How Are Solder Masks Used in SMT Circuit Boards?

How Are Solder Masks Used in SMT Circuit Boards?

Solder Masks Used in SMT Circuit Boards

In order to ensure that the copper traces on a smt circuit board are not soldered together during assembly, they must be covered with a layer of non-conductive material. This layer is called solder mask and is applied in several ways, depending on the size and complexity of the board.

One method is to silk-screen the solder mask, using a stencil with different color areas to represent different traces on the board. This method is very expensive, but it is capable of producing extremely high-quality results. Another method is to apply the solder mask with an imaging process that uses a photo film aligned with the board, and which is then exposed to UV. This causes the ink to harden where it is opaque, protecting those areas of the board that need to remain un-solderable.

Once the solder mask is applied to the circuit board, it must be inspected and tested. If any defects are identified, they will be corrected at this stage. Once the testing and inspection stages are complete, the PCB can be moved on to the reflow soldering process.

In the reflow soldering process, the components are placed onto the board and then reflow-soldered to it. Solder mask protects the surface of the board from contamination by hazardous chemicals used in this process, and prevents solder bridges from occurring on the plated copper features. Solder mask also ensures that the plated vias on the PCB stay open and do not become closed. If the solder mask is not sufficiently thick, reflow solder can melt and flow through the holes, causing unwanted connections to be made.

How Are Solder Masks Used in SMT Circuit Boards?

The most important function of the solder mask is to protect the underlying copper from corrosion and oxidation. The corrosive action of the molten solder is accelerated by environmental factors such as temperature, humidity, and airborne contaminants. A quality solder mask is designed to protect against these factors, and is usually made of a non-conductive resin such as epoxy.

The final step in the smt circuit board production process is to coat the circuit board with a surface finish, such as hot air solder leveling (HASL) or electroless nickel immersion gold (ENEPIG). This will protect the etched copper from further oxidation and degradation. The ENEPIG process is very environmentally friendly, as it requires less energy and produces far fewer volatile substances. It is recommended to choose a conductive copper plating that matches the color of the solder mask, to ensure that the plated areas are not obscured.

In this way, the integrity of the smt circuit board is maintained, and it will be able to perform as intended. During this process, the solder mask is cleaned to remove any residue from previous steps, and then a final inspection and test are conducted. This will determine whether the PCB is ready for assembly and shipping. If not, the smt circuit board will be sent back for touch-up or repair. During the manufacturing process, errors in component placement will often go undetected, and can lead to costly and time-consuming rework.

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