TI OMAP Placement-Experience Counts!
BESTProto has experience in assembling the TI OMAP processor PoPs for a variety of prototype assembly applications. We use a two-pass assembly which has an intermediate step in which the memory is first mounted ontothe OMAP processor. Then these two parts are placed in a carrier tray and reflowed. These joined devices are then mounted on the circuit board and the finished board is reflowed a second time.
Some assembly houses specify a slightly larger than normal opening to overprint the paste. However, for the BeagleBoard, because the pads are soldermask-defined, our experience tells us that no overprinting is required.
A critical aspect of PoP assembly such as this is the type and amount of warpage that can occur during reflow of the processor and the memory. BGA package warpage during reflow soldering can cause open solder jointfailure. With PoP, it is more critical at the top solder joints (memory to processor) than at the solder joints between the processor and the circuit board. Correct circuit board design prevents most common problems at that interface because with correct circuit board design, sufficient solder paste and properly sized pads between the paste and the BGA are all that are required. However, in most cases, only flux is applied to the memory device before it is reflowed. Thus, there is no additional paste volume to help fill the void as the two packages separate during reflow.
Minimizing this warpage is a trade-off among materials, temperature control and time. Warpage is always present, and it may not always be repeatable from batch to batch.
Placement of PoPs can be repeatable at the prototyping level as long as good assembly practices are used and shops with the right level of experience so that when the inevitable problems crop up they can be handled.
Call BESTProto today to see how we can be of assistance in making your proto build with PoPs a success!