Papers about solder
by Eric Bastow
Low melting-temperature alloys are vital to successful electronics assembly.
Solder is a critical material that physically holds electronic assemblies together while allowing the various components to expand and contract, to dissipate heat, and to transmit electrical signals. Without solder, it would be impossible to produce the countless electronic devices that define the 21st century.
Solder is available in numerous shapes and alloys. Each has its particular properties, providing a solder for nearly every application. Many times, solder is an afterthought in the design and engineering process. However, by considering the soldering step early in the design process, problems can be minimized. In fact, with the proper information, the characteristics of a solder can be part of an optimal design.
solder
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Posted on 1 Dec 2005
by Dr. Ning-Cheng Lee , Dr. Hong-Sik Hwang, Dr. Benlih Huang
Addition of Al into SAC alloys reduces the number of hard Ag3Sn and Cu6Sn5 IMC particles, and forms larger, softer non-stoichiometric AlAg and AlCu particles. This results in a significant reduction in yield strength, and also causes some moderate increase in creep rate. For high Ag SAC alloys, adding Al 0.1-0.6% to SAC alloys is most effective in softening, and brings the yield strength down to the level of SAC105 and SAC1505, while the creep rate is still maintained at SAC305 level. Addition of Ni results in formation of large (Ni,Cu)3Sn4 IMC particles and loss of Cu6Sn5 particles. This also causes softening of SAC alloys, although to a less extent than that of Al addition. Addition of Al also drives the microstructure to shift from near-ternary SnAgCu eutectic toward combination of eutectic SnAg and eutectic SnCu. Addition of Ni drives shifting toward eutectic SnAg. For SAC+Al+Ni alloys, the pasty range and liquidus temperature are about 4°C less than that of SAC105 or SAC1505 if the addition quantity is less than about 0.6%. Addition of Al and Ni also results in a slight decrease in modulus and elongation at break, although the tensile strength is not affected.
Ni, Al, creep resistant, compliant, soften, SAC, lead-free, solder
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Posted on 1 Jan 2009
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
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Posted on 1 Jan 2009
by Paul A. Jaeger, Dr. Ning-Cheng Lee
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.
lead-free, pb-free, nitrogen, flux, reflow, soldering, low-residue, no-clean, solder paste, solder
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Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee , Dr. Benlih Huang, William Manning, Dr. Yan Liu
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.
pb-free, soldering, BGA, CSP, void, voiding, SMT, solder, lead-free, microvia, profile, reflow
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Posted on 2 Mar 2010
by Paul Socha , James Slattery
Solder preforms are manufactured shapes of solder or braze metal designed to fit a specific joint configuration. Preforms contain precise and predetermined quantities of an alloy or a pure metal. They have been used in a variety of applications, such as hybrid and discrete component assembly and surface mount technology. Used in place of traditional solder forms such as wire and ingot, preforms offer advantages for different applications, including versatility enhanced production economies and flexibility.
preforms, solder
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Posted on 10 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
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Posted on 15 Mar 2009
by Dr. Benlih Huang, Dr. Ning-Cheng Lee
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.
pb-free, tombstoning, solder, soldering, solder paste, flux, lead-free, surface mount
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Posted on 4 Mar 2010
by Dr. Benlih Huang, 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
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Posted on 4 Mar 2010
by Dr. Arnab Dasgupta, Dr. Benlih Huang, Dr. Ning-Cheng Lee
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.
pb-free, reflow, surface mount, microvia, voiding, void, lead-free, soldering, solder
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Posted on 4 Mar 2010
by Dr. Ning-Cheng Lee , Dr. Mikolaj E. Jozefowicz
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.
lead-free, pb-free, no-clean, surface insulation resistance, EM, flux, soldering, solder, electromigration, SIR
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Posted on 1 Jan 2009
by Dr. Anu Maria, K. P. Rangan, Rajkumar B. Raj, Dr. Ning-Cheng Lee , Dr. Xiaohua Bao
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.
lead-free, pb-free, viscosity, flux, tack, slump, print, rheology, solder paste, soldering, solder
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Posted on 1 Jan 2009
by Chris Anglin
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Establishing a Precision Stencil Printing Process for Miniaturized Electronics Assembly
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The advent of miniaturized electronics for mobile phones and other portable devices has required the assembly of smaller and smaller components. Currently 01005 passives and 0.3mm CSPs are some of the components that must be assembled to enable these portable electronic devices. It is widely accepted that about 65% of all end of the line defects occur in the stencil printing process. Given all of the above it is critical that a precision stencil printing process be developed to support miniaturized electronic assembly.
