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FONA 3G Cellular LiPo chip failure
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FONA 3G Cellular LiPo chip failure

by Smeds on Mon Jul 27, 2020 11:54 am

Hi,

I have purchased 3 "FONA 3G Cellular + GPS Breakout" boards over the last several months. I am using these in the "Rotary Cell Phone" project designed by Justine Haupt. The battery I use in the project is the one recommended (Adafruit product 258) and this battery has never been connected incorrectly. I have now had two of these boards fail with what seems to be burn out of the the MCP73831 LiPo charger. The first failure I attributed to perhaps shorting something inadvertently (not that I thought I did that) and it wasn't clear to me that the LiPo chip was at fault. But, with a second one now failing, I investigated a bit and realize that the boards are functional with the exception of the LiPo chips, which have failed. My phone build is complete and the phone is/was functional but I've been debugging software on the phone. I don't believe I've done anything "wrong" and have only had the boards connected to computer Usb ports for debugging the phone software. The failure is that the battery just stops charging. I've discussed this issue in the Rotary Cellphone forum (may need to request access):
http://skysedge.us/forum/viewtopic.php?f=4&t=1496

It was recommended there that I contact Adafruit and request exchange/repair of these parts. It was also recommended that I request, if possible, the current programming resistor (R3) be changes from 2k to 3k.

Thanks for any help you can provide!

Smeds
 
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Re: FONA 3G Cellular LiPo chip failure

by adafruit_support_mike on Thu Jul 30, 2020 12:54 am

Could you post a photo showing the LiPo you're using please? The whole thing, including the wiring and JST-PH2 connector.

800x600 images usually work best.

adafruit_support_mike
 
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Re: FONA 3G Cellular LiPo chip failure

by Smeds on Thu Jul 30, 2020 9:48 am

Hi -- photo is attached
FONA and battery, 800x600.jpg
FONA and battery, 800x600.jpg (176.7 KiB) Viewed 13 times

Smeds
 
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Re: FONA 3G Cellular LiPo chip failure

by adafruit_support_mike on Fri Jul 31, 2020 4:43 am

Thank you. I wanted to make sure the LiPo's JST-PH3 plug was wired with the correct polarity. There are two ways to arrange the positive and negative wires in a JST-PH2 connector, and unfortunately you can find both in the market. The one shown above does match the FONA.

In the other forum, you mentioned the first FONA getting hot. That's almost always a sign of a short circuit, and often a sign of power being connected backwards. Since the LiPo has the right connector polarity, we'll have to call that an accidental short to GND somewhere.

You also mentioned replacing the MCP73831. The good news is that soldering SOT-323 parts isn't too hard. To remove the existing chip, cut the pins with a flush cutter to get rid of the package, then desolder what's left of the pins one by one. Use some solder wick to pull most of the remaining solder off the pads, then tin the pad on the upper right corner in the photo above. Place the new device over the footprint, trying to line the pins up with the pads reasonably well, then heat the tinned pad to tack that pin to the PCB.

If the pin at the opposite corner (lower left above) needs to move more than about half the width of its pad to be in the right place, reheat the tacked joint and nudge the pins into better position with a toothpick. Once you're within half a pad of the correct location, make a solder joint on the lower left corner. Keep the solder hot and use a toothpick to torque the pin into its correct position, then remove the iron and let the joint cool. Go back to the tack joint in the upper right corner and reheat it to release any stress on the pins.

Once you have two opposite corners soldered down, doing the rest of the joints is easy. Surface-mount soldering is all about the surface tension of the molten solder, so the parts are designed to form good joints automatically. If you get a bit too much solder, or a solder bridge between two pins, use a piece of solder wick to pull away the excess. The pins and pads will hold enough to make good joints as you do.

Regarding the second FONA, it doesn't sound like there was any obvious reason for the MCP73831 to fail. Two boards failing the same way for unrelated reasons is unusual, but not impossible.

When did you get the second FONA?

adafruit_support_mike
 
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Re: FONA 3G Cellular LiPo chip failure

by Smeds on Sat Aug 01, 2020 1:17 pm

Hi -- I'm sure there is a short but I suspect it is internal to the LiPo charger chip. The FONA gets hot in the vicinity of that chip. I'm reluctant to reconnect this chip, as is, to USB because of that. If I do connect it, the FONA is otherwise fully functional -- I can connect to the serial port and make and receive calls as long as the battery has charge. Only missing functionality is the LiPo charging.

The second FONA was purchased from Digi-Key on 7 May 2020. This one has failed similarly but there is no noticeable hot-spot on this board, suggesting, I think, that the LiPo chip on this has burned-out internally but has no lingering internal short.

Smeds
 
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