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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