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March 2002

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Date:
Thu, 14 Mar 2002 11:42:39 +0800
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Was this strength test written into the requirements when the job was taken
on? 20 in pounds is quite a torque to apply to an electronic component.
Does your customer have any first-hand experience of circus board
manufacture? - it's sounds like "no", or he'ld understand for himself why
it's a bad idea to perform a test like this. Is it intended to be 100%
inspection?

The process to stick the heatsinks to the BGA's should have been confirmed
as part of a product qualification test, as should the means and method of
testing the adhesion. Properly specified, this process can then be passed
onto CM's or whoever assembles the boards to repeat.

For process monitoring purposes, to ensure adequate adhesion is being
maintained, batch sampling can be done, but I wouldn't do it on expensive
"live" product - I would load the occasional board with appropriate dummy
components, mount the heatsink and test those. If there's a problem, stop
and review the processes.

Otherwise, what is the test going to prove? Either the heatsink comes off,
or the BGA does (especially if it's one of Earl's illigitimate TI things),
or they both stay on the board but the solder joints (ball to board or
balls to device or both) have been unnecessarily stressed, which is bound
to reduce working life by anyone's reasoning - by how much, should have
been one of the HALT test results, using a non-torqued set of pieces for
comparison.

How to change the customer's mind? Ask what qual tests were done and what
results were obtained along the lines I suggested above, and judge for
yourself if they were adequate. If not explain why you think so, and if
he's still adamant about carrying out the test on live product, get him to
sign a disclaimer removing your liability for failures now or later for any
boards on which this test has been carried out.

Can't suggest anything else, but good luck.

Peter




                    Carl Ray
                    <carlr.ray@SANMIN        To:     [log in to unmask]
                    A-SCI.COM>               cc:     (bcc: DUNCAN Peter/Asst Prin Engr/ST
                    Sent by: TechNet         Aero/ST Group)
                    <[log in to unmask]>        Subject:     [TN] Solder Joint Mechanical Stress


                    03/14/02 12:17 AM
                    Please respond to
                    "TechNet E-Mail
                    Forum."; Please
                    respond to Carl
                    Ray






I have a customer who is requiring that I verify the bond strength of a
heat sink that is glued to the top of PBGA. The customer has supplied me
with a torque tool to verify the strength of the bond between the heat
sink and the BGA. I have concerns that testing the strength of the glue
bond may affect the solder joint reliability of the BGA. The tool
supplied by the customer is set at 20 in pound torque. Any ideas how I
can "change" my customer mind?

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