How Does Temperature Control Affect the Coverlay PCB Process?

Temperature Control Affect the Coverlay PCB Process?

Coverlay is the protective layer that protects the copper layers of flex PCBs from physical damage. Its primary function is to provide insulation, but it also offers durability, flexibility and conductor protection that solder mask cannot offer. While adding coverlay to a flex circuit does increase manufacturing time, it is an essential step to ensure that the resulting flex PCB will meet quality and performance requirements.

Adding a coverlay layer to a flex circuit requires additional processing steps, including surface treatment, adhesive application, lamination and trimming. Choosing the right coverlay material and adhering it properly will ensure that it provides durable, flexible insulation to the conductor layers. Coverlay can be a polyimide film or liquid photoimageable material, and it may be available in a range of thicknesses. Its choice is based on the operating environment and specific needs of the flex circuit.

The plated through hole (PTH) and surface mount technology (SMT) features on the flex circuit are patterned into the coverlay using mechanical NC drilling or punching, laser ablation, knife cutting or CNC drilling depending on the complexity of the design and feature size. Unlike the photolithography used to pattern solder mask, these methods do not impose the same IPC tolerances for pad exposure as does flex LPI.

However, they do impose larger minimum annular rings to allow for material and manufacturing tolerances as well as potential adhesive squeeze out during the lamination process. Also, they do not provide isolated “island” type features as these would fall out of the coverlay during machining.

How Does Temperature Control Affect the Coverlay PCB Process?

Once the coverlay is patterned, it is aligned to the copper circuit layer and laminated under heat, vacuum and pressure, curing to form a strong bond between the layers. A stiffener is typically added to help prevent the flex circuit from moving or becoming loose during this process.

Despite the best efforts of flex PCB manufacturers to apply and cure coverlay pcb with great care, some issues can occur that impact quality. Wrinkles and delamination of the coverlay can result from inadequate lamination temperature, pressure, venting or surface preparation. Delamination can expose conductors, and it is often difficult to re-adhere such areas without risking loss of insulation integrity.

Other defects, such as dents, bumps, discoloration or debris in the coverlay, are usually cosmetic but can still affect quality perception. Careful inspection is needed to identify these flaws in the finished product, and they should be rectified by re-adhesiveing, re-machining or stripping and re-applying the coverlay.

This can be costly, especially when these issues are extensive and require re-application of the entire coverlay layer. The best way to avoid these issues is careful control of the coverlay manufacturing process, from initial fabrication through assembly. In addition to the coverlay itself, the selection of a suitable assembly adhesive is important for long-term reliability. This is because the adhesive must be able to resist thermal cycling, vibration, bending and other stresses. Insufficient adhesion between the flex circuit and the coverlay will result in premature failure.

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