Changing, Removing, Desoldering - Rfm95 radio module

[Rocketman4992]

Hello all,

Is it possible to remove the rfm95 radio from an Adafruit rfm95 breakout board and solder it onto a Moteino? I have a few of the Adafruit breakout boards and would like to save some money since I'm no longer using them. They are the same freq.

[Felix]

It's possible but you will need good tools and some skill.
Some of the pads are GND and those are thermally much slower to heat up.
I do this type of thing with wide chisel tips (as wide as the radios) on a dual iron station, and I have to be somewhat swift otherwise the boards or their pads can be damaged.
I would also recommend adding some solder to all the soldered pads to help the heating transfer.

[perky]

I do this as well, except I use a heat gun with Kapton tape to protect the radio and surrounding parts. As long as the board is preheated that works well.

Mark.

[Felix]

I would generally avoid using a heat gun because the QFN package used by HopeRF is a custom package of an unknown spec, and could be (not obviously) damaged by high heat if it's not MSL 1 spec as other QFNs. This is because of moisture trapped in the package. Shielding it well would obviously help but the whole PCB beneath has to heat up to ~250C for the pads to come loose.

[perky]

I would generally avoid using a heat gun because the QFN package used by HopeRF is a custom package of an unknown spec, and could be (not obviously) damaged by high heat if it's not MSL 1 spec as other QFNs. This is because of moisture trapped in the package. Shielding it well would obviously help but the whole PCB beneath has to heat up to ~250C for the pads to come loose.

You use the heatgun in a circular pattern over the pads, it's quick enough not to heat up the whole substrate. If it did the solder on all the components would flow, this doesn't happen. You need several layers of Kapton tap over it and the surrounding area so only the pads are exposed, there is a thermal resistance between the pads and the substrate. I've used this technique many times and on other radio modules, the key is to do it fast and make sure the only heat that can get into the substrate is via the pads, then the thermal resistance of the pads and the thermal inertia of the substrate won't allow it to heat up to those temperatures in that timeframe.

Mark.

[Felix]

Mark,
Yes quite an art.
I do have a heat gun and from my memories, I remember trying it to desolder radios, and while I learned the "art" of doing it that way the result was delaminated or deformed PCBs, or burned edges. In some way or another they always seemed to sustain some damage because the heat was always more than a certain limit. So I was not doing a good job, or my gun was too hot or something. I haven't tried kapton shielding but I always used soda can aluminum to shield other parts from heat.
If you only have 1 radio (your first ) to desolder it sounds like a gamble either way
Don't get me wrong, I also damaged one or two desoldering with the chisel tips until I learned that art.

[perky]

I certainly agree that if you haven't done it before the chances are you'll damage something! BTW for things like TQFPs I thread some of that ultra thin laminated mod wire through the gaps between the pins and the package and make it into a pyramid shaped cradle, then use the heatgun in a circular fashion and when flowing apply a small force upwards on the cradle. Pads are damaged by lateral forces when hot, pulling it up vertically is best. Also never attempt to tidy up the pads until it's cooled down otherwise the glue that holds them on is still weak and the pads are likely to lift off.

Mark.

[Felix]

Ok I think Rocketman4992 has some plenty detailed info here,
Thanks for the discussion and sharing the experience