, and equipment. The key data is the quality of a flux to promote wetting of various alloys on a variety of surface finishes. [1] This will be calculated as a change in solder diameter after reflow. Although solder spread is the numerical outcome of the testing, cleanability of water-soluble fluxes and post reflow residue of no-clean fluxes may become apparent to the technician involved in this testing. It is a good way to get a feel for a material set in a very short time.
Solder Melting, Solder Basics, solder alloy, solder, pb-free, Flux Cleaning, flux, BGA, ball attach
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Sniket A. Bhave, Dr. Ronald C. Lasky , Dr. Daryl Santos, Aniket A. Bhave, Wang Ming
Numerous studies have shown that greater than 60% of end of line defects in SMT assembly can be traced to solder paste and the printing process. Reflowing adds another 15% or so. In light of this fact, it is surprising that no simplified procedure for solder paste evaluation has been documented. This paper is about such a procedure.
By using designed experiments and the measurement
of critical solder paste related process metrics, we were able to develop a solder paste evaluation procedure that maximizes information about the solder paste and its processability while minimizing experimentation. While using only 12 stencil printed PWBs, we were able to generate statistically significant results that enabled us to rank solder pastes according to their performance. Response metrics that were investigated were print volume and definition before and after pause, squeegee hang up, slump, tack, release from aperture, and solder joint quality.
In addition, we found such variation in solder paste volume repeatability that this criterion alone can be used as a screening procedure.
stencil printing, solder paste, solder paste evaluation, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 9 Mar 2010
by Wanda B. Hance, Paul A. Jaeger, Dr. Ning-Cheng Lee
Solder beading is a special phenomenon of solder balling when using solder paste in certain SMT applications. In brief, solder beads are large solder balls near components with very low stand-off (see scheme below). With more attention being drawn to no-clean paste applications due to CFC concerns, a better understanding of this event becomes indispensable. In this study, the data indicate solder beading was caused by flux outgassing which overrode the paste cohesive force during the preheat stage. The outgassing promoted the formation of isolated paste aggregates underneath the low clearance components. At reflow, the isolated paste melted and , once emerged from the underside of the components, coalesced into solder beads. Processingwise, this problem can be remedied by slowing down outgassing via a milder preheat profile, or by reducing print thickness. Materialwise, solder beading can be corrected by enhancing the paste cohesive force via cold welding of solder powders during the preheat stage. This in-turn can be accomplished through the use of lower activation temperature flux, coarser solder powder, higher metal load, and solder powders with lower oxide content. Other parameters which could affect the performance will also be discussed.
solder beading, beading, solder paste, flux, SMT, solder balling, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Benlih Huang, Dr. Ning-Cheng Lee
Several unique solder paste systems have been developed and tested for 63Sn/37Pb solder bumping for wafer, CSP, and BGA with the low cost print-detach-reflow process. The results indicate that the bump height achieved is very adequate and consistent for all three area array package systems. Microstructure of solder bumps appears normal. The yield is also very high for both before reflow and after reflow condition, and is dictated by printing performance. With the unique high slump resistance exhibited by those newly developed pastes, the paste transfer efficiency at printing stage becomes the most critical performance for this process. The transfer efficiency increases with increasing area ratio, increasing taper angle, decreasing pitch, decreasing stencil thickness, decreasing challenge, with adoption of square aperture design, and is not sensitive to aspect ratio of aperture to solder particle size. The paste systems appear to have more potential for depositing a larger amount of paste per unit pitch, as evidenced by the linear relation between expected paste volume and the deposited paste volume. Increasing metal content helps improving bumping performance. The bottleneck of increasing bumping performance for wafer applications appears to be developing a stencil manufacturing technology capable of providing an aperture pattern with spacing considerably smaller than the stencil thickness. Slow print speed is also essential for adequate printing. A non-shiny non-smooth stencil surface is considered beneficial for aiding paste rolling. The flux residue of those pastes is cleanable with solvents.
solder, soldering, area array package, Flip Chip, BGA, CSP, sphere, Bumping, paste, flux, fluxless, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Timothy Jensen
As the July 1, 2006 Pb-free deadline approaches, many electronics assemblers are beginning to fathom the changes and process demands required. The two biggest material concerns involve solder paste and components. This document provides practical recommendations for evaluating Pb-free solder pastes and ensuring that the selected solder paste will deliver assembly yields comparable to, or better than, the incumbent Sn/Pb solder paste.
