Material expansion control: Temperature changes can cause the circuit to deviate

The thermal expansion coefficients of P1 substrate and copper foil are different (PI is about 50ppm/℃, copper is about 17ppm/℃), and stress is generated when the temperature changes, resulting in circuit offset. The solution is to reserve a "compensation amount" in the design: the line direction forms a 45 ° angle with the length direction of the substrate to reduce the impact of expansion and contraction. In the temperature cycling test of a certain FPC from -40 ℃ to 85 ℃, the line offset of the 45 ° angle wiring is only 5um, which is one-third of the parallel wiring.

The production workshop must maintain constant temperature and humidity (25 ℃+2 ℃, humidity 50% ± 5%). A certain factory once experienced a summer air conditioning malfunction, causing a temperature fluctuation of 10 ℃ in the workshop, resulting in a deviation of more than 30um in the FPC circuit produced that day, with a scrap rate of 10%.

Line fracture prevention: "stress concentration" during bending is a killer

When FPC is bent, the corners and solder joints of the circuit are prone to breakage due to stress concentration. In terms of technology, stress is dispersed through "rounded corner treatment" (using rounded corners of 0.1mm or more instead of right angles at corners) and "line thickening" (increasing the width of the line near the solder joint by 20%). After increasing the corner radius of a folding screen phone's FPC from 0.05mm to 0.2mm, the fracture rate decreased from 3% to 0.1%.

The selection of rolled copper is also crucial. In areas with frequent bending such as hinges, rolled copper must be used, and the thickness of the copper foil must be controlled below 18um (thinner, more flexible). A certain test shows that the lifespan of 18um rolled copper at a bending radius of 1mm is twice that of 35um rolled copper.

Inter layer alignment: precise stacking of multi-layer FPC

Multi layer FPC (2-4 layers) requires alignment and bonding of multi-layer circuits, with an alignment accuracy requirement of+25um, which is more stringent than the ± 50um requirement for rigid PCBs. A certain 4-layer FPC adopts "optical positioning" to identify the reference points of each layer, adjust the position with a robotic arm, and control the alignment deviation within ± 15um. The interlayer short circuit rate is reduced from 1% to 0.1%.

The thickness of the adhesive layer used for bonding should be uniform (25-50um). If the adhesive layer is too thick, it will cause gaps between multiple layers, and if it is too thin, the bonding will not be firm. A certain multi-layer FPC achieves a thickness deviation of ± 5um by controlling the amount of adhesive applied, and the interlayer peel strength reaches 0.6N/mm, meeting the requirements for repeated bending.

The manufacturing process of FPC is an art of balancing flexibility and rigidity, which not only allows the circuit board to bend like paper, but also ensures the conductivity and reliability of the circuit. With the development of wearable devices, flexible displays, and other fields, the manufacturing process of FPC will continue to advance towards thinner (thickness<0.05mm) and more bending resistant (over 500000 times) directions. In the future, we may see circuits woven like fabric, making electronic devices truly ubiquitous.