Bridging During PCB Assembly

As the world continues to cram in electronics that are smaller and faster, the challenges of a successful printed circuit board (PCB) assembly grow. One of the most difficult is solder bridging, which occurs when unintended connections are formed between pads or leads on a PCB during the assembly process. It’s not just a problem for PCB manufacturing; it can also lead to product failure and unsafe use.

Solder bridges are the result of a number of factors, some of which can be prevented during the pcb design phase and others during the manufacturing process itself. The most common causes include:

The increasing need to cram in more and more technology into ever-smaller packages is creating a myriad of challenges for the people designing, testing, and manufacturing electronic products. As a result, there’s sometimes a need to compromise on the layout of a new product in order to get it out the door as quickly as possible. This can often mean sacrificing proper component placement and layout, which can lead to the formation of unintended solder bridges.

Solder Bridging During PCB Assembly

The shape and size of the stencil used to apply the solder paste to the PCB surface can impact the occurrence of solder bridges. Using stencils that are too small for the pad sizes on the PCB will result in excessive paste volume, which can create bridges between adjacent pads. A properly designed stencil will ensure that there is enough space between the pads to allow for adequate flow of solder.

Contaminants such as dust, moisture, oil, and flux residue can all contribute to solder bridging by preventing the solder from wetting the pad or lead. Cleaning the PCB and stencil thoroughly prior to soldering will help to eliminate these issues.

When reflowing a PCB with solder paste, the board is heated to a specific temperature to melt and flow the solder to form the joint. If the preheat temperature is not set correctly, this can cause the solder to not fully melt or flow, which may lead to solder bridges.

Solder mask coating prevents molten solder from adhering to places where it’s not intended. A well-designed PCB will include a solid layer of solder mask between all exposed components and their pins, with the exception of those that require it for functionality. A thin layer of solder mask between pins of through-hole components is essential, as long leads can easily cause bridging during reflow.

Using fiducial marks on a PCB to enable automated component placement is another key way to help reduce the likelihood of solder bridges during assembly. These marks help machines locate the board, align it correctly, and place components accurately. The best way to maximize the benefit of these markers is to follow IPC guidelines on optimal number and positioning. A poorly placed fiducial mark will increase misalignment, which increases the chance of solder bridging during reflow.