What SMD means
Surface mount device. Each LED package is soldered onto the PCB as a separate component, then the module gets a mask. The lamp is a discrete part with a lens on top. This is the standard for most LED displays today.
What COB means
Chip on board. The bare LED dies are bonded directly to the PCB and covered with a layer of encapsulant. There is no separate lamp package. The surface is flatter and more uniform, and there is no lens to catch light at an angle.
What you see on the wall
| Property | SMD | COB |
|---|---|---|
| Surface | Slightly textured, visible lamps | Flat, continuous |
| Contrast | Good | Better, less stray light |
| Close viewing | Lamp structure visible | Cleaner at very close range |
| Repair | Replace single lamp | Often replace whole module |
| Price (P1.5) | Index 100 | Index 125 – 145 |
Where COB genuinely wins
Fine pitch below P1.2, where SMD lamps get difficult to place and the failure rate rises. Also in rooms where people get very close: control rooms, studios, museums. And in humid environments, because the encapsulant protects the die better than a lamp lens protects an SMD joint.
Where SMD is still the right call
P2.5 and above, outdoor, and any project where repairability matters. A single failed lamp on SMD is a five-minute fix. On COB it is a module swap, and on a large wall that changes the service model and the spare parts plan.
The question that decides it
How close do people get, and how often will someone have to repair it? If the answer is 'very close' and 'rarely', COB. If it is 'normal distance' and 'we want easy repair', SMD. Both are correct answers; the mistake is buying COB for the wrong reason.
A note on the word itself
Some suppliers use COB to mean any module with a protective coating. Ask to see the module edge. Real COB has no lamp packages on the surface, just a continuous encapsulated layer. If you can see individual lamps with lenses, it is SMD with a coating, which is a different product.