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The Significance of Solder Mask in Rigid Flex Circuits

Solder Mask in Rigid Flex Circuits

Rigid flex circuits offer a combination of rigid PCB materials with a flexible substrate. This allows for high-density connections with greater flexibility than traditional rigid PCB designs. In addition, this combination offers lower total system weight due to the reduced circuit traces and cables. Many of the world’s most popular products incorporate rigid-flex technology. Rigid flex is used in laptop computers, smartphones, tablets and many other consumer electronic devices.

Printed circuit boards (PCBs) require a wide range of processes to produce them, from initial design and layout, through fabrication, drilling, and etching to the final assembly and testing. As such, a PCB can be an extremely complex and time-consuming process to manufacture.

The most significant component in this multi-step process is the solder mask layer. Often considered an unseen feature, the solder mask is one of the most critical components in a PCB and plays an important role in a board’s reliability.

As the name suggests, the purpose of solder mask is to mask the copper surfaces on a PCB and prevent them from being exposed during the manufacturing process and subsequent soldering. In order to do this, the masking material must be highly durable. The solder mask must also be able to withstand the heat of a reflow oven without degrading or flaking off.

The Significance of Solder Mask in Rigid Flex Circuits

This durability is achieved through the use of a high-density polyimide material, known as coverlay. The thickness of the coverlay must be sufficient to withstand the mechanical stresses caused by the board’s bend requirements. In addition, the thickness must allow the masking to withstand the reflow temperature without degrading or flaking off.

The coverlay is applied to the flex or rigid flex circuits using an adhesive. Depending on the specific requirements of the design, this can be an epoxy or acrylic based flexible adhesive. The bonding of the coverlay to the flex or rigid-flex circuit is essential because it provides an impermeability barrier between the conductive copper surfaces and the soldering material.

A photo-imageable solder mask is then applied to the surface of the coverlay. This can be a dry or liquid application and is available in different colors, such as green, blue, red, yellow, black, purple, Grey, Orange, Matte black and transparent/clear. Green solder mask is most commonly used because it creates the smallest possible solder dam between adjacent pads, and it is also visible in daylight.

The solder mask must be sufficiently durable and transparent to allow the copper surfaces to be plated. As such, the tolerances of the solder mask are quite strict and must meet IPC-A-600 standards for optical acceptance. These tolerances are not as stringent as the tolerances for the copper plating, however, and slight scratches and rework are permissible to a certain extent. This is because the function of the solder mask is technical rather than aesthetic. However, the surface of the solder mask must be free of dirt, oil, and lubricants in order to work effectively. The good news is that Epec offers a comprehensive set of materials for flex or rigid-flex PCBs that can be customized to suit your design needs.

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