What is the Solder Mask?

Solder mask or solder resist is a coating applied to a Printed Circuit Board (PCB) to protect the copper traces on the board and to stop electrical shorts. The solder mask is usually applied on both sides of the PCB i.e Top and Bottom to ensure reliability and performance. It protects the traces from oxidation and other environmental factors like contaminants, dust, and dirt which might affect its overall performance. 

In addition to the environmental factors, there is a risk of electric shorts due to the formation of a solder bridge. A solder bridge is an unintended electrical connection between solder pads and is responsible for creating an electric short. In the image below, the light green line is the solder mask layer. As you can see, the solder mask covers the entire trace, leaving only the pads exposed which are required to make electrical connections.

What material is used for solder mask?

Polymer is the primary choice as the resin material for a solder mask because it provides excellent insulation, humidity, and high-temperature resistance. The solder mask layer is primarily green in color because it is the preferred choice of designers as it aids the inspection procedure of the PCBs. However, they are also available in red, yellow, white, black, and blue. This is something that needs to be specified to the PCB manufacturer before fabrication. Click here to learn more about PCB solder mask colors.

Types of Solder Masks

Epoxy Liquid

Epoxy is the most cost-efficient solder mask material. Epoxy Liquid is a thermosetting polymer that hardens when subjected to heat. Epoxy liquids are used as solder mask as it has minimum shrinkage post hardening. It is a type of polymer that has many applications.  

Liquid Photo Imageable (LPI) Solder Masks

Liquid Photo Imageable (LPI) solder masks are a mixture of different solvents and polymers. These liquids are only mixed together before application of the mask layer to a enable longer shelf life. LPI Masks are available in various colors such as green, red, yellow, white, black, and blue. LPI masks can be used for making different colored PCBs which will help in the identification and visibility of PCBs in essential designs.

Dry Film Solder Masks (DFSM)

The Dry Film Photo Imageable Solder Mask is applied using vacuum lamination. The dry film is encased over the PCB board and sealed with the help of a vacuum. They are available in the form of thick as well as thin sheets. Since the film is dry in nature, there is no liquid present that might flow into the holes present on the board which might result in irregularities in copper plating. Boards with flat surfaces benefit the most from Dry film solder masks because it provides them with uniform thickness. However, it is more expensive when compared to Liquid photo imageable (LPI) solder masks.

Steps for Applying Solder Mask onto the PCB

Here is a descriptive video of how a solder mask is applied to a PCB board along with the steps.

Step 1: Cleaning the Board

Before starting the procedure, one must ensure that the board is clean, dry, and free from dust.

Step 2: Resin Coating the Solder Mask

After the board is cleaned, the board must be placed in a vertical coater for solder mask ink coating where the ink completely encapsulates the copper tracking of the board. The thickness of the coating is determined by the number of electronic components and board thickness.

Step 3: Pre-hardening

Pre-hardening is the process to ensure that the ink coating is dried by a conveyorized dryer where at this point the board is now checked for a complete and even coating. 

Step 4: Imaging and Hardening

This process involves covering the boards with a transparent film. The Transparent film is printed with the exact circuit imagery of the board which will help encase the copper traces when they are subjected to Ultra Violet (UV) light. Here, the solder mask is hardened along with the circuit traces onto the board by means of Ultra Violet (UV) exposure. It is essential for the transparent film to have a correct alignment with the copper layers of the board to ensure the successful completion of this process.

Step 5: Developing

The Freshly hardened panels are then subjected to the developer, which is a machine that consists of high-pressure water sprays which help peel off the transparent film and strip off the unhardened and excess ink coating.

Step 6: Hardening and Cleaning

Post Developing, The Hardening Procedure (Step 4) is repeated again to provide a robust and permanent coat of the solder ink. The board is then cleaned to remove any dust particles which might have gathered on its surface

Post hardening and cleaning, the surface of the board is then smoothed and the electronic components are then assembled.