In a message dated 06/08/2000 2:35:48 PM US Eastern Standard Time,
[log in to unmask] writes:

> Does pre-heat temperature influence SIR test results?

I'll give my favorite answer - It Depends.  If you are talking about flux
residues, running too cold will increase the likelihood that you have
unreacted or unactivated flux residues, which tend to be more conductive,
especially around humidity.  As such, SIR would tend to be lower, all other
factors being equal.  On the other hand, if you run the preheats too hot, you
run the risk of taking any benign flux residues and causing them to thermally
break down (pyrolize).  Such residues are generally corrosive and cause SIR
failures.  Most flux manufacturers that I know of, during product
development, will run SIR on flux samples that are nominal temp, nominal-20C,
and nominal+20C to examine the window.

If you are talking about bake operations, I would recommend them if you are
doing aqueous cleaning and have low standoff parts.  I have seen IPC-B-36
boards with 4 LCCs on them (5 mil standoff) go past three 15 HP blower motors
and still have water under them.  Such water shoots SIR immediately.
Prebakes are a good idea.

Doug Pauls
CSL

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