Constantino,

The failure at the corner of the BGA package is the expected place for a
ball crack or pad/trace crack.  A possible retrofit for your existing
(unshipped) assemblies would be an underfill for the BGA.  The description
of the field failure suggests a vibration type stress.  Is the BGA located
in a position on the board that is the farthest distance to the PCB mounting
methods?  If the board flexes from vibration, the greatest flex will happen
at the mid-point between the support points.  If this is the root cause of
the stress, some stiff rubber glued between the PCB and the case at the
center point could dampen the vibration and lower the stress that your BGA
is subjected to.  The teardrop design change to the pad/trace interface will
help, but is just treating the symptom.  If the stress is great enough to
tear a pad loose from the base material (picture #1), a teardrop design
change and/or underfill may not be enough.  Has the packaged design (PCB
assembled in the equipment case) been tested for vibration (like MIL Spec)?
This should have shown this type of failure, if it is a vibration cause.

Harold Hostetter
Electronic Assembly Manufacturing Engineer
Lester Electrical
-----Original Message-----
From: Constantino Gonzalez [mailto:[log in to unmask]] 
Sent: Monday, December 22, 2008 9:04 PM
To: [log in to unmask]
Subject: [IPC-600-6012] Pad to trace interface on BGAs.
Importance: High

Hello, I have a customer with a problem described below, Could you PLEASE
HELP ME?
The problem is that several boards are being returned to the plant after
failing in the field.
 
After Failure Analysis the engineers have found that the reason for the
failure is that one of the traces of the PCB connected to a BGA pad is
becoming cracked (Micro crack at BGA Pad AJ29 in the corner of the BGA) as
showed in picture below:
 
image001.gif   image002.gif
 
They have concluded that Copper qty at PCB layer is not enough to support
the normal Mechanical Stress in the unit chassis during the Transportation
resulting in the trace and pad crack.
Based on Jedec JESD22B113 standard, they are suggesting that the customer
change design on the traces connected to BGA pads as shown below: Does the
IPC7351 suggest anything related to this...
 
image003.gif
 
I have not heard before about this type of junction from traces to pads, and
being honest I did not know the Jedec STD mentioned.
 
Have any of you ever heard about this special union from traces to BGA pads
to avoid stress or any other recommendation to reinforce trace to pad
joints?
Could you please enlight me with your thoughts or opinions about this issue?
 
Regards.
 
Constantino J. González, ACME, INC. - GALUSAMEX REPAIR Co.
President / Consultant Engineer - Official IPC MASTER INSTRUCTOR - IPC-A-610
Committee Chairman
USA Cell Phone: 605-381-5963 - Mexico Cell Phone: 55-2709 - 6392 -
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