What is COB led
COB is a chip on board, a bare chip that is attached to an interconnecting substrate with conductive or conductive tape and then bonded electrically with leads. There are three types of packaging: strip, square and round.
The COB led strip is first covered with thermally conductive epoxy resin on the substrate surface, then the silicon is placed directly on the substrate surface and heat treated until the silicon is firmly fixed to the substrate, followed by a direct electrical connection between the silicon and the substrate by wire bonding.
There are two main forms of bare chip technology: COB technology and Flip Chip technology. In chip-on-board (COB), the semiconductor chip is mounted on a printed circuit board, and the electrical connection between the chip and the substrate is made by lead stitching, while the electrical connection between the chip and the substrate is made by lead stitching and covered with resin to ensure reliability. Although COB is the simplest bare chip mounting technique, it is not as densely packed as TAB and flip chip soldering techniques.
COB LED soldering methods
- Thermocompression soldering. Using heat and pressure to press the wire and solder area together. The principle is that by heating and applying pressure, the weld zone undergoes plastic deformation while destroying the oxide layer at the interface of the press weld, thus creating an attractive force between the atoms to achieve the purpose of "bonding". This technique is generally used for chip COG on glass plates.
- Ultrasonic welding. Ultrasonic welding is the use of ultrasonic energy generated by the generator, through the transducer in the ultra-high-frequency magnetic field induction, rapid expansion and elastic vibration, so that the knife corresponding vibration, and at the same time in the knife apply a certain pressure, so the knife in the joint action of these two forces, drive the AI wire in the welded area of the metalised layer such as (AI film) surface friction quickly, so that the AI wire and AI film surface plastic deformation, this deformation also destroyed the AI layer interface. This deformation also destroys the oxide layer at the interface of the AI layer, bringing the two pure metal surfaces into close contact to achieve interatomic bonding, thus forming a weld. The main welding material is the aluminium wire tip, which is generally wedge-shaped.
- Gold wire welding. Ball soldering is the most representative soldering technique in lead bonding, as AU wire ball soldering is used in semiconductor packages for diodes and triodes. And it is easy to operate, has flexible, strong joints (diameter of 25UM AU wire welding strength is generally 0.07 ~ 0.09N / point), and non-directional, welding speed that can be up to 15 points/second or more. Gold wire welding is also called thermal (pressure) (ultrasonic) welding the main bonding material is gold (AU) wire welding head is spherical so for ball welding.
Cob-led packaging process
- Crystal expansion. The entire LED wafer film provided by the manufacturer is evenly expanded using an expander, so that the LED grains attached to the surface of the film in close alignment are pulled apart for easy cobbing.
- Adhesive backing. The expanded ring is placed on the surface of the backing machine, which has been scraped with a silver paste layer, and backed with silver paste. Dotting silver paste. Suitable for bulk LED chips. Use the dispensing machine to dispense the appropriate amount of silver paste on the PCB-printed circuit board.
- Place the silver paste-prepared crystal expansion ring into the crystal stabbing holder and have the operator stab the LED wafer onto the PCB with a crystal stabbing pen under a microscope.
- The PCB printed circuit board with good crystal stabbing is placed in a thermal cycle oven at a constant temperature for a certain period of time, to be removed after the silver paste has cured (not for a long time, otherwise the LED chip plating will be baked yellow, i.e. oxidation, causing difficulties for bonding). If there is LED chip bonding, the above steps are required; if only IC chip bonding is cancelled above steps.
- Sticky chip. Use a dispensing machine to apply the right amount of red glue (or black glue) to the IC position on the PCB printed circuit board, and then use anti-static equipment (vacuum pen or sub) to place the IC bare chip correctly on the red or black glue.
- Drying. Place the glued die in a hot-cycle oven on a large flat heating plate and leave it for a while, or it can be cured naturally (longer).
- Bonding. The wafer (LED die or IC chip) is bridged to the corresponding pad of aluminium wire on the PCB board using an aluminium wire soldering machine, i.e. the inner lead of the COB is soldered.
- Pre-testing. The COB board is tested using special testing tools (different equipment for different COB applications, simply a high-precision voltage regulator) and any unqualified boards are reworked.
- Dispensing. A dispensing machine is used to place the appropriate amount of AB glue onto the bonded LED die, while the IC is encapsulated with black glue and then encapsulated according to the customer's requirements.
- Curing. The sealed PCBs are placed in a thermal oven with a constant temperature, and the drying time can be set according to the requirements.
- Post-testing. The encapsulated PCBs are then tested for electrical performance with special testing tools to distinguish between good and bad.
What is thermally conductive double-sided tape
Double-sided thermal tape is made of transfer thermal conduct glue or glass fibre coated with thermal conduct glue.
Thermally conductive double-sided tape is made of acrylic polymer filled with thermally conductive ceramic powder and compounded with silicone adhesives. It is highly thermally conductive and insulating and is flexible, compressible, conformable and strongly adhesive. It is suitable for a wide temperature range, can fill uneven surfaces, and can fit closely and firmly to heat source devices and heat sinks to conduct heat out quickly.