Circuits Assembly Online Magazine - Successful Lead-Tinning

Reasons for the surge in use, and the basics of a well-designed process.

Lead tinning has experienced a surge in popularity recently for a number of reasons, a leading one being the effect of RoHS on SnPb lead-finished component supplies and high-reliability electronics manufacturers’ need for these hard-to-find components.

The need for lead tinning has been around for nearly 25 years. The original need arose when the military decided that plated finishes – which are not fused – were not suitable or acceptable for high-rel environments. The problem at the time was plated finishes were found not to be sufficiently robust to withstand oxidation encroachment to the base lead, and could result in solder joint failure in the field. That process gradually diminished in necessity over the years, but for that and other reasons has now returned.

There are primarily four reasons why hot solder dipping is back in vogue. The first is for legacy components: those that might have plated finishes on their leads and need to be dipped for the same original reason; i.e., applying a robust, fused solder finish. The second is it is a good way to “scrub” the gold off component leads. Gold is initially plated on to help them stand up to the rigors of the burn-in process. That gold must be removed because it can cause embrittlement in the solder joint if it remains. Removal is achieved by hot solder dipping to “wash” off the gold.

The third reason is mitigation of tin whiskers, something everyone seems rightfully concerned about, thanks to the conversion to high tin content Pb-free alloys. Even NASA has published papers that say, in effect, that the only reliable way to mitigate tin whiskers and prevent their growth is to dip the leads in molten alloy. This creates a fused intermetallic finish unlike the non-fused electroplated finishes, which are a lot like a coating of sand – not fused or connected and prone to tin whisker development under certain conditions.

The fourth reason, for the high-reliability environment, is to convert components to be compatible with SnPb assembly. Since RoHS, fewer components are in stock with SnPb lead finishes, and those that aren’t must be hot solder dipped in a SnPb bath to make them suitable for assembly in high-reliability products. Hot solder dipping, done properly, will wash off the tin so the leads can be properly re-plated with SnPb.

In automated lead tinning, this must be done with a two-pot system. One pot absorbs the contamination, and its purity must be monitored, and it must be exchanged for a fresh pot when the level of gold or unwanted material reaches a certain saturation point. Then a second, or virgin, pot must be used to re-plate the leads with fresh and uncontaminated material. One does not try all this with one pot! Of course, the reverse is also common – SnPb-plated components are scrubbed of lead and re-plated with tin for use in RoHS-compliant assemblies, using two tin pots in succession.

There are a few basic but critical requirements for a successful automated lead-tinning process. As mentioned, dual solder pots are a must: one for the cleansing or scrubbing, and a second virgin pot for re-plating with the correct alloy.

The second requirement is a flowing, not static, solder pot. Flowing solder, particularly in the scrubbing pot, removes contaminated or scavenged material such as gold from close proximity to the leads, so that the contamination will not be pulled back to and deposited onto the component lead when the part is pulled from the solder. It’s also important to have some sort of agitation in the first, or scrubbing, pot, as this actually helps remove gold or lead solder from the leads. This can easily be accomplished with mechanical manipulation of the device holding the component in the bath.

The use of nitrogen blanketing or inert atmosphere in the second, or finishing pot, is desirable because it promotes a lustrous finish, and mitigates the formation of dross, icicles and bridging.