This paper will be a summary of a significant amount of experimental data and process optimization techniques that were employed to establish precision SMT printing process. Our results indicate that the industry standard stencil aperture aspect ratio requirement of > 0.66 is an excellent rule of thumb. However, by optimizing printer setup with vacuum support, foil-less clamps, squeegee edge guards etc and assuring cleanliness and squeegee and stencil quality, we have been able to obtain acceptable stencil printing results with area ratios of 0.5 with Type III solder pastes. The work that was performed to achieve these results will be discussed in detail in the paper.
halogen-free, solder paste, solder, solder reliability, flux, solder quality, stencil printing
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Posted on 11 May 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
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Posted on 4 Mar 2010
by Dr. Ning-Cheng Lee , HongWen Zhang
Although lead-free soldering has been the main stream of industry since 2006, with the replacement of eutectic SnPb system by SnAgCu system, the development of drop-in lead-free alternatives for high melting high lead solder alloys is still far from mature. BiAg alloy exhibits acceptable bulk strength but very poor ductility and wetting, therefore it is not acceptable as an option. In current work, a mixed powder BiAgX solder paste system has been developed as a viable alternative, high temperature lead free solder. The metal powder in the paste is composed of a high melting first alloy powder as majority and the additive powder as minority. The additive contains a reactive element to react with various metallization surface finishes. The additive will melt and react on the parts before or together with the melting of the majority solder. The reactive element in the additive is designed to be converted completely into IMCs during the reflow process, hence resulting in a high melting solder joint. In the mixed powder paste system, a melting temperature above 260°C was verified by both DSC and TMA data. The mixed powder solders show a significantly improved wetting comparing to Bi11Ag. The voiding and TCT performance are comparable with high lead solders. The IMC layer thickness of the mixed powder system is insensitive toward thermal aging at 175°C, while the high lead ones do show a considerable increase.
BiAg, voiding, wetting, mixed alloy, solder joint, solder paste, solder, lead-free, high temperature
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Posted on 20 Oct 2011
by Mike Bixenman D.B.A., Steve Stach, Dr. Ning-Cheng Lee
Ionic Cleanliness testing machines are designed to determine the total ionic content extractable from the printed wiring board for purposes of process control. The conductivity of the extract solution is measured and the results are expressed as sodium chloride equivalence per unit area. The problem with this method is two fold: 1.) Many of today’s low residue flux and lead-free flux residues are not soluble in the extract solution. 2.) Contamination of concern is with site specific components, from which contamination does not correlate to the area of concern. The purpose of this study is to research low residue and lead-free flux structures, identify solvent compositions that will dissolve these residue types, and offer options for performing both bulk and site specific ionic cleanliness testing methods.
lead-free, Cleaning, flux residue, flux, soldering, solder, solder paste, SMT
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Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee
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Lead-Free Flux Technology and Influence on Cleaning
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Lead-free flux technology for electronic industry is mainly driven by high soldering temperature, high alloy surface tension, miniaturization, air soldering due to low cost consideration, and environmental concern. Accordingly, the flux features desired included high thermal stability, high resistance against burn-off, high oxidation resistance, high oxygen barrier capability, low surface tension, high fluxing capacity, slow wetting, low moisture pickup, high hot viscosity, and halogen-free. For each of the feature listed above, corresponding desired chemical structures can be deduced, and the impact of those structure on flux residue cleanability can be speculated. Overall, lead-free flux technology results in a greater difficulty in cleaning. Cleaner with a better matching solvency for the residue as well as a higher cleaning temperature or agitation are needed. Alkaline and polar cleaner are often needed to deal with the larger quantity of fluxing products. Reactive cleaner is also desired to address the side reaction products such as crosslinked residue.
lead-free, flux, flux residue, solder, soldering, cleaner, Cleaning, SMT
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Posted on 1 Jan 2009
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.
