How to securely solder a wire to a vibrating SMD PCB?

I have a problem with a "4in1" control unit for a RC helicopter. Due to some problems with helicopter setup, it is currently vibrating heavily which makes things even worse.

The 4in1 unit basically has two PCBs with electronic speed control, accelerometer, radio receiver and servo controller.

The receiver has short (say 2.5 cm) piece of wire which acts as a 2.4 GHz antenna and is soldered to an SMD pad. It is also glued to the PCB with hotglue. The SMD pas is around 2 mm away from the edge of the board towards which the antenna is supposed to point.

My problem is that after a while, the solder joint breaks and so does the glue joint and the antenna falls off. Fortunately, I haven't had any crashes due to that, but I'd rather fly with antenna than without.

So I'm looking for tips how to securely solder a wire to a SMD pad on a vibrating board. I already tried just resoldering the wire and gluing it with hotglue, but it seems that it doesn't help much.

Here's the photo of the part. The solder ball is where the antenna should be attached. I removed the layer of hotglue that was over it. Here's full size image, for those who dare click it.

• Any chance you could post a photo of the board and where the wire solders to? Jun 25 '11 at 22:56
• @Matt Jenkins I added a photo. Jun 25 '11 at 23:19
• Solder it back on and just enclose the enitre wire in a silicone / glue. this wont affect the signal. So basically glue the antena to the board.enclose it in glue so it does not vibrate. Jun 27 '11 at 17:18
• The original antenna was made of stranded wire. I switched to solid wire and that helped a lot with antenna falling off. It is a bit heavier, but the mass difference hasn't made any measurable impact from what I can see. Jul 2 '11 at 13:42

A SMD pad is not a good way to attach a wire. That should be done with a thru-hole pad. You might even want two thru-hole pads to loop the wire thru for better strength.

If the wire is vibrating a lot, then maybe a few zig-zags in the wire near the PCB will cushion or decouple the wire motion from the PCB a bit.

• Unfortunately, redesigning the PCB is a bit beyond my capabilities at the moment and the board looks like it has several layers, so I'm not too comfortable drilling a hole. There isn't much space for zigzaging on the board (pad is just 2 mm from the edge and board is heavily populated), but there is a thin plastic cover over the PCB over which I could loop the antenna. How critical would the antenna length be? Jun 25 '11 at 20:24
• I was told it was called stress relieving the wire when you run it through holes before you solder it.
– Dean
Jun 25 '11 at 20:31
• @Andrejako: I didn't mean to zig-zag the wire on the board. I meant to do this shortly after the wire comes off the board, a bit like a mechanical spring. It will act like a strain relief and soften the forces on the solder joint. Excessive zig-zag can change the properties of the antenna, so just a little. Maybe 2-4 segments each a mm or two long. Jun 25 '11 at 21:31
• @Olin - I know every mm counts in antennas, but it's not at all my field (mind the pun! :-)). Can he play with the wire's diameter? Jun 26 '11 at 8:13
• After some experimenting, I determined that in my particular case this answer helped the most. Jul 2 '11 at 13:40

For better vibration performance use silicone instead of hot glue.

• Silicone will stay more flexible.

• Taper the silicone so that it is a big blob at the PCB end and tapers down the wire.

• This reduces vibration and doesn't focus the flexing.

• It's standard procedure in mil-spec connector wiring for this very reason.

What kind of silicone : not acid cure, it eats the board.

• non-conductive epoxy would probably work as well.
– Mark
Jun 26 '11 at 23:26
• Epoxy is hard and acts as a force concentrator. The softness damps vibrations gradually, and by tapering the silicone, the damping slowly damps the wire and it doesn't snap off at the edge of the silicone. Jun 28 '11 at 0:16

I second David's idea of supporting the antenna to minimize vibrations. I would take a 2cm piece of stiff paper the length of the antenna, fold it lengthwise so that both halves are perpendicular, glue it on the housing of your receiver so that it stands up and glue the antenna to the fold.

Not for stiffness, but better soldering might help as well. If the wire has been soldered well the solder would flow up against it a mm or so, and not be a nice round blob like you have now.

I'm guessing that the tip of the antenna wire is free to vibrate back and forth, until the antenna breaks loose. I suspect the hotglue near the edge of the board is breaking first, and then the solder joint breaks.

Perhaps you could use hotglue (or superglue, or wire dots, or tape dots) on some or all of the antenna wire so it doesn't vibrate as much. Perhaps hotglue a larger piece of cardboard or plastic sheet to the opposite side of the PCB for the antenna to rest on. (Hopefully a piece of plastic a couple inches long won't be too heavy ...)

  +---------+
+-*-*-*     |
|     |     |
|   o=*=*==*|
|PCB  |     |
+-*-*-*     |
| card    |
+---------+


Where

= : antenna
o : soldered end of the antenna
* : hotglue dots or superglue dots or tape dots


I'm sure you can invent a smaller, more lightweight shape for the card, and cut the card into that shape after the glue hardens.