How to re-solder fine broken flex cable connection to pcb

Question

I have a phone where the volume button button flex cable has been partially pulled away from the PCB. See this photo for what it would have looked like before being damaged:

http://guide-images.ifixit.net/igi/noJObJXR2ZSfniZV.large

It's the orange flex cable connection at the center top of that image. There are three connections on the flex cable, the top two have separated from the pcb, the bottom one is still attached.

Can someone advise me what I need to re-solder this? The phone has other problems which may not be fixable so I'd like to attempt this myself as a learning experience rather than sending to a workshop. As well as the partially attached volume flex cable I also have a second identical (but not working) PCB with its volume button still attached. So if I remove that I also have a second chance to solder a volume flex cable to the good PCB if I mess up reconnecting the partially disconnected one.

Some questions that come to mind:

1. Should I detach the partially detached flex cable completely with a soldering iron? (i.e. separate the remaining third connection on the flex from the PCB), or should I just attempt to re-solder the two broken connections?

2. The tiny amount of solder that had made the connection on the two broken connections looks to still be attached to the flex cable, can I just try to re-solder by melting this to the pads on the PCB or do I need to remove this from the flex cable and start again with fresh solder?

3. What kind of solder do I need? Do I need separate flux or just electronics solder wire?

Below is a video of someone doing exactly what I need to do with a near identical PCB and volume button flex cable, but there are no instructions/commentary to elaborate on what the video shows, so I'm not sure if I understand correctly.

The video shows the repair of both a volume flex cable and power button flex cable on a single PCB. First the damaged volume/power flex cables are removed from the PCB, then a good volume flex cable is removed from a donor PCB and attached to the target PCB. Then a good power button flex cable is removed from the donor PCB and attached to the target PCB. In addition to my original three questions above, could you say if my understanding of the different points of the video explained below are correct and answer the queries where I don't understand?

http://www.youtube.com/watch?v=1F6C13O76qk

0:10 Add solder to iron tip, gently remove old volume flex cable with iron tip.

0:30 Clean bare solder pads on PCB by gently scraping with iron tip (and some solder?)

1:01 Apply something to the bare pads with a small paintbrush, glue or flux? If it's glue, what kind?

1:16 Attach volume flex to PCB pads, solder connection (i.e. using the remnants of solder left on the flex cable from its removal from the donor PCB board?)

1:31 Apply some extra new solder if required to make the connection secure?

3:04 Here the power button flex cable is being attached. It looks like before this he has followed the same procedure as with the volume button but then he adds something more with the paintbrush after doing some soldering, and then does some more soldering. What is on the paintbrush at 3:04? Why does he do this in this instance for the power button flex cable but not previously when attaching the volume button flex cable (in that case he added something with the paintbrush to the pads before attaching the flex cable but not halfway through soldering the connection).

If you have any other hints/tips, please let me know. I have to buy the soldering iron and solder and anything else I need (would soldering wick and/or flux be useful for example)? So any other suggestions welcome.

Thanks.

Answer 1

In answer to your questions,

1. Remove the third lead, this will let you clear/prepare the pads for re-soldering.

2. It's probably best to flick the old solder off the leads; do this by heating and bending the lead with the tip of the iron and letting it spring back, DO NOT DO THIS TOWARDS YOUR FACE!

3. You can buy solder with a flux core. Also, if you can get it, buy lead solder, it is MUCH nicer to work with.

As regards the vid:

Yes the guy tins his iron to remove the leads, you should always tin (add solder to your iron tip) before you do anything, it stops the tip oxidising. It also helps when melting old solder.

To clean the pads generally I would put a bit of new solder on, then wick it off to give a nice clean and shiny surface.

He is applying flux, this will stop the solder "balling up" and sticking to stuff it shouldn't. You can do this, but if you are careful, and have flux core solder, you wont need to.

As regards the rest of your questions, this is just about technique. The guy seems to tack solder (a connection made to hold it in place) the leads at first. When the whole lead is tack soldered he goes around and tidies up the job.

So what YOU need to do.

1. Remove the lead - Tin and clean the pads and flick the old solder off the end of the lead (NOT TOWARDS YOUR FACE!)

2. Add a little solder to each of the pads

3. Place each lead above the pad and push down gently whilst touching the tip of the iron to the solder you placed on the pad. Make sure you remove the iron before removing the pressure.

4. When all connections are made, get some tweezers and push down on each lead in turn, melting the solder on the pad with the tip of the iron so the lead is properly flat on the pad.

Remember! Before you touch the iron to anything, make sure you have applied some solder to it and wiped it off on a sponge.

Also, if you are buying new equipment, when you first turn the iron on, keep adding solder to the tip and wiping it off. You need to thoroughly tin/protect the tip before you start using it. You can get little pots of hard flux, this is useful and it will help remove the crud on the tip due to oxidation. Also, tin it and DON'T wipe it off when you are done, the tip will still be hot for a while after you switch it off.