What is Solder Bridging?

Solder Bridging is an issue that arises at the time of soldering. It occurs when component pads on a PCB get connected due to the application of excess solder. Normally, each pad or component pin is intended to be separated from each other, but the occurrence of a solder bridge connects them and forms a conductive path between them, which can cause the component and thus the circuit to malfunction. Solder bridging is a microscopic phenomenon. Any solder bridge, which goes undetected during inspection, may cause a short-circuit in the component traces. Solder bridging is defined as a defect under IPC-A-610D.

The solder mask layer or solder resist helps to keep the solder on pads and it is applied top of the PCB during production to protect unwanted solder application. For example, during assembly, a customer would want to apply solder on the pads only, and not between them, and this what the solder resist/mask should do. If it appears between the pads, it will cause a solder bridge problem.

Solder bridging can be repaired. The excess solder can be drawn off by dragging the tip of hot iron between the two solder joints. If there is too much solder, a solder sucker or solder wick can also be used.

Causes of Solder Bridging:

• Incorrect stencil dimensions (thickness) can cause too much solder application on the SMT pads
• Designing bigger components pads relative to the gap between them, aspect ratio is very important
• Incorrect way of component placement with component lead and pad size mismatch
• Having an insufficient amount of solder resist applied between the pads 
• Inadequate solder paste volume

Preventive measures to avoid solder bridging

• Perform design rule checking to identify the places where solder mask application could be avoided. 
• Always maintain solder resist dam (minimum 2 mils) between the pads to avoid the flow of solder to the nearby pads/ areas
• Place fiducial marks on the PCB for better component and stencil alignment, whenever required.
• Solder reflow profile analysis is important since liquid solder tends to flow towards the areas having higher temperature, resulting in undesired solder accumulation. Modifications in the solder profile can help with this.
• Always clean the stencil after every use
• Pick apt solder paste quantity
• Never mix old and new flux for preparing solder paste