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April 1998

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Subject:
From:
Jana Carraway <[log in to unmask]>
Reply To:
Date:
Thu, 23 Apr 1998 08:38:18 PDT
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"Engelmaier" <[log in to unmask]> Wrote:
|
| Hi Scott,
| You have a legitimate question, but the reponses you
| got....well.
| The intermetallic compounds (IMCs) of concern for solder
| joints are all tinbased; thus we have AuSn, AuSn2, AuSn4,
| Ag3Sn, Cu3Sn, Cu6Sn5, Ni3Sn2, Ni3Sn4,Ni3Sn7.
| All of these IMCs are brittle, the important ones CuxSny
| and NixSny arestrong, wheras AuxSny and AgxSny IMCs are
| weak, leading to what has beentermed 'gold embrittlement'
| when concentrations get too high. The dissolutionrates
| into Sn are for Au:Ag:Cu:Ni about1000:200:30:1.
| Originally, Ni was used as a barrier layer for
| Ag-metallizations to preventthe formation of Ag3Sn IMC. On
| PCBs the Ni layer provide the possibility toprovide a
| single surface treatment that is both solderable
| (albeitsignificantly less so than Cu) and wirebondable. I
| do not no where the notionof "Cu diffuses into the Au" or
| that"Ni acts as a barrier layer to prevent theformation of
| CuAu intermetallics" comes from, but that is not the case.
| The Au on the Ni has the primary function of preventing
| the oxydation of theNi (much the same as the solder
| coverage of Cu pads); it also helps thespreading łnot the
| wettingłof the liquid solder over the Ni surface.
| Toproperly wet to Ni you need higher reflow temperatures
| (about 15C) than forCu.


| If you do not need wirebonding, you are definitely better
| of with soldercoated Cu.

        Werner, I have not heard a really good explanation of the difference
between hard gold and soft gold for wire bonding.  How are the two
significantly different and how does that affect the mechanism of bonding?

Thank you for your informative and indepth answers.
Jana Carraway

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Jana Carraway
Maxtek -- the Maxim/Tektronix Multichip Module facility
tel 503.627.2063, fax 503.627.4651
email [log in to unmask],  www.maxtek.com

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