Papers about flux
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
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Sunil Chhabra, Sergio Porcari, Steven Rocco Marongelli, Dr. Richard Ludwig, Dr. Ning-Cheng Lee
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.
lead-free, pb-free, piston, Archimedes, fine dot, pump, SMT, surface mount, flux, solder paste, dispensability, dispensing
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ronald C. Lasky , Paul Socha , Carol Gowans
Although many have predicted the demise of through-hole components, they
are alive and well with tens of billions assembled each year. In many cases
these components are assembled by wave soldering. However, in many mixed
product technology (i.e. SMT and through-hole on the same board) products,
it makes sense to consider assembling the through-hole components with the
pin-in-paste (PIP) process. PIP has been successfully used for several decades
now; however in many cases it is not possible to print enough solder paste to
obtain an acceptable solder joint. In addition to this “solder starved” condition,
the large quantity of solder paste used to form the though-hole joint results in
excess residual flux. This residual flux can lead to difficulties in in-circuit testing
and potential surface insulation resistance concerns.
In light of the above need, solder preforms have been developed. These slugs of
solder typically come in the same sizes as 0402, 0603, and 0805 passive components.
The solder preforms are placed by the component placement machines
onto the solder deposit. This additional solder assures that an adequate solder
joint is formed with a minimum of solder paste and its residual flux.
Although PIP was an early application of solder preforms, more recently other
“solder starved” applications have emerged such as radio frequency (RF)
shields and connectors. In addition, the use of ultra thin stencils in the assembly
of miniaturized components can result in some other components being solder
starved and, hence are candidates for solder preforms.
This paper will cover the design and assembly techniques for using of solder
preforms in the “solder fortification™” needs described above. Several successful
applications will be presented. In some of these applications, defects were
reduced by 95% after implementing solder preforms.
Apex 2011, solder starvation, flux, PIP, pin-in-paste, through-hole, SMT, solder fortification, solder paste, solder preforms
[Permanent Link to this Paper ]
Posted on 11 Apr 2011
by Dr. Ronald C. Lasky , Carol Gowans
As early as the 1990s, people were predicting the end of through-hole components, but they are alive and well with the numbers of dual in-line packages (DIPs) and connectors still measured in the tens of billions per year. Many of these components are assembled by wave soldering; however, in mixed technology (SMT and through-hole on the same board) where the through-hole count is low, it is often advantageous to consider selective soldering or the pin-in-paste process (PIP). PIP is a process in which solder paste is printed over or near the PWB through-holes. The through-hole components are then placed and the solder joint is formed during the reflow process. PIP has the advantage of eliminating the wave soldering process step. In many cases it is difficult to print enough solder paste to make an acceptable through-hole solder joint. Solder preforms were developed to meet this need.
These solder preforms are typically shaped in the form of 0402, 0603, or 0805 passive components. The preforms are placed on the appropriate printed solder paste deposit by a component placement machine. Preforms come in tape & reel packaging.
Today solder preforms are also used in other “solder starved” applications such as radio frequency (RF) shields, connectors, and under QFN thermal pads. In all cases, the extra solder delivered by the preform is vital to the reliability of the assembled product.
In this paper, process, design, and assembly methods for solder fortification using preforms will be discussed. Four successful solder fortification examples will be presented along with the associated defect reductions.
solder preforms, pin-in-paste, solder fortification, solder starvation, mobile phone shields, QFN packages, flux
[Permanent Link to this Paper ]
Posted on 14 Oct 2011
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 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
[Permanent Link to this Paper ]
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
[Permanent Link to this Paper ]
Posted on 4 Mar 2010
by Nicole Palma, Dr. Ronald C. Lasky
With the advent of RoHS and WEEE and the concern of some companies to eliminate halogen-containing compounds from their products, it is vital to have an understanding of halogen compounds and how to detect them. Halogens are a series of nonmetal elements from Group 17 in the periodic table. These elements are fluorine, chlorine, bromine, iodine, and astatine. A halide ion is a halogen atom bearing a negative charge. Halides can be part of the flux activator system that aid in oxide removal in either a solder paste or flux for wave soldering.
Halide content can be determined by qualitative or quantitative tests. The silver chromate method is a quick and inexpensive qualitative test method used to determine halides in a flux. This test is performed by placing the flux on silver chromate test paper. The halides in the flux react with the silver chromate and produce a characteristic color change on the test paper. A quantitative measure of halides is done by ion chromatography. This quantitative test is quite expensive and time consuming.
Test methods have also been developed to determine the activity of the fluxes in solder paste and wave solder. Most commonly used are the copper mirror and surface insulation resistance (SIR) tests. Copper mirror testing determines the activity of the flux by the effect the flux has on bright copper mirror films, which have been vacuum deposited on clear glass. Based on J-STD 004B, the flux can be classified based on its activity levels as determined by this test.
