Story Time

Raspberry Pi Repair

Two years ago, I blogged about an issue with my Raspberry Pi 3 A+. I accidentally shorted out the 3.3V voltage rail with a GPIO pin which was probably set to put out 5V and since then, the Raspberry Pi stopped working properly. In my last post I wrote, that I was going to purchase the corresponding voltage regular IC and attempt to repair the Raspberry Pi on my own.

Unfortunately, this story happened two years ago and I was not updating my web site for a very long time. Thanks to a visitor called Christoph who wrote me an email asking for a status update on my repair, I now decided to give you all an update on my repair.

As already stated in my last post about this topic, I was buying the replacement PMIC (Power Management IC) with the part number MXL7704-R3 from AliExpress. There were a lot of offers but only a few of them were actually the” R3″ version and the reviews of other buyers confirmed, that this chip is working with the Raspberry Pi.

As usual with AliExpress orders just a few weeks later, the chip arrived well packaged and I had a look at it. It was quite terrifying to solder such a small QFN32 package with just hobbyist tools at home, but for the price of a complete new Raspberry Pi I would for sure just give it a try. Since the 3.3V rail was gone and a measurement with the multimeter just showed no voltage at all at the relevant 3.3V pins, I was at least sure to have found the culprit of my issue.

I have the usual cheap lab tools, nothing special. A regular soldering iron – at that time of the repair, I was using the popular TS-100 soldering iron from MiniWare with a beefy power supply which can supply up to 4A at around 19V. Additional to that, I have a cheap hot air reflow station and a desoldering gun. All very basic and cheap stuff, no big brands. However, I was still very confident to get the job done with my tools.

Since we are talking of replacing a QFN32 IC that has a huge exposed pad on the underside it was obvious to use the hot air station for the job of removing the IC from the Raspberry Pi. For some reason, I was not considering the fact, that the Raspberry Pi is not one of the usual 2-layer boards I usually deal with. Instead, I guess it is a 4 or even 6-layer board with probably dedicated voltage planes – at least one for Ground. That implies, that there is a huge thermal mass and a lot of heat required to get the chip desoldered and soldered as well.

It took me an unusual long time, until the chip became loose but I got it finally removed from the board. I then used the regular soldering iron to clean up the solder pads with a lot of flux from the remaining solder. This was the most difficult part. The soldering iron just couldn’t compete with the thermal mass of the Raspberry Pi board and it was incredible hard to get an even surface with solder wick and flux. However, somehow I managed it to get most of the remaining solder removed without ripping of pads from the board and I finally had a state, where I could start soldering on the new replacement IC.

First, I prepared the IC itself with a little bit of flux and solder and I pre-tinned all pads on the underside. I carefully placed the IC on the pads and used again my very cheap hot air station to solder the IC on the board. Still struggling with the huge thermal mass, it took me an unusual long time to get the chip soldered. Right after I thought “this is enough heat for this board, otherwise I roast to much of the surrounding component” (which were “protected” by some fake Kapton tape), the solder job didn’t look as promising. Obviously I used way to much solder for my pre-tinning and a lot of solder was around the chip bridging some pads.

To solve that issue, it took a lot of effort and patience – and heat in particular. I was really worried about the board taking additional damage from the excessive heat I was putting onto the pads and the new replacement voltage regulator.

To be honest, I did not expect, that the replacement of this IC would be that challenging. I would rate myself as an experienced hobbyist soldering guy with a lot of experience in also SMD soldering down to 0603 parts without any issues. These QFN chips without or only very tine exposed pads however, are still very challenging for me to this date.

So, the new IC was soldered on the Raspberry Pi and it somehow looked ok. I did some basic continuity testing with the multimeter and checked for shorts using a magnifier or even a USB microscope. I was confident enough to plug the Raspberry Pi in – let’s give it a change, it was already dead… And to my surprise, it actually worked. The Raspberry Pi 3 A+ was revived and back alive working without any issues. That was a very happy moment and the whole effort was worth it.

As a small conclusion, if you have the right tools (at least a very good soldering iron and a hot air reflow station) and you have some experience in SMD soldering, I would encourage you to at least give it a try to replace the PMIC. The MXL7704-R3 ICs are not too expensive, if you get them in China for example through AliExpress and you only can loose a little bit of time and a few bucks. On the other hand, you might save a (currently very valuable) Raspberry Pi!

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