Rigid flex PCB are used as the "core main force" in the fields of communications, medical treatment, industrial control, security, automobiles, electric power, aviation, military industry, and computer peripherals. The functions of the products are getting higher and higher, and the PCB is getting more and more sophisticated. The production of PCB proofing is becoming more and more difficult.

The multilayer circuit has various special requirements for high speed, thick copper, high frequency, and high Tg value. The requirements for inner layer wiring and pattern size control are getting higher and higher. For example, the ARM development board has a lot of impedance signal lines in the inner layer. To ensure the integrity of the impedance increases the difficulty of the inner line production.

There are many signal lines in the inner layer, and the width and spacing of the lines are basically about 4mil or less; the thin production of multi-core boards is prone to wrinkles, and these factors will increase the production of the inner layer.

Suggestion: design the line width and line spacing above 3.5/3.5mil (most factories have no difficulty in production).

For example, a 6-layer PCB board, it is recommended to use a fake 8-layer structure design, which can meet the impedance requirements of 50ohm, 90ohm, and 100ohm in the inner layer of 4-6mil.