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April 1998

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Subject:
From:
Alvin Leong <[log in to unmask]>
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Date:
Fri, 10 Apr 1998 21:06:31 +0800
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Hi, FYI
Alvin

Engelmaier wrote:
>
> 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.

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