The order of decisions
Design is cheap to change early and expensive to change late. The sequence below puts the expensive decisions first, which is the only reason the process has an order at all.
1. Requirements
What the product does, its environment, its life, its compliance regime and its volume. These decide the laminate, the layer count, the finish and the acceptance class — and they are usually written down last.
2. Schematic
The electrical intent. Two checks matter downstream: every net that carries power or a sensitive signal should be visible in the schematic as such, and every connector should have a defined pinout before layout starts.
3. Stack-up
Layer count, copper weight, dielectric thickness, impedance targets. This is where the fabricator should be involved, because the stack-up is also the build instruction: a dielectric thickness that is only a CAD number will not survive lamination.
4. Placement
Placement decides routability, thermal behaviour, EMC and test access. Three rules that pay for themselves: keep high-current loops small, keep the crystal and its load capacitors together, and leave test access on the nets you will need.
5. Routing
Routing is the part everybody watches, and the part most constrained by the four decisions above. Two habits prevent most DFM findings: never route closer to the board edge than the process allows, and never leave a copper island unconnected.
6. The three gates
Before the Gerber leaves, three reviews:
- DFM — can this be fabricated inside the process window? Annular ring, mask slivers, feature size, panel fit, copper balance.
- DFA — can this be assembled? Courtyards, polarity marks, fiducials, component clearance around connectors, panel support.
- DFT — can this be tested? Test points, probe access, fixture clearance, power rails and ground reference.
The first review is the fabricator's, the second and third are the assembly house's, and all three are cheaper than a re-spin.
7. Prototype and bring-up
A first build exists to find the mistakes that survive the reviews. Prototypes run in 1–3 days, which makes a two-pass approach practical: build the risky board early, learn, then commit to volume.
What to hand over
- Gerber set (copper, mask, legend, outline) and drill file with units stated.
- Stack-up drawing with copper weight per layer and dielectric build.
- Fabrication notes: finish, acceptance class, impedance targets, tolerance callouts.
- For assembly: BOM, assembly drawing with polarity and keepouts, test specification, and the panel preference if a fixture already exists.
The limits behind these decisions are on the capability page; the review gates are described in the quality control note, and the fabrication sequence in the process note.
