There are two normal ways of doing POP
1) Place and solder lower device on PCB
Place and solder upper device on top of lower device
2) Place lower device on PCB
Place upper device on top of lower device
Solder both in one run
The option you suggest soldering the two devices first and then mounting
both to the PCB I have never heard about. That being said it might work, but
I have never heard about anybody using that technique... It might be hard to
predict the exact placing of the lower device balls when it's "picked up" by
the upper device?
Any ordinary modern PCB assembly house should be able to do POP, but they
simply just don't know, since they haven't tried. It really doesn't contain
a lot of magic except of the fine-pitch control and the ability to place a
BGA at another Z-axis location than 0 mm...
I hope this helps you in the right direction? - Best regards - Good luck
Søren
Clint W Cooley
Circuitco Electronics LLC
675 North Glenville #195
Richardson, TX 75081
main 214.466.6690
fax 214.466.6694
mobile 214.683.7309