4 Common Errors In SMT Assembly

Surface Mount Technology (SMT), is a commercially used assembly process because of its many benefits, but it is one of those processes that, in time, will experience manufacturing errors.

That being said, most of these errors can be avoided.  This article will discuss the four most common SMT errors, their causes and solutions, and how a surface mount soldering training course can help your staff manage each step of the SMT assembly process, eventually reducing the likelihood of these errors.

1. Bridging

Solder bridging happens when a solder accidentally connects two electrical conductors that should not be connected, often resulting in a short circuit.

Among its many causes, excess solder is the most common culprit.  Other causes are high temperatures, moistures, cold slumps, or a problem during the solder paste printing process where the stencil arrangement of the PCB pad configuration is off its mark.

To avoid solder bridging, the first thing to ensure is the correct positioning of the stencils.  The proper component placement pressure should also be maintained.  It is also advisable to avoid working in an extra moist environment.  Another solution that could help is to reduce the thickness of the stencil.

2. Solder Balling

As its name suggests, solder balling happens when tiny ball-like particles appear on the board.  Much like solder bridges, solder balls could cause electrical disruptions, especially those big enough to connect two conductors.

A possible culprit of solder balling is moisture because when moisture saturates during reflow, it leaves solder spheres slightly outside of the solder joint.  During the printing process, a smeared solder paste on the bottom of the stencil can also lead to solder balls forming.

Like solder bridging, the first immediate solution is to reduce the humidity and moisture in the facility.  Another solution that can work is cleaning the bottom of the stencil to make sure that there’s no smearing.

3. Tombstoning

Also known as the “Manhattan Effect,” tombstoning occurs when the component is partially or entirely lifted off from the pad, making it rise on one end, and looking like a tombstone, hence its name.

Uneven heat distribution is one of the leading causes behind the lift because when the solder melts at different rates, one side may reflow quicker than the other.  This temperature difference, and also in most cases, humidity in the solder paste, causes the component misalignment and creates that vertical or horizontal lift.

Improper placement of components before heat reflow can also create this tombstoning problem because it causes unbalanced soldering.  To avoid this imbalance, check component placement. Make sure its body covers at least 50% of the pads on both sides.  Like with bridging and solder balling, less exposure to moisture and heat could help tremendously.

4. Non Wetting Or De Wetting

Non-wetting is an SMT defect that occurs when the base metal surface on the board doesn’t accept the molten solder, creating a poor joint condition where a component’s terminals can’t stick properly to the PCB pads.

On the other hand, De-wetting occurs when the molten solder coats the component terminals and PCB pads successfully at first but then recedes, leaving thin areas of solder on the base metal and thick clumps.

One of the main reasons for this problem is that either the PCB pads or the component terminals are oxidized, preventing proper contact between the solder and the base metal surface.  An expired solder paste is also a common cause of this defect.

To prevent Non-Wetting and De-Wetting, make sure to clean the boards first, don’t use expired solder paste, and don’t use a PCB stored for more than a year without a protective cover sheet.

Conclusion

Reducing SMT board errors is the key to providing the most efficient PCB assembly services.   The right SMT soldering course can help you achieve this goal. By following this guide and investing in surface mount soldering training, your assembly staff can have tighter control over the soldering process, which will then elevate the quality of your output to higher levels.

Want to learn more about our SMT soldering course?  Contact Blackfox today at (888) 837-9959 or at sharonm@blackfox.com.