SIR is an electrical test that measures a change over time in the electrical current between electrodes on the surface of a PCB. It is performed at high temperature and humidity levels, typically 85°C and 85% RH. Ionic residue, left on the PCB after reflow from flux activators, may cause low (i.e., poor) SIR readings.
This paper will discuss the theories behind these test techniques, their differences, and how the presence of halides in the flux activators will affect the SIR and copper mirror results.
Apex 2012, silver chromate, flux, copper mirror, SIR, halides, halogens, ion chromatography
[Permanent Link to this Paper ]
Posted on 14 Oct 2011
by Mario Scalzo , Todd O'Neil
Although many predicted the demise of through-hole components, they are alive and well with tens of billions used each year. In mixed SMT/through-hole PCBs, through-hole components, and especially connectors, are often used for their mechanical robustness. A typical example would be a USB connector in a laptop PC. Typically an SMT connection just doesn't have the mechanical robustness needed to support multiple connector plug-in and removals. However, performing a full wave soldering process to assemble a few through-hole components on a mostly SMT PCB doesn't usually make economic sense and may damage the PCB. In such situations, the best option is often to assemble the through-hole components and connectors with a selective soldering process.
This paper touches on identifying favorable flux properties, down-selecting low solids fluxes for lead-free selective wave soldering, the selective soldering process itself, and testing criteria. Topics reviewed will be the flux selection, optimizing the selective soldering process by varying the flux concentration, pre-heat parameters, soldering temperatures, and dwell time. The paper will finish with a summary of the work and a systematic process to select a flux and optimize the selective soldering process for high yields and quality.
flux, pb-free, lead-free, selective soldering, SMT, through-hole, PCB assembly, Apex 2011
[Permanent Link to this Paper ]
Posted on 11 Apr 2011
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
[Permanent Link to this Paper ]
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
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Chris Anglin
White Paper Video
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
[Permanent Link to this Paper ]
Posted on 11 May 2009
by Dr. Ning-Cheng Lee
In general,
new lead-free solder alloys with the following characteristics are desired in order to enable the continuation of miniaturization trend: (1) alloy with a reduced melting temperature, (2) alloy with a better solder spread, (3) alloy with a slower wetting speed at melting temperature, (4) a softer alloy, or alloy with a reduced voiding tendency or greater ductility, (5) alloy with a refined grain size, (6) alloy with low tendency to form large IMC plate, (7) alloy with a higher resistance toward corrosion and electrochemical migration, (8) alloy with a greater oxidation resistance. On the other hand, no-clean fluxes with the following features are needed: (1) reduced volatile, (2) halide-free, (3) greater fluxing
capacity, (4) higher residue resistivity, (5) more resistant to oxidation and charring, (6) more efficient oxidation barrier, (7) lower activation temperature, (8) slower wetting speed when solder begins to melt, (9) less spattering, (10) higher probe penetratability, (11) capability of inducing nucleation of solder upon cooling, and (12) greater resistance against slump.
SAC, solder joint, soldering, flux, solder alloy, lead-free
[Permanent Link to this Paper ]
Posted on 4 Mar 2010
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
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee
White Paper Video
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
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Paul Flynn, Jeff Schake, Jim Hisert
The wafer-level microsphere process is an accessible system of bumping a wafer with solder, which focuses on achieving high output at a low cost. This process begins with a wafer that has undergone front and back end-of-line procedures and is ready to accept solder as means of later interconnection. Flux is printed on the wafer UBMs (under bump metallizations) in a standard wafer paste type printing operation. This operation employs either a mesh screen or stencil to align flux deposits directly over the UBM. It is very common to use solder to act as an interconnect, while the UBM provides an attachment point for the solder, as well as a barrier to unwanted diffusion. The UBM also controls intermetallic formation. One common under bump metallization stack is titanium/nickel/gold. Each material has a purpose. In this example, titanium is used as an adhesion layer, nickel limits diffusion, and gold passivates the nickel to limit oxidation. After the flux is deposited, spheres of the correct size (typically 60 – 300um) are placed into the flux deposits and sent through reflow. The temperature for wafer reflow depends on the alloy, which is selected for the application. Tin/silver/copper alloys are very popular, although many people still use tin/lead and other low melting point alloys. The main consideration for choosing a certain alloy is often driven by processing restrictions during packaging or assembly. Sometimes a particular alloy is needed to endure life cycle testing or in-use conditions. Other lower temperature alloys are, at times, needed to allow the joining of die, which can not endure standard processing temperatures. The resulting solder formations should be spherical, with minimal height variations and maximum metallurgical attachment to the UBM. The flux can then be cleaned from the wafer surface if desired.
flux, spin coating
[Permanent Link to this Paper ]
Posted on 15 Oct 2009
by Seth Homer , Dr. Ronald C. Lasky
According to Prismark Partners, the use of quad-flat no-leads (QFNs) is growing faster than any package type except for flip-chip CSPs. Prismark projects that by 2013, 32.6 billion QFNs will be assembled worldwide, which represents 15% of all IC packages.
