Leland, you have to relate the warpage on those boards to the distance over
which you are measuring in the XY plane.

Did you switch to lead free in that time frame?

John

 
 
John Burke
 
(408) 515 4992
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leland Woodall
Sent: Thursday, July 12, 2007 9:03 AM
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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