Part of the equation for reliable solder joint formation is time, but
temperature is also a factor.  Wave soldered joints are made in 3-5
seconds and a re very reliable, but the temperature is typically a minimum
of 240C and more commonly 250C or higher.  The higher temperature
accelerates the intermetallic formation process, which is the heart of
reliability in solder joints.  At issue in reflow are also the questions of
how high of a temperature can the parts and/or the flux take, how fast
of a temperature ramp rate can the paste endure without causing solder
balls, what are the thermal shock limits of the parts, etc.  These issues
tend to dominate the reflow process and the net effect is slower ramps,
lower peaks and longer dwells in order to achieve the reliability that
comes with good intermetallic formation.

Jon A. Moore
TRW

>>> David Whalley <[log in to unmask]> 01/28/98 09:19am
>>>
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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