However, QFNs can be a challenge to assemble, especially when it comes to voiding. In most QFN assembly processes, solder paste is used as a means of attachment. This approach can be problematic, as excessive voiding often occurs due to the lack of standoff on the component and the high flux content of the paste. The addition of a solder preform can reduce such voiding by increasing the solder volume of the joint without adding flux volume.
Adding preforms to an assembly process is very easy. Preforms are packaged in tape & reel for easy placement by standard pick and place machines, right next to your components. The focus of this paper will quantify the preform requirements and process adjustments needed to use preforms in a standard SMT process. In addition, experimental data showing void reduction using preforms will also be presented.
Apex 2012, solder preforms, flux, QFN packages
[Permanent Link to this Paper ]
Posted on 14 Oct 2011
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
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
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.
lead-free, pb-free, SMT, vapor phase, infrared, convection, soldering, solder paste, flux, defect, profile, reflow
[Permanent Link to this Paper ]
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
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Karl Pfluke
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Photovoltaic Module Assembly Using SMT Assembly Materials and Processes
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EMS providers specializing in SMT are seeking to diversify and fill capacity. Photovoltaic module assembly is a popular choice. PV cell stringing in solar module assembly is achieved using many common SMT materials and processes. Solders, fluxes, and reflow technologies produce electrical interconnects in a-Si and c-Si photovoltaic assembly technology.
flux, bus ribbon, solar, tabbing ribbon, CIG, Copper Indium Gallium, photovoltaic
[Permanent Link to this Paper ]
Posted on 1 Jul 2009
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
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Yan Liu, Pamela Fiacco, Derrick Herron, Dr. Ning-Cheng Lee
Consideration and selection of dip transfer fluxes and solder pastes for PoP assembly are described, based on process considerations. The crucial properties vital for successful dip transfer include homogeneity, open time on the flux/paste bed, volume and consistency of the dip transferred material, open time after the dip transfer before reflow, and solder joint formation. For each property, one or more practical, recommended test methods are described. Overall, this work should provide the assembly house with an easy way to select a flux or solder paste adequate for dip transfer of PoP assembly applications.
PoP, package-on-package, flux, solder paste, dip transfer, soldering, SMT
[Permanent Link to this Paper ]
Posted on 24 Jan 2011
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 Dr. Ning-Cheng Lee , Paul A. Jaeger, Wanda B. Hance
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.
lead-free, pb-free, solder balling, SMT, flux, solder paste, beading, solder beading
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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 Maria Durham , Chris Nash
The electronics industry trend toward miniaturization of electronic assemblies has created the necessity for smaller pitch and bumps on chip-scale-packages (CSP) and package-on-package (PoP) components. Smaller pitch and bumped packages have led to the need for novel solder paste material characteristics. The flux vehicle rheology, powder size, and metal load all play a crucial role in the solder material’s dipping and reflow performance. It is equally important to understand the role of the actual dipping and reflow processes with regard to performance of the material.
electronics assembly, solder paste, chip-scale packages, package-on-package, dipping and reflow process, flux
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Posted on 22 May 2012
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
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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
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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 Jim Hisert
The interconnection of solar cells is a technology that has been around for hundreds of years, but is a relatively new application of the soldering process. By combining the metallurgical knowledge of solder joints (which has been developed through other applications) and new materials designed specifically for solar manufacturing, solar cells and cell strings can be effectively connected with high throughput, conductivity, and reliability.
Stringing of solar cells is used across the solar industry, and is a process that newcomers to the solar industry should be familiar with. However, it is a process that even experts still need to optimize. The top layer of a solar cell is a transparent conductive oxide (TCO) to which solder will not adhere. Therefore, a metallization paste is used to bond to the TCO and provide a solderable surface for strips of solder-coated copper called tabbing or stringing ribbon. These ribbons are commonly applied as parallel strips that weave from the top of one cell to the bottom of the next to connect the positive and negative sides of the cells in series. Once connected, the tabbing ribbon channels electrical current to larger solder-coated copper strips, known as bus ribbon. Bus ribbon serves as an input/output for the entire solar array to the module junction box.
bus ribbon, flux, solar, tabbing ribbon
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Posted on 15 Oct 2009
by Chris Nash
In this article, I will present a basic overview of soldering for those who are new to the world of soldering and for those who could use a refresher. I will discuss the definition of soldering, the basics of metallurgy, how to choose the proper alloy, the purpose of a flux, soldering temperatures, and typical heating sources for soldering operations.
soldering, metallurgy, flux, Solder Basics
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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 Eric Bastow , Chris Nash
Many electronics manufacturers perform SIR testing to evaluate solder materials and sometimes the results they obtain differ significantly from those stated by the solder material provider. The difference in the results is typically the result of SIR coupon preparation. This paper will discuss the issue of SIR coupon preparation, board cleaning techniques, and how board cleanliness directly affects SIR results.
solder paste, flux, SIR, surface insulation resistance, solder materials
[Permanent Link to this Paper ]
Posted on 20 Jun 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
