Commonly Used in Coverlay PCB

The type of coverlay material selected depends on the application and environment of the flex PCB. It is also a factor in the cost of the overall assembly. The choice of coverlay also impacts the flex circuit’s flexibility and ability to withstand dynamic stresses, such as bending and twisting.

Coverlay rework can be costly and time-consuming. Proper design considerations can prevent coverlay rework and maintain production throughput. The best way to minimize rework is by selecting a suitable coverlay material from the start. For example, specifying a higher-temperature or abrasion-resistant coverlay will prevent damage caused by vibration and mechanical stress.

A wide range of coverlay materials is available, including polyimide, polyester, and liquid photoimageable (LPI) covers. Polyimide is the most common and provides high performance for rigid-flex applications. Its tensile strength is up to 200 MPa and can withstand extreme temperature and humidity conditions. Polyimide coverlay pcb is also abrasion-resistant and is not affected by chemical solvents.

Other coverlay options include polyester, which offers superior mechanical properties and insulation. It is available in different grades, including high-temperature and low-cost options. Polyester is easy to process, has excellent dimensional stability, and is resistant to moisture. It can be laminated to copper foil for use in flex circuits.

Choosing the right coverlay material and adhesive is crucial to ensuring the quality of the final product. Insufficient adhesion between the adhesive and coverlay can lead to delamination, resulting in open circuits and other defects. This can occur when the adhesive is not cured properly or when there are imperfections on the surface of the substrate or the substrate dielectric.

What Materials Are Commonly Used in Coverlay PCB?

Before the coverlay is applied to the board, it must be cleaned to remove any contamination and ensure a strong bond. This can be done using a chemical etchant or plasma cleaning. The coverlay material is then cut and punched to create openings for pads, vias, and other exposed features. This can be done manually or using a laser cutter. A thin layer of acrylic, epoxy, or polyurethane adhesive is then applied to the surface of the coverlay and the flex dielectric.

The coverlay is then laminated to the flex PCB using heat and pressure to form a strong bond. During this step, any air bubbles are removed to ensure a consistent surface. The flex circuit is then routed and any required finishing, such as edge rounding or bevels, is completed.

Testing is an important step in determining the quality of the flex circuit. The flex PCB must be subjected to a series of environmental tests, such as bending endurance, soldering thermal shocks, and temperature cycling. These tests evaluate the flex circuit’s ability to withstand harsh environmental conditions and ensure a strong bond between the coverlay and substrate dielectric layers. Only a coverlay that passes these tests can be considered a viable option for the flex PCB assembly.

To create the desired coverlay pattern, a bare flexible PCB is first printed with solder mask or etch resist in a standard PCB layout. Then, the coverlay is cut to size with a CNC router or other precision cutting methods. These can include drilling, routing, laser cutting, punch & die, or knife cutting.