In a message dated 2/12/99 11:53:38 AM Pacific Standard Time,
[log in to unmask] writes:

<< Has anyone done a study on the correlation of board reliability vs. the
number
 of heat cycles a board sees?  Particularly, if a board has been through 2 smt
 processes and wavesolder, how much smt rework can one do to the board and
still
 feel confident that the product won't fail in the field?  I'm especially
 interested in large boards, .100" thick, 18 layer, OSP finish. >>


Hi Jason!

Holy Moly! 18-layer and only a hunert thou? You must have got things squished
pretty flat in there!

Me thinks you might have a bit of trouble getting anybody to give you a
statement of how many times you can rework a board like that and still have a
warm n' fuzzy...I could give you one; "None"...I'd feel a little more
confident saying that, but is that real? ...probably not.

There's a lot of varibles that will determine what one would want to put as
guidleine, or limit, of how many times you can rework something and it still
be okie-dokie. First thing, is what is this? A control board for a pinball
machine, or the target aquisition board for an ICBM? The end use is something
that'll affect how much you want to risk. I've heard things like 5 reflow
cycles, down to only 3...it just depends on what it's doing.

The the type of parts will play a big factor too as I'm sure you're aware.
Maybe what you should do is find out what your biggest failure has
been...determine if it's part or rework related, and then set some sort of
limit from that.

-Steve Gregory-

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