BMX160 Dev Board Transfer

[polygnomial]

I am planning to de-solder some BMX160s from a couple of dev boards and throw them onto a PyCubed v5. Wondering if anyone has done this before and has advice! Either way I will update with how it goes.

[cadenhillis]

That was one of the things I had to change for my fork. I couldn't find any 3-in-one IMU's and the BMX160 is just out of stock forever. I had to do a redesign of the IMU ckt with 3 different IC's, magnetometer, gyro, and accelerometer.

I've done similar things during the chip shortage for other parts/applications, so as long as you are comfortable soldering (hot air highly recommended) I don't see a problem.

[maholli]

I've always wanted to add a hot air rework section to my soldering tutorial: https://maholli.com/soldering but still haven't found the time 😞

Here's my 30 second run-down. Let us know how you do!

When doing this type of rework, you need both hands so make sure the board is held in place (but avoid having whatever is holding the board act as a big heat sink).

1. have flux, tweezers, and hot air tool near by
2. set hot air to something like 400C with air flow around "medium." If you have more than one nozzle for the tool, pick one that is slightly larger than the BMX160 QFN package. Too big and the heat isn't properly focused, too small and the heat isn't properly distributed.
3. I use paste flux, so I start by getting some on the tip of my applicator (wooden stick) and hold it with my left hand.
4. Holding the hot air tool in my right hand, I start to apply heat directly over the package
5. After about 3 to 5 seconds, raise the hot air tool up enough such that you can apply flux to one or two edges of the package. Since that area is locally warm the viscosity of the flux will decrease and you let capillary forces pull the flux under the device and to the other sides. Coax it with heat. You want to avoid globing flux on all sides and trapping air under the part which would make it harder to reflow the solder. Add a bit of extra flux (if needed) to get good coverage
6. put the direct heat back down on the part and the flux should activate (you'll see vapor)
7. put the flux in your left hand down and pick up tweezers
8. after consistently heating the part for ~30 seconds (this varies a lot) you might see the parts solder joints start to look more-shiny. with the slightest amount of force you manage, use your tweezers and touch one side of the package and see if it moves. if the whole thing moves (and likely "snaps" back into place), you're ready to remove the part. don't force things if it acts like it's still stuck to the board - just be patient and keep applying heat
9. in one controlled motion, grip the package with your tweezers and raise it directly up
   • if you spend too long trying to get the right grip on the part with your tweezers, they can locally cool some solder joints and you'll end up ripping pads.
   • spend too long heating everything and you cause unnecessary damage to the PCB laminate (pads will be easier to damage, etc... )
   • spend too little time heating things up and you risk ripping pads
   • use excessive force and you risk ripping pads
   • if the part isn't showing signs of reflowing a minute or two AFTER you've applied flux, it may need fresh flux to get the job done. this is something that's hard to gauge.
   • too much flux and it'll get cooked into a "glue" that makes your life hell. this is also something you'll end up getting a feel for with time.

Good luck! be patient! :)

[polygnomial]

Thanks for the detailed response!

[polygnomial]

No luck tonight, just a couple of toasty fingers. I'm able to get the BMX160s off the dev boards just fine, but putting them on the PyCubed was giving me trouble. First, I tried by applying solder using a metal stencil with a BMX 160 that I cut up with a little tab to remove it. Then I applied paste while holding the stencil down with tweezers—I was able to get up to 3/4 rows of pins with good paste coverage. I was unsuccessful with soldering the part on, so I switched to using an iron and solder wire (leaded) applied to the pads. Then I applied a bit of flux to stick the part on, and applied heat from above. I was able to get the part soldered on, and it swam around just a little bit. Also tried heating it up and wiggling it around bit. Any suggestions for soldering the BMX onto PyCubed?

[maholli]

Ok you got the BMX160s off, which is good. Putting them on is a whole other beast. I'll try and give you a quick run down.

Personally what I'd do in your situation is solder bump the PyCubed board and not use paste. Here's how:

1. first you need to clean and prep the bmx160 you removed ->
2. once it's room temp, use IPA and clean any junky flux remanining from the desoldering process.
3. now you have to hold it somehow (even just placing it on something with enough friction to prevent the part from sliding away on you) and use solder wick to clean the pads.
   • this step has the biggest potential for damaging the part since all the heat from the iron has to go into the part/package and there's no where else for the heat to go so it very quickly degrades the plastic packaging of the QFN. try to be as brief as possible.
   • Here's a little video I filmed about using solder wick: https://twitter.com/maholli404/status/1151332270109687808 it may or may not be helpful
   • there can't be excess solder on the bottom of the QFN or it's likely to give you trouble when reflowing
   • finish up by cleaning with IPA again
4. with the part clean, you need to prep the PCB
5. like you tried originally, use solder to create little "bumps" on each of the pads.
   • the bumps MUST be as close to the same size as possible, otherwise you open yourself for a bunch of trouble getting all the pads soldered during reflow
   • one trick is that once you get solder on all the pads, add generous amounts of flux and then go over it with a very clean iron again. surface tension will dictate how much solder wants to stay on the board vs jumping ship to your iron.
6. with the PCB solder bumped, clean very well with IPA
7. once the PCB is clean and dry, add a very small amount of flux onto the PCB footprint and position the BMX160 component with tweezers (no heat yet). make sure to get the orientation correct - verify pin 1 fiducial
8. set your hot air tool to ~400C with air flow low/medium-low. too much air flow and the part will blow away
9. same as removal, start heating the part with the tool. work your way to having the nozzle directly over the package (as close as you can get without bumping anything) over the course of ~10 seconds.
10. at this point, if you added too much flux, you'll see the whole thing float up and away. you'll have to try and nudge it in place with tweezers while maintaining adequate heat
11. after ~60 seconds, you may need to add more flux at this point, but you'll see one or two of the solder joints get molten.
12. for this solder bumping technique, this is your cue to GENTLY apply force with your tweezers down on top of the part. this is "brute-forcing" contact between the bumps and QFN pads since it's hard to get everything completely equal/level. be gentle with the force from the tweezers and make it as directly downward as possible
13. the BMX160 should more or less be soldered at this point, but the hardest pads to solder are the ones that takes the most heat, which in this case could be the ground pad in the middle of the part. So keep applying heat for ~10 seconds longer and hope you get everything soldered.
14. remove heat
15. allow to cool to room temp. unless you're going to spend the time to bake the board out (hour or two at 60C), don't use excessive IPA for cleaning since it's unlikely you'll get it all out (capillary forces are intense).

• while it's cooling, you can soak up some flux with a dry brush & chem wipe if you think it's needed

16. once you're confident things have cooled to room temp, plug the board in and give it a try

• if PyCubed can't see the IMU but the solder job looks decent, unplug the board and give the area a thorough cleaning with IPA. use a brush that can reach proper spaces between the QFN castellations. Dry best you can with wipes then bake out at 60C for an hour or two.

Let me know how it goes!