At risk of raising a topic that has already been discussed at length, if that is the case then please point me in the right direction.
We have a Lead-Free assembly that has BGA devices on both sides of the board. Any recommendations as to how we might go ahead assembling this board. My concern off course is the secondary reflow of the underside BGA.
First piece of advice: build the board as you would any other board. Probably, you will be surprised at the holding force of the solder of the connections of the BGA, because the surface area of the solder is very large compared to the weight of the component.
Additional points are: * Gravity increases the stand-off of a first-side BGA slightly as the solder goes liquid during the second reflow cycle. Increased stand-off is a good thing, it helps make the solder joint more compliant. * Keep the heavy-weight BGA on the second side to be reflowed. * If the above fails; print paste, dispense glue [your favorite chipbonder] dots at the 4-corners of the device, place the part, then cure/reflow.
Thanks Dave. What you say makes complete sense. We ran a job a short while back in this way (though without bonding) and found we had a much higher than normal failure of the BGAs which I can only attribute to the secondary reflow. As I have only got on-board now - I must still investigate the failure mode of the affected boards.
I was just hoping for a few general pointers from people who have perhaps already been through the mill so to speak on this topic.