Case study · 400G · Data center optics
400G Optical Module PCB Case Study: 12-Layer 4-Step HDI with Segmented Gold Fingers
A 12-layer, 4-step stacked HDI board on TU-933+ for a 400G data center optical transceiver, with 4-level, 8-segment gold fingers, ±6% impedance tolerance and ±0.1 mm board thickness tolerance.
- Data rate
- 400G
- Stacked HDI
- 4-step
- Impedance tolerance
- ±6%
- Max. finger gold
- 30 µin
Overview
What this 400G optical module PCB is.
A 400G optical module PCB is the small high-speed board inside a pluggable transceiver that links the host connector to the optical engine. This AstroPCB build is a 12-layer, 4-step stacked HDI board on TU-933+ with segmented gold fingers, held to ±6% impedance and ±0.1 mm thickness tolerance.
| Parameter | This build |
|---|---|
| Application | Data center optical-electrical conversion module (400G) |
| Layer count | 12 layers |
| Blind via steps | 4-step stacked HDI |
| Laminate | TUC TU-933+ |
| Gold fingers | 4-level, 8-segment |
| Surface finish | ENIG + hard gold fingers |
| Maximum gold thickness | 30 µin |
| Board thickness tolerance | ±0.1 mm |
| Impedance tolerance | ±6% |
Key takeaways
- 400G modules combine high-speed HDI routing and a mechanical edge connector on one small board.
- Segmented gold fingers (4 levels, 8 segments) need precise etch-back and plating control at the board edge.
- ±6% impedance is tighter than the common ±10% and depends on stable dielectric thickness through four lamination cycles.
The challenge
What made this board difficult.
Optical transceiver boards are small, dense and mechanically demanding. The high-speed lanes run from the gold-finger edge connector to the DSP and optical engine through very short, impedance-critical paths. Any discontinuity at vias, pads or the finger transitions shows up as loss and reflection at 400G.
The board also has to plug into a cage repeatedly. That puts tight limits on board thickness at the fingers (±0.1 mm here), on finger geometry and on the hardness and thickness of the gold. Combining 4-step stacked microvias, a low-loss laminate, ±6% impedance and multi-level gold fingers in one panel is what made this project demanding.
Stack-up
Stack-up, vias and finish.
| Element | Detail |
|---|---|
| Total layers | 12 (L1–L12) |
| HDI structure | 4-step stacked blind microvias (4 build-up levels per side over the core, by layer arithmetic) |
| Laminate | TU-933+ low-loss system |
| Surface finish | ENIG on pads; hard gold on fingers |
| Gold finger layout | 4 levels, 8 segments |
| Max gold thickness | 30 µin on fingers |
| Thickness tolerance | ±0.1 mm |
| Impedance tolerance | ±6% |
With 12 layers and four blind-via steps, the layer budget leaves a 4-layer core with four build-up layers on each side. Stacking the microvias lets high-speed lanes leave fine-pitch pads and change layer immediately, keeping via stubs and transition length to a minimum. That matters more at 400G than almost any other factor on such a short board.
Gold fingers
Segmented gold fingers explained.
Standard edge connectors use one row of equal-length fingers. Segmented or multi-level fingers split contacts into different lengths and rows so that some pins make contact before others when the module is inserted (for example ground and power before signals), or so that two rows of contacts fit on the same edge. This board uses 4 levels and 8 segments.
Manufacturing segmented fingers means plating hard gold only where contact is made, removing the plating bus cleanly between segments, and holding the finger-to-edge bevel geometry. Gold thickness here goes up to 30 µin, which balances wear resistance for repeated insertion against cost.
| Feature | Why it matters |
|---|---|
| Hard gold up to 30 µin | Wear resistance over repeated insertions |
| ENIG on component pads | Flat, solderable surface for fine-pitch parts |
| Segment isolation | No residual plating tails between segments |
| ±0.1 mm thickness | Correct fit and contact force in the cage |
How we built it
Manufacturing controls on this build.
4-step stacked microvias
Four sequential lamination and laser cycles with copper fill, aligned level by level for stacked vias.
Microvia±6% impedance
Dielectric thickness and trace-width compensation tuned to the TU-933+ construction, verified on TDR coupons.
Controlled impedanceSelective finishes
ENIG on pads and selective hard gold on fingers, with bevel and segment isolation at the edge.
Surface finishVerification
How builds like this are verified.
- Gold thickness measurement on fingers (XRF)
- Board thickness measured at the finger area against ±0.1 mm
- Bare-board electrical test (flying probe or fixture) against the netlist
- Impedance coupons measured by TDR where impedance is specified
- Microsection of coupons to confirm microvia fill, plating and layer registration
- Visual and dimensional inspection to IPC-A-600 for the specified class
Design takeaways
Design notes for optical module PCBs.
- Put impedance targets and the tolerance (e.g. ±6%) on the fab drawing, per layer and reference plane.
- Define finger segments, lengths, bevel angle and gold thickness on a dedicated edge-connector drawing.
- Specify board thickness at the fingers separately from overall thickness if the cage requires it.
- Keep high-speed via transitions short; stacked microvias help on small modules.
- Confirm the laminate (TU-933+ or similar low-loss) before routing so impedance widths are right.
- Ask for a stack-up proposal with modelled impedance before layout.
Quote a similar build
What to send for an optical module PCB quote.
- Fabrication data and stack-up intent
- Impedance targets and tolerance
- Edge-connector drawing: segments, lengths, bevel and gold thickness
- Finish split (ENIG + hard gold)
- Quantities and schedule
Upload files on the quote page. See also AI & data center PCB and high speed PCB.
FAQ
Questions about this build.
What PCB is used in a 400G optical module?
Typically a compact high-speed HDI board on a low-loss laminate with a gold-finger edge connector. This one is 12 layers, 4-step stacked HDI on TU-933+.
What are segmented gold fingers?
Edge-connector fingers split into different lengths and rows so some contacts mate first or more contacts fit on one edge. This board uses 4 levels and 8 segments.
How thick is the gold on optical module fingers?
On this build the maximum gold thickness is 30 µin of hard gold, balancing wear resistance and cost.
What impedance tolerance can you hold?
This build was held to ±6%. Achievable tolerance depends on the stack-up and geometry and is confirmed during DFM.
Why is board thickness tolerance important for transceivers?
The fingers must fit the cage connector with correct contact force; this board is held to ±0.1 mm.
Which finish combination is used?
ENIG on component pads and selective hard gold on the gold fingers.
Do you build 800G modules?
Send the design for review; feasibility depends on the stack-up, laminate and via structure rather than the label.
Ready when your files are
Building a similar board? Send it for a stack-up review.
Upload Gerber, ODB++ or IPC-2581 data. Every quote includes a written engineering DFM review, typically within one business day.