One of the nice things about solder mask defined pads on top side joints
makes seeing fillet lifting of the lead-free joints difficult on selective
or wave soldering. So adding solder mask will prevent your inspectors
worrying about the issue shown by my old photo in IPC610 page 5-21
Mr Burke will recalls the nice examples of cracking on joints with solder
mask defined pads on our first BGA project together many years back.
Many thanks
Bob Willis
2 Fourth Ave, Chelmsford, Essex, CM1 4HA England
Tel: (44) 1245 351502
Fax: (44) 1245 496123
Mobile: 07860 775858
www.ASKbobwillis.com
www.SolderingStandards.com
New Package on Package Workshops 24th November
www.ASKbobwillis.com/PoPWorkshops.pdf
PCB Design for Manufacture & Assembly Workshop 15th October
www.ASKbobwillis.com
PCB Inspection & Quality Control Workshop 3rd November
www.ASKbobwillis.com/faworkshops.pdf
Book Bob's "Step by Step Failure Analysis Workshop" 4th November
www.ASKbobwillis.com/faworkshops.pdf
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier /*
Sent: 17 September 2009 20:00
To: [log in to unmask]
Subject: Re: [TN] fractured SJs due to soldermask compression
Hi Chris,
This depends on the details.
Werner
-----Original Message-----
From: [log in to unmask]
To: TechNet E-Mail Forum <[log in to unmask]>; [log in to unmask]
Cc: [log in to unmask]
Sent: Thu, Sep 17, 2009 2:53 pm
Subject: Re: [TN] fractured SJs due to soldermask compression
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]>
Werner Engelmaier /* <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
09/17/2009 02:48 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
[log in to unmask]
To
[log in to unmask]
cc
Subject
Re: [TN] fractured SJs due to soldermask compression
Indeed, it is.
Werner
=0
A
-----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=2
0by 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 cyc
lic
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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