solder paste, evaluation, response to pause, reflow, stencil printing, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Gregory Evans, Dr. Ning-Cheng Lee
This paper focuses on many of the problems facing process engineers today. The experiments used in this study were designed to find the true causes of the problems and headaches which plague SMT assembly today. Data indicate that wicking is caused by a relative hotter component and is aggravated by non-coplanarity. It can be reduced by slower heating rate or more bottom-side heating. Bridging is caused by slumping, and is aggravated by smaller pitch dimension and slower flux wetting speed. Tombstoning is a result of uneven heating. It can be reduced by optimizing pads spacing and by using fluxes with slower wetting speed, or by a smaller print thickness. Problems such as slumping, clogging, solder balls, and white residue are also discussed.
solder paste, SMT, slumping, clogging, wicking, bridging, tombstoning, solder balling, white residue, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee
Spheres are manufactured via sequential flow/quench or reflow processes, then followed by degreasing and classification. Surface contamination or mis-handling can aggravate sphere solderability. Sphere attachment onto BGA typically is achieved via vacuum-transfer or gravity-dispensing processes, and the spheres are held in place by flux or solder paste before reflow. Welding process also in use. Bumping can be achieved via confined solder paste during reflow. Bumping with Sn62/Sn63 spheres & paste yields excellent results. Bumping with Sn62/Sn63 spheres & flux desires high viscosity, high volatility, large pitch, low print thickness, low flux activity, & small pads. Bumping with Sn10 sphere & paste exhibits no missing, and the yield increases with decreasing print thickness, decreasing volatility, increasing sphere solderability, increasing flux activity, increasing pad size, increasing metal load, & increasing pad solderability. The yield is not affected by viscosity, pitch, and reflow profile. For bumping with paste alone approach, easily releasable paste is crucial for regular print-release-reflow process. Bumping with integrated preform is promising. Reducing the thickness & width of solder links is essential for better yield.
solder, sphere, ball, BGA, Bumping, attachment, flux, solder paste, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by William Casey, Dr. Ning-Cheng Lee
Soldering is the primary interconnection technology for area array packages. Methods for solder bumping for area array packages can be categorized as follows: (1) build-up process, (2) liquid solder transfer, (3) solid solder transfer, and (4) solder paste bumping. The first group includes both evaporation and electroplating processes, while the second group includes meniscus bumping and solder jetting. The third group includes wire bumping, sphere welding, decal solder transfer, tacky dot solder transfer, integrated preform, and pick and-place solder transfer processes, with the last one (pick & place solder transfer) being the current prevailing option. Solder paste bumping exhibits great potential to reduce bumping costs dramatically, and includes the print-detach-reflow, print- reflow-detach, and dispense approaches. For an area array package attachment process, depending on the type of packaging, either
flux, fluxless soldering or solder paste printing may be used as the attachment medium. Although area array packaging generally offers a robust process, attention should be paid to reduce defects such as delamination, misalignment, elongated joint, voiding, bridging, opens, cracking, poor wetting and various attachment interactions.
fluxless, flux, paste, Bumping, sphere, CSP, BGA, Flip Chip, area array package, soldering, solder, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 10 Mar 2010
by Andy C. Mackie PhD , Jim Hisert , Adrian Low
Although there are some unsubstantiated claims that the history of solder reaches back 7000 years (Ref. 1), it seems more likely that the first gold-tin solders were used in jewelry in the Egyptian Early Dynastic Era, around 5000 years ago (Ref. 2).
Why is solder still the overwhelming choice for interconnects when high-tech alternatives abound? The answer is simple: Solder is the only electrically conductive
joining material that is so compatible with the metal surface it is joining to that it intermingles on the atomic level.
