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Subject:	Re: Shelf life of circuit board plating
From:	Werner Engelmaier <[log in to unmask]>
Reply To:	(Leadfree Electronics Assembly Forum)
Date:	Wed, 3 May 2006 12:20:01 EDT
Content-Type:	text/plain
Parts/Attachments:	text/plain (40 lines)

In a message dated 5/2/06 3:10:07, [log in to unmask] 
writes:
> Just a question to Werner:
> what's about soluting of Cu in Sn even at room temperature but regarding a
> longer time ? May the storage time be limited due to that effect ? Compared
> to SnPb you will not find an enrichment of Pb within the tin- /
> solder-covering. But the imSn cover is much thinner than (normally) SnPb by
> HASL.
> 
Hallo Wolf-Dieter,
The dissolution rate of Cu in Sn follows an Arrhenius relationship with some 
activation energy—it is highly temperature dependent. Thus, while there is 
some dissolution and thus IMC growth at RT, it is slow. For example, HASL layers 
of SnPb solder are time-limited by their thickness—about 1 micron will be 
penetrated by Cu6Sn5 in 3 to 5 months--that is why HASL layers of thicknessses 
exceeding 3 micro-m are recommended. 
Also correct is the fact, that no layer with a high concentration of the 
Pb-rich solder phase will form next to the Cu6Sn5 layer.
On the other hand, while the imSn layer [1 to 1.5 microns] is thinner than a 
HASL layer, it is not as metallurgically in intimate contact with the Cu and 
it is porous--another factor slowing IMC growth.

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com


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