1. Introduction. LEDs for the LED strip
2. Samples of COB strips
3. Removing the phosphor from the COB strip
4. Blue light
5. Damage to the individual LED chips and the effect on the operation of the strip
6. Comparing SMD and COB chips. What’s inside the SMD chip?
7. Chip density on LED strips
8. What i will do in the future
An LED (light-emitting diode) is a semiconductor that emits light by transmitting an electric current. LEDs are made from a silicon crystal (i.e., a chip). LEDs have both advantages and drawbacks. The most important ones are fragility (silicon is a very brittle material) and the inability to sustain temperatures over 200 to 250 °F.
The LED chips are protected from mechanical damage by a housing usually made from plastic. This housing also dissipates the heat from the chip to the surrounding objects (e.g., an additional heat sink).
The LED chip is placed inside such housing. There are miniature copper conductors inside the housing for the electrical connection of the chip. On the outside of the housing, there are copper conductors that a used to connect the housing to the surface (circuit board, tape, wires, etc.).
Enclosures can be different. SMD (Surface Mounted Device) housing technology is traditionally used for the mass production of LED strips.
Different shapes of housings
The SMD housings are soldered to a flat long base with copper conductors to make a strip.
Notice the yellow-orange spots. This is a phosphor, a substance that absorbs and emits (re-emits) light creating a glow of the desired color or combination of colors.
What is it used for?
White light is created by blending several colors from dark red to violet. A simple case is RGB. These are three colors (red, green, and blue). But this kind of white light isn’t a good solution for lamps, as the lighted objects look faded and gray.
To get high-quality white light, you need to mix many individual shades of color. The more color shades are mixed, the greater the quality of the resulting white light.
The LED chips can only shine in one color. Red light is emitted by one LED. To emit dark red, a second LED is needed. Light-green needs the third LED, dark green — one more LED, and more, and more of them… You need many individual LEDs to create a quality white color.
At the same time, the efficiency of conversion of an electric current into light in LEDs is low for the red color and high for blue one. This means that the red LED will consume significantly more energy than the blue LED with the same brightness.
It would require a massive structure with many LEDs and additional electrical circuitry. It would be complicated and expensive.
But scientists have come up with a phosphor. This is a mixture of different chemicals that absorbs light of only one color and re-emits light in many different colors. This allows only one LED chip (with one color) to be used to create white light from dozens of shades of different colors. What color to choose for a single LED? Since the efficiency of a blue LED is much higher than a red LED, it is possible to use a blue LED as the base-emitter, getting a lot of light from a small amount of electricity.
In SMD technology, the phosphor is applied on top of the housing above the blue LED chip.
COB (chip-on-board) technology is the most modern. It means that the LED chip does not have a separate plastic housing. The chip is soldered to the built-in copper conductors on the strip base directly. The phosphor is applied to the strip from above in a continuous layer. Much more phosphor is required in production, but considering the advantages of the technology, it is not important.
The base of the COB phosphor is silicone. It is a soft material to keep the tape flexible. Also, silicone is almost not affected by high temperatures — it remains neutral up to 400–480 °F.
In high contrast, you can see where the LED crystals are under the phosphor:
I purchased 4 popular LED strips from Amazon as samples. Three kinds of COB strips and 1 kind of regular LED strip (SMD) with high chip density (149 chips/ft) were used for comparison.
COB strip phosphor is a mixture of phosphor substances and a silicone base. Silicone is resistant to temperatures up to 250 degrees Celsius, hydrophobic, dielectric, chemically inert, and elastic. It almost does not react to conventional solvents (ethanol, acetone, gasoline), so it can only be removed mechanically or with some specific aggressive destructor.
I used a scalpel. I cut off the silicon phosphor very carefully without damaging the LED chips. Silicon chips without a casing are very delicate and are easily damaged by the tip of the scalpel.
I connected the strip samples to the power supply. I saw a natural blue glow of the LEDs in places where there was no phosphor.
I knew that damaging the chips and resistors (mechanically or due to high temperature) turns off only the segment of the LED strip (between the cut lines). But how would the segment be affected if the individual chips were destroyed?
I purposely damaged (removed) some of the LED chips to find out how their removal affects operation. I needed to damage several chips to make the segment stop working (the location of the chips on the segment is also important).
Why? Because SMD and COB LED strips do have their own circuitry. The type and complexity of this circuit are determined by many factors. For example, the supply voltage, type of LED chips, chip selection, temperature mode, the required cost of goods, etc.
Our strips were built according to figure 2.
To get a single SMD chip i heat the strip with a heat gun. The solder melts and i can easily separate the LED chip.
I could have torn the LED crystal from the COB strip, but i didn’t do that. I just put the SMD LEDs side by side :-)
I measured the dimensions of the chips. I also broke the SMD chip housing and saw the LED chip the same size as the COB !!!
LED strips of different types, supply voltages, and light temperatures are made from a very small range of chips, as this is the most high-tech and expensive part. All the differences in the strips are due to the phosphor, the housings, and the electronic circuit power chips.
146–150 chips / ft is the density of SMD LEDs type 2216 on the LED strip (in one row), close to the limit density.
For the same chips (silicon crystals) in COB technology — the density easily exceeds this limit and has some margin. I know of COB strips with such chips with a density of 180–200 chips /ft.
I want to add to the information i have. This is what i interested in:
Heating.
How hot are LED strips of different types and wattages?
How hot are LED strips under different conditions (chipboard, aluminum profile, air)?
Is there any correlation?
Is it possible to glue the LED strip to the chipboard without the aluminum profile?
I will measure the total and spot heating with a professional thermal imaging camera. I will find out how the heat will be distributed in the material.
Power.
Does the electrical wattage declared by the manufacturers match the actual wattage?
I will measure the voltage and amperage.
Luminous flux.
Does the manufacturers’ stated luminous flux match the actual luminous flux?
I will measure the comparative luminous flux with a Luxmeter.
Pulsations of light.
Why does light pulsation from LEDs appear? Are light pulses harmful to our eyes?
Why is the cost of power supplies for LED strips different (up to ten times)?
I will measure high-frequency changes in the output electrical current with a professional oscilloscope. I will take apart the power supplies and specify their circuitry. I will describe their advantages and disadvantages.
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