The ideal solder joint for through-hole components should resemble the diagram below.
A Disturbed joint is one that has been subjected to movement as the solder was solidifying. The surface of the joint may appear frosted, crystalline or rough.
Often called a 'Cold Joint'. They can look similar to a true cold joint, but the cause is different.
Repair: This joint can be repaired by reheating and allowing it to cool undisturbed.
Prevention: Proper preparation, including immobilizing the joint and stabilizing the work in a vise can prevent disturbed joints.
Cold JointA 'Cold Joint' is one where the solder did not melt completely. It is often characterized by a rough or lumpy surface. Cold joints are unreliable. The solder bond will be poor and the cracks may develop in the joint over time.
Repair: Cold joints can usually be repaired by simply re-heating the joint with a hot iron until the solder flows. Many cold joints (such as the one pictured) also suffer from too much solder. The excess solder can usually be drawn-off with the tip of the iron.
Prevention: A properly pre-heated soldering iron with sufficient power will help prevent cold joints.
Overheated JointAt the other extreme, we have the overheated joint. The solder has not yet flowed well and the residue of burnt flux will make fixing this joint difficult.
Repair: An overheated joint can usually be repaired after cleaning. Careful scraping with the tip of a knife, or little isopropyl alcohol & a toothbrush will remove the burnt flux.
Prevention: A clean, hot soldering iron, proper preparation and cleaning of the joint will help prevent overheated joints.
Insufficient Wetting (Pad)These two joints both show signs of insufficient wetting of the solder pad. The solder has wetted the leads nicely, but it has not formed a good bond with the pad. This can be caused by a dirty circuit board, or by failing to apply heat to the pad as well as the pin.
Repair: This condition can usually be repaired by placing the tip of the hot iron at the base of the joint until the solder flows to cover the pad.
Prevention: Cleaning the board and even heating of both the pad and the pin will prevent this problem.
Insufficient Wetting (Pin)This solder in this joint has not wetted the pin at all and has only partially wetted the pad. In this case, heat was not applied to the pin and the solder was not given adequate time to flow.
Repair: This joint can be repaired by re-heating and applying more solder. Be sure that the tip of the hot iron is touching both the pin and the pad.
Prevention: Even heating of both the pin and the pad will prevent this problem.
(Surface Mount)Here we have three pins of a surface mount component where the solder has not flowed onto the solder pad. This is caused by heating the pin instead of the pad.
Repair: This is easily repaired by heating the solder pad with the tip of the iron, then applying solder until it flows and melts together with the solder already on the pin.
Prevention: Heat the pad first.
Solder StarvedA solder starved joint simply does not have enough solder. It may make good electrical contact, but it is hard to verify by inspection. In any case, it is not a strong joint and may develop stress cracks and fail over time.
Repair: Re-heat the joint and add more solder to make a good strong joint.
Too Much SolderThis might be a perfectly good joint, but we can't tell for sure. It is entirely possible that this blob of solder wets neither the pin nor the pad and is not a reliable electrical connection. The best evidence of proper wetting (and good electrical contact) is a nice concave surface as on the joint on the far left.
Repair: It is usually possible to draw off some of the excess solder with the tip of a hot iron. In extreme cases, a solder-sucker or some solder wick can be helpful as well.
Untrimmed LeadsLeads that are too long are potential short circuits. The two joints on the left are an obvious danger of touching. But the one on the right is long enough to be dangerous as well. It would not take much force to bend that lead over to touch an adjacent trace.
Repair: Trim all leads just at the top of the solder joint.
Solder BridgeThe left two solder joints have melted together, forming an unintended connection between the two.
Repair: Sometimes the excess solder can be drawn off by dragging the tip of a hot iron between the two solder joints. If there is too much solder, a solder sucker or solder wick can help get rid of the excess.
Prevention: Solder bridges most often happen between joints with too much solder to begin with. Use only enough solder to make a good joint.
Lifted PadThis photo shows a solder pad that has become detached from the surface of the circuit board. This most often occurs when trying to de-solder components from the board. But it can result simply from overworking the joint to the point where the adhesive bond between copper and the board is destroyed.
Lifted pads are especially common on boards with thin copper layers and/or no through-plating on the holes.
Repairing a Lifted PadIt may not be pretty, but a lifted pad can usually be repaired. The simplest repair is to fold the lead over to a still-attached copper trace and solder it as shown to the left. If your board has a solder-mask, you will need to carefully scrape off enough to expose the bare copper.
Other alternatives are to follow the trace to the next via and run a jumper to there. Or, in the worst case, follow the trace to the nearest component and solder your jumper to the leg of that. Not exactly pretty, but functional.
Stray Solder SpattersThese bits of solder are held to the board only by sticky flux residue. If they work loose, they can easily cause a short circuit on the board.
Repair: These are easy to remove with the tip of a knife or tweezers.
All of the Above!Don't panic. Take your time. Most joints can be repaired with patience. If the solder refuses to flow the way you want it to:
Stop and let the joint cool.
Clean and tin your iron.
Clean off any burnt flux from the joint.
Let the iron come back up to temperature.
- Then reheat the joint and try again.