February, 2013
www.us-tech.com
Page 73
Hot Solder Dip Component Lead Refurbishing
By Alan Cable, President, ACE Production Technologies, Inc., Spokane Valley, WA
their components to lead-free solder compatibility, leaving those with exemptions, e.g., Hi-rel and military electronics suppliers, with no source for components for tin-lead soldered assemblies.
I The only reliable way
to mitigate tin whiskers and prevent their growth is to dip the leads in molten alloy.
To remedy this, companies have
taken to hot solder dipping, a process, whereby aged components are refurbished in preparation for re- qualification to “High Reliability” standards. The purpose is to remove the pure tin finish from the compo- nent terminations, and refinish with tin-lead solder. All of the pure tin must be removed to avoid tin whisker growth. To do this, the com- ponent’s terminations must be dipped in hot tin-lead solder right up to the body of the component. Published research has shown that the only reliable way to mitigate
n the years since RoHS took effect, component manufacturers have converted virtually all of
tin whiskers and prevent their growth is to dip the leads in molten alloy. This creates a “fused” inter-
These components may have been inventoried for years under uncer- tain conditions, making the solder-
The whole operation, i.e., fluxing, dipping, and cleaning, must be pre- cisely controlled not only to ensure repeatable results, but also to pre- vent thermally-induced internal damage to the component. If the external terminations become too hot for too long, heat transferred into the body can cause lead fingers to delam- inate from the mold compound, set- ting the stage for a future electrical failure.
The heat may also cause a crack Power semiconductor is typical of devices that need re-tinning.
metallic finish that is 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. In addition to removing tin, we
have the need to ensure that older or “legacy” components are solderable.
ability of the terminations question- able. The molten solder dipping process will remove the old finish while applying a fresh “fused” SnPb finish.
Thermal Stress Dipping means that the body of
the part may be thermally stressed.
in the mold compound itself. The duration of the hot solder dip needs to be precise, and the immersion depth of the terminations also needs to be precise. Also, leaving even a small band of the original pure tin solder on a termination opens the way for tin whisker growth in the field. The only way to achieve consis- tent quality and reliable results is through a controlled, automated process. This ensures controlled dwell times, depths, fluxing, and other process variables to ensure repeatability.
Two Solder Pots In simplest terms, an automat-
ed lead tinning system consists basi- cally of a central fluxing station with
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