ball attach, BGA, flux, Flux Cleaning, pb-free, solder, solder alloy, Solder Melting
[Permanent Link to this Paper ]
Posted on 15 Oct 2009
by Eric Bastow , Timothy Jensen
The advent of the EU’s RoHS law has encouraged a significant amount of research to find an alloy, for electronic assembly that will satisfy RoHS’s lead-free requirement and have optimum process ability and field reliability. The resulting research, much of it lead by iNEMI, resulted in the near eutectic tin-silver-copper alloy SAC387 (Sn95.5Ag3.8Cu0.7) as an initial favorite to fill this need in the early 2000s. By 2004 or so, many people were using SAC305, partially because of its greater resistance to tombstoning. It appeared that SAC305 would become the de-facto lead-free standard alloy for RoHS compliant electronic assembly. However, with the dramatic increase in silver prices in the last few years, SAC105, having 2% less silver was being evaluated and used for its obvious cost savings. Reliability testing of SAC105 also showed that although it did not perform as well as SAC305 in thermal fatigue cycle testing, it was better than SAC305 in drop shock tests. The explosive growth of mobile phone sales, over 1 billion per year, made SAC105’s superior drop shock performance attractive for these and other portable devices.
In addition to research relating to SAC305 and SAC105, much work has been performed on the study of the effects of small quantities (<0.1%) of alloying metals on lead-free alloys’ process ability and reliability performance. These "dopants" can dramatically affect an alloy’s performance.
All of the above work has resulted in what many are calling lead-free alloy proliferation as more and more alloys are being considered for implementation. This proliferation drives up solder paste cost as manufacturers cannot achieve economies of scale. In addition, with so many alloys to consider, it is difficult for researchers to develop extensive data bases of process and reliability performance.
This paper is an overview of this lead-free alloy proliferation and an outlook on how alloy convergence might occur.
solder, SAC, pb-free, dopants, Reliability, thermal cycling, drop testing
[Permanent Link to this Paper ]
Posted on 15 Oct 2009
by Ross B. Berntson, Dr. Ronald C. Lasky , Karl Pfluke
Surface mount assembly has dominated its through-hole predecessor since the early 1990s. The higher density and lower ultimate cost of SMT makes it a preferred assembly technology. However, the mechanical strength of through-hole connections continues to make through-hole the technology of choice in assembling connectors. This presentation will describe the primary methods currently used for through-hole connector assembly: 1) selective wave solder, 2) pin-in-paste (PIP)i reflow, 3) hand soldering and 4) solder preforms. We will show how solder preforms are an excellent alternative when PIP provides insufficient solder.
The wave solder method requires specialized equipment and processes to solder connectors. Pin-in- paste reflow evolved as a way to accomplish through-hole assembly without additional equipment or process steps. In the PIP method, the additional solder required to fill the though-hole barrel is deposited by overprinting the pad in the area of each connector pin, using standard SMT equipment. During reflow, the solder wicks to each pin forming the solder fillet.
This paper explains why pin-through-paste reflow methods based on overprinting solder paste have become more challenging due to an increasing use of Organic Solderability Preservative (OSP), fine- feature devices (e.g. fine pitch connectors) and densely populated PCB layout designs that conflict with requirements for successful use of step-stencils. This paper also shows an example where solder preforms were used to provide extra solder volume for each pin. This work demonstrates how solder preforms provide a viable manufacturing solution to ensure complete through-hole solder joints.
solder preforms, pin-in-paste, intrusive reflow, mixed technology, selective wave soldering, through-hole connectors, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Steve Dowds
As long as we are using components which are finished in Tin-Lead, which we certainly will over the next three to five years, Bismuth-containing solders cannot be used due to the long-term reliability problem associated with Lead-contamination. Despite fears to the contrary there are no problems with the long-term availability of Bismuth.
pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 10 Mar 2010
by Dr. Chingchen S. Chiu, Dr. Ning-Cheng Lee , Kimbela Randle, Christopher Parrish
Voiding in BGA at Sn63 solder bumping stage typically occurs at the interface of eutectic solder and the BGA pad, due to the tendency of forming minimal molten solder surface area at bubble surface . At low voiding level, Pb90 bump systems exhibit more voiding than eutectic Sn-Pb bump systems, primarily due to the sandwich effect which entraps fume bubbles for Pb90 systems. However, at high voiding level, Pb90 bump systems exhibits less voiding than eutectic Sn-Pb bump systems, due to the radius of curvature effect which compresses the bubble size of Pb90 bump systems. In general, the voiding in BGA at solder bumping stage increases with decreasing flux activity, decreasing flux or paste deposition thickness, increasing oxide level of spheres or pads, increasing pad dimension, increasing reflow profile length, and increasing metal content. The sphere oxide effect is more pronounced for Pb90 bump systems than for eutectic Sn-Pb bump systems, due to the immobilized oxide for the former systems as well as the sandwich effect . Voiding also increases with decreasing flux/paste viscosity, presumably due to a decrease in the flux capacity. No correlation can be identified between voiding and flux volatility. The mechanisms of voiding unveiled suggest that the preferential location of voids at interface is inevitable, and use of high melting point sphere for solder bumping helps confining the void size. Surface tension is the most crucial property dictating voiding. It influences the voiding phenomena via tendency of forming minimal liquid surface area at bubble surface and radius of curvature effect .
