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June 2009

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Subject:
From:
Paul Reid <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Paul Reid <[log in to unmask]>
Date:
Fri, 12 Jun 2009 07:33:05 -0400
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Hi Genny,

If you are concerned about the cratering occurring in the fabrication of the PWB I would look for lifted pads on the bare unassembled board. BGA pads do not tend to lift in PWB fabrication but the pads around the PTH do tend to lift. Find a lifted pad on a PTH and prepare a microsection. You should see a 45°C crack in the material at the knee of the hole and the material still attached to the pad. That means the adhesion of the copper foil is stronger than the material.

Of course this is predicated on the boards having a surface finish that requires a thermal excursion.

Do not rely on Tg as a good indicator of material robustness. It is a PWB myth that high Tg means a better material. A high Tg is an indicator of a high reliability material but not the definitive measure of material robustness. Reliability testing is the better way to rank material robustness.

Paul Reid 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Genny Gibbard
Sent: Thursday, June 11, 2009 4:10 PM
To: [log in to unmask]
Subject: [TN] Corner ball pad cratering

Hello all,
If you have identified a BGA with corner ball pad cratering, what would be your suggestions to correct this condition?
Our thoughts:
1.  We plan to evaluate our handling processes to reduce/remove excessive flex/strain occurring around this part 2.  We plan to evaluate corner bonding or underfilling for the BGA.

Which do you feel would be the best direction to go?  Do you have other suggestions?
Thanks in advance for all your help

Genny 

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