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June 19, 2013

PoP Assembly Options

Which of the following is suggested for (PoP) package on package assembly?

Option 1: Use a fixture to support the lower package, reflow the top package on the bottom package. Then place the PoP on the main circuit board and reflow with all other components. 

Option 2: Place all components, including the bottom package of the PoP on the circuit board and reflow. Then place the top package of the PoP and reflow with modified ramp and zone reflow profile. 

Any suggestions or references would be appreciated.


Experts Comments

I would go with option 1, the assembly process first, putting all the components together and verifying they are ok prior to placing them as an assembly on the printed board. This also allows the  use of a different solder alloy to mount the two components together, which would be problematic with mounting them all at the same time.

Assembling them together would allow many changes of misalignment due to vibration of the equipment and conveyor.
Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
Consider Option 3 for high volume processes - Place all components in normal SMT, including the bottom package. Then place the top package after a flux dip. Reflow in a normal SMT process.

The flux method for the top package would be similar to a pin transfer method using gel flux. I believe there are some pick n place manufacturers who are making equipment for this type of process.
Stephen Schoppe
Process Sciences, Inc.
Stephen Schoppe is President of Process Sciences, Inc., and has 19 years experience providing SMT services to electronics manufacturers. Stephen provides consulting to several Fortune 500 clients on solder and SMT processes, and is a frequent guest speaker at SMT industry events.
Of the two options presented, Option 1 results in one less reflow cycle for the entire assembly, and that is a good thing. I would suggest that there is also a third option: mount the top POP package (using tacky flux) during the same placement program as the bottom package, and reflow together. This would require a film fluxer or other means of rapidly, accurately fluxing the component.

I do not know which approach is most commonly used today, however if I were implementing the process, I would prefer this third option due to the combination of process flexibility, reduction of reflow cycles, and reduction of labor/time.
Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
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