Need Help with BGA Soldering in PoP Assembly

Unless the PoP BGA has considerable thermal mass, then it's highly doubtful there is much of a temp delta from the circuit board to your ball interface.

There are several things that can play into an insufficient solder situation.

A few things to think of: Are you certain your screen-printing process is under control? Is the component remaining planar to the PCB during reflow? Do you have solderability issues with your PCB... In other words is all of the solder paste wicking up onto the ball because of poor PCB solderability ?

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It sounds like the lower BGA isn’t getting enough heat during the reflow process, likely due to the upper component in the PoP assembly blocking some of the heat. First, double-check the thermal profile to make sure it matches the solder paste specs—specifically the preheat, soak, and reflow stages. You might need to tweak the ramp rate to ensure the heat is transferring evenly without causing thermal shock. If the profile looks good, try extending the time in the soak or reflow zones or slightly increasing the peak temperature to help the heat reach the lower BGA.

Another area to look at is the solder paste application and stencil design for the lower BGA. If there’s not enough solder paste or if the stencil apertures aren’t optimized, you won’t get proper solder joints. Make sure the paste volume is correct and evenly applied. Also, check if the lower BGA or the PCB might have any warping issues that could interfere with the soldering process.

You can also try slowing down the conveyor speed to give the assembly more time in the oven, especially in the soak and reflow zones. If your reflow oven has bottom-side heating, increasing the intensity there can help get more heat to the lower BGA. Another option is exploring vapor-phase reflow, which can be more effective for PoP assemblies since it ensures even heat distribution.

Lastly, the X-ray inspection results should give you clues about the root cause. Look for signs of cold joints, insufficient wetting, or solder bridging. If tweaking the profile and reflow parameters doesn’t work, consider trying a different solder paste or flux designed for challenging PoP applications. If all else fails, reaching out to the oven or solder paste manufacturers for advice can often point you in the right direction.

This message was posted the Electronics Forum @ SMTA

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Consider the thermal mass of the PoP assembly. The upper BGA might be shielding the lower BGA from adequate heat. You could test with an experimental setup that pre-solders the lower BGA separately to confirm whether heat transfer during reflow is the issue. slope

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