Common Soldering Mistakes
Soldering errors are easy to make, especially when you have hundreds of printed circuit boards to modify. Consequently, you could end up experiencing various types of soldering defects while handling these tasks. Even more advanced workers sometimes make mistakes like over- or underheating a pin or pad, or covering it with too little or too much solder. To avoid these mistakes and master the ideal solder joint, it is crucial to know how to identify bad solder joints.
Table of Contents
Insufficient Wetting of the Surface Mount
Insufficient Wetting of the Pad
Insufficient Wetting of the Pin
Too Much Solder
Solder Repairs at Global Electronic Services
A trained eye can readily spot most types of soldering defects. A properly soldered joint will have a concave shape and be symmetrical, clean and orderly compared to its neighbors. An improperly soldered joint will be messy and have some physical abnormality, such as an overly swelled shape or a long stem. The following problems result from the 12 most common soldering mistakes.
1. Disturbed Joint
One of the most common problems along rows and columns of solder joints is the disturbed joint, which has a bloated, distorted and sometimes flaky appearance. The primary cause of a disturbed joint is a movement of any sort at the time of solder application. If you have a board placed on an unstable surface and it happens to shake or swerve just as you are applying the solder, the joint could get disturbed and lose its composition. A disturbance can also occur if the board is subject to movements or vibrations while the solder is drying.
People often confuse disturbed joints with cold joints, which have a similar appearance, yet result from different problems. In some cases, you can correct this issue if you reheat the joint and allow it to harden without any further disturbances. To prevent the creation of disturbed joints in the future, make sure you have each joint immobilized and stable at the time of solder application.
2. Cold Joint
Another obvious problem often spotted on newly soldered boards is the cold solder joint, which usually has a lumpy and sometimes shapeless appearance. Cold joints typically form during applications where the solder fails to come to a sufficient melting state. Aside from their odd appearance, cold joints are problematic because they are incapable of fulfilling their intended purpose. Cold joints generally lack sufficient bonding ability and tend to develop cracks over time.
In some cases, you can repair a cold joint by reapplying heat to it. You can accomplish this by applying a hot iron to the joint until the solder flows into place. In some cases, cold joints will also have an excess amount of solder. If so, you can remove it with the hot tip of the iron. To prevent the formation of cold joints in the future, make sure to heat the iron to the proper temperature before applying the solder.
3. Overheated Joint
Across a newly soldered set of joints, one of the most significant possible eyesores is the overheated joint, which forms during applications where the solderer has applied too much heat. Whereas cold joints result when the iron lacks sufficient heat, overheating occurs when the iron gets excessively preheated just before the application. Overheated joints often have a burnt look with typically lumpy, malformed joints.
To undo the problems caused to a joint and its surrounding area by an overheated solder application, scrape off the burnt flux with the tip of a PCB working tool. In some cases, you might need to use a solution, such as isopropyl alcohol, to remove the burnt flux. The most effective way to apply alcohol is with a small toothbrush.
Overheated joints are a challenge to repair and therefore crucial to avoid at all costs. To prevent this problem in the future, make sure to heat your iron to the proper temperature and double-check that the joints are clean before the soldering application.
4. Insufficient Wetting of the Surface Mount
If you heat the pin but not the pad during a solder application, you could end up with an insufficiently wetted surface mount, where the components of the mount have failed to flow onto the solder pad. The site is easy to identify if you see irregularities across a row of mounts. If several of the mounts have properly flowed onto the pad, but one or two additional mounts have retained their original pin composition, an insufficiently wetted pad most likely caused the problem.
To rectify a pad that was improperly wetted at the time of the original mounting application, preheat the iron tip to the necessary heat level, then apply it to the solder pad. Put more solder on the pad, giving it time to flow into place and melt into shape with the solder of the pin. To prevent this problem from happening in the future, heat the pad before heating the pin.
5. Insufficient Wetting of the Pad
If a solder pad doesn’t get enough wetting during the soldering process, the joint will have an uneven look that would indicate a lack of bonding potential. In most such cases, a generous quantity of solder will have wetted the leads, but the pad will lack the necessary bonding strength. Often, the wetting problem can occur if you apply solder to a dirty circuit board. Due to the presence of dirt on the board, the solder will not be able to bond as needed. In some cases, a pad will end up with insufficient wetting if the pad and the pin lack proper heat at the time of the application.
To reverse the problems of an insufficiently wetted pad, heat the tip of the iron bar to the proper temperature and place it at the joint’s base, then let the solder flow over the pad. To prevent this problem in the future, make sure the board is clean before the application, and that you have preheated the pin and pad to the necessary temperature level.
6. Insufficient Wetting of the Pin
In cases where the pad has been poorly wetted and the pin has no wetting at all, you will likely have a case where the pin has not been heat-treated properly, hence an insufficiently wetted pin. This problem typically happens if you fail to treat the pin with sufficient heat, therefore not allowing enough time for the solder to flow into place.
