Marcelo,

Ideally you want to keep the time at temperature as short as possible in
order to minimise intermetalic formation (and/or other damage to the
assembly). Solder paste vendor guidelines are likely to be based on what
they have found to work i.e. what gives reliable results, rather than on
a detailed understanding of the process. Once the solder is fully molten
joint formation can be extremely rapid, but there has been very little
work done that really quantifies the minimum time required to succesfully
form a joint. By reducing your time you may get good joints, but you may
also have reduced your process window so that when the process changes a
little (e.g. when you get PCB's made from another batch of material, or
the weather changes, or the oven calibration drifts (or it gets full of
flux residue)), you may end up getting process defects.

David Whalley

At 08:49 AM 1/28/98 -0500, you wrote:
>Based on the responses received, I may have been too vague.... My solder
>paste vendor recommends approximately 30 seconds from the time the
>solder becomes liquidus, 183 deg. C, to the peak temperature which in
>most cases is from 210 to 235 deg. C.... In my case our peak temperature
>is 215 deg. C.... My question is in regards to this 30 seconds...If I
>know that I have wetting within 10 seconds or so, what are the
>consequences to decreasing the reflow time from 30 to say 20 or even
>10.... If intermettalics formation is an issue, please highlight how
>thick should it be for BGA assemblies and methods to measure it....The
>surface finish on this PWB is Im. Au...
>
>Thanks
>Marcelo

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