The problem is that the unpopulated boards meet the requirements -- they
do not warp until the assembly process -- this does not violate the
(6012) bare-board or the (4101) raw material acceptability specs...I
suspect that the root-cause lays in the lamination, however there are
many other factors that can influence of course -- but spec-wise, the
bare-boards pass...Ian
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee Parker
Sent: Thursday, July 12, 2007 2:27 PM
To: [log in to unmask]
Subject: Re: [TN] PCB Fab Question Regarding Bow and Twist
Leland
I would first characterize the boards using the parameters in the 620
document. If the boards are out of spec, I would inquire as to why these
defective boards are being shipped. The cause is often best left to the
manufacture (you would probably have to do an in-plant audit to begin to
identify the root cause). The first place I would look is at the raw
material; does it meet your specification.
Best regards
Lee
J. L. Parker Ph.D.
JLP Consultants LLC
(804) 779 3389
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leland Woodall
Sent: Thursday, July 12, 2007 12:03 PM
To: [log in to unmask]
Subject: [TN] PCB Fab Question Regarding Bow and Twist
Everyone,
I have a product that ran for almost a year with little or no problem.
Recently we've started seeing unsoldered pins on a 256 pin QFP, and
found the cause to be excessive twist in the board.
We have verified that no changes have been made to our process. Reflow
temps, cooling rates, conveyor speeds, rail parallelism, and transfers
are all good. We've checked incoming raw material and found only slight
deflection in a few specimens.
A couple of examples:
Incoming: 0.13 MM
B reflow: 0.66 MM
A reflow 1.98 MM
We've identified assemblies that have excessive twist after each of the
reflow operations, and our once good product is becoming an expensive
stepchild. We cannot mount twisted boards into cases because of too
much strain, and are forced to scrap this material.
What in the PCB manufacturing process might have changed that would
cause this scenario?
Thanks in advance for your input.
Leland
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