Papers about wetting
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 Rick Lathrop, Karl Pfluke
For crystalline silicon solar cell front contact metallizations, silver thick film formulations are ubiquitous. For backside contact pads, either silver or silver/ aluminum formulations are common. The trend for back contact metallizations is towards low lay down formulations, resulting in thin fired films. Although there are many different reflow methods used to “string” cells together, the need for fast wetting, leach resistant and well adhered front and rear contact metallizations are common to all methods. In order to accurately predict a material's compatibility with the module assembly process, quantitative tests needed to be developed due to an absence of industry standard tests. Classic thick film solder pot leaching and wire peel adhesion tests do not emulate the solar module assembly process well. Although more similar in process, SMT solderability tests also lack close correlation. To fill this gap, several solderability tests specifically designed for module assembly and cell metallizations have been developed and will be discussed in detail. These tests are, in fact, a hybrid of SMT and thick film tests but tailored, for the solar module assembly process. Wetting assessment is accomplished by measuring the reflowed area and height of a precise volume of solder using a confocal measuring system. For ribbon adhesion, manual and automated methods are compared, as well as various peel angles. From these studies, a ribbon attach method and adhesion test emerges suitable for benchmarking contact metallization formulations. Recommendations on how to recognize and prevent silver leaching are also discussed.
tabbing ribbon, metallization, silver leaching, adhesion, wetting
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Posted on 20 Oct 2011
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
