Aluminum-trace PCB design rules
Trace, space and via rules for aluminum-trace boards, and why they differ from copper.
Quantum PCB
Draw the board you want to receive. Your layout should show as-fabricated trace and space values. Enigma's CAM department makes the layout adjustments needed for aluminum; the details below are for information.
Preliminary notes. Aluminum-trace technology is still developing, so values on this page may change. Confirm the details for your design with our engineering team.
As fabricated
Trace and space
Routing, drilling and solder mask rules are the same as for copper PCBs. Trace and space are not as tight: going below the minimum results in shorts and opens.
| Etched, DC | Space | Trace |
|---|---|---|
| Minimum | 6 mil | 4 mil |
| Relaxed | 7 mil | 5 mil |
| High yield, large area | 8 mil | 7 mil |
| Laser-structured, DC | Space | Trace |
|---|---|---|
| Minimum | 2 mil | 4 mil |
| Relaxed | 3 mil | 5 mil |
- Too costly for large areas; use only where needed. Layout and as-fabricated sizes are the same.
| Etched, controlled impedance | Space | Trace |
|---|---|---|
| Minimum | 6 mil | 4 mil |
| Relaxed | 9 mil | 7 mil |
| ±2% impedance control | 12 mil | 10 mil |
Smaller spaces are possible in rare cases, such as thin (0.5 mil) aluminum or a short coplanar-waveguide taper at the board edge for wire bonding to an on-chip line. We review these case by case.
Why 5 mil?
Etched aluminum edges aren't vertical
Aluminum etches isotropically. A 25 µm layer etches back about 30–35 µm from the edge defined by the dry film, so a 2–2.5 mil space on the film becomes 5–6 mil after etching. Our CAM team makes spaces smaller and traces wider in the layout to compensate. Narrow spaces etch more slowly, so the edge offset (EO) shrinks a little as the space gets smaller.
Wet-etched aluminum. Aluminum etches isotropically, so the sidewalls slope and the edge moves back under the etch mask.
Laser-structured aluminum. Straight walls and narrow spaces, but the laser always cuts somewhat into the substrate.
Trace as fabricated = layout − 2 × EO; space as fabricated = layout + 2 × EO.
| Example 1 | Example 2 | Example 3 | Example 4 | Example 5 | |
|---|---|---|---|---|---|
| Layout trace / space (mil) | 20 / 16 | 16.8 / 5.9 | 15.2 / 4.3 | 13.2 / 2.7 | 10.9 / 1.3 |
| As-fabricated trace / space (mil) | 15.6 / 20.4 | 12.4 / 10.3 | 11 / 8.5 | 9.2 / 6.7 | 7.2 / 5 |
| Etch offset (mil) | 2.2 | 2.2 | 2.1 | 2.0 | 1.85 |
Gerber layers
Naming aluminum and copper layers
There are two physical copper layers: base (seed) copper under the aluminum, and surface or "patch" copper on top of it. One copper Gerber defines both, depending on context.
- Use AL1, AL2 for aluminum on layers 1 and 2, and CU1, CU2 for copper.
- CU1 and AL1 together specify surface copper. Pull CU1 back 1.5–2 mil from the AL1 edge.
- CU1 without AL1 specifies bare base copper. Keep it at least 4 mil from the nearest aluminum; a copper Gerber can't cross an aluminum edge.
- Bare base copper helps thermalize the board, since bare substrate conducts heat poorly. Connect it to cold copper on the fridge; putting it under mounting screws so the screw head and bracket clamp onto CU1 and CU2 is good practice.
- Inner layers follow the same rules. Where a via passes through, the drill landing pad must be base copper only, with aluminum pulled back 2–3 mil from the hole, so no aluminum is exposed to the plating chemistry.
- Pull aluminum back 2–3 mil from plated edges and from the board outline.
- Solder mask can go anywhere.
Multilayer vias
Via pads in aluminum
A drilled hole must expose copper for the connection, but never aluminum.
Ground-plane vias
For a via from layer 1 to layer 4, open a hole in the AL layer larger than the drill, and keep base copper under it with the CU layer. For example, for a 12 mil drill:
- 18 mil hole in the AL layer (3 mil pull-back)
- Circular CU patch of at least 26 mil, overlapping the aluminum by at least 4 mil (5 mil is better)
- With a full ground plane, the CU layer already covers the hole in the AL
Signal vias
Surface signal pads are etched after aluminum plating, so they can be smaller than inner-layer pads. For a 12 mil drill:
- 18 mil aluminum signal pad
- Optional copper surface on the pad, slightly smaller (16 mil), pulled back 2 mil from the aluminum edge
- Via fill is not possible
Laser structuring
Spaces under 5 mil
Laser structuring is the only way to get aluminum spaces below 5 mil. It costs roughly 10–30 times more per area than etching, and every board on the panel is lasered, including ones that end up scrapped.
- Use one standard laser width (2 mil, 51 µm) and the same space everywhere to minimize setup.
- Best practice is a hybrid: laser only the tightest-pitch areas, typically near a chip, and wet-etch the rest.
- Where an etched space meets a laser-cut space, the laser cuts deep into bare substrate (5–10 mil), and can go through thin dielectric. Put metal on the opposite side of the board under these transitions.
- Stagger the transitions so they don't line up in a single row and weaken the board, and fill them with solder mask to recover stiffness.
| Software | Z, Zeven, Zodd | Wavelength | Loss | Structures |
|---|---|---|---|---|
| Rogers Corporation MWI-2019 | Yes | Yes | Dielectric + conductor | CPWG, MS, ECMS, SL, ECSL, BCSL |
| QucsStudio transmission line calculator | Yes | Yes | Dielectric + conductor | MS, SL, CPW, CPWG, SLTL, coax, twisted pair, rectangular waveguide, ECMS, ECSL, differential CPW/CPWG, arbitrary |
| Agilent TXLINE 2003 | Yes | Yes | Yes | MS, ECMS, SL, CPW, CPWG, coax, ECSL, SLTL |
- MWI-2019 includes detailed conductor properties such as surface roughness, and a large substrate library.
- QucsStudio handles arbitrary cross-sections once you script them.
- TXLINE is the simplest, but can back-calculate trace dimensions from a target impedance.
- For checking substrate Dk at room and cryogenic temperature, use the Rogers Corporation Ring Resonator software (see technologies).
MS microstrip · ECMS edge-coupled microstrip · CPW coplanar waveguide · CPWG coplanar waveguide with ground · SL stripline · ECSL edge-coupled stripline · BCSL broadside-coupled stripline · SLTL slotline
Ready to lay out an aluminum board?
Send us your files and we'll review them against these rules.