Magnin,
It is not the flux that destroyed the gold layer. Gold very much likes to
combine with the Tin in the solder paste. In the few days that the solder
paste was on the PCB, before cleaning, the Gold was "dissolved" into the
tin.
By the way, the Gold is a sacrificial layer, meant to completely dissolve
during soldering, allowing the solder to wet to the Nickel underneath.
Without the Gold, the Nickel will oxidize, becoming unsolderable.
Ryan Grant
-----Original Message-----
From: Marie-Elisabeth MAGNIN [mailto:[log in to unmask]]
Sent: Monday, January 27, 2003 2:05 AM
To: [log in to unmask]
Subject: [TN] dammage of N1/Au layer by solder paste
Dear Technetters,
I receive today from a customer a strange report.
A few weeks ago, he provided us a batch of bare PCB's for assembly.
We first did screen printing with solder paste.
Because of some procurement and planning problems, the PCB remained with
this solder paste on them for a few days and finally, we washed them and
gave them back (unpopulated) to our customer.
He (and we) noticed that there remained some grey areas on the pads,
corresponding to the surface where solder paste was deposited.
The PCB manufacturer was consulted, he sent a PCB for analysis, and the
answer came:
"There remains solder paste residues on the gold, which can be eliminated
with desoldering braiding
However, the chemical gold layer has been partially desolved, and thus, the
Nickel layer is no longer garantied reliable"
I accept that cleaning could leave some paste residues on the pads (trapped
into the uneven surface of the gold layer)
But :we use a no clean solder paste, and we had never heard that the flux
could destroy the gold layer !!
Who met such a problem or has a technical answer ?
Many thanks for your help
ME Magnin
QA Manager
FIRSTEC SA
Rue du Grand Pré 70
CH 1211 Genève 2
Tél. : +41(0)22 918 36 85
Fax : +41(0)22 918 36 93
mailto:[log in to unmask]
http://www.firstec.ch
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