Specifying a PCB fabrication order correctly is the difference between a board that works first time and a costly re-spin. This guide covers the key decisions — layer count, material, copper weight, surface finish, and impedance control — that procurement and engineering teams must get right before releasing a fabrication package to a supplier.
1. How Many Layers Does My PCB Need?
Layer count depends on signal density, impedance requirements, and component complexity. Each additional layer adds approximately 15–25% to fabrication cost and 2–4 days to lead time.
| Layers | Typical Application | Notes |
|---|---|---|
| 1–2 | Simple consumer electronics, LED boards, power supplies | Lowest cost. Avoid for designs with controlled impedance or dense BGA. |
| 4 | MCU-based designs, moderate signal density | Most common for industrial control boards. Enough for basic impedance control. |
| 6–8 | FPGA boards, DDR memory interfaces, high-speed digital | Controlled impedance routing recommended. Dedicated power and ground planes. |
| 10–16 | Complex FPGA, backplanes, RF/digital mixed-signal | Higher cost and lead time. Blind/buried vias often required. |
| 18–32 | Telecom backplanes, server motherboards, advanced FPGA | Specialist fabrication. Expect 3–5 week lead times. |
Key principle: Add layers when signal integrity demands it, not just to simplify routing. A well-designed 6-layer stackup with dedicated reference planes often outperforms a poorly-planned 8-layer board.
2. FR-4, High-Tg, Rogers, or Aluminum?
| Material | Tg (°C) | Best For | Relative Cost |
|---|---|---|---|
| Standard FR-4 | 130–140 | Consumer, general industrial | 1× |
| High-Tg FR-4 | 170–180 | Lead-free soldering, automotive, industrial | 1.2–1.5× |
| Rogers 4350B | >280 | RF/microwave, antenna, high-frequency digital | 3–5× |
| Aluminum (IMS) | — | LED lighting, power converters, thermal management | 1.5–2× |
| Polyimide | >250 | Flex and rigid-flex, aerospace, high-temp | 4–8× |
Rule of thumb: If your board uses lead-free solder (reflow peak ~260 °C), specify High-Tg FR-4 as a minimum. Standard FR-4 can delaminate under repeated lead-free reflow cycles.
3. How Much Copper Do I Need?
Copper weight is specified in ounces per square foot (oz/ft²). Standard is 1 oz (35 µm).
1 oz (35 µm): Standard for signal layers in most designs
2 oz (70 µm): Power supply boards, motor controllers, moderate current (<10 A)
3–4 oz (105–140 µm): High-current power distribution, automotive power modules
6–12 oz (210–420 µm): Heavy copper for bus bars, welding equipment, high-power inverters
Heavy copper (>3 oz) significantly increases cost and may require wider trace/space minimums — specify your current requirements to the fabricator before finalizing the design.
4. Surface Finish: HASL, ENIG, or OSP?
| Finish | Shelf Life | Flatness | Best For |
|---|---|---|---|
| HASL (Lead-free) | 12 months | Uneven — poor for fine-pitch | Through-hole, low-density SMT |
| ENIG | 12+ months | Excellent — flat pads | Fine-pitch BGA, QFN, gold wire bonding |
| OSP | 6 months | Very flat | High-volume consumer, short shelf-life acceptable |
| Immersion Silver | 6–12 months | Flat | RF/microwave, high-frequency |
| Immersion Tin | 6 months | Flat | Press-fit connectors, flat requirements |
For fine-pitch BGA (≤0.5 mm pitch), ENIG is the standard recommendation. HASL's uneven surface can cause coplanarity issues with small BGA balls.
5. Controlled Impedance: When and How
Controlled impedance is required when trace lengths exceed a critical fraction of the signal wavelength — typically when edge rates are below 1 ns or clock frequencies exceed 50 MHz. Common impedance targets: 50 Ω single-ended, 100 Ω differential.
You must provide the fabricator with your target impedance, the layers requiring control, and your preferred stackup. The fabricator will adjust trace widths and dielectric spacing to meet the target — do not assume your CAD tool's impedance calculation matches the fabricator's actual process.
6. DFM Checklist Before You Release
☐ Minimum trace/space meets fabricator capability (typically 4/4 mil for standard, 3/3 mil for advanced)
☐ Minimum drill size specified (0.2 mm mechanical, 0.1 mm laser)
☐ Annular ring ≥ 0.125 mm for mechanical drills
☐ Solder mask clearance and expansion defined
☐ Silkscreen text height ≥ 0.8 mm, line width ≥ 0.12 mm
☐ Controlled impedance layers marked in fabrication drawing
☐ Board outline on a mechanical layer, with dimensions
☐ Layer stackup diagram included in fabrication package
☐ IPC class (2 or 3) specified
For PCB sourcing with ADD Components, send your Gerber files and fabrication notes to sales@add-components.com. We'll review your stackup, recommend cost optimizations, and return a quotation within 24 hours.