Why Do Circuit Boards Fail PCB Assembly?
For prototyping, low-volume, and high-volume production, PCB assembly is an important stage of manufacturing. It is one of the last times build errors are discoverable before shipment. Therefore, you should work with a CM that optimizes quality during assembly to achieve the highest and most accurate yield rate for your project. To collaborate effectively with your CM, be sure your development team is aware of the most common PCB assembly defects.
Common PCB Assembly Defects
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Solder bridges
Solder bridges result when two solder joints are incorrectly connected. This failure mode causes a short that may cause component or board damage.
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Tombstoning
Tombstoning occurs when one side of a component detaches from the board. This introduces an open circuit that can be fixed during rework if discovered.
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Position shifts
Another common issue is the shifting of a component such that its pin(s) does not align with footprint pads. This can cause an open circuit and unpredictable operation.
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Webbing
Solder mask webbing can prevent shorts or solder bridges between adjacent pins, typically for multi-pin ICs. Solder webbing, on the other hand, is caused by pollutants and can present problems with the solder joint quality.
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Holes
These usually show up at via locations and may extend into or through the entire length of the PTH. The cause of these blow or pinholes may be gas attempting to escape due to internal moisture buildup.
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Non-wetting and De-wetting
Non-wetting—where part of the pad does not contain solder—can occur if not enough solder remains after liquid solder removal. De-wetting describes the opposite scenario where too much solder remains.
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Skips
The automated soldering process can skip an SMD pad and leave it without solder. Because no fixed connection with the component pin and footprint pad exists, operation can be erratic.
The list above is not exhaustive, and other solder joint quality issues can arise during PCB assembly. However, the list above describes defects that may be addressed with good design practices.
Designing to Avoid Common PCB Assembly Defects
Your team can take design steps to help facilitate an optimal PCB assembly process for your CM. The most important of these are:
| Design Guidelines to Avoid Common PCB Assembly Defects | |
| Design Action | To Avoid PCB Assembly Defect |
| Balance component weights across board | Solder bridging |
| Orient like components in the same direction | Solder bridging Pinholes Blowholes |
| Leave sufficient gap between pads and solder mask | Solder bridging |
| Use components with similar solder thermal profiles | Tombstoning |
| Ensure procured components are within their solderable shelf life | De-wetting Non-wetting |
| Use correct lead to hole size ratio | Pinholes Blowholes |
| Ensure pad sizes are the same for a specific SMD | Skips |
By employing the design actions above, you can ensure that your design will be well-prepped to facilitate an efficient and high-quality PCB assembly process.