Daan

A good question. But no real answer, other than do tests under your conditions and see what happens. I wouldn't be worried, in your position, but I'd have sufficient concern that I'd put it somewhere in the middle of my "to do" pile. One never knows when it might reach the surface.

"d. terstegge" wrote:

> Hi Brian,
>
> If temperature is so important for the SIR then how about the difference between top and bottom side of a wave soldered board with a no-clean flux ? I mean there's always some flux applied to the the top of the board (with a foam fluxer it's even possible that the gap underneath IC's becomes completely filled with flux beacuse of capillary force). The temperature on the top is much lower than on the solderside. Is this something to be worried about ?
>
> Kind regards
>
> Daan Terstegge
> Unclassified mail
> Personal website: http://surf.to/smtinfo
>
> >>> Brian Ellis <[log in to unmask]> 06/09 9:39 am >>>
> 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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