Solder Mask

What is solder mask?

The solder mask (also called solder mask) is a thin layer of polymer material applied to the surface of the PCB (printed circuit board). Its main function is to protect the copper traces and prevent solder from flowing into areas where soldering is not required during soldering. In order to make the soldering more perfect, the entire circuit board, except for the pad area, will be coated with a solder mask.

Solder mask is applied on both sides of the PCB. Resin is the main component of solder mask because it has good moisture and high temperature resistance and is non-conductive. Initially, most PCBs used green solder mask, so it is often called "green oil". However, solder mask also has many colors, such as green, white, yellow, red, blue, black, etc. The specific color to use depends on the different needs of customers.

Function of solder mask

The solder mask on the PCB has the following functions:

• Prevent oxidation of the copper layer;
  
• Prevent short circuits caused by bridges during soldering;
  
• Prevent physical breaks in the conductor line;
  
• The solder mask has high insulation properties, making it possible to design high-density PCBs.
  
• Prevent short circuits between conductive lines and solder joints during reflow soldering, wave soldering and manual soldering;

Types of Solder Mask

There are different types of solder mask on PCB. Regardless of the type, it needs to be heat cured after the pattern is determined. Common types of solder mask are as follows:

• Dry Film Photosensitive Solder Mask:

  DFSM is vacuum bonded to the PCB, then exposed and developed.
  

• Liquid Epoxy:

  Depending on the application requirements, the solder mask can be made of different media. The lowest cost is the liquid epoxy type, which prints the solder mask pattern onto the PCB by screen printing.
  

• Liquid Photosensitive Solder Mask:

  LPSM can be screen printed or sprayed on the PCB, then exposed and developed to form openings so that components can be soldered to the copper pads.

Solder Mask vs. Stencil

Solder mask is a key process in PCB manufacturing. The colored surface layer on the PCB is the solder mask. The solder mask is a "negative output", so when the solder mask pattern is applied to the board, the copper is exposed in the pattern opening instead of being coated with solder mask ink.

The steel mesh layer is actually a template for SMD device packaging, corresponding to the pads of SMD components. It can be directly understood as a steel sheet mold designed and made according to the steel mesh layer. In the SMT mounting process, a steel mesh is usually used to punch holes at the corresponding positions of the PCB pads, and solder paste is scraped on the steel mesh. When the PCB is placed under the steel mesh, the solder paste will flow down through the holes and evenly cover the pads. Therefore, the opening of the steel mesh layer should not be larger than the actual pad size, and it is better to be slightly smaller or equal to the pad.

The core differences between the solder mask layer and the steel mesh layer are as follows:

Layer limit

Usually, we can only manufacture single-layer aluminum substrates and double-layer aluminum substrates. Due to the limitations of the manufacturing process, multi-layer aluminum substrates are difficult to manufacture, so they cannot meet the needs of complex multi-layer designs.

Poor flexibility

Metal aluminum materials have high rigidity and low softness, and are not as flexible as polyimide or polyester substrates. Therefore, they are not suitable for applications that require repeated bending.

Mismatched thermal expansion coefficients

The thermal expansion coefficient of aluminum substrates is relatively high, which is different from some components and solder materials. The mismatch of the thermal expansion coefficients of the two can easily lead to solder joint damage or delamination, affecting overall reliability.

Aluminum substrates bring additional process requirements

Compared with ordinary substrates, the metal properties of aluminum substrates require more time to consider during manufacturing and assembly, which will increase process complexity and cost.

High cost

Although aluminum substrates have significant advantages in thermal management, compared with traditional FR4 materials, aluminum-based PCBs have higher material costs, special manufacturing processes and surface treatment requirements, so the overall manufacturing cost increases.

Application fields of aluminum-based PCB

• The openings of the solder mask layer are free of solder mask ink, while the openings of the steel mesh layer are used for solder paste deposition;
  
• The solder mask layer is used to apply solder mask ink, while the steel mesh layer is used to apply solder paste;
  
• The solder mask layer belongs to the PCB manufacturing stage, while the steel mesh layer belongs to the PCB assembly stage;
  
• The solder mask layer has a variety of colors to choose from, while the steel mesh layer is usually gray.
  
• The solder mask layer is part of the PCB, while the steel mesh layer is not, it is just a template used for patching;