Electronics > Manufacturing & Assembly

Issues with soldering QFN components in reflow

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richardmartino:
Hi all,
I've been having trouble soldering some QFN packages with exposed pads on my boards. I got a stencil and have been using ChipQuik SN63 solder and I'd like to think I have been applying the paste properly? See attached picture, one of my paste-ing jobs.

I have been using both a reflow plate and a toaster oven I got from Amazon. Both of them have reflowed the solder nicely, I just can't get the devices to respond to programming (they are ESP32 Pico D4). I'd say it was a design issue, but I've gotten 1 out of 4 to take a program. Not only does it take a program, it works as expected for the most part.

Does anyone have advice for a beginner to reflow soldering?

ataradov:
Your smaller device has more paste than necessary, but generally this does not look too bad. Pads do not need that much paste. The exposed pad itself is typically small, and the excess solder tends to lift the chip.

The common solution in a non-production environment is what Louis Rosmann is doing - hot air on the chip until it reflows, press on the chip so that excess solder squeezes out, collect it with soldering iron, reflow again to finally seat the chip. It is labor intensive, so for production you tune the amount of paste.

Have you checked for shorts? Do you have pictured of the solder joints? Do they look ok?

If you have one reference board, it should be easy to tell what is wrong with the others.

48X24X48X:
It looks like you pushed in the paste instead of holding the squeegee at 45 degree angle and moving across the board resulting in too much of paste on the pads.

richardmartino:

--- Quote from: ataradov on May 18, 2021, 05:21:26 am ---Your smaller device has more paste than necessary, but generally this does not look too bad. Pads do not need that much paste. The exposed pad itself is typically small, and the excess solder tends to lift the chip.

The common solution in a non-production environment is what Louis Rosmann is doing - hot air on the chip until it reflows, press on the chip so that excess solder squeezes out, collect it with soldering iron, reflow again to finally seat the chip. It is labor intensive, so for production you tune the amount of paste.

Have you checked for shorts? Do you have pictured of the solder joints? Do they look ok?

If you have one reference board, it should be easy to tell what is wrong with the others.

--- End quote ---

On visual inspection, it is kind of hard to see where the issue is. I think the issue might be too much solder on the EP or other pads. The joints look good, but I do not have an X-ray inspection device  :( Do you think the cheap-o 858D would be powerful enough? I tried it before and actually did the press down method, but that was before I bought a Hakko soldering iron with a plethora of fine tips, so I might try that again. Attached is a picture of a not working board.


--- Quote from: 48X24X48X on May 18, 2021, 06:15:41 am ---It looks like you pushed in the paste instead of holding the squeegee at 45 degree angle and moving across the board resulting in too much of paste on the pads.

--- End quote ---

I will try getting less paste on there! Thank you!

thinkfat:
It is hard to see, but I think the result after reflow is not OK. It seems the fillet has not formed as it should. C13 for example looks like cold joints. Does your toaster do any temperature profile? Or do you just heat up until the solder melts and then switch off? It looks a little like it took too long to heat up and the flux had evaporated before reaching peak temperature.

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