COB

Chip on board describes an integrated circuit or LED die mounted directly onto a printed circuit board with no intermediate package. The silicon itself, typically 2 to 10 millimetres square, is bonded to the substrate using epoxy or solder, then connected via thin gold or aluminium wires to contact pads on the board. This bare-die assembly is then encapsulated in a protective resin coating. The result is a much smaller footprint and lower cost than a traditional IC in a plastic DIP or QFP package.

COB is common in two distinct applications. In LED lighting, COB arrays pack dozens or hundreds of individual diodes onto a single substrate, creating high brightness uniform light sources for spotlights, downlighters, and panel fixtures. The resin lens cast over the dies also acts as a diffuser. In consumer electronics, COB is used for microcontrollers, memory, and analogue circuits in space-constrained products: kitchen scales, LED drivers, audio amplifiers, and battery chargers often hide a COB assembly under their plastic shell.

Assembly and reliability

The assembly process uses wire bonding, where a fine wire is ultrasonically fused from a pad on the die to a board trace, usually in an array of 40 to 200 bonds per chip. The encapsulation resin must have a coefficient of thermal expansion close to the silicon to avoid stress cracking during temperature cycling. Moisture ingress under the potting layer can cause corrosion of the bond wires and board metallization; quality COB is assembled in controlled humidity and sealed with conformal coating if needed for harsh environments.

Failure modes are specific. Bond wire lift, caused by thermal or mechanical shock, opens a connection with no visible sign. Delamination between the die and board, or between the encapsulant and the substrate, traps moisture and leads to slow degradation. Solder ball voids at the die attach point can cause intermittent electrical faults. Rework or repair is difficult because the assembly is buried under resin.

The name COB emerged in the 1980s as a shorthand for this cost-saving alternative to dual in-line packages. It sits between fully integrated circuits (which include internal bonding and packaging) and discrete components on one end, and multi-chip modules (which house several packaged ICs in a single enclosure) on the other. For cost-sensitive high-volume manufacturing, COB trades testability and replaceability for density and simplicity.