Hi, Josee,
My often-repeated philosophy is never subject a board t more heat than is
absolutely necessary, as the effects are cumulative. My answer to your
question is that it's not the number of (undefined) thermal cycles that
kill a board, but the effect of cumulative heat exposure on your product.
Time and temperature increase the thickness of the brittle intermetallic
layer that is the solder joint, rendering the solder joints increasingly
susceptible to fracture. The less heat you pump into a board (except for
necessary heat exposure for soldering) the thinner the intermetallic layer
will stay and the longer the joints will last.
There are no rules for how many times a board can be heated - there are
guidelines, but like all guidelines, they are not specific for your
product. Testing, recording and experience will tell you how much heat your
product can withstand before reliability is affected. Some people say you
increasingly risk de-wetting or oxidation problems if you expose a given
area of board to more than 3 extreme thermal excursions, others say that
they've reworked a site 15 times without encountering any problems. Me, I
would start to be more cautious after the third rework. If the thing still
solders OK, no problem. If it doesn't then you will be faced with a bit of
cleaning up to do.
The main danger is physical damage to the pads - they come off with too
much attention, but even then, they're repairable and there are some good
PCB repair kits around for this purpose. It would be a rare thing to scrap
an assembly simply because it's had a few thermal cycles, but you may face
a bit of recovery work, depending on what happens.
Peter
Josee Wolfe <[log in to unmask]> 15/02/2003 12:51 AM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to Josee Wolfe
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] [Fwd: TechNet question ...]
Hi,
I am looking for a guideline as to how many heat cycles on a product is
considered acceptable? Once it is beyond this number, does the product
need to be scrapped? Also, what is considered one cycle? For example, is
a single side reflow with one BGA component rework considered 3 cycles;
one from the oven, two from rework (removal, & attachment).
Thanks,
Josee
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