Hi Genny,
Bythe time you can underfill, you already have pad cratering—so this does not help.
Why pad cratering?
Remember, ‘pad cratering’ results from a number of factors coming together.  Those factors are: 1) SAC solders are about 40% stiffer generating higher forces on cooling; 2) SAC solders creep significantly slower and thus do not provide stress relief on cooling; 3) the delta-T between solder solidification and the glass transition temperature, Tg, doubles for SAC-solders thus doubling the thermal expansion mismatch and the resulting stresses;  4) for some PWB base materials the fillers and curing agents used to make the resin RoHS-compatible make the resin more brittle with less cohesive strength.


 Solution Options: 1) reduce cooling rate from soldering to no more than 1.5°C/sec; 2) use PWB base materials with higher Tg; 3) increase solder joint thickness between component and soldering pad; and 4) my favorite, go back to environmentally friendly SnPb solder.
Werner


 

-----Original Message-----
From: Genny Gibbard <[log in to unmask]>
To: [log in to unmask]
Sent: Thu, 11 Jun 2009 4:09 pm
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=2
0do 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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