Hi Vladimir,
We are talking about a 'reflow' process; therefore 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 have a rather unconventional and uneconomic '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].
You further need to remember, that 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.

Werner

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