Cara

OH!!! but yes! With any flux/paste, there is an optimum preheat profile (time and
temperature) to obtain a max SIR (of course, different for each flux/paste). This is
something that the lead-free enthusiasts have not yet cottoned on to, that the flux
formulation must be tweaked for optimum results and that the SIR will drop, partially
because the preheat profile will need modifying for optimum results, and partially
because of the higher liquidus. This will certainly apply to "no-clean" and also
probably to cleaned assemblies, because, by the same token, the residues will be that
much more difficult to remove.

One of the major factors is the time that the assembly is above the substrate Tg. The
Tg itself is not a sharply defined parameter, even if laminate manufacturers quote it
as such. It depends on a number of factors, such as the MW of the resin which varies
typically in a 3:1 ratio, the absorbed humidity, etc. This means that if the quoted Tg
is, say, 130 deg C, and the humidity is high, the real Tg may be over a range of, say
90 - 130 deg C, with a random distribution over the surface. (Dry, it may be 110 - 150
deg C.) Above the Tg, there is more likelihood of ad/absorption of flux components,
especially oxygenated long-chain molecules such as some carboxylic acids (activators)
and alcohols (vehicles, such as polyglycols). This is one reason why amino-acid
hydrohalide fluxes in a rosin matrix, correctly used, often exhibit superior SIR
values, because the rosin molecules, being tricyclic, are simply too big to enter too
significantly into the resin matrix, even above the Tg.

In practice, changes of say +/- 10 deg C or +/- a few seconds will probably have little
effect, but it may be that, beyond these limits, the drop will start to accelerate.

Brian

"Cara.Startek" wrote:

> Does pre-heat temperature influence SIR test results?
>
> Cara Startek
>
> Process Engineering
> http://www.leitch.com
>
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