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September 2009

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Subject:
From:
Werner Engelmaier /* <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Thu, 17 Sep 2009 14:48:31 -0400
Content-Type:
text/plain
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text/plain (219 lines)
 Indeed, it is.
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 2:36 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 st
ress 
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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