Assuming the problem is not design related (excessive deflection as 
mentioned), I would focus on the PCB base material as mentionned by 
Werner in his point 4. However, I doubt that using higher Tg materials 
is a good fix as I have also seen post-reflow pad cratering with 
high-performance high-Tg materials. Even if they are very thermally 
robust, they can be also brittle.
The kind of BGA may be also an aggravating factor.
Curious to know which PCB material and BGA are involved in your case. 
Could you share these details?
Best regards,

A. Grivon

Genny Gibbard a écrit :
>  That may be the case sometimes that pad cratering will occur just from the reflow process.  We have some reason to suspect that it could occur later, due to some unique mounting of mechanical components, the size of the PCB (the potential for deflection), and when the defect seems to show up in the test process.  
>
>
> Genny 
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier /*
> Sent: June 11, 2009 3:33 PM
> To: [log in to unmask]
> Subject: Re: [TN] Corner ball pad cratering
>
>  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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