Chip-on-board (COB) packaging

Chip-on-board is a way of building the optical front end without individually packaged parts: the bare laser and photodiode dies are glued and wire-bonded directly onto the module's printed circuit board (or a small carrier), then covered by a lens array that couples them straight to the fibre ferrule. It is the reason parallel short-reach optics and active optical cables are as cheap and as dense as they are — and the reason they cannot be repaired.

Classic packaging vs COB

Where COB is used

• Parallel multi-mode optics — 40G SR4, 100G SR4, 200G/400G SR8, 800G SR8: a 4- or 8-element VCSEL array and a matching PIN array, each behind a moulded lens array, coupled to an MPO-12 or MPO-16 ferrule.
• Active optical cables (AOC) — a COB engine at each end with the fibre permanently attached; no receptacle at all, which removes coupling loss and cost.
• On-board optics / COBO — optical engines mounted on the host board near the ASIC rather than in a pluggable cage.
• Co-packaged optics (CPO) — the same idea taken to its limit: optical dies packaged on the switch ASIC substrate.
• Some single-mode designs use COB too (edge-emitting DFB/EML dies on a carrier with a planar lightwave circuit), but hermeticity concerns keep most single-mode optics in cans or in silicon-photonics packages.

VCSELs suit COB especially well: they emit perpendicular to the wafer, so an array can be tested before dicing, placed face-up on the board and coupled with a simple lens — no facet cleaving, no edge coupling (Laser types).

How a COB engine is built

1. Substrate — the module PCB itself, or a carrier (ceramic, silicon, glass) that is later soldered to the PCB; high-speed traces run to the driver and TIA ICs placed millimetres from the optical dies.
2. Die attach — VCSEL array, PIN array, driver and TIA dies fixed with conductive epoxy or AuSn solder; alignment fiducials on the substrate.
3. Wire bonding — gold wires from die pads to substrate pads; bond length is kept minimal because it is the main parasitic at 25–100G.
4. Lens array — a moulded glass or polymer part with one lens per lane and alignment pins, placed by machine vision to the fiducials (passive alignment) or actively while monitoring coupled power.
5. Encapsulation — a transparent gel or glob-top over dies and bonds, or a lid; the optical path stays open through the lens.
6. Ferrule — an MT ferrule with the fibre ribbon plugs onto the lens array's guide pins, presenting an MPO interface at the module faceplate (or the fixed fibre of an AOC).
7. Test — only after assembly: eye diagram and power per lane. A failed lane means a scrapped or down-binned engine.

Advantages

• Cost at volume — no hermetic cans, one alignment step for eight lanes.
• Density — eight lanes in the footprint of two cans; enables SR8 and AOC.
• High-speed performance — short bonds, driver and TIA next to the dies, low parasitics.
• Thermal path — dies sit on the board copper rather than inside a can.
• Automation — pick-and-place and vision alignment scale to millions of units.

Drawbacks

• Non-hermetic — moisture and contaminants reach the dies; VCSELs and PINs tolerate this well, edge-emitting lasers less so. Encapsulant quality decides long-term reliability.
• No repair — a single degraded VCSEL in an 8-lane array retires the whole module or cable; with an AOC that means re-pulling the cable (DAC vs AOC vs transceivers).
• Yield risk — alignment and bonding are done before the engine can be fully tested.
• Multi-mode only in practice — reach is bounded by the VCSEL/MMF combination.

COB and diagnostics

Because the eight lasers share one board and one thermal environment, they age together — but not identically. The classic COB failure is one lane of an SR4/SR8 array losing power while the others are fine, hidden by LACP or by a module-level "OK". Per-lane Tx bias and Rx power are the only early warning (Per-lane diagnostics, Failures). COB modules carry the standard SFF-8636 or CMIS memory on the same board; their identity is coded like any other module.

In CodingBox

CodingBox reads COB-based modules and AOC ends like any QSFP/QSFP-DD: per-lane DDM on the DDM screen shows the lane that is dying, and the EEPROM editor codes the identity a host expects — the one thing about a COB module that can be changed after it leaves the factory.

COB against TO-cans and box packages, with alignment tolerances and reliability: Optical packaging; COB engines as the ends of AOCs: Cable internals; the factory steps COB removes: Manufacturing & testing.