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July 2008

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Subject:
From:
"McGee, Michael" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, McGee, Michael
Date:
Fri, 18 Jul 2008 15:28:23 -0400
Content-Type:
text/plain
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text/plain (103 lines)
Ian,
I would never dream of manually cracking a v-groove panel with SMT
parts. Get a "pizza slicer" style depaneler. They can be had relatively
cheap and work great. They put zero stress on the board. Even an
inexpensive manual one is faster and more ergonomic than manually
breaking them.

-Mike

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Friday, July 18, 2008 1:48 PM
To: [log in to unmask]
Subject: [TN] Mechanical damage to BGA

Dear Technos,

 

I have this BGA that gives me a headache. In many instances the boards
do not pass test, we press on the BGA and they start working; a
classic... The problem solder joints are always on the same side of the
component and on the outer row; needless to say, we have a V-groove line
that is manually cracked. So the cause is most likely mechanical stress
that causes the pads to detach from the PCB. After prying off some BGAs
we see the soldering is good, all the pads come off the board and stay
solidly attached to the balls.

 

The specifics of this board are a bit special and I would need your
opinion on what to improve in the design in order to eliminate the
propensity to damage.

 

PCB is 18.55x28.65mm, FR4, 0.55mm [0.021"] thick

The BGA is 17x17mm, 1mm pitch, 0.65mm balls assembled on 0.25mm circular
pads. In the troubled zone the pads are connected to 0.050mm traces.

 

My opinion is that the board flexes very easily and even if we fix the
depanelization (I will modify the panel) all the subsequent handling,
including the integration of this module, puts the joints at high risk.

 

I think the pads are too small so they do not have sufficient adhesive
force to stay put on the PCB. On top of it, the traces are so minute, so
the slightest stress will be destructive.

 

Would increasing the pad size help? Is this construction (thin PCB +
superfine traces) viable?

 

Could the CTE mismatch be strong enough to cause the damage, instead of
the mechanical stress from bending the board?

 

Thank you,

 

Ioan

 

 


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