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May 2003

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Subject:
From:
"Valerie St.Cyr" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 22 May 2003 09:38:07 -0400
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Daan,

Boards which are badly warped then post-baked for stress relief will warp
again once heat is applied. If these see more than one heat cycle, the
second pick and place operation could be a very large problem. We have also
been concerned (although we have no data) that boards with high pin count
small ball BGAs may suffer fractured joints once the board warps again in
assembly and then cools post assembly.

There are 2 possibilities about the material and the "inspections" - one is
that the material itself did not meet the specifications. I have found that
a board shop needs to have vigilance regarding the quality of the incoming
laminate. Yes, it should all be ship-to-stock, but I have seen too many
times when the laminate or prepreg was out-of-specification, it was shipped
to the fabricator, and the fabricator used it. All of a sudden, on a part
that was running fine, there is warpage. That should signal a review of the
material; some testing; some tracability to see how many boards and which
boards used that material etc. The fabricator should have a audit or
quarterly (monthly? semi-annual?) test and qualification retention
procedure for the laminate. Also, arrangements where the fabricator has
turned over the laminate stores to the laminate supplier (they manage
min/max levels based on counts or some other measure; and they are
responsible for managing shelf-life and establishing a FIFO system)
absolutely need to audit that department *as though it were not within it's
4 walls*. I have audited many shops and have found out-of-date prepreg on
more than one occassion (sorry laminate makers - but you need tighter
controls).

The other possibility about "inspections" is that the layup person didn't
"inspect" the material that was kitted for the job; an operator is
responsible to pull all the materials (cores and prepregs) require to build
up the panel, and those materials are grouped together and sent into the
layup area. It is highly possible to kit the wrong prepregs when it is a
manual operation. In that case the layup person may or may not have a
procedure to verify the materials prior to making the multilayer. So, that
could be the inspection step referred to. In that case the corrective
action belongs entirely to the fabricator; they should have the  controls
to keep that from occuring.

Prepregs that are "cross-plied" to each other will warp and that warp will
only be temporarily disguised by a wieghted bake and will return again.

You need to go deeper and have them say more about the root cause besides
"inspections"? At what stage in the process? What are the inspections
designed to find ?

Valerie






"d. terstegge" <[log in to unmask]>@listserv.ipc.org> on
05/22/2003 04:13:45 AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>;
       Please respond to "d. terstegge" <[log in to unmask]>

Sent by:    TechNet <[log in to unmask]>


To:    [log in to unmask]
cc:

Subject:    [TN] Bad laminate and warped board


Hi Technet,

I recently received some boards that were warped outside the IPC
specifications and I complained about it to the manufacturer. Their
answer is that the problem was caused by variations in laminate and
prepreg, where the operater didn't do the prescribed
material-inspections.
They will rework the boards by applying pressure at a temperature
higher then Tg, to release the stresses in the material.

Some questions:
Whats' the proper way for a bare board manufacturer to control the
quality of laminates and prepregs with regards to flatness ?  Is it
common practice to rely on an incoming inspection ?
If a board is flattened, will it remain flat after soldering ?  I know
that with an unblanced design the problem will always re-occur, but I'm
not sure what will happen when it's the laminate-quality that caused the
problem.

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

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