To correct the issue of an insufficiently wetted pin, reheat the iron tip to the necessary temperature level and apply more solder. As you do this reapplication, make sure to touch both the pin and the pad with the tip of the hot iron, as this will be a necessary measure to condition these components for a proper application. To prevent this issue from recurring in the future, make sure to heat both the pin and the pad to the appropriate level when you apply solder.
7. Solder Starved
If a joint lacks sufficient solder, it falls under the category of solder-starved. A joint that does not have the right amount of solder will lack the strength to function as intended. Without the proper amount of solder, a joint could crack under stress and cause the board to fail. While a solder-starved joint might have good conductivity, it would still render the board unsafe for use unless you rectify the problem with a reapplication of solder to the joint in question.
To repair a joint that lacks the necessary amount of solder, preheat the joint with the tip of your iron and apply more solder to the same spot as the prior application. Once you have completed this step, the joint should be stronger and more consistent with the others on the board. Now, the joint should be able to handle stress and make the board safe for use in computing devices.
8. Too Much Solder
Another big problem involving solder quantity involves an excessive amount. Such joints are easy to spot on a board because the joint will typically be round and swelled, like a bubble. By contrast, a healthy joint with the right amount of solder will have a concave shape. When someone has covered a joint with too much solder, it will not likely have sufficient electrical conductivity. Though the joint would probably not be susceptible to stress cracks, it will not be able to fulfill its intended purpose.
To fix an overly soldered joint, you will need to remove some of the solder and pare the joint down to a proper concave shape. To pare down the solder, heat the tip of your soldering iron and use it to trim away some of the excess solder. Do this until the joint no longer looks round and inflated. A solder-sucker might make the job of removing excess solder easier.
9. Untrimmed Leads
One of the most dangerous mistakes found on soldered joints is the presence of untrimmed leads, which stick out like horns and are easy to spot with the naked eye. Whereas normal leads have a concave shape, untrimmed leads stick out higher and will often have a slight sideways tilt. If two of these untrimmed tips come into contact with one another, you could end up having a short circuit on your hands. Even if the leads are standing upright, they could easily get bent over time and make contact.
Untrimmed leads are easy to rectify by trimming them to an appropriate size for a standard joint. Take the same tool you use for solder trimming and pare down each overly long lead to bring the joint down to the same height as the others on the board.
10. Solder Bridge
If an excess amount of solder melts between neighboring joints on a board, you could wind up with a solder bridge. On printed circuit boards, joints are separate for a reason. If you unintentionally form a contact between two joints while performing a solder application, you should rectify the problem immediately, as the board will otherwise be unusable.
To remove a solder bridge and separate the unintentionally connected joints, heat the tip of your soldering iron and use it to cut through the bridge from the bottom. Create a canal through the bridge, then widen the gulf until the two joints are at an appropriate distance from one another. After the bridge is gone, use the iron to correct the shape of the joints, if necessary. To prevent this problem from recurring, only use the proper amount of solder during each application, as too much solder can easily melt sideways and collide with neighboring joints.
11. Lifted Pad
Beyond any problems with the joints, you might notice spots where the solder pad separates from the circuit board. In spots where you have overworked a joint or given the solder several treatments with a hot iron, you might wind up with a lifted pad. The problem generally results from repeated, excess stress on the board. For example, if you remove a solder bridge with a hot iron and scale the lateral bleed-over away from each joint, you might accidentally lift too much and dislodge the solder joint from the board altogether. The problem is most common on boards where the copper layer is thin and through-plating is nonexistent.
It’s possible to repair a lifted pad, though the task is challenging. The most practical repair method is to fold the solder lead over and bond it to a still-intact copper trace. If solder mask is covering the nearby trace, you would need to remove that to reveal the copper.
12. Stray Solder
In some instances, you might end up with random bits of solder on the board as messy oversights during the soldering process. These unintended bits are unattached to copper trace and are bonded with flux residue. Even if you have not joined these bits to anything else on the board, you should still remove them before you put the board to use. Otherwise, these stray solder bits might cause a short circuit in the board.
To remove stray solder from a printed circuit board, use the tip of a sharp PCB tool, such as a small knife or pin. Apply the point of the tool to the stray solder bits and scrape each of them off from the surface of the board. Double-check the board to make sure no additional pieces of solder remain when you use the board.
Solder Repairs at Global Electronic Services
Properly soldered joints on a printed circuit board are essential for it to safely conduct currents and function as intended inside an electronic or computing device. In many cases, however, some of the biggest companies end up with boards that fail or short circuit due to soldering mistakes. At Global Electronic Services, we repair industrial electronics for companies across the private sector. Contact us today for more information on our electronic repair services.