It took two gurus with good  insight in the crack  world to  make me 
understand the weakness that follows such rounds.
/Inge


----- Original Message ----- 
From: "Stadem, Richard D." <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, September 17, 2009 8:36 PM
Subject: Re: [TN] fractured SJs due to soldermask compression


> Thanks, Werner.
> I think I understand this a little better. If the soldermask-defined pad 
> does not allow the solder to flow down around the sides of the pad, but 
> rather induces a stress riser where the BGA solderball meets the top of 
> the pad, the stress is focused at the point where the solder contacts the 
> pad, at the top of the pad. Therefore it is easier for the crack to form 
> as opposed to a solder ball that is "anchored" with the pad fully embedded 
> inside the solder ball. So the CTE-induced stresses are not from the mask, 
> but the CTE delta between the BGA package and the substrate. Being that 
> this stress riser is circumferential around the point where the solder 
> makes contact with the pad, the initiation point and the direction of the 
> crack propagation can be anywhere and go in any direction. Is that 
> correct?
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier /*
> Sent: Thursday, September 17, 2009 1:22 PM
> To: [log in to unmask]
> Subject: Re: [TN] fractured SJs due to soldermask compression
>
>
> Hi Dean,
> Let us be clear about this.
> If you have soldermask-defined pads (SMDs), what results is solder joints 
> with geometries defined by that solder mask. These geometries cause stress 
> concentrations which typically will cause cycles-to-failure to be cut by 
> about half. It has nothing to do with CTE expansion and contraction of the 
> solder mask—unless, you do solder mask repair after assembly (!?) and the 
> solder mask fills the space between component and PCB.
> Werner
>
>
>
>
>
>
>
> -----Original Message-----
> From: Stadem, Richard D. <[log in to unmask]>
> To: [log in to unmask]; [log in to unmask]
> Sent: Thu, Sep 17, 2009 1:49 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 c
> ompression 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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