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July 2008

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Subject:
From:
Bev Christian <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Bev Christian <[log in to unmask]>
Date:
Mon, 21 Jul 2008 20:20:39 -0400
Content-Type:
text/plain
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text/plain (144 lines)
Richard,
I just wanted to compliment you on one fine TechNet reply.  Well done.
Bev
RIM

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, July 21, 2008 9:02 AM
To: [log in to unmask]
Subject: Re: [TN] Mechanical damage to BGA

Ioan,
One thing you can do to help reduce the amount of stress seen directly on
the BGA solder joints is to underfill the BGA prior to depaneling. Once
cured, the underfill will help alleviate the stresses seen directly on the
solder joints/pads caused by board flexure. This is especially true when you
have a thin circuit board, as you most certainly do. I am not talking about
stresses causing the solder joint itself from cracking, but the pad
attachment to the PWB from cracking.
Please note this will also help prevent cracking due to CTE mismatch,
however, the underfill epoxy must be carefully selected such that its rate
of CTE closely matches the rate of CTE of the PWB. 
An excellent article on this subject was written by Werner Engelmaier. I
have attached it here.
http://www.globalsmt.net/regions/japan/content/view/33/65/ 

I don't believe there is any point in performing dye and pry, etc., as I
believe you have already determined the root causes of the cracking, which
are essentially:
1. pad cratering due to mechanical stresses during depanelization.
2. Handling issues.
3. Possible CTE stresses during service cycling.

All of which are contributed to by the thin board.

Two other possible fixes are looking at the board material itself. Some
circuit board materials crater much easier than others. It is also possible
you can use an .062" board rather than the .028" thickness, but that depends
on whether the board must fit into a sliding frame, whether it has edge
connectors that must line up, etc.

So, you have three options you can try without any drastic changes to the
overall design.
1. Epoxy underfill
2. Board material
3. Board thickness

If reworkability of the BGA is an issue such that you don't want to do a
full underfill, then try a combination of cornerbonding and edgebonding of
the BGA using Loctite 3609. While perhaps not as strong as a full underfill,
this may be enough to prevent the cracking. If you need to rework the BGA
you can heat the Loctite 3609 to 150 C and it will easily chip off with a
small exacto knife, allowing you to remove/replace the BGA. See examples of
this in the pictures in this article, also by Werner Engelmaier
http://www.globalsmt.net/regions/uk/index.php?option=com_content&task=view&i
d=1971&Itemid=131


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Monday, July 21, 2008 6:25 AM
To: [log in to unmask]
Subject: Re: [TN] Mechanical damage to BGA

I see your point Eric.

The size of the balls is actually a manufacturing error, the balls were
intended to be 0.35mm in diameter, so the pad size was designed at 0.25mm on
both the component and the PCB.

So, since nobody comments on the size of the traces and the pad size from
the mechanical and stress and CTE point of view, I guess the design is OK...

Thanks to everybody that answered,

Ioan

-----Message d'origine-----
De : TechNet [mailto:[log in to unmask]] De la part de Eric CHRISTISON Envoyé :
July 21, 2008 4:14 AM À : [log in to unmask] Objet : Re: [TN] Mechanical damage
to BGA

Measure the size of the pad on the underside of the BGA device.

The size of the pad on the PWB should be about the same size to make the
joint of equal strength at both interfaces.

0.25 for a 0.60mm dia ball sounds far too small to me.

Agreed though that you may still get failure at depanelisation due to board
flexure. Best to avoid that.

Regards,

Eric Christison Msc
Mechanical Engineer
Consumer & Micro group
Imaging Division

STMicroelectronics (R&D) Ltd
33 Pinkhill
Edinburgh EH12 7BF
United Kingdom

Tel:	+44 (0)131 336 6165
Fax:	+ 44 (0)131 336 6001

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