Modern LED technology
COB, IMD and fine pixel pitch
Screens you can stand right in front of without seeing dots require an extremely fine pixel pitch. Two technologies make that possible: COB, in which bare chips are encapsulated, and IMD, in which several pixels sit in one package.
To make a screen that is super sharp, even when you stand right in front of it, the points of light have to be extremely small and close together.
There are two clever ways to do this. With COB the bare chips are placed directly on the board and protected with a transparent layer. With IMD a few dots are put together in one very small block. Both let the dots come much closer together than before.
Both technologies aim at fine pixel pitch (about P1.0 and smaller), where ordinary SMD runs into limits.
COB (Chip-on-Board)
With COB, the bare LED chips are bonded directly onto the circuit board and covered together with a protective resin layer, without separate packages. That enables very fine pitch (P0.9 and smaller, up to 4K and 8K resolution), with high robustness (resistant to impact and touch), good contrast and better heat dissipation.
IMD (N-in-1)
IMD (Integrated Mounted Device) brings several pixels together in one small package, for example ‘four-in-one’ or ‘six-in-one’. It likewise reaches very fine pitches (down to about P0.4, with P0.7 in stable mass production) and has the advantage that the packages can be individually tested and sorted, which benefits colour consistency. Below P1.2, IMD often offers a more favourable price than COB.
COB eliminates the individual package: bare dies are bonded onto the PCB and collectively encapsulated. Advantages: high impact/moisture resistance, better thermal spreading, a wide viewing image and strong contrast. Drawback: mixed-chip encapsulation does not allow individual binning/sorting, and the optical spread can produce larger colour differences at wide viewing angles; repair is moreover more difficult.
IMD, MIP and the choice
IMD bridges SMD and COB: several SMD-like pixels in one matrix package, with full testability and good colour consistency, and a favourable cost for sub-P1.0. Finer still, one goes with MIP (micro/mini-LED in package) and micro-LED. Related variants are GOB (a protective coating over an SMD screen). The choice between SMD, IMD and COB follows from the desired minimum viewing distance, the budget and the serviceability.