Surface Finish in Flex PCB Fabrication

In PCB fabrication, surface finish plays a major role in the cost, complexity and assembly efficiency of a flex PCB. It can also impact the reliability of a flex circuit board, depending on the type of finish used. During the design process, it is important to consider the effect of the chosen surface finish on your project and choose the best one for its intended use.

The stack-up of a flex PCB is formed by multiple layers of dielectric films and copper-clad laminates (CCL). A CCL can be made from different types of materials, including flexible plastic (Mylar or polyester), polyimide, Kapton, and fluoropolymer (PTFE). These are typically combined with a rigid core of FR-4 or Rogers 4003 material. In order to fabricate a flex circuit, the conductive layers must be printed on the substrate and then etched. After the etching process, small holes are drilled and then plated with copper. The copper plating connects all the layers of the flex circuit and makes them electrically connected. The next step is to apply the solder mask, which protects the exposed copper traces from corrosion and oxidation during assembly.

A common choice of surface finish for a flex pcb fabrication is HASL or High-Area Solderability Level. This process uses a photoimageable solder mask to transfer the solder mask pattern onto the production panel using collimated UV light. The solder mask then encapsulates the copper, protecting it from contaminants and providing an easy surface for components to be mounted.

The Role of Surface Finish in Flex PCB Fabrication

Layer count: The number of layers in your flex circuit will significantly increase manufacturing costs. This is because each additional layer requires more complex lamination, resulting in higher labor and materials costs. Likewise, tighter trace and space sizes require more precision and add to the overall cost.

Conductor thickness: Thicker conductive layers can add to the overall cost of your flex circuit. In most cases, this is necessary for proper performance.

Stiffeners: Some flex circuit designs may require stiffeners, which are areas of rigid materials (can be FR4) added to specific parts of the flex circuit to increase dimensional stability. These can be expensive, but they can improve the lifespan of your flex circuit by adding durability and reliability.

Adhesives: In the past, adhesives were a common part of the flex circuit manufacturing process. However, these are no longer a standard and have been replaced with better alternatives. For example, the Bondply® composite is made from a polyimide film coated with B-staged acrylic adhesives on both sides to encapsulate the etched details in heavy copper multi-layered constructions of flex and rigid-flex boards. Other examples of non-adhesive technologies that have been developed include FR-4 prepregs and thermosetting epoxies. These technologies reduce the number of steps in manufacturing, making a more cost-effective solution to the use of adhesives for flex and rigid-flex boards.