Dumbass. READ THE POST. It is stated right there.
There's IMHO no reason to become offensive. I was reading your reply
carefully including the two words "surface tension" but it was and
still is too short for me to make all doubts go away.
I'm just not sure if surface tension is enough to solder chips to BOTH
sides of a PCB which still is and was my original question. I'm well
aware that surface tension is responsible for the chips to align
propprely during reflow soldering. Maybe I'm wrong but so far I
thought that either the chips have to be glued to the PCB (at least if
they reside on the bottom) or else it can't be done. The question
arose when I first looked at one of those high density 2GB PC memory
modules where there are BGA chips on both sides of the PCB.
I simply figured that if one side of the PCB is solderd, those chips
would fall off, or could be misaligned if the PCB is brought back into
the oven with those already soldered chips on the bottom. I also could
figure that the weight of the PCB itself would have a negative impact
on the chips on the bottom and that there is special care needed if
the chips on the bottom are different ones and not of the same height
etc.
So either "SURFACE TENSION" is not everything there is to say about
it, or then you are right but you then may understand now where I
have/had my doubhts and alas why I ask again.
I don't care wether X-Ray equipement is $200K or $200 zillions. I just
wonder how they do it for personal interest. Sometimes I just see
something and wonder how it's done. I'm under the impression that this
question is on topic in sci.electronics.basics. Please accept my
apologies if not.
Could you or someone else therefore either confirm that even for
soldering BGA chips to both sides of a PCB "surface tension" is enough
or then eventually be kind enough to elaborate one the topic a bit
more?
TIA
Markus