pb-free, alloy, soldering, sn, Pb, solder, lead-free, lead
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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
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Posted on 1 Jan 2009
by Dr. Benlih Huang, Dr. Ning-Cheng Lee
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.
lead-free, pb-free, tombstoning, solder, solder paste, SMT, solder alloy, pasty range, soldering, reflow
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Posted on 1 Jan 2009
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.
lead-free, pb-free, paste, flux, soldering, solder, no-clean
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Posted on 1 Jan 2009
by Dr. Chingchen S. Chiu, Dr. Ning-Cheng Lee
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® preforms 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® preforms 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.
lead-free, balling, bump, Bumping, BGA, solder sphere, solder paste, integrated preforms, flux, defect rate, pb-free, solder
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Posted on 1 Jan 2009
by Jeff D. Sigelko, K.N. Subramanian
Pending legislation and global marketing pressures driven by environmental concerns, along with the need for solders with higher temperature capability for severe service environments, have resulted in significant activities to find substitutes for lead-bearing solders for microelectronics.
solder, lead-free
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Posted on 9 Mar 2010
by Dr. Ning-Cheng Lee , Dr. Benlih Huang
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.
pb-free, tack time, shelf life, solder appearance, solder balling, wetting, flux, paste, reflow, soldering, solder, lead-free
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Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee , Dr. Arnab Dasgupta, Dr. Runsheng Mao, Dr. Yan Liu
Electronic industry has been driven toward lead-free by RoHS (Restriction of Hazardous Substances Directive) which is in force since 2006. Recently REACH (Registration, Evaluation and Authorization of Chemicals) further drives the industry toward halogen-free. As a result, solder pastes for PCB assembly are required or desired to be both lead-free and halogen-free. Lead-free solder alloys in general wet poorer than tin-lead due to the higher surface tension of the former alloys. In the mean time, halogen-free fluxes typically also wet poorer than the more powerful halogen-containing fluxes. Consequently, the lead-free and halogen-free solder paste products that emerged inevitably suffer from a considerably inferior soldering performance than that of conventional halogen-containing tin-lead solder pastes. The deficiencies include poor wetting, solder balling, voiding, graping, head-in-pillow, etc. This gap is particularly significant for fine-pitch applications where the impact of oxidation is more profound. Furthermore, the higher soldering temperature of the higher melting lead-free alloys also aggravates the challenge of in-circuit test for no- clean processes, mainly due to the difficulty for probe to penetrate through the toughened flux residue. Although use of inert reflow atmosphere may alleviate some of the problems, the higher cost of it is prohibitive for most of the manufacturing firms. In this work, a halogen-free lead-free no-clean solder paste system, Indium8.9HF series, has been developed. It exhibits superior oxidation tolerance, thus assures superior resistance against graping, head-in-pillow, solder balling, voiding, and poor wetting for miniaturized electronic applications. In spite of the immense challenge in material science, this system also shows outstanding probe testability, in addition to its very good printability, non- slump, SIR, and ECM performance. The superior performance of this Indium8.9HF system effectively sealed the gap caused by lead-free and halogen-free requirements.
lead-free, halogen-free, no-clean, solder, solder paste, miniaturization, graping, head-in-pillow, voiding, solder balling, probe testability, ICT, oxidation
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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
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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
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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 Amanda Hartnett , Dr. Ronald C. Lasky , Timothy Jensen
Chinese version of Soldering Challenges in a Halogen-Free PCB Assembly Process
halogen-free, halide-free, solder, soldering, graping, flux, head-in-pillow, hole-fill, CHINESE LANGUAGE
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Posted on 13 May 2011
by Timothy Jensen , Dr. Ronald C. Lasky , Amanda Hartnett
Flame retardants have played an important role in the safety of many products. It is safe to say that thousands of lives have been saved by flame retardants. Flame retardants are used in products as varied as children's pajamas to electronics assemblies. Some of the more successful flame retardants are halogenated compounds. Halogenated materials are found in polyvinyl chloride (PVC), brominated flame retardants (BFRs), chlorinated flame retardants (CFRs), as well as in fluxes used in the electronics assembly industry. Product does not contain any halogenated compounds. However, that is not exactly how the term is used for soldering fluxes. A flux that is classified as
halide-free by the IPC/J-STD-004 is actually only free of ionic halides.
