Bill,
If you have adequate HASL thickness then this finish should be good for
significantly longer than a year. We routinely ran HASL boards that were 2
years old and more. Of course maybe that is why we no longer do any internal
assembly. There are some important parameters that need to have been
controlled. The minimum thickness should show no readings below 60
microinches and we required at least 95% of the readings over 100
microinches. If you have XRF readings that are in the 40 microinch range
then I would be concerned, as part of what the XRF picks up as solder
thickness is the tin atoms in the intermetallic layer. This also shows up as
a higher tin percentage in the XRF reading. Also, the boards need to have
been stored in a controlled environment. With today's lean manufacturing a 6
month shelf life may be considered adequate, but a good HASL coating
properly stored will last much longer.
Thanks,
Robert Furrow
Printed Circuit Board Engineer
Supply Chain Networks
Lucent Technologies
978-682-2260 [log in to unmask]
-----Original Message-----
From: Bill Page [mailto:[log in to unmask]]
Sent: Friday, April 25, 2003 4:49 PM
To: [log in to unmask]
Subject: [TN] Raw card solderability
Consider this a hypothetical question :)
We have some fabs that, due to reduced production requirements, have been in
house for an extended period of time (more than a year). In investigating
some solderability concerns, the board house suggested that with thinner
HASL coatings, copper migrates through the tin-lead coating, reducing
solderability. The vendor seemed to think that a six-month shelf life is
now an industry standard...indeed, he involved the magic name of IPC.
It has, admittedly, been a long time since I considered myself up to date on
PCB fabrication issues.
a. Is this a legitimate observation from our supplier?
b. Should we expect only a six month shelf life on PCB?
c. What rework method is recommended to bring these back in spec?
Thanks,
Bill Page
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