through-hole connectors, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Wanda B. O’Hara, Dr. Ning-Cheng Lee
Voiding in BGA assembly using Sn63 solder bumps is primarily introduced at board-level assembly stage. On the pretinned PCBs, voiding of BGA joints increases with increasing solvent volatility, increasing metal content, and increasing reflow temperature, and with decreasing powder size. This can be explained by a viscosity dictated flux-exclusion-rate model. In this model, a higher viscosity in fluxing medium at reflow temperature could hinder the exclusion of flux from the interior of molten solder, hence increase the chance of outgassing due to the increasing amount of entrapped flux, and consequently result in a higher voiding in BGA assembly. Flux activity and reflow atmosphere appear to have negligible effect on voiding when the solderability of the immobile metallization is not a concern. Increase in void content is accompanied by an increase in fraction of large voids. This suggests that, similar to voiding phenomena in SMT process, factors causing voiding in BGA will have an even greater impact on the joint reliability than what shown by the total-void-volume analysis results.
void, BGA, viscosity, volatility, flux-exclusion-rate, soldering, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee , Wanda B. Hance
The mechanisms for void formation are investigated for applications involving solder paste in SMT. Generally the voids are caused by the outgassing of entrapped flux in the sandwiched solder during reflow. The voiding is mainly dictated by the solderability of metallization, and increases with decreasing solderability of metallization, decreasing flux activity, increasing metal load of powder, and increasing coverage area under the lead of the joint. Decrease in the solder powder particle size shows only a slightly negative effect toward voiding. The data indicate that voiding is also a function of the timing between the coalescing of solder powder and the elimination of immobile metallization oxide. The sooner the paste coalescing occurs, the worse the voiding will be. Increase in voiding usually is accompanied by an increasing fraction of large voids, suggesting factors causing voiding will have an even greater impact on the joint reliability than what shown by the total-void-volume analysis results. Preliminary data show that certain predry treatment and flux solvent with higher boiling point appear to cause increased voiding.
solder, soldering, solder paste, flux, void, voiding, SMT, solder joint, reflow, solderability, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee , Dr. Hyoryoon Jo, Benjamin E. Nieman
Microvia technology is a critical element in high density
interconnect development. It allows realization of low cost, high density, high speed and miniaturization for electronic devices. However, accompanied with all of the advantages described above is the observation of a high occurrence rate of voiding in the solder joints. Presence of voids in the solder joints often affects the mechanical properties of joints and deteriorates the strength, ductility, creep and fatigue life, due to the growth in voids, which could coalesce to form ductile cracks and
consequently lead to failure. The deterioration could also be due to the enhanced magnitude of the stresses and strains of solder caused by voids. In addition, voids could also produce spot overheating, hence lessen the reliability of joints. Although voiding in typical solder joints has been studied extensively, very little work has been done on the emerging microvia applications which appear to be more prone to voiding problems. In this study, the effect of materials and processes on voiding in
microvia, such as printing process, solder particle size, metal content, solderability of pads, reflow profile, and flux chemistry are studied. Results of investigation indicate that voiding was found to decrease with increasing number of print, increasing flux activity, decreasing solder powder size, decreasing metal
content, decreasing peak temperature, and use of linear ramp profile instead of profile with a soaking zone. Voiding is affected by variation in flux chemistry. But the second pass for air reflow does not suffer deterioration in voiding. Among all, profile effects are relatively moderate, and double print, powder size, and flux activity effects are more pronounced. The voiding mechanisms for microvia applications are mostly similar to that using regular pads. Hole-filling capability is a new element contributing to voiding in microvia. Lead-free soldering does not introduce new voiding mechanism here.
lead-free, pb-free
[Permanent Link to this Paper ]
Posted on 10 Mar 2010
