Two conditions that lead to PCB failure

Insufficient solder joints or electrical open circuits

When two electrical connections are separated, or an area on the PCB that interrupts the intended design of the circuit is called an electrical open circuit.

Potential Causes

The solder paste printing stage of the surface mount process contributes the most to this defect.

Insufficient solder on the joints can lead to an open circuit. This can occur if the solder paste clogs the holes in the stencil.

Even if the amount of solder is sufficient, an open circuit can occur if it does not make contact with both the leads and the pads during reflow. This is called component lead coplanarity.

An open circuit can also be the result of the PCB manufacturing process itself.

Possible solutions

First, the solution is to correct the aspect ratio. Aspect ratio is defined as the ratio of the width of the aperture to the thickness of the stencil. A blocked hole in the solder paste may be due to a too small aspect ratio.

Extreme environmental conditions are strictly prohibited during the manufacturing process. Solder paste contamination is avoided by controlling the environment.

Investigation of co-planarity is also very important when solving electrical open circuit problems.

Manufacturing must be checked with the PCB supplier.

Solder Balls

The development of very tiny spherical solder particles isolated from the body forming the joint. This is an important issue for no-clean processes because a large number of solder balls can create a false bridge between two adjacent leads, which can cause functional problems for the circuit. For water soluble processes, solder balls are not as much of a concern because they are removed periodically during the cleaning process.

Potential Causes

Moisture contamination of the solder paste is one of the main causes of solder balls. Moisture saturates during reflow, leaving solder balls behind.

Lack of proper reflow profile can also lead to solder balls. Fast preheat rates do not provide enough time for solvent to evaporate gradually.

Excessive oxides on the solder powder in the solder paste can also form solder balls.

Solder balls can be caused by poor alignment of the solder paste printing and by the solder paste printing on the solder resist layer instead of the pads.

Solder paste applied at the bottom of the stencil during printing is also an attribute.

Possible solutions

It is recommended to use a coarser powder size as fine powder sizes have more oxide and are more likely to collapse.

The reflow process should be selected according to the solder paste.

Interaction of the solder paste with moisture and humidity should be avoided.

Check the minimum print pressure to be used.

Print alignment should be verified on a consistent basis prior to reflow.

Ensure proper and frequent cleaning of the bottom of the stencil.

Conclusion

With the miniaturization of PCBs, there are more and more issues related to PCB manufacturing. Tighter control of the soldering process is required for the smaller components used on PCBs. Reducing common errors in SMT is an important step in improving the efficiency of SMT manufacturing, leading to more efficient PCB assembly services.

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