Proper Wetting During Soldering in PCB Assembly

PCB assembly is a complex process that involves a great deal of careful preparation and meticulous care. A key step in the process is soldering, which is when copper pins on a circuit board are joined together using tin and lead to form strong metallurgical bonds that make up the conductive structure of the circuit board. One of the most common defects in soldering is known as non-wetting, which refers to a failure of the solder paste to bond properly to the PCB surface. It is important to understand the causes of this defect so that you can take steps to avoid it in your manufacturing processes.

Non-wetting can occur when the surface of a pcb printed circuit board assembly is polluted with contaminants such as oils, oxides, or residues from previous processes. These contaminants prevent the solder from wetting properly, leading to failure. Another cause of non-wetting is oxidation, which can occur when the solder or substrate material are exposed to excessive temperatures. This leads to a loss of surface energy and the formation of oxide layers that interfere with proper wetting.

Soldering is a highly temperature-controlled process, and it is necessary to maintain the temperature in the reflow zone within specified limits for optimal wetting and metallurgical bonding. If the temperature in the reflow zone is higher or lower than specified, it can result in non-wetting or other defects.

How to Ensure Proper Wetting During Soldering in PCB Assembly

Another cause of non-wetting is insufficient plating thickness. If the copper pins on a PCB are not plated with enough tin and lead, they will be more likely to experience non-wetting because the tin/lead will interfer with the solderability of the copper. Long storage periods can also lead to non-wetting, as the tin/lead coating may break down during this time.

Stencil design and placement can also contribute to non-wetting. If the stencil size is too small, it will not be possible for the solder paste to cover the entire pad and component leads. Likewise, if the solder paste is placed too close to components or the copper pads on the board, it will be difficult for the solder to wet these surfaces.

Finally, if the reflow process is allowed to run too long, it may exhaust the flux and lead to non-wetting. The reflow process typically lasts between 30 and 60 seconds, and it should not exceed this limit to ensure that the solder does not begin to freeze.

Non-wetting can lead to poor electrical conductivity, intermittent connections, or even complete failure of the joint. In addition, it can increase mechanical stress on the solder joint and cause cracking or fracturing.

The good news is that preventing non-wetting can be as simple as ensuring the correct temperature in the reflow zone, maintaining proper flux application, and increasing soak times for the reflow zone. By following these guidelines, you can eliminate this defect and ensure high-quality solder joints for your electronic devices.