Help: RP2040 gets hot

[girishji]

I created pcb using RP2040 mcu just as Pico guide recommended. But when I plugged into computer it does not get recognized as UF2 disk. More worrisome is that both mcu and crystal instantly gets got and crystal makes buzzing noise.  USB is drawing 600ma. Looks like there is a short but I can't find. Is it possible mcu has short inside?

[pcprogrammer]

It will help if you post information about your pcb, like schematic, pcb layout and pictures of the assembled board.

[girishji]

I hand soldered the MCU. I buzzed out a short b/w 2 adjacent smd pins (3v3 and 1v). Maybe this is the cause. I desoldered the mcu completely. I am going to try soldering again. These qfn packages are a nightmare to solder.

Does this type of short cause permanent damage to rp2040?

[Berni]

This might have sent 3.3V into the core supply and this might have killed the chip indeed. Id solder in a new chip rather than reusing it.

QFNs are actually the easiest small pitch component to solder as long as you apply a generous amount of flux before soldering. The solder bridges between pins fix themselves by simply running the soldering iron along the whole side.

[MikeK]

Doesn't the USB boot disk functionality exist as part of the firmware?

[girishji]

This might have sent 3.3V into the core supply and this might have killed the chip indeed. Id solder in a new chip rather than reusing it.

QFNs are actually the easiest small pitch component to solder as long as you apply a generous amount of flux before soldering. The solder bridges between pins fix themselves by simply running the soldering iron along the whole side.

How do you deal with the ground pin at the center?
(I didn't use flux before and this caused the bridge)

[eutectique]

How do you deal with the ground pin at the center?

Oven. Hot plate. Hot air gun.

[SiliconWizard]

UNless there's something wrong with your schematic, the culprit is very likely the soldering indeed. Yes soldering QFN packages is tough if you don't have the right technique. What kind of equipment do you have?

[girishji]

Thank you everyone for advice. It finally worked after couple of tries. I could not get the solder ball rolling with the soldering iron. Molten solder would break off and stick everywhere even with lot of flux. Then I tried hot air but the solder paste would not melt when it got mixed with flux. Then I tried another brand of solder paste (says lead free) and it worked flawlessly.

[Berni]

Thank you everyone for advice. It finally worked after couple of tries. I could not get the solder ball rolling with the soldering iron. Molten solder would break off and stick everywhere even with lot of flux. Then I tried hot air but the solder paste would not melt when it got mixed with flux. Then I tried another brand of solder paste (says lead free) and it worked flawlessly.

Solder paste goes bad over time just like food. We have to buy new solder paste about once a year and we keep it in a fridge. Yours has likely gone past its expiration.

Perhaps you don't have the right flux or the right kind of soldering iron tip. Normally with a QFN you can just drag a ball of solder around the whole chip and it only small parts of it flow over the pin nicely. There are plenty of youtube videos on how to solder small chips.

For going with hot air the easiest method is to tin the pads by covering them in flux and running a soldering iron over them. With a bit of skill or trial and error you can get all the pins to have a similar quantity of solder on it (center pad should have minimal solder tho as it has nowhere to go). Then you clean the old flux, put new flux on, put the chip on, then take hot air and start heating the area of the PCB around the chip (don't point directly at the chip, it will get hot anyway, the PCB is bigger and so harder to heat up). Once it gets hot enugh you can see the chip center itself into place, give it a gentle tap with tweezers to make sure it sat down well and you are done.

[girishji]

You maybe right on the solder paste. It is from aliexpress and there is no expiry date.

I have seen videos of people using solder ball technique. My skills are below average. I used a thicker tip (hoping it holds heat better) but I got too many bridges.

Your hot air technique seems cleaner. I used the following video. It is quite forgiving, but I had to remove excess solder (bridges) using a wick just like he did.

https://youtu.be/QCh0t2bu5kk

[tszaboo]

You don't need paste for a QFN Package. Just tin the pads with your regular iron and solder, maybe add a bit of flux if you have, heat up the pad with hot air (~20s 10-20mm distance, based on heat output), place the chip (keep heating) and then when the solder is melted, squish down the chip with tweezers. It will push excess solder to the outside, making small balls out of it. Touch up the pins from the side if needed and done. Works better with lead solder, and if you have temperature control for the hot air, it's even better.
You can easily solder QFN with those 2000W heat guns that are sold for 30 EUR.
I think once I'll try doing it with a hair dryer just for the giggles.

[girishji]

I soldered another QFN. For those who suggested tinning first and then using hot air, well thanks. It is the most reliable technique so far. Chip floats into place even after squishing.

[tooki]

And remember the other important lesson here: don’t buy cheap Chinese solder (paste or wire) or flux. Name brand solder is a lot more expensive, but you use so little of it that the cost per board is basically irrelevant. Your sanity and the cost of components, on the other hand, are very much relevant, and it’s not worth jeopardizing either one to save €0.003 on solder.

Good quality solder paste, stored refrigerated, lasts quite a while. I’m using up a tube of MG Chemicals paste I bought in 2016 or so and it’s still totally fine.

You’re in Poland, so ordering from TME should be cheap and easy. Buy solder from there.

[m98]

Also, for flux, you really want to use a flux gel, as it stays active longer and seems to be more effective in the prevention of bridging.