hole-fill, head-in-pillow, flux, graping, soldering, solder, halide-free, halogen-free
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Posted on 10 Mar 2010
by Karl Pfluke
Increasingly, electronics manufacturing services (EMS) providers specializing in SMT are seeking to diversify and fill capacity. Photovoltaic (PV) solar cell module assembly is becoming a popular choice to meet those goals. PV cell stringing in solar module assembly is achieved using many common SMT materials and processes. Solders, fluxes, and common reflow technologies produce electrical interconnects in both a-Si and c-Siphotovoltaic technology.
CIG, flux, solder, photovoltaic solar cell assembly, solar module assembly, SMT
[Permanent Link to this Paper ]
Posted on 6 Jun 2011
by Dr. Ning-Cheng Lee , William Casey
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.
lead-free, pb-free, solder, soldering, area array package, Flip Chip, BGA, CSP, sphere, Bumping, paste, flux, fluxless
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Posted on 10 Mar 2010
by Adrian Low, Jim Hisert , Andy C. Mackie PhD
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.
Solder Melting, solder alloy, solder, pb-free, Flux Cleaning, flux, BGA, ball attach
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Posted on 15 Oct 2009
by Dr. Ning-Cheng Lee , Dr. Yan Liu, Pamela Fiacco
Head-in-pillow (HIP) is ailing the electronic industry when assembling BGAs or CSPs onto PCBs. It is caused by warpage of components or boards at reflow process, and is aggravated by oxidation. Methods for assessing the potential for occurrence of HIP are highly desired by the industry. Besides using BGA rework station followed by tedious dye and pry treatment, two other simpler methods are introduced in this work, Tiny Dot Paste method and Ball Onto Paste method. The Tiny Dot Paste method is stressed on the assessment of oxidation barrier capability of solder paste, while Ball Onto Paste method assesses combined capability of oxidation resistance and excessive fluxing capacity. Both methods are quick, easy, and close simulation, with the latter being better in real process simulation. Prevention of HIP can be accomplished by (1) designing packages without warpage, (2) printing more paste, (3) dipping solder paste or flux, (4) using inert reflow atmosphere, (5) reducing reflow temperature, (6) placing heat shield on BGA or CSP, (7) avoiding using water soluble solder paste for BGA bumped with no-clean process, (8) using solder bumps or solder powder with oxidation resistant alloy, (9) using fluxes with high oxidation barrier capability and high fluxing capacity. Among all options listed above, using solder paste with high oxidation barrier capability and high fluxing capacity is considered the most easily implemented approaches.
head-in-pillow, solder, soldering, reflow, SMT, solder paste, BGA, CSP
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Posted on 24 Jan 2011
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 Paul Bachorik, Dr. Ning-Cheng Lee , Dr. Weiping Liu
SAC-Ti alloys exhibited significantly improved drop test performance over not only SAC alloys, but also 63Sn37Pb for ENIG/OSP, NiAu/OSP, and OSP/OSP surface finish systems. The superior performance is attributed to (1)the increased grain size and dendrite size, therefore reduced hardness of solder, (2) inclusion of Ti in the IMC layer, and (3) reduced IMC layer thickness. DSC data indicate that the melting temperature and range were not affected by Ti, but the undercooling was almost completely suppressed. The creep properties of SAC-Ti alloy were comparable with those of SAC alloy, strongly suggesting the gain in drop test performance was not achieved by compromising the thermal fatigue performance. SAC-Mn alloys were also found to outperform SAC alloys and Sn63 for the X/OSP finish combinations studied. In general, SAC-Ti performed equally to or better than SAC-Mn alloys.
