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Reflow using Maker Paste and hotplate, questions...
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Reflow using Maker Paste and hotplate, questions...

by westfw on Wed Jul 31, 2019 4:43 am

So I've now made my first foray into reflow soldering a fine pitch QFN (0.5mm; I guess not THAT fine), using some of the low-temperature "Maker Paste" from Adafruit, and an old hot plate...

I have no stencils, but based on some prior experience with two-lead components, I masked off the QFN pads with some painter's tape to keep things from getting too out of control.

I still got the large solder balls shown in the photo; is this just from having too much paste? The layer I wound up with was pretty thick compared to what a stencil would give (but nicely held the part in place!) The "maker paste" isn't terribly consistent - I got pure flux out of it at the start, and I suspect it could use mixing before application, somehow.

If my paste later will be thicker, can I dilute the paste with other flux instead of trying to make it thinner?

Is the collection of these solder balls likely to have sucked away solder from someplace else that needs it?

The balls cleaned up pretty easily with an iron, and the board looks pretty good (as much as I can see, anyway.) Is there likely to be similar extra solder under the chip, shorting things out, or will that "magic surface tension" trick squeeze any extra mostly to the outside?

Do you have any tips for checking for shorted pins on a micrcontroller, before the "apply power and see if it smokes" phase?

Thanks... (this has worked better than I expected, but not quite as well as I'd hoped...)
reflow.jpg
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Re: Reflow using Maker Paste and hotplate, questions...

by adafruit_support_mike on Wed Jul 31, 2019 12:59 pm

Yeah, that’s a classic symptom of excess paste. The total volume of solder in the deposited paste is larger than the amount pulled into the joints by capillary forces, and the excess does the next best thing it can: get as much volume inside as little surface area as possible.

The balls aren’t likely to have starved any of the joints. The surface energy and capillary forces are stronger than the squeeze-out. As long as there was paste close enough to wet every pad and pin, you’ll probably have good joints.

Cleaning up the excess with an iron is pretty normal for paste applied without a stencil. It’s basically the same as pulling solder bridges away from SOIC pins with solder braid, just at a smaller scale.

Thinning the paste with additional flux could lead to some minor problems. The chips float on flux, so the layer of flux under the chip lifts the pins off the pads. Too much flux will can lift the chip too high, possibly far enough that joints won’t form properly.

You can try thinning the paste with alcohol, then letting it evaporate back to near its original consistency before placing the chips.

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Please be positive and constructive with your questions and comments.