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Can Coverlay PCB Bend Without Damage?

Coverlay PCB Bend Without Damage

While flexible PCBs are a critical component in modern electronics, they must be rugged and durable enough to protect circuits from damage during use and transport. To do so, manufacturers must consider the coverlay used and its ability to withstand bending. The best covers are made of biaxially stretched polyimide (PI) film that demonstrates excellent tensile strength and flexibility.

In addition to its tensile strength, PI covers are also able to resist thermal degradation and chemical exposure, making them suitable for many different operating environments. They also exhibit high dielectric constants and low losses, allowing them to support high-speed, high-density designs in demanding applications.

As the name suggests, a coverlay pcb is a thin layer of plastic or metal that wraps around the copper circuit layers to insulate and protect them from contaminants during the manufacturing process. In addition, it can improve the signal transmission of flex PCBs and provide additional electrical protection for components like power chips. Compared to rigid FR-4, a coverlay can be up to 40% lighter and has much higher flex life.

The coverlay is typically a sheet of PI or PET film with an adhesive on both sides. It is bonded to the flex dielectric using heat and pressure. It may be pre-cut to a precise size or laser cut before lamination. The flex board fabricator will then drill holes in the coverlay for plated through hole and surface mount technology pads, depending on the design. The drilling method is mechanical, so the holes are round and smaller than the corresponding flex PCB pin spacing. To ensure a tight bending radius, the holes must be positioned away from critical bend areas in the layout.

Can Coverlay PCB Bend Without Damage?

A coverlay behaves similar to a solder mask in terms of providing a patterned opening for each pad on the Gerber view, but it is not imaged with photolithography like a solder mask. Instead, the flex PCB manufacturer must cut the coverlay with a tool such as a drill, router, punch, or knife to create the pad openings before stackup assembly. Depending on the complexity of the layout and feature sizes, this can be difficult and time-consuming.

When evaluating the bending capabilities of a flex PCB, it is important to consider both the coverlay and the solder mask. The transition between the two materials could potentially concentrate mechanical stress and degrade flex circuit performance if placed over tight bend zones. The flex PCB should be designed with a gap between the overlapping coverlay and solder mask area to minimize this impact.

In addition to ensuring a proper coverlay-to-solder mask gap, it is important that the pad design is accurate for the fabrication process and the fabrication equipment. Specifically, the holes must be sized to accommodate the machining diameter of the mechanical NC drill used by your flex PCB fabricator. Oversized holes can cause overlapping and prevent the insertion of a solder joint. To avoid this issue, the oversized hole should be made at least 1 mil larger than the pad size and 2 mil larger than one side of the pad to ensure sufficient clearance.

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