Flexible printed circuit board
Solder mask is a thin lacquer-like layer of polymer that is applied to the copper traces in a printed circuit board. This protects the traces from oxidation and prevents unintended connections between solder pads (short circuits). It also allows components to be placed on the pads, which are then connected with solder paste during reflow soldering. This process produces a robust metallurgical bond between the leads and the pads.
There are many different types of flexible printed circuit board masking materials. The choice of material depends on the design requirements and bending requirements of a specific project. Epec’s team will provide guidance on the most suitable material to use for your PCBs. Increasingly complex designs often require the use of both flexible coverlay and flexible solder mask. This is typically to address the challenges of tight bend areas and high component density. The combination of these two layers ensures that your flex circuit boards can meet the required design and production standards.
The most common type of solder mask is liquid photoimageable (LPISM) which is silkscreen printed on the PCB surface. It is then exposed to UV light in a pattern which hardens the areas that are coated, leaving openings for the copper pad and component locations.
How is solder mask applied in Flexible printed circuit board?
Dry film photoimageable (DFSM) solder mask is also used. This is similar to the LPISM technique, but instead of using a liquid ink, manufacturers apply the material as a sheet that is vacuum laminated to the copper PCB surface. It is then exposed to the same UV pattern and developed, resulting in openings aligned to the copper pads and component locations on the PCB.
Both LPSM and DFSM are available in various colors. Green is the most popular color because it creates the smallest solder dam and is easiest to see in daylight. However, there are purple, Pink, Grey, Orange, Matte black and transparent/clear solder mask available as well.
The solder mask is important for protecting the copper traces from damage during manufacturing, assembly and operation. It is also important for preventing unintended short circuits between adjacent traces. For this reason, it is critical that the dielectric strength of the mask be sufficient to withstand the voltage differences between the conductive elements on the PCB.
In addition to the aforementioned benefits, solder mask is a critical safety feature of a PCB because it helps to prevent solder bridges, which are unintended connections between adjacent pads and traces. The bridges may occur due to a number of reasons, including poor PCB design and misalignment of the surface mount pads and their copper traces. In order to avoid solder bridges, a PCB should be designed in accordance with IPC standards.
Solder mask can also be used to control the placement of surface mount devices on a circuit board. This is possible because of the ability to print a “solder mask defined pad” or SMD pad. These pads are usually smaller than the copper pad they expose and are intended for use with special components.
