We’re all familiar with LED flexible strips – a thin copper FPC with SMD components soldered onto the surface: LED chips and resistors and in some cases ICs. Of course this makes sense since an LED is a single thing, a single component, a single chip. Right?
Well yes and no. Like my Facebook status, it’s complicated.
You see, an LED is not actually that rectangular white thing with a little yellow something in the middle. That is the LED package – the LED is actually inside that package. The package contains an LED epiwafer which is then soldered to the legs of the LED package using very thin wire typically gold wire, with a phosphor coating sealing everything in place. This is your typical SMD LED chip which was available in different “packages” e.g. 2835, 5050, 4040 – there are a number of packages available. Packages chips are very convenient – they make it easy for a pick and place machine to place the chips on the FPC in the same manner they would place a resistor or IC on the FPC – and if manufactured well, every chip will deliver a consistent light output meaning your flexible strip has the same colour / temperature and brightness throughout.
As wonderful as these SMD LED chips are they have a few downsides :
* They need to manufactured by a chip packager
* They illuminate as visible dots
* They get hot and need heat management
What we’re now starting to see are what we’re calling COB strips. COBS – Chip-On-Board – have been around for a few years now – usually on a rigid board and used for things like flashlights where a very high brightness is required in a concentrated area. Chip on board is where multiple LED epiwafers are mounted directly onto a board and then the phosphor coating is applied over all the epiwafers – a single splodge of phosphor if you like – which then gives you the effect of a single large “LED chip” ; of course behind the scenes there are multiple epiwafers. In the case of some of the COB boards you can have dozens of epiwafers behind a single COB.
The COB board has now evolved into a COB strip – where the LED epiwafers are soldered directly onto an FPC in a straight line and then a long solid line of phosphor is poured over the epiwafers. This has a lot of benefits:
* It removes the need for chip packaging
* It delivers a more constant light ie fewer if any dots
* It is usually more efficient
* Better heat management
Because you’re working with small epiwafers instead of larger chip packages you can get a much higher chip density and ensure that no dots are ever visible – even without a diffuser. We currently offer a range of COB strips ranging from 384 LEDs per meter (using the more correct term of LED here) to 512 LEDs per meter. This level of chip density was never possible previously with SMD LED chip packages. Packaged inside a diffused IP 67 sleeve you can achieve what was only achievable with neon flex (more expensive than flexible strips) or with an additional diffused aluminium profile.
COB strips are currently more expensive than the more traditional SMD LED chip package strips – but as they become more prevalent the cost will inevitably drop.
My prediction? That in time we will generally only see COB flexible strips being sold.