Then you were in a better situation :-) as in my case even bare boards twist :-)
Vladimir
SENTEC Testing Laboratory Inc.
11 Canadian Road, Unit 7.
Scarborough, ON M1R 5G1
Tel: (416) 899-1882
Fax: (905) 882-8812
www.sentec.ca
-----Original Message-----
From: Leif Laerum <[log in to unmask]>
Sender: TechNet <[log in to unmask]>
Date: Mon, 3 Oct 2011 12:02:11
To: <[log in to unmask]>
Reply-To: TechNet E-Mail Forum <[log in to unmask]>,
Leif Laerum <[log in to unmask]>
Subject: Re: [TN] Board question
Vlad,
We had a similar thing happen. The board had 40 double stacked SOPs on
both sides. It bowed in the direction of the pins (parallell to the
pins) after reflow of the fist side, then straightened out when we built
the second side. Un populated boards never bowed. The conclusion was
that this issue was a CTE mismatch problem. The pins of these chips
resisted the PCB contracting after the expansion in reflow. We were
never able to prove this by turning the chips 90 degrees like I wanted
Did not work layout wise), but we had this issue from multiple board
vendors, batches and revisions. The board was balanced. Like I said,
blank boards never bowed.
Good luck.
--
Leif Erik Laerum
-----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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