fragility, drop test, SAC, tin-silver-copper, lead-free, solder
[Permanent Link to this Paper ]
Posted on 10 Mar 2010
by Derrick Herron, Dr. Yan Liu, Dr. Ning-Cheng Lee
Quad-flat no-leads (QFN) package designs are receiving more and more attention in the electronics industry. This package offers a number of benefits including (1) small size, such as a near die size footprint, thin profile, and light weight; (2) easy PCB trace routing due to the use of perimeter I/O pads; (3) reduced lead inductance; and (4) good thermal and electrical performance due to the adoption of exposed copper die-pad technology. These features make the QFN an ideal choice for many new applications where size, weight, electrical, and thermal properties are important. However, the adoption of QFN often runs into voiding issue at SMT assembly. Upon reflow, outgassing of solder paste flux at the large thermal pad has difficulty escaping and inevitably results in voiding. It is well known that the presence of voids will affect the mechanical properties of joints and deteriorate the strength, ductility, creep, and fatigue life. In addition, voids could also produce spot overheating, lessening the reliability of the joints. This is particularly a concern for QFN where the primary function of thermal pads is for heat dissipation. Thermal pad voiding control at QFN assembly is a major challenge due to the large coverage area, large number of thermal via, and low standoff. Both design and process were studied for minimizing and controlling the voiding. Eliminating the thermal via by plugging is most effective in reducing the voiding. For unplugged via situations, a full thermal pad is desired for a low number of via. For a large number of via, a divided thermal pad is preferred due to better venting capability. Placement of a thermal via at the perimeter prevents voiding caused by the via. A wider venting channel has a negligible effect on voiding and reduces joint continuity. For a divided thermal pad, the SMD system is more favorable than the NSMD system, with the latter suffering more voiding due to a thinner solder joint and possibly board outgassing. Performance of a divided thermal pad is dictated by venting accessibility, not by the shape. Voiding reduction increases with increasing venting accessibility, although the introduction of a channel area compromises the continuity of the solder joint. Reduced solder paste volume causes more voiding. Short profiles and long hot profiles are most promising in reducing the voiding. Voiding behavior of a QFN is similar to typical SMT voiding and increases with pad oxidation and further reflow.
voiding, thermal pad, solder, solder paste, SMT, flux, Apex 2011, QFN assembly
[Permanent Link to this Paper ]
Posted on 11 Apr 2011
by Dr. Ning-Cheng Lee , Dr. Yan Liu, Derrick Herron
Quad Flat No Leads (QFN) package designs receive more and more attention in electronic industry nowadays. This package offers a number of benefits including (1) small size, such as a near die-sized footprint, thin profile, and light weight; (2) easy PCB trace routing due to the use of perimeter I/O pads; (3) reduced lead inductance; (4) easy PCB trace routing; and (5) good thermal and electrical performance due to the adoption of exposed copper die-pad technology. These features make the QFN an ideal choice for many new applications where size, weight, electrical, and thermal properties are important. However, adoption of QFN often runs into voiding issues at SMT assembly. Upon reflow, outgassing of solder paste flux at the large thermal pad has difficulty escaping and inevitably results in voiding. It is well known that the presence of voids will affect the mechanical properties of joints and deteriorate the strength, ductility, creep, and fatigue life. In addition, voids could also produce spot overheating, lessening the reliability of the joints. This is particularly a concern for QFN where the primary function of thermal pads is for heat dissipation. Thermal pad voiding control at QFN assembly is a major challenge due to the large coverage area, large number of via, and low standoff. Both design and process were studied for minimizing and controlling the voiding. Eliminating the via by plugging is most effective in reducing the voiding. For an open via situation, a full thermal pad is desired for a low number of via. For a large number of via, a divided thermal pad is preferred due to better venting capability. Placement of a via at the perimeter prevents voiding caused by via. A wider venting channel has a negligible effect on voiding and reduces joint continuity. For divided thermal pada, the SMD system is more favorable than the NSMD system, with the latter suffering more voiding due to a thinner solder joint and possibly board outgassing. Performance of a divided thermal pad is dictated by venting accessibility, not by the shape. Voiding reduction increases with increasing venting accessibility, although introduction of a channel area compromises the continuity of solder joint. Reduced solder paste volume causes more voiding. Short profiles and long hot profiles are most promising in reducing the voiding. Voiding behavior of a QFN is similar to typical SMT voiding and increases with pad oxidation and further reflow.
solder paste, reflow, SMT, solder, void
[Permanent Link to this Paper ]
Posted on 21 Feb 2011
by Wanda B. Hance, Dr. Ning-Cheng Lee
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.
lead-free, pb-free, solderability, reflow, solder joint, SMT, voiding, void, flux, solder paste, soldering, solder
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
