How to connect external switches to this circuit board

Question

I am circuit bending a walkie-talkie to move some of the switches around and embed it in a shoe.

One part I'm not sure about is how to replace some momentary switches on the front with external switches I bought. I will be hand soldering this, and I don't want to fry the circuit board.

Here's the front of the circuit board.

One can peel up the white film that adheres to the board to reveal the leads.

The back of the circuit board is quite dense, and I'm not sure where to find the leads from those switches on the front.

Here is the momentary switch that I want to connect in place of the white film stuck on the front.

Answer 1

Here's how to solder it neatly:

As Polynomial suggested, First rub the gold pads with a little fine sandpaper (1200 grit), or a salmon Garyflex block. Clean it with some methylated spirits. Run a little hot solder onto the circular ring, and the spot in the middle, taking care not to bridge them.

Trim both ends of your wires to 1.5mm, then tin them. You may find that the insulation shrinks back a little. Twist the wires together. Lay the wires on the PCB so that the tinned ends sit nicely over the solder. Tape them in place. Add a tiny amount of flux to the solder. With tweezers, position one wire over the solder on the ring. While pressing it with your thumb, melt the solder, so that the tinned part of the wire sinks into the solder. Do the same thing for the wire going to the centre spot.

Test that the wires work by touching the bare ends together. Does this do the same as a button push?

Now, clean the solder joints with some flux cleaner and a toothbrush. And lastly mix up a little potting compound or other epoxy, and spread it over the solder joint and a little over the wires so that they are safely glued down to the PCB. Once the glue has set fully (be patient), you can attach the button. Don't forget to use some heat shrink tubing.

This will create a strong, reliable solder joint that should last a lifetime.

Answer 2

From the looks of it, the buttons on the front are pressure buttons (not sure of the technical name), which simply consist of two conductive rings with a metal plate hovering over them. When you press the button, the plate hits the rings and bridges the circuit.

I'd guess that the outer ring is connected to a transistor, since the trace from it goes to the middle of the three gold spots below the "KEPC" text, and one of the other vias goes to what looks like a ground plane. You could try to figure out which component that is on the other side. It also looks like the center ring goes all the way through the board (or at least attaches to an internal via), so that might be tricky to find.

You'd probably be best to just solder directly onto the rings themself. Just give them a quick rub over with some fine sandpaper to remove any oxidised surface, and carefully solder a wire to the surface. It can be fiddly work, so you may need a clamp to keep it all in place.