|Ask the Experts|
December 14, 2011
Solder Hardness Comparison
While running a solder paste comparison through our SMT and ICT processes we found that one solder was harder than another. Pin impact into the solder on test points was less on one type than on the other. The two solder pastes use the same powder, 63-37, but different carriers and actives. One paste is designed to over come poor wetting, head in pillow effects that come with soldering Pb Free components in a SnPb SMT process. The other is a long standing SnPb solder paste. What could be causing the differences in solder joint hardness?
|Expert Panel Responses|
As they have been run through the same process one would assume that they have both seen the same operating conditions and so outside influences should have had little effect. Therefore we could assume it is the product that is slightly different. Although both products are 63/37 they may have slightly different, additional metals can be added to the alloy to impart a benefit, historically there were two specs for Sn63 or 63/37 the 'American' spec had an addition of antimony whereas the European spec did not. The question is would any of these additions make a noticeable change in the hardness of the reflowed solder. It could also be that the flux of one solder paste is significantly more active on certain metals than the other, this would promote better soldering and could reduce the risk of Head In Pillow (HIP), this could also mean that the paste is also then dissolving more of the PWB metal finish during soldering and this could in turn slightly change the characteristics of the two reflowed solders. A significant dissolution of copper or silver would make the final alloy slightly harder. You can always get a cross section of the finished joint carried out and from this a good metallurgist should be able to give you a more definitive answer as to whether there is a structural change in the two alloy surfaces which could explain the hardness change you are seeing.
Global Product Champion
There are a few possible causes for this difference in solder joint hardness. First, if the alloys are from different carriers, they will have different impurities, and that might affect hardness. Differences in wetting characteristics and flux residue could also change the appearance of the dimpled solder joint. For instance, if the flux residue is in a thick layer on the surface, it might hinder penetration of the probe, and impact how far the probe penetrates into the solder. Flux residues could also create the illusion that one is dimpled more than the other. To determine whether there is an actual difference, it might be valuable to do some die shear or tensile testing to see if there is an actual difference in mechanical properties.
Technical Education Program Manager
IPC - Association Connecting Electronics Industries
This is an interesting question and could create a good topic for a metallurgical evaluation. I would surmise the issue is due to the grain structure of the residual homogeneous solder alloy and the change in grain structure over time. I would recommend the paper titled "The Superior Drop Test performance of SAC-Ti Solders and Its Mechanism" as a start of trying to learn how all of this comes about. It includes information on the Intermetallic thickness, the grain structure and the operating temperature. There is also information on http://www.boulder.nist.gov/div853/lead_free/part1.html which discusses the "Database for Solder Properties with Emphasis on New Lead-free Solders" You can see the comparative information between the lead-free materials and the leaded solders. Hope this information is useful.
Vice President, Technical Director
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