Rigid Flex Circuits
The solder mask is an essential component of the printed circuit board (PCB) that protects the copper traces during fabrication. It also functions to prevent oxidation of the copper surfaces and enhances the electrical conductivity of the traces. A correctly specified and applied solder mask is crucial to achieving high quality, reliable PCB assemblies.
A PCB can be made of a rigid or flex circuit. rigid flex circuits are hybrids of the two, combining elements of both rigid and flexible PCBs to meet a specific set of design requirements. They are more complex than traditional rigid PCBs and require specialized manufacturing processes, including solder mask application and conformal coating. In order to ensure a successful and repeatable manufacturing process, it is important for the designer to understand the requirements of each process and how they interact with one another.
The solder mask on a PCB is the layer of ink covering the surface of the PCB that is not covered by the pads. It is commonly referred to as the green layer and it is used to protect the copper on the PCB from tin during wave soldering. A properly designed solder mask will prevent tin bridging between pads and also prevent short-circuiting caused by excess solder.
Solder Mask Requirements for Rigid Flex Circuits
When designing a solder mask, the thickness of the ink must be taken into account. The solder mask ink should be at least half of the width of the narrowest copper traces on the PCB. This will provide adequate coverage and protection for the traces while minimizing the possibility of solder mask detachment and other defects.
A common method for applying the solder mask is through the silkscreen process. This is a semi-automatic printing technique that can be used to apply a variety of materials, including solder mask and etch resist. This process is able to deposit a wide range of material thicknesses, making it an ideal solution for producing high-quality and affordable PCBs.
The selection of the type and color of the solder mask is a critical part of the design process. The choice of color depends on the use case for the PCB and whether it needs to be compatible with other assembly processes, like conformal coating or appearance requirements from end customers.
There are a few different options for the type of solder mask, including dry film and wet film. Wet film solder mask is usually a liquid epoxy resin, which is silkscreened and then chemically developed with high-pressure sprays of water. This is an inexpensive and reliable option, but it can produce smudges, smears, and uneven coatings. Dry film solder mask is a more expensive option, but it is very durable and has a good level of chemical resistance.
When choosing the colour of the solder mask, it is important to consider the UL approvals of the base material and the dye. Although UL has more than 1,600 paint types, most factories only have a few colour and type approvals. Therefore, it is important to discuss these issues with the factory in advance and make sure they have the right UL approvals for your design before sending them out for production.