Solder Mask in SMT Assembly
The solder mask is a critical component of the printed circuit board (PCB) assembly process, and it can be used to prevent the accidental bridging of adjacent pads or tracks during the soldering process. This is accomplished by using a chemical layer that contains openings for the areas that are to be soldered and masking the rest of the surface. It also controls the placement of components on the board by defining the space for them and preventing them from touching other circuit features.
Solder mask is available in several different colors, but the most common is green to match the color of the copper traces on the PCB. The mask should be thick enough to protect the underlying copper, but not so thick that it makes it difficult for the soldering equipment to see the traces. The ideal thickness is between 20 and 50 microns.
There are two basic types of solder masks: dry film and liquid photo-imageable. Dry film is applied as a sheet of dry material that is exposed to ultraviolet light and developed afterward. The resulting pattern is then used to transfer the mask to the PCB, and it is thermally cured afterward. This method is commonly used for high-density boards because it requires less solvent and is faster than liquid photo-imageable.
The most important function of solder mask is to protect the exposed copper on the surface of a PCB from the elements. Copper is a highly conductive metal, but it is susceptible to corrosion from moisture and oxygen in the air. When the metal oxidizes, it loses its electrical properties and becomes brittle, which can result in shorts during the assembly and rework processes. Solder mask seals the copper away from the atmosphere and keeps it functional, reducing the amount of time required to perform rework on boards that have experienced shorts.
The Importance of Solder Mask in SMT Assembly
Solder mask can be etched or screened to create a mask that fits the size and shape of the pads on the PCB. Pads are small, flat areas on a PCB where solder paste is applied to support the pins of SMD components. A stencil is often used to apply the solder paste in the predetermined locations on the circuit board.
Traditionally, through-hole technology (THT) was the dominant assembly technique for PCBs. THT involved drilling holes in the PCBs for each lead. This was a time-consuming and costly process, especially for large-scale production. SMT allows PCBs to be fabricated with minimal to no THT, cutting assembly costs and the overall time to market for electronic devices.
Solder mask is a crucial part of the smt assembly process, and the quality of the mask can significantly impact product reliability and manufacturability. PCB fabricators use a number of standards, including IPC-SM-840C, to guide the specification and testing of solder mask materials. This standard outlines requirements for the material’s physical properties, such as its thickness and curing temperature, as well as its chemical resistance to chemicals used during the assembly process.
