I believe the rule for 5 rework cycles came from the rework qualification testing in MIL-PRF-55110 where samples were required to withstand 5 heat cycles from a soldering iron.  As you point out, modern assembly processes are different and often better controlled than manual application of a soldering iron.  IMO sample boards should still be periodically qualified against a company's rework process.  I'm not aware of any standard test protocols that simulate rework or that there is a published standard for qualification.  I know that many rework cycles exceed the qualification testing performed by organizations like U/L.  I also know that number of heat excursions, temperature ramp rates, dwell times, and peak temperatures affect the integrity of a board.  Fabrication variables like laminate type, board construction, plating thickness, and plating bath control also enter the equation.

There are others on Technet with better first-hand experience with these factors, but at the end of the day my guess is that the answer will be the Doug Pauls standard of .... it depends.

Don Vischulis

-----Original Message-----
From: Don Vischulis <[log in to unmask]>
Sent: Aug 9, 2004 8:51 PM
To: [log in to unmask]
Subject: FW: [TN] Complications re: Rework Cycles



-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Phil Bavaro
Sent: Monday, August 09, 2004 2:43 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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