Bottom line first: 1.6T optical modules push per-lane rates into the 200G class, and pluggable modules and co-packaged optics (CPO) will coexist for the near term — but the two place different demands on the PCB. Pluggables compete on ultra-low loss inside a small footprint plus very tight tolerances; CPO competes on substrate flatness and fine-line capability. What buyers should do is not wait for the standards to converge, but put material grade, tolerances and acceptance method into the specification in advance.
Three transmission chains behind the speed increase
- The loss budget is compressed: when the channel rate doubles, the insertion-loss target for a trace of the same length tightens; laminate Df has to reach the 0.002 level at 10 GHz, and copper foil roughness has to stay in the sub-micron range.
- Finer traces, tighter spacing: space inside the module is limited, so line width and spacing often land in the 50–75 μm band; moving on to mSAP takes line width below 30 μm.
- Tighter tolerances: the module has to mate with the connector and the optical engine, so board thickness, outline and gold-finger position tolerances are all squeezed, and general outline tolerances are frequently no longer sufficient.
Four hard requirements for pluggable modules
- Layer count is typically 6–12, and the stackup must reserve a symmetrical construction for the ultra-low-loss material so that warp after lamination does not affect insertion;
- the finish is usually a mix of hard gold on the fingers and ENIG or OSP on soldering areas, with hard gold thickness selected by mating cycle count;
- impedance tolerance is tighter than on a general board and can be held at ±8%, with measured data and sampling records for the impedance coupon supplied alongside;
- state the reflow count explicitly: modules carry many components, and at high reflow counts neither OSP nor immersion tin is suitable.
What CPO changes is the substrate, not just the traces
Co-packaging moves the optical engine next to the switch chip. The high-speed electrical channel gets shorter, but the substrate inherits new requirements: flatness and coplanarity directly affect optical coupling efficiency, fine-line density rises, and the thermal path has to be redesigned. The consequence is that fine-line (mSAP) capacity and glass or ceramic substrates take a larger share, while growth in conventional large-format high-speed multilayer boards may slow. On trend, CPO penetrates gradually over the medium and long term, but maintainability, standardisation and yield keep pluggables the dominant shipping form in the near term.
A procurement calibration checklist
- Do not write "low loss" in the specification; write the test frequency and value for Dk/Df, the copper foil type and roughness, and the glass cloth style;
- ask for a measured report on the stackup impedance coupon plus the sampling ratio in volume, not first-article data alone;
- for fine lines, define minimum line width and spacing, etch tolerance and AOI criteria, so that "the drawing can be built" does not turn into "volume cannot hold it";
- dual-source qualification: at least two materials or two stackup options qualified for the same function, to cut delivery risk from being locked to a single part number;
- align delivery dates against project milestones, confirm the cycle-time gap between qualification samples and production parts in advance, and schedule expedites on a 24-hour prototyping rhythm;
- confirm early whether the finish at wire-bond locations is compatible (ENEPIG or soft gold) rather than discovering it just before assembly.
Joint advice for design and procurement
The most common waste during a speed upgrade is not unit price but repeated redesign caused by designing to a high-grade material while purchasing to a general one. Lock material grade, tolerances and acceptance method once, before prototyping, and put the capability envelope directly into the RFQ — 1–44 layers, a maximum panel size of 610×1100 mm, aspect ratio up to 20:1 (25:1 for backplanes), line-width accuracy ±8 μm, impedance tolerance ±8%, finished copper up to 13oz and 24-hour expedited prototyping — and the front-end communication cycle shortens noticeably. Engineering and quotation: info@ipcba.com.cn, +86 138 2310 9230.
Send your Gerber files and stackup requirements; a 24-hour expedite channel is available.