Hi Mike,
There is no test method for HASL solder thickness nor is there a general
specification for a minimum thickness—and perhaps there should not be. The
minimum solder thickness that will assure solderability will depend on the
time between the HASL-process and the reflowing of the assembled PCB, because
thin solder coatings have a limited shelf life which is also temperature-
dependent. This comes from the fact that the intermetallic compound (IMC)
layer Cu6Sn5 will grow and thus consume the available tin. This diffusion
process is temperature-dependent. As the tin is consumed, the layer above the
IMC layer becomes increasingly lead-rich and therefore there will be an
increase in the Liquidus temperature making subsequent reflow soldering more
difficult if not much original near-eutectic solder remains above it. In the
worst case, the IMC can grow to the surface—upon exposure to oxygen the
formerly solderable IMC becomes non-solderable.
So, you have to judge for yourself, what minimum solder thickness is
appropriate for your needs. Anything less than 2 micrometers has a rather
limited shelf life, while above 5 micrometers a long shelf life for most
reasonable storage conditions is assured.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]


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