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September 2009

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Subject:
From:
Chris Ball <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Thu, 17 Sep 2009 14:53:52 -0400
Content-Type:
text/plain
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text/plain (1 lines)
Hello to All,



I asked this with thoughts unrelated to BGA's. We have selective wave

solder process on some assemblies with pth connectors. As we move to

leadfree, we review our processes and practices and have thought to change

to soldermask defined pads for PbFree selective solder of pth conectors.



I was reviewing a document about this and reading my technet email at the

same time, so the possibility of a problem with soldermask defined pads

alarmed me.



Does a soldermask defined pad reduce solder joint cycles-to-failure in a

pth, selective wave scenario?



Thanks again,

-Chris







                                                                           

             Werner Engelmaier                                             

             /*                                                            

             <[log in to unmask]                                          To 

             OM>                       [log in to unmask]                     

             Sent by: TechNet                                           cc 

             <[log in to unmask]>                                             

                                                                   Subject 

                                       Re: [TN] fractured SJs due to       

             09/17/2009 02:48          soldermask compression              

             PM                                                            

                                                                           

                                                                           

             Please respond to                                             

              TechNet E-Mail                                               

                   Forum                                                   

             <[log in to unmask]>                                             

             ; Please respond                                              

                    to                                                     

             [log in to unmask]                                             

                     M                                                     

                                                                           

                                                                           

                                                                           









 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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