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March 2002

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Subject:
From:
Dave Pahlas <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 11 Mar 2002 16:00:00 -0500
Content-Type:
text/plain
Parts/Attachments:
text/plain (219 lines)
Hans, these were actually boards from PDQFabs when I used to work for ACME.
Interesting thoughts on the root cause, not really sure what was going on
except we were probably getting what we were paying for. (But the wear and
tear was free.)

-----Original Message-----
From: Hinners Hans M Civ WRALC/LUGE [mailto:[log in to unmask]]
Sent: Monday, March 11, 2002 12:00 PM
To: [log in to unmask]
Subject: Re: [TN] Flattening a board during assembly


Hi Dave,

Hey, I used to make stuff for Jabil - and your panels always came out flat,
honest.  Your example is a Lamination Engineer's dream - I never would have
seen a double sided board in lamination - unless it's the UL Cert. panels.
Two guesses at your double sided problem - either the circuit design top to
bottom gave it an induced twist or the material had some flaw from the
manufacturer.  Diagonal twist makes me think grain direction, No?

Normally, you laminate a nice balanced mlb with a reasonable cooldown cycle
and the panels come out flat.  It really saves wear and tear on your Quality
Engineers that way.

Hans

AC-130 Gunship Motto: "You can run but you'll only die tired."
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Hans M. Hinners
Electronics Engineer
Warner Robins - Air Logistics Center (WR-ALC/LUGE)
226 Cochran Street
Robins AFB GA 31098-1622

mailto:[log in to unmask]

Com: (478) 926 - 5224
Fax:   (478) 926 - 4911
DSN Prefix: 468


-----Original Message-----
From: Dave Pahlas [mailto:[log in to unmask]]
Sent: Friday, March 08, 2002 5:14 PM
To: [log in to unmask]
Subject: Re: [TN] Flattening a board during assembly


Sounds like in a past life we were the recipients of boards "not done
properly". We had 14"x16" double sided PCBs that would twist pretty bad
during first assembly reflow making stencil gasketing at second assembly
pretty tough. We ran a sample of unpopulated boards through the reflow oven
riding flat on the mesh and the diagonal corners actually curled up. We
weren't very knowledgeable on PCB fabrication but thought maybe the internal
stresses created in the board shop's flattening process were being released
in our reflow oven. We never had this problem with the previous (more
expensive) supplier. Hmmmm. The product went EOL before resolution was
reached, but ...

Enjoy your weekend.

-----Original Message-----
From: Hinners Hans M Civ WRALC/LUGE [mailto:[log in to unmask]]
Sent: Friday, March 08, 2002 11:27 AM
To: [log in to unmask]
Subject: Re: [TN] Flattening a board during assembly


Hey Daan,

Here's a generic dewarp I've used in the past.  You or preferably the Fab
house should have an oven large enough to load the panels or boards.  If
done properly you won't see re-warp during the assembly process.  Dewarp is
not a board friendly process and I shudder to consider the longer term
reliability issues but it gets them flat when nothing else will.  It's
usually better to dewarp boards instead of panels because of stress relief
that occurs at Route.  On the other hand a 2 or 4 up panel cuts your cycle
time down considerably.

1.  Load warped panels into oven.  (Keep stacks reasonable sized: ~500 mils
of board stock or less - 6-10 panels depending on thickness)
2.  Place several Caul plates (that match or overlap panel size)  ~50 pounds
of weight & Use Kraft Paper (Sulfur Free) to protect panels
3.  Bake at 340+ F for 4-6 hours  (Material must be at or slightly above Tg)
4.  Cool slowly to room temp.
5.  Manually assist (Bend & Twist) the dewarp at Final Inspect.  (Great
upper body workout on those 200+ board shipments)
6.  Inspect Panels on Flatness (Granite) Table.  ("Tap" test the 4 corners.
If the board's corners don't touch the table you've got more flexing to do.)

Hans

AC-130 Gunship Motto: "You can run but, you'll only die tired."
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Hans M. Hinners
Electronics Engineer
Warner Robins - Air Logistics Center (WR-ALC/LUGE)
226 Cochran Street
Robins AFB GA 31098-1622

mailto:[log in to unmask]

Com: (478) 926 - 5224
Fax:   (478) 926 - 4911
DSN Prefix: 468



-----Original Message-----
From: d. terstegge [mailto:[log in to unmask]]
Sent: Thursday, March 07, 2002 11:48 AM
To: [log in to unmask]
Subject: [TN] Flattening a board during assembly


Hi Technetters,

We've got a batch of boards here that's really twisted (about 1.5 % when
measured according to IPC-TM-650) due to imbalanced design. The customers is
not willing to change the design, and we discussed with them the possibility
of using a carrier-frame for flattening these boards while they are
processed through our SMT-line.

My worry is that this results in unacceptable stress in the solder joints as
soon as the board is released from the carrier. Fortunately there's no BGA
on this board, but it does have a small QFP and some 40 mm long
SMT-connectors. It's supposed to be a high-reliability military product.

Does anyone have any data on the effects of such internal stresses ?
Also other comments are welcome.

Best regards.

Daan Terstegge
SMT Centre
Thales Communications
Unclassified mail
Personal Website: http://www.smtinfo.net

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