Soldering Play in Coverlay PCB
Coverlay is the thin, flexible film that protects traces and pads from mechanical wear during production, shipping, and assembly. It also provides insulation and a stable base for component attachment on the flex circuit board. Coverlay is a critical step in ensuring the reliability of your flex PCB design. The choice of material and thickness must be balanced with cost and flexibility.
Adding a coverlay pcb layer to a flex circuit requires extra processing steps including surface treatment, adhesive application, lamination and trimming. This adds time to your production and imposes additional costs. However, the added reliability and durability of your flex PCB is well worth the extra effort.
The most common problem with coverlay is delamination, which is caused by improper lamination pressure or temperature. It causes small areas of the coverlay to separate from the underlying substrate and impairs dielectric strength.
Another coverlay issue is overlapping holes, which can be due to drill bit size or design constraints. These holes can cause a lack of alignment and registration between the coverlay and the copper conductor layer. They can also impact the quality of your finished product.
A final common coverlay issue is the presence of dings, dents and scratches. While these are cosmetic and only impact the appearance of your product, they can still impose a negative perception of quality.
What Role Does Soldering Play in Coverlay PCB?
One of the primary functions of a coverlay is to resist solder and prevent track scars, which are created when the metal conductors on a circuit board are heated during the soldering process. During the manufacturing process, the copper foil circuit and the coverlay are exposed to heat and vibrations from the soldering tools. This can create circuit scars, which are areas of uneven or brittle copper that affect the conductors’ ability to conduct current.
When a flex circuit is subjected to repeated bending, the creases in the covers can become brittle and tear away from the conductor layers. This can damage the connections and lead to intermittent failures. A brittle coverlay can also interfere with the flex circuit’s reflow soldering capability. A brittle coverlay can also inhibit the correct transfer of heat from the solder to the copper conductors.
Coverlay issues can be avoided by choosing a material and thickness that will suit your flex circuit’s flexability requirements. The ideal coverlay for a flex circuit is a polyimide film, which is more robust than an acrylic or epoxy adhesive. Acrylic and epoxy can be used to adhere coverlay to the copper circuit layer, but polyimide is the preferred material for flex PCBs that will see tight bending and dynamic flexing.
Another key factor to consider when choosing a coverlay is the temperature range that the flex circuit will be subjected to. The coverlay should be able to withstand the temperatures that will be encountered during normal use of your flex circuit.
After lamination, the PCB with the coverlay is subjected to a curing process to ensure that the adhesive sets properly. This typically involves heating the PCB assembly in an oven for a specified period, depending on the adhesive properties. Curing solidifies the bond between the coverlay and the PCB, enhancing the durability and stability of the final product.
