Estimated reading time: 4 minutes

Quest for Reliability: New Solder, Same Old Testing

Solder is inarguably one of the required building blocks for electronic assemblies and, apart from a few exotics, every assembly in the world has it. Solder, in general, has been around for over 5,000 years, by some accounts, and used for weapons, jewelry, and stained glass, among other items. Fast forward about 4,900 years (give or take), and now it’s in pretty much everything with a battery or a plug.

For the first 100 years, tin-lead solder dominated the electronics assembly process with proven results. In 2006, everything changed for the vast majority of contract manufacturers with the RoHS directive that effectively removed lead from the soldering process for all products to be built or imported into the EU. There are plenty of opinions on both sides of this issue to this day on whether this was a good idea, or even necessary. When it comes to meeting the requirement, those opinions and historical reliability data are not taken into consideration.

Now, tin-lead is used for very few assemblies, and those are almost always high-reliability applications like medical and aerospace. Directive 2002/95/EC ushered in the era of lead-free solder, as well as questions about any impact removing lead may have on solder joint quality. Removing lead increased the amount of thermal energy required to melt the solder and create a good IMC. Removing lead also increased the risk of tin whisker formation.

I talk a lot about dendrite growth, but although whiskers have the same failure mechanism as a dendrite, they don’t require moisture or conductive residues. According to the NASA Goddard website, the first published reports of tin whiskers dates back to the 1940s. Since removing lead promotes tin whiskers, 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 formulations are adding bismuth or zinc for lowering melting points, but those additions come with the trade-off of more risk for oxidation—particularly 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, providing 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 acceptance criteria are found in the IPC-A-610 standard 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 regarding 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 solder is wetting to the pad and look for cracks and voids and any number of other conditions 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. Without that proof, you have nothing but unanswered questions about reliability.

This column originally appeared in the May 2020 issue of SMT007 Magazine.