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.
To read this entire column, which appeared in the May 2020 issue of SMT007 Magazine, click here.