Converting from Surface to Leaded Capacitors
Beginning in the late 1980's, manufacturers began switching to surface-mount capacitors. Many of these game and computer systems are now experiencing failures due to either improper construction or improper processes used during board manufacturing. See Category:Bad Capacitors for systems with known issues.
The following is a rough guide to removing, cleaning up, and replacing surface mount capacitors with traditional leaded capacitors. As you get some experience, you may find that some methods work better for you than others. While practice makes perfect, patience is a decent substitute for experience. If you have tips to share, please send them along!
The process in a nutshell
In this picture, we can see the stages of replacing a defective surface mount cap that refuses to be desoldered in a normal fashion.
From upper-left to lower right
- The defective part is located. Capacitance, voltage, and polarity are noted. In this example: 100uF, 6V, left lead negative.
- The body of the cap is removed.
- The plastic spacer is removed.
- The area is cleaned with isopropyl alcohol, or other safe PCB cleaner. Fresh solder is added to assist in removing the old legs.
- All solder is removed to expose the mounting pads. The area is swabbed one final time to ensure removal of electrolyte.
- Fresh solder is added to the solder pads.
- The new capacitor is installed. In this example, I wanted to replace the original RF shield, so I formed leads in such a manner so that the capacitor would be installed as flush with the board as possible.
Removing the old capacitor
This is probably the hardest part, mainly because if damage is going to be done, it's going to happen here.
SMD caps don't leave much of their solder exposed. If they've leaked, the solder that is exposed may be too corroded to be removed without adding some fresh solder. You probably want to avoid converting the leaked fluid into vapor, too.
I've tried all sorts of techniques, and I'll list them here so you can pick and choose your own strategy:
- My Preferred Technique: Twist the cap to loosen or break the leads.
- This technique, for me, has had the lowest healthy solder pad mortality rate. I hold the cap with pliers and slowly twist it clockwise and counterclockwise. Just a little bit in each direction at a time, slowly increasing the range of rotation as things begin to loosen up. Eventually the cap legs break off opening up the PCB for rework. Sometimes this will break the pad as well. Personally, I would rather break a compromised solder pad during removal than try to save and reuse every pad. If the system is prone to caps that leak and damage the PCB (like the Sega PAC-S10), leaving a pad in place with a compromised connection may set you up for gremlins later. If the pad breaks, you can trace it to it's source and bypass an entire trace that could be bad. I've had the misfortune of reworking several boards that had compromised traces that showed continuity but failed under load.
- Desolder the part normally, using a desoldering tool
- If there is enough exposed, uncompromised solder, this can work if your tools are fine enough. With very small SMD caps, you may not have much access to the leg. If the cap has leaked, whatever solder is present may not easily melt. Adding fresh solder may or may not work.
- Use an iron to heat the solder and lift each leg one at a time.
- This works pretty well, but again only if the solder is still clean. Pick the most accessible solder pad, apply heat, and gently apply pressure to the part away from the pad that you're heating. Once the solder melts, the part will detach from the board, giving you greater access to the still-attached leg of the part.
- Grab the cap with pliers, pull from board
- If the solder pads have been compromised even a little, they will tear right off the board and remain attached to the cap. This is a hail mary move. DO NOT DO THIS!
- Chop the cap in half with cutters
- Sometimes this works, and sometimes it doesn't. The pinching action provided by the cutters can sometimes disturb the solder pads. The head of the cap will go flying and you'll probably never find it again.
- Rock the cap back and forth to loosen or break the leads.
- This works reasonably well. The rocking motion should be perpendicular to the leads. Eventually the leads will break, or they will become extracted from the capacitor body, allowing you to break & remove the plastic wafer and have access to the legs.
I have also seen boards where the solder was so compromised that I could peel the cap legs right off of the board.
Prepping the board for a new capacitor
When the original cap has leaked, it's important to ensure that the area is clean and capable of supporting the new cap. Be sure to keep the area clean of leaked fluid as best as possible during the removal process. If your soldering iron touches the old fluid, a pretty horrible smell is released as you boil it away and the tip of the soldering iron will become contaminated.
Observe the condition of the copper traces connected to the original cap. Corrosive agents, if left unchecked, have a habit of following traces, slowly munching on them. Sometimes you can replace a cap and still have a problem with the board if the damage has extended to the next component, or made it's way through a via on the board. These problems can even be intermittent, making them very frustrating to troubleshoot, so it pays to give the area a brief inspection.
As much of the original solder should be removed as possible. Sometimes the only way to do this is to add new solder first.
Once all of the solder is removed, the exposed solder pads should be inspected for damage. Sometimes the old electrolyte will have corroded the trace straight through the board. If a solder pad has been too compromised, you may have to reroute, which leads us to the next section...
Routing around damaged pads & traces
If you replace enough leaked SMD caps, eventually you will encounter a broken trace or lifted solder pad. Depending on the damage, a few ideas for repair include:
- Wide traces can have their coating removed, and the component leg attached to the bare copper. A scratch pen, hobby knife, or medium grit sandpaper can be used to remove the coating and expose the copper underneath. Buy a scratch pen, it's the least clumsy way to effect this type of repair.
- Follow the trace to it's next destination.
- If it's another component, the capacitor leg can be soldered directly to it.
- If it's a plated through hole, see if the cap leg will fit through it. If so, remove the coating from the top & bottom of the through hole, and solder the leg on both sides.
- If the broken trace is connected to a ground plane or voltage source, removing the PCB coating and solder it directly to the nearest ground/voltage plane.
This is a bit of a free-form exercise, my advice here is:
- Keep the legs as short as possible
- Form the legs to match the original solder pads
It might be tempting to just cut the leads to length and solder them in place. This method works and will save some time, but I like to form the leads into little feet to increase the contact area. Unless you tape or glue the replacement component in place, the solder is the only mechanical connection the board, so I like to be sure that the replacement part isn't going anywhere.
Another benefit to forming leads that match the original pads is that you reduce the chances of solder bridges & shorts. If the solder resist layer has been removed, leads that match the original pads have less of a chance of wandering.
You can actually buy special pliers for this operation, called lead forming pliers. They're not cheap.
Soldering the new component in place
I find it's easiest to first add two fresh solder lumps to the board. Then one at a time heat the solder and insert a leg into the molten solder. If the area on the board is crowded, I use a hemostat to hold the cap by one of it's legs, and solder the other leg into place first (if you hemostat the leg you're attempting to solder, it may act as a heat sink and slow the process down). Once the first connection has been made, the capacitor will stay in place, allowing you to release the hemostat and solder the other leg.