Hi Ioan & All,
You need to consider what is actually happening when reflow soldering the SJs
underneath a BGA.
(1) the BGA is a 'cold' heat sink;
(2) the PCB pad underneath the BGA gets heated by conduction of the PCB;
(3) the paste heats up with the PCB, the fluz activates, the tiny solder
paste balls, become pasty above Solidus [at this time the BGA solder ball is way
below solidus];
(4) the solder paste melts and forms a puddle/pillow/whatever-you-call-it,
taking on the shape of the still solid BGA ball;
(5) conduction heat transfer takes place between the molten solder paste
solder, and the solid BGA solder ball;
(6) the BGA solder ball reaches Liquidus on the bottom and gets pasty [the
rest of the BGA ball is still below Solidus;
(7) the BGA ball reaches Liquidus on it surface and melts;
(8) of the consistency/surface tension of the molten BGA ball solder is
right, the solder from the paste and the solder from the BGA ball join together to
form a solder joint.

Ioan, you got stuck somewhere between (4) and (7).

Werner

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