The COB literally means that the chip is directly attached to the substrate. It can be seen that the most important type of COB is the type of substrate, and different types of substrates have different types of COB. 
Substrate material
Aluminum substrate: composed of an aluminum substrate, an insulating layer and a copper foil layer. Line fabrication and surface treatment are performed on the copper foil layer to form a package area.
Copper substrate: similar to an aluminum substrate except that the substrate is copper. Copper has a higher heat transfer rate and can produce COB light sources with higher power than aluminum substrates.
Ceramic substrate: The substrate is made of a ceramic substrate, and the ceramic material may be alumina, aluminum nitride or glass (in a broad sense, glass is a kind of ceramic). There are generally two types of circuit fabrication processes for ceramic substrates: thick film processes and thin film processes.
The insulating properties of the ceramic substrate are easier to pass the withstand voltage test than the metal substrate. The alumina content in the alumina ceramic substrate can reach 96% or more, and the LED chip substrate material sapphire has a thermal expansion coefficient close to that. Lower reliability is better than metal substrates.
Aluminum nitride has a higher thermal conductivity and can produce high optical density and high power products. However, the color of aluminum nitride is gray, which is not conducive to the light output of COB.
The transparent nature of the glass substrate is often used to make filament-like products that form 360° full-angle illumination. However, glass has a low thermal conductivity and is not suitable for making high-power products.
Substrate process
Press-bonded substrate: A plate process that uses two different materials, the middle is assisted by film, and combined by a pressing process.
Currently the most common use is: the substrate is highly reflective aluminum, and the upper material is BT material. The positive-loading chip is fixed on the highly reflective aluminum to improve the side light of the chip. The aluminum substrate can accelerate the heat transfer, and the chip-to-chip soldering method is finally soldered to the upper BT material line to realize the thermoelectric separation structure.
A copper or ceramic press-bonded substrate is also used for the bottom substrate.
Injection molded substrate:
The metal substrate and the conductive frame form a stable connection by injection molding. A thermoelectric separation and high power COB substrate can be fabricated. The metal substrate used is typically copper.
Machined substrate:
On a common aluminum substrate, a packaged area is formed by milling. This method can also achieve good structural characteristics of thermal conduction and hot spot separation. The package area formed by the machining method has a low reflectance, and the ability to collect light on the chip side is limited. In the machining process, it is necessary to pay attention to factors such as overheating of the metal material and influence of the metal characteristics on the milling cutter. In the process of processing, the tool speed is accelerated and the cooling measures are assisted to obtain a better package area.
What substrate is best?
A substrate with good thermal and reflective properties and suitable for use in the environment of use.
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