Hello Phil,

Spoke with Steve Stach, whose more familiar with rework than I.  Steve
basically suggested the origin of the guidelines is probably found
somewhere between practical experience and traditional practices.  But,
rather based appropriate mechanical stress test, rather than the coupon
float testing.  The coupon float testing is looking for laminate
discoloration.  Steve suggested looking for stress failure.

Determine the appropriate testing method to match the end use (flexure
or thermal).  Then, baseline the failure analysis of a non-reworked and
multi-reworked (or multiple thermal excursion) assembly/test vehicle.
When the failure analysis of the two are significantly different,
there's an indicator that the assembly may have reached an over the
limit number of reworks.

Basically, a reworked assembly should behave under test similarly to a
non-reworked one.

Hope this is helpful,

Shean Dalton
Austin American Technology
www.aat-corp.com <http://www.aat-corp.com>

-----Original Message-----
From: Phil Bavaro
Sent: 8/9/2004 2:54 PM
To: [log in to unmask]
Subject: [TN] Complications re: Rework Cycles


We are discussing the maximum number of rework cycles that an assembly
can
withstand before being considered a reliability risk.
Historically we have always used five rework cycles as our guideline
which
permitted the part to be taken off, and put on twice during rework.

1. Reflow oven exposure
2. Removal cycle
3. Resolder cycle
4. Removal cycle
5. Resolder cycle.

The first question is where does this requirement actually come from and
the answer usually is give based on the standard coupon float testing.

But in today world, with one piece shields incorporated, such a simple
approach is usually not the case.

1. Reflow first side oven exposure
2. Reflow second side oven exposure
3. Remove shield cycle
4. Remove part cycle
5. Resolder part cycle
6. Resolder shield cycle
7. Remove shield cycle
8. Remove part cycle
9. Resolder part cycle
10.Resolder shield cycle

Comments anyone?

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