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Inge, solder is ductile ("soft") but it's not a "plasticine" (and Pb-free particularly). So a crack (once been formed) doesn't become blunt and it grows. All you might need is a tiny stress concentrator to create one and the thickness of solder mask doesn't matter in that situation.

Regards,

Vladimir
SENTEC
11 Canadian Road, Unit 7.
Scarborough, ON M1R 5G1
Tel: (416) 899-1882
Fax: (905) 882-8812
www.sentec.ca


-----Original Message-----
From: Inge <[log in to unmask]>

Date:         Thu, 17 Sep 2009 21:43:22 
To: <[log in to unmask]>
Subject: Re: [TN] fractured SJs due to soldermask compression


Richard,
we have seen so many microcracks start at the edge of the defined 
soldermask, that we do only non-defined boards. One thing I don't 
understand: how can the small percentage of the thin soldermask cause damage 
to the much bigger area of the solder ball, especially when the solder is a 
little 'flexible' (I know W does not like the word)? It does not make sense 
to me.
/Inge


----- Original Message ----- 
From: "Stadem, Richard D." <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, September 17, 2009 7:49 PM
Subject: Re: [TN] fractured SJs due to soldermask compression


> Werner,
> What I am trying to say is that issues of ground plane configuration will 
> probably not lead to a reliability issue directly, unless they cause 
> issues during reflow of the BGA. But if the ground pour is done using 
> soldermask-defined pads, the soldermask-defined pads can lead to premature 
> failure of the SJ. My understanding (gained from a direct question I asked 
> you in your class) was that the soldermask can constrict the solder during 
> CTE expansion and contraction. Did I not understand you correctly?
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier /*
> Sent: Thursday, September 17, 2009 12:32 PM
> To: [log in to unmask]
> Subject: Re: [TN] fractured SJs due to soldermask compression
>
>
> Hi Chris,
> First, never heard of "fractured SJs due to soldermask compression during 
> cyclic operation"—solder mask is much too soft and too thin; now, if you 
> talk encapsulants that is a different story.
> Second, "all of the other issues pale in the face of fractured SJs due to 
> soldermask compression during cyclic operation"—even if it were an issue, 
> a failure is a failure is a failure—'nough said.
> Werner
>
>
>
>
>
>
>
> -----Original Message-----
> From: Chris Ball <[log in to unmask]>
> To: [log in to unmask]
> Sent: Thu, Sep 17, 2009 1:07 pm
> Subject: [TN] fractured SJs due to soldermask compression
>
>
>
>
>
>
>
>
>
>
> Hello,
>
> Starting a new branch here.
>
> "All of the other issues pale in the face of fractured SJs due to
> soldermask compression during cyclic operation."
>
> Is this concern specifically for BGAs, or the same for any soldermask
> defined pad, even including selectively waved through-hole (PbFree)?
>
> Thanks,
> -Chris
>
>
>
>
>
>
>
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