Deep Dive:
The Complete World of a Circuit Board

Price is mainly determined by layer count, substrate material, process complexity and quantity. A standard 2-layer FR-4 prototype of 5 pieces costs only ¥50–100; a 6-layer board runs ¥400–800; an 8–10 layer board with high-Tg, impedance control and blind/buried vias costs ¥2,000–5,000; and IC-substrate-grade 20+ layer boards easily exceed ¥10,000. Specialty processes such as ENIG, gold fingers, heavy copper and HDI all add cost.

HASL (Hot Air Solder Leveling) is low cost with excellent solderability, but its uneven surface makes it unsuitable for fine-pitch QFN/BGA packages and press-fit connectors. ENIG (Electroless Nickel Immersion Gold) offers a flat surface, low contact resistance and long shelf life — ideal for keypads, BGAs and precision components — but costs ~30% more and carries a "black pad" failure risk. Consumer products default to HASL; premium products prefer ENIG.

When signal frequencies exceed 100 MHz, traces are no longer simple "wires" but "transmission lines" that require controlled characteristic impedance (typically 50Ω single-ended, 100Ω/90Ω differential). Impedance discontinuities cause signal reflections, ringing and degraded eye diagrams, ultimately leading to communication errors. Fabs achieve impedance matching by controlling trace width, copper thickness, dielectric thickness and Dk (dielectric constant), and provide impedance test coupons for verification.

HDI (High Density Interconnect) boards are defined by laser-drilled blind/buried vias and fine trace/space (≤3 mil / 3 mil). Conventional mechanical drills have a minimum diameter of ~0.2 mm, while laser vias can reach 0.1 mm or smaller, freeing up substantial routing space. Smartphone motherboards use nearly all Any-Layer HDI; the iPhone motherboard employs 10–14 sequential HDI laminations, representing the pinnacle of consumer PCB technology.

Flexible Printed Circuits (FPC) use polyimide (PI) as the substrate and can be bent and folded repeatedly; they are indispensable in phone camera modules, foldable hinge interconnects, TWS earbuds and smartwatches. Rigid-Flex boards laminate rigid and flexible sections into a single assembly, eliminating connectors and reducing size and weight. They are widely adopted in aerospace, military electronics and premium foldable phones (e.g., Huawei Mate X series, Samsung Galaxy Fold) and cost 5–10× more than standard boards.

DFM (Design for Manufacturability) is the fab's process audit of your design files: whether trace/space exceeds the fab's capability, whether annular rings are sufficient, whether solder mask dams are appropriate, and whether panel utilization is optimized. Almost every beginner's board has DFM issues — from trivial ones like silkscreen over pads, to critical ones like traces routed too close to the board edge causing exposed copper. Reputable fabs issue a DFM report for approval before production; skipping this step often results in unusable boards.