Introduction to Solder Mask
Solder mask, which refers to the part of the board to be painted with Solder resist. In fact, solder mask uses negative film output, so after the shape of the solder mask is mapped to the board, it is not painted with solder mask, but the copper is exposed. Usually in order to increase the thickness of the copper, make solder mask open to remove the solder resist, and then add tin to increase the thickness of the copper wire.
Requirements for solder mask
The solder mask is quite vital in controlling soldering defects in reflow soldering. PCB designers should minimize the spacing around the pads.
Although many process engineers would rather separate all pad features on the board with solder mask, the pin spacing and pad size of fine-pitch components require special considerations. Although solder mask openings that are not partitioned on the four sides QFP may be acceptable, it may be more difficult to control the solder bridges between the component pins. For the solder mask of BGA, many companies provide a kind of solder mask that does not touch the pads, but covers any features between the pads to prevent solder bridges. Most surface mounted PCBs are covered with solder mask, but if the thickness of solder mask coating is greater than 0.04mm, the application of solder paste may be affected. Surface mounted PCBs, especially those using fine-pitch components, all require a low-profile photosensitive solder mask.
Solder mask materials must be used through liquid wet process or dry film lamination. Dry film solder mask materials are supplied in a thickness of 0.07-0.1mm, which can be suitable for some surface mounted products, but this material is not recommended for fine-pitch applications. Few companies provide dry films that are thin enough to meet the fine-pitch standards, but there are a few companies that can provide liquid photosensitive solder mask materials. Generally, the solder mask opening should be 0.15mm larger than the pad. This allows a gap of 0.07mm on the edge of the pad. Low-profile liquid photosensitive solder mask materials are economical and are usually specified for surface mounted applications, providing precise feature sizes and gaps.
Introduction to paste mask
The paste mask is used for soldering SMD packaging and corresponds to the pads of SMD components. In SMT processing, usually, use a steel plate, laser holes on the steel corresponding to the PCB component pads, and apply solder paste on the steel plate. When the PCB is under the steel plate, the solder paste leaks, and it happens that every pad can be stained with solder. Therefore, the solder paste layer usually should not be larger than the actual pad size, preferably less than or equal to the actual pad size.
The role of solder mask and paste mask
The solder mask is mainly to prevent the PCB copper foil from being directly exposed to the air and play a protective role.
The paste mask is used to make stencil for the stencil factory, and the stencil can accurately put the solder paste on the SMD pads that need to be soldered when printing solder paste.
Both layers are used for soldering. It does not mean that one is soldered and the other is coated with solder resist, but:
1. The solder mask layer means to opening on the whole piece of solder mask solder resist, the purpose is to allow soldering;
2. By default, the area without solder mask layer must be covered with solder resistor;
3. The paste mask is used for soldering SMD components.