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February 2008

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Subject:
From:
Ioan Tempea <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Ioan Tempea <[log in to unmask]>
Date:
Fri, 15 Feb 2008 08:18:44 -0500
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Dear Technos,

 

We're having an FR-4 PCB on which we will assemble a few SMT parts and
then this board will be bonded onto a flex cable. The bonding will be
done on about 140 x 50microns wide "gold fingers". The smallest SMT
features on the board are the 232 pads of a BGA, 250 microns in
diameter, so a little less than 10 microINCHES.

 

The PCB plating spec that was proposed is 10 to 20 microINCHES of Au
(per an ASTM spec, not sure, maybe 448?) on 150 to 250 microINCHES of
Ni. Unfortunately, I don't know whether we talk about ENIG or rather
hard or flash Au.

 

Is this plating OK for bonding?

 

What about the gold embrittlement on the SMT components side, the spec
might be OK for the Ni, but strongly contradicts the 2 to 5 microinches
of Au as per IPC-4552?

 

Is it common practice, or at least acceptable to have the same finish on
the SMT pads and the surfaces to be bonded?

 

What would be a proper specification for this kind of PCB?

 

Thank you,

 

Ioan

 

 


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