The quality of soldering has a huge impact on the overall quality of the PCB. Through soldering, different parts of the PCB are connected to other electronic components to make the PCB work properly and achieve its purpose. When industry professionals evaluate the quality of electronic components and equipment, one of the most prominent factors in the evaluation is the ability to work in soldering.
To be sure, welding is very simple. But this does require practice to master. As the saying goes, “practice can be perfect.” Even a novice can make functional solder. But for the overall life and function of the equipment, clean and professional welding work is necessary.
What is a perfect solder joint?
It is difficult to include all types of solder joints in a comprehensive definition. Depending on the type of solder, the PCB used or the components connected to the PCB, the ideal solder joint may change dramatically. Nevertheless, most perfect solder joints still have：
- Completely wet
- Smooth and shiny surface
- Neat recessed corner
In order to obtain the ideal solder joints, whether it is SMD solder joints or through-hole solder joints, an appropriate amount of solder must be used, and the appropriate soldering iron tip must be heated to an accurate temperature and be ready to contact the PCB. Oxide layer removed.
The following are the nine most common problems and errors that can occur when welding by inexperienced workers:
- Solder bridge
PCBs and electronic components are getting smaller and smaller, making it difficult to manipulate around the PCB, especially when trying to solder. If the tip of the soldering iron you use is too large for the PCB, an excess solder bridge may be formed. Soldering bridge refers to when the soldering material connects two or more PCB connectors. This is very dangerous. If it goes undetected, it may cause the circuit board to short circuit and burn. Make sure to always use the correct size soldering iron tip to prevent solder bridges.
- Too much solder flux
Novices and beginners often use too much solder flux when soldering, and large bubble-shaped solder balls are formed at the solder joints. In addition to what looks like a weird growth on the PCB, if the solder joint is functioning properly, it may be difficult to find. There is a lot of room for errors under the solder balls. The best practice is to use solder sparingly and add solder if necessary. The solder should be as clean as possible and have a good concave angle.
- Cold stitch
When the temperature of the soldering iron is lower than the optimal temperature, or the heating time of the solder joint is too short, a cold solder joint will occur. Cold seams have a dull, messy, pock-like appearance. In addition, they have a short life and poor reliability. It is also difficult to evaluate whether cold solder joints will perform well under current conditions or limit the functionality of the PCB.
- Burnt out node
A burnt joint is the exact opposite of a cold joint. Obviously, the soldering iron works at a temperature higher than the optimal temperature, the solder joints expose the PCB to the heat source for too long, or there is still a layer of oxide on the PCB, which hinders the optimal heat transfer. The surface of the joint is burnt. If the pad is lifted at the joint, the PCB may be damaged and cannot be repaired.
When trying to connect electronic components (such as transistors and capacitors) to the PCB, tombstones often appear. If all sides of the component are properly connected to the pads and soldered, the component will be straight. Failure to reach the temperature required for the welding process may cause one or more sides to lift up, creating a tomb-like appearance. The tombstone falling off will affect the life of the solder joints and may have a negative impact on the thermal performance of the PCB. One of the most common problems that cause tombstone cracking during reflow soldering is uneven heating in the reflow oven, which may cause premature wetting of solder in certain areas of the PCB relative to other areas. The self-made reflow oven usually has the problem of uneven heating. Therefore, it is recommended that you purchase professional equipment.
- Insufficient wetting
One of the most common mistakes made by beginners and novices is the lack of wettability of solder joints. Poorly wetted solder joints contain less solder than the solder required for proper connection between the PCB pads and the electronic components connected to the PCB by solder.
Poor contact wetting will almost certainly limit or damage the performance of electrical equipment, reliability and service life will be poor, and may even cause a short circuit, thereby seriously damaging the PCB. This situation often occurs when insufficient solder is used in the process.
Jump welding may occur in the hands of machine welding or inexperienced welders. It may happen due to lack of concentration of the operator. Similarly, improperly configured machines may easily skip solder joints or part of solder joints. This leaves the circuit in an open state and disables certain areas or the entire PCB. Take your time and check all the solder joints carefully.
- Pad lift
Due to the excessive force or heat exerted on the PCB during the soldering process, the pads on the solder joints will rise. The pad will lift up the surface of the PCB, and there is a potential risk of short circuit, which may damage the entire circuit board. Be sure to reinstall the pads on the PCB before soldering the components.
- Webbing and splash
When the circuit board is contaminated by contaminants that affect the soldering process or due to insufficient use of flux, webbing and splashing will occur on the circuit board. In addition to the messy appearance of the PCB, webbing and splashing are also huge short-circuit hazards, which may damage the circuit board.