Hi Gordon,
As I have said in another e-mail, we need to be specific about what we are
talking about. what can be done in a lab under controlled conditions is not an
indication of what is possible in production.
For a 'reflow' process the time parameter is fixed by the belt speed which in
turn is fixed by the required through-put.
Sure, you can solder just above Liquidus, but in order to assure that enough
heat is delivered, you you need to crank up the peak temperature in a 'reflow'
process.
For the typical through-put in a reflow oven, you need to reach Liquidus +20C
for about 3 to 5 seconds at THE ONE solder joint in the whole assembly that
heats up the slowest [typically the center-SJ of the largest BGA].
What most people do not realise is it is conduction heat transfer through the
PCB that heats the center-SJ of any BGA, and it is the solder paste that
reaches its Liquidus long before the BGA ball.
Sure, for SnPb solder paste you can make a metallurical contact at somewhat
lower T's than for SAC-paste, howerver you get very odd localized compositions
as Dave hillman has demonstrated in his experiments that have been published.
Indeed, the Pb-ban of this EU-RoHS directive has resulted in a big mess, and
there is no easy way around it.
Unfortunately, I do not have a company paying my way, otherwise I would be
there.

Werner

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