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36 SMT007 MAGAZINE I MAY 2020 lished reports of tin whiskers dates back to the 1940s. Since removing lead promotes tin whis- kers, it was necessary to add other metals to help mitigate whisker growth, and the most popular solder on the market is SAC305 (tin/ silver/copper). There are other variations of content, but pure tin solder is rarely used at this point. A lot of research is being done to formulate a better solder regarding solder joint strength and tin whisker mitigation as part of ongoing advances in soldering. Other paste formula- tions are adding bismuth or zinc for lowering melting points, but those additions come with the trade-off of more risk for oxidation—partic- ularly with zinc—so you really have to take a good look at your product's intended purpose and end-use environment to determine which formulation is best for you. Solder itself is only one part of the overall advances in soldering; other major factors are the dispensing and inspection equipment. One of the biggest advancements we have seen around here is jet-printable solder paste. This is certainly not a new technology, but as of today, we have not seen widespread adoption of jet printing, so we would consider it fairly new. There are already a few companies using this technology, and from a reliability standpoint, we see the biggest benefit in the way it reduces bridging and/or skips from dirty stencils and excessive solder applications, which reduces the amount of flux applied. When less flux is applied, you are always better off, provid- ing you see acceptable solder joints, of course. That is what I want to remind everyone here with this column. You can use the latest and greatest solder and application methods, but that will never preclude them from needing to be tested for quality. As always, unless you have contracts in place that state otherwise, the best place to start will usually be IPC for guidance on how to best determine which test is applicable to your product. The basic visual inspection and accep- tance criteria are found in the IPC-A-610 stan- dard that covers all types of solder joints. The criteria found in the 610 covers parameters like end joint width, solder thickness, fillet height, etc. What it does not cover is what the IMC should look like, as that would be impossible to determine in a non-destructive manner. Cross-section is the best tool available for determining solder joint quality. The process is covered in IPC TM-650 2.1.1. I say this is the best tool because it is really the only way to see if your assembly process is forming a solder joint that is acceptable. The old leaded or even lead-free thermal profiles may or may not be sufficient with new metallurgy regard- ing IMC and hole fill for PTH. Cross-section with SEM inspection is also a crucial step in qualifying any soldering process because you need to get in there and see how well the sol- der is wetting to the pad and look for cracks and voids and any number of other condi- tions that weaken the joint. This is on top of the vibration testing and thermal cycle/ shock test to determine the fatigue effect on the joint. No matter what your material choices are, or the reasons you have for making those choices, you still need to do the work to verify that those choices will yield a reliable solder joint. With- out that proof, you have nothing but unan- swered questions about reliability. SMT007 Eric Camden is a lead investigator at Foresite Inc. To read past columns or contact Camden, click here. Solder itself is only one part of the overall advances in soldering; other major factors are the dispensing and inspection equipment.

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