Hi, Hans,
Uncontrolled or too rapid cool-down during lamination is what I thought had
probably caused the warp in the first place, and you're right - the trick
with de-warping is not getting the warp back again as soon as you go near
reflow or wave soldering. I suspect a lot of finger crossing and whispered
prayers are likely the main means of passing that hurdle successfully, as
there are no guarantees that a flattened board will stay flattened.
I'm going to follow Jeff's guideline - it's not so different from what I
used years ago on "conventional" technology boards - to try and straighten
out some of the boards we have as a back-up in case we can't get good
boards even after a second re-make. I'm three and a half months into a 2
week delivery on these little b----rs!
Peter Duncan
Hinners Hans M Civ
WRALC/LUGE To: [log in to unmask]
<Hans.Hinners@ROBI cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
NS.AF.MIL> Aero/ST Group)
Sent by: TechNet Subject: Re: [TN] Board Warp
<[log in to unmask]>
11/17/01 05:33 AM
Please respond to
"TechNet E-Mail
Forum."
Hello Peter,
Symmetrical stack-up is crucial and I've seen a few designs where it just
couldn't be helped (impedance). Rapid or uncontrolled cool down during
lamination can also contribute to bow/twist in a balanced construction.
I've successfully used post lamination dewarp procedures similar to Jeff's
suggestion but that was for Class 2 stuff. The trick is making the dewarp
last through subsequent high temp. assembly steps (soldering).
Hans
Integrity First - Service Before Self - Excellence in All We Do
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Hans M. Hinners
Electronics Engineer
Warner Robins - Air Logistics Center (WR-ALC/LUGE)
Special Operations Forces System Program Office (SOF - SPO)
Gunship Team
226 Cochran Street
Robins AFB GA 31098-1622
mailto:[log in to unmask]
Com: (478) 926 - 5224
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DSN Prefix: 468
-----Original Message-----
From: <Peter George Duncan> [mailto:[log in to unmask]]
Sent: Thursday, November 15, 2001 9:11 PM
To: [log in to unmask]
Subject: [TN] Board Warp
Hi, All,
We have received a number of 12 layer ENIG-finished MLB's with blind via
hole groups, and which are destined to have BGA's (1.27mm pitch) populated
onto them. The boards measure 8.2" x 5.7" and they are warped to the extent
of one corner being raised by between 1.5mm and about 3mm. The axis of the
warp is the long side of a triangle whose other two sides measure about 5.7
x 5.7.
In this age of sensitive boards, does a technology or technique exist for
straightening the boards back into spec - they are class 3 boards - or is
this an area where angels still fear to tread?
The boards are desperately urgent, which is why I'm asking this rather than
simply rejecting them and having them re-made (which takes about a month).
TIA for any help.
Peter Duncan
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