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March 2002

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From:
Hinners Hans M Civ WRALC/LUGE <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 22 Mar 2002 16:53:42 -0500
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Hi Michael,

I know that board flattening procedure isn't board friendly thing but what's
a better alternative?  It came down to either do the bake & twist procedure
or scrap the panels.

You are 100% correct that excesses could cause irreparable damage to the
panels but the procedure was only done by the PE (me), QEs and the Final
Inspect Leads.  Your caution/warning could apply to any manual operation.
At least a flattened board stayed flat and didn't curl up when it saw an
elevated temp. (smt reflow) at the assembly house.  If I could have figured
a way to laminate an unbalanced construction flat the extra procedure
wouldn't have been necessary.

Hans



-----Original Message-----
From: Mcmaster, Michael [mailto:[log in to unmask]]
Sent: Thursday, March 21, 2002 7:10 PM
To: [log in to unmask]
Subject: Re: [TN] Flattening a board during assembly


Rumor has it that we've reached the bottom of the recent economic downturn.
That may or may not be true.  All I know is that I'm busy enough that I've
fallen a few weeks and a few hundred Technet e-mails behind.  Maybe I need
to introduce my Technet E-mail Backlog Index and see how it corresponds to
the other financial indicators.

All kidding aside, Hans your "manual assist" process is not advised.  I've
seen boards with cracked soldermask that were traced down to someone
"flattening" a warped board before placing on a driller or router bed.  I
don't recommend flexing cold boards to remove warpage.

> ----------
> From:         Hinners Hans M Civ
> WRALC/LUGE[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Hinners Hans M Civ WRALC/LUGE
> Sent:         Friday, March 08, 2002 10:27 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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