It could be the ORIENTATION of the PWB with respect to the direction of the glass bundles. If the boards were cut from the blank with the long dimension oriented with the strand orientation at one vendor and the short dimension with the other, the warpage can occur at one and not at the other.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Vladimir Igoshev. PhD
Sent: Thursday, September 29, 2011 11:50 AM
To: [log in to unmask]
Subject: [TN] Board question

Hi TechNetters,

I have a question for board gurus, as I'm almost lost.

Here is a story. A panel of four simple 4 layers FR4 twists after
going through the first side re-flow and flatten out after the second
side is re-flowed.   It does the same thing with and without the
components placed.

The panels are from  new vendor and the old ones had never exhibited
the problem. What confused me is that both boards (from both of the
vendors) were built identically: dielectric layers and  Cu
layers are look identical thickness- wise.

There are no signs of resin recession or delamination in
cross-sections which could have pointed out at incomplete curing.

Glass bundles look also identical.

What else it could be?

Any suggestions are deeply appreciated.
-- 
Best regards,

Vladimir Igoshev. PhD                          mailto:[log in to unmask]

SENTEC Testing Laboratory Inc.
11 Canadian Road, Unit 7.
Scarborough, ON M1R 5G1
Tel: (416) 899-1882
Fax: (905) 882-8812
www.sentec.ca


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