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June 2007

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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Tue, 19 Jun 2007 09:09:16 -0500
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I totally agree with Werner. I have attempted to develop profiling
matrixs for given sizes/types of boards, but they seldom worked very
well. My experience has been there are too many surprises (unknown
factors) such that you can never really look at an assembly drawing and
guess at which profile to use, they must always be fully profiled with
TCs all over the assembly to really dial it in properly, and the matrix
is only good for getting you somewhat close to a starting point, maybe.
Especially for Pb-free, as the higher the overall temperature, the more
of a delta exists between the coolest (defined as longest time to reach
liquidus, not necessarily the lowest maximum reflow temperature) and the
hottest (again defined in terms of shortest time required to reach
liquidus, but not necessarily the hottest MRT on the PWB).
Recently I needed to validate the reflow oven recipe for an assembly
that was 24" high by 36" wide, with 48 layers, 24 on layer one of a
separate PWB soldered to a second 24-layer PWB much like a giant BGA.
This PWB had 6 miles of traces alone, not including the internal power,
ground, and heatsink planes. Just the PWB alone weighed 40 lbs.

Boy, did I learn a lot about temperature deltas and profiling.

To obtain a profile (process window) that minimized the longest TALTS
and yet provided sufficient TALTs for the coolest portions was very
challenging, but we pulled it off. We then built a validation assembly
and sacrified it for microsectioning to validate the profile. 47
microsections of the hottest, coolest, and intermediate SJs were made at
an outside laboratory, 100% displayed excellent solder joint formation
and good IMF. My concern was oxidation of the hottest MRT (longest TALT)
portions (much like the dross formation on a solder pot), and
insufficient IMF formation on the coolest. Neither of these manifested
itself.

But in general, for any given PWB, the effects of factors such as copper
weight, power/ground plane quantity and size and distribution, type of
material used for the substrate, the PWB thickness, and the effects of
the components themselves are all very, very significant. You can have
two PWBs of the same size, type of PWB substrate, but slightly different
component types and different copper structure and the "optimized"
profiles for the two assemblies can be completely different.
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Friday, June 15, 2007 9:13 AM
To: [log in to unmask]
Subject: Re: [TN] Reflow Ovens for Printed Circuit Boards

Hi Ted,
DataPaq would have to have a huge component library for these kind of
estimates to come even close to being accurate. I would need to see data
developed with DataPaq and actual measurements that are ratrher close
before I would trust this.

Werner



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