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April 1998

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Subject:
From:
John Waite <[log in to unmask]>
Reply To:
John Waite <[log in to unmask]>
Date:
Fri, 10 Apr 1998 07:32:17 -0400
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Good Morning Werner,
    Very good input on the solder thickness dilemma.  I agree that I have
yet to see any real "objective" criteria on solder thickness.  I believe
that IPC has or is putting together a spec for this situation.  The old
standard of .0003" (back in the ages of tin/lead fusing/reflow) was a before
fusing requirement and after fusing, the standard was "to meet the
solderability of IPC-S-804".  In our industry, we have come to a "generic"
number of 50 millionths minimum at the knee to aid inhibition of the "copper
migration issue".  On SMT pads, we have seen specs not only for minimum
thickness of 50 millionths, but also for planarity callouts within a smt
component area.
    Measurement has been performed with XRF (X-Ray Fluorescent) equipment
and statistical data gathered from that.
    I believe that solder is becoming less of a viable solution for assembly
and other alternatives (OSPs', Tin/nickel, palladium, electroless
Ni/Immersion AU, etc.) are on the forefront of replacing tin/lead (solder)
compounds.  I personally like the OSP process due to the ability to easily
rework in the event of shelf life situations.
    That is my 2 cents worth (in today's age of inflation, it's worth less)
and hope that others will make this worth a dollar.   JOHN WAITE
-----Original Message-----
From: Engelmaier <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Thursday, April 09, 1998 11:41 PM
Subject: Re: [TN] [TN IPC Test methodes


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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