Everything You Need to Know About PCB Solder Masks

Printed Circuit Boards (PCBs) consist of copper layers, which serve as the base of the PCB. All components are soldered onto this board using a soldering machine. These components are connected to each other with thin lines of copper known as traces. Finally, a protective film is applied on the copper traces. The solder mask is coated on both sides of the board.

The solder mask forms the last layer step in PCB manufacturing. These steps are repeated to create multi-layered PCBs. In multi-layered PCBs, the layers are connected to each other using specific inter-connectors.

Material Used

A variety of materials are used depending on the budget, size of the board, the density of copper traces, number and size of holes, etc. Below are mentioned two popular materials used:

1. Epoxy Liquid
   It is the most commonly used material for solder masks, since it is very reasonably priced and provides the PCB with reliable protection. It is efficient even in the presence of many components and connections.
2. Dry Film Resist
   A dry film is used when the surface of the PCB is even.

Resolution Range of Solder Mask

The performance of solder mask will depend greatly on the following factors. The range varies depending on the number of components, traces, and number, thickness and material of layers:

1. Solder Mask Thickness
   0.2mil – 1.6mil
2. Solder Dam
   4mil for green colored solder mask, and 5mil for solder mask of other colors.
3. Solder Mask Hole Plug Diameter
   10mil – 25mil

Colors & Finishes of PCB Solder Masks

Initially, solder mask was available only in green color. Now a variety of colors can be used to make your PCBs looks attractive. When PCBs feature different solder mask colors, it is easy to distinguish between them as well. The colors are available in glossy as well as matt finish. The array of colors available comprises:

1. Glossy Finish
   • Green
   • Black
   • Red
   • Yellow
   • White
   • Purple
   • Blue
2. Matt Finish
   • Green
   • Black

Importance of Solder Mask on Your PCB

1. It prevents oxidation of the copper traces.
2. Special PCB inter-connectors are used at particular places on the PCB to connect one layer of a PCB to another. Any other kind of connection is unwanted. Since, the layers are very close to one another; even the smallest bump of solder could cause unneeded connections between the layers. At this point, a layer of polymer, which is the solder mask, helps greatly in making sure that the solder doesn’t provide unnecessary connections.
3. It prevents unexpected connections between components even where tolerances are tight.
4. Solder mask keeps dust and dirt away from the electric connections and the substrate.
5. It prevents the electric connections from corroding.

Solder mask is used for both hand as well as automatic assembly. It is an essential step in the entire process of PCB fabrication. For efficient connections and performance, solder mask is of immense importance.