As a carrier of heat and air convection, the heat conductivity of power LED encapsulation substrate plays a decisive role in LED heat dissipation.
DPC ceramic substrate, with its excellent performance and gradually reduced price, has shown strong competitiveness in many electronic packaging materials, which is the future development trend of power LED packaging.
With the development of science and technology and the emergence of new preparation technology, the high-heat conductive ceramic material as a new type electronic packaging substrate material has a broad application prospect.
With the continuous improvement of the input power of LED chip, the high heat generated by the high dissipation power has put forward newer and higher requirements for LED packaging materials.
In the LED heat dissipation channel, the encapsulated base plate is the key link connecting the internal and external heat dissipation path, and it has the functions of heat dissipation channel, circuit connection and physical support to the chip.
For high power LED products, the encapsulated substrate is required to have high electrical insulation, high thermal conductivity and thermal expansion coefficient matching the chip.
Resin - based packaging substrate: supporting high cost and popularity is still difficult
EMC and SMC have high requirements for molding equipment, and the price of a molding production line is around 10 million yuan.
In recent years, the spliced LED bracket generally adopts the high-temperature modified engineering plastic material, and takes PPA(polyphthalamide) resin as the raw material. By adding the modified filler, certain physical and chemical properties of PPA raw material can be enhanced, thus making PPA material more suitable for injection molding and the use of spliced LED bracket.
PPA plastic has low thermal conductivity. Its heat dissipation is mainly carried out through metal lead frame. Its heat dissipation capacity is limited, and it is only suitable for small power LED packaging.
As the industry takes LED cooling seriously, two new types of thermosetting plastics - epoxy plastic seal (EMC) and sheet molded plastic (SMC) - are introduced into the patch LED holder.
EMC is a powder molded plastic with high performance phenolic resin as curing agent, high thermal conductivity silicon powder as filler and a variety of additives.
SMC is mainly composed of about 30% unsaturated resin, about 40% glass fiber, inorganic filler and other additives.
These two kinds of thermosetting molding compound heat curing temperature over 150 ℃ or so, after modification coefficient of thermal conductivity of 4 w/(m K) ~ 7 w/(m K), compared with PPA plastic has improved greatly, but the disadvantage is that liquidity and thermal conductivity is more difficult to two or morethings, curing at high hardness, easy to produce cracks and burr.
The curing time of EMC and SMC is long, the molding efficiency is relatively low, and the requirements for molding equipment, molds and other supporting equipment are quite high. The price of a molding and supporting production line is around 10 million yuan, which makes it difficult to popularize on a large scale.
Metal core printed circuit board (PCB) : complex manufacturing process with few practical applications
The manufacturing process of aluminum substrate is complex and costly. The thermal expansion coefficient of aluminum differs greatly from the chip material.
With the development of LED packaging toward thin and low cost, the technology of on-board chip (COB) packaging is gradually emerging.
At present, the COB encapsulated substrate is mostly printed circuit board with metal core, while the high-power LED encapsulation is mostly based on this substrate, whose price is between the middle and high price.
At present, high-power LED heat dissipation substrate is produced generally. The thermal conductivity of the insulation layer is extremely low, and due to the existence of the insulation layer, it cannot withstand high-temperature welding, which limits the optimization of packaging structure and is not conducive to LED heat dissipation.
How to improve the thermal conductivity of epoxy insulation has become a hot spot in the research of aluminum substrate.
At present, a modified epoxy resin or epoxy glass cloth adhesive sheet mixed with highly heat-conductive inorganic filler (such as ceramic powder) is used to bond copper foil, insulator and aluminum sheet through hot pressing.
At present, a kind of "all-gum aluminum substrate" has been developed internationally. The thermal resistance of the all-gum aluminum substrate can reach 0.05k /W.
In addition, a company in Taiwan recently developed DLC, a kind of carbon drilling material, and applied it to the insulating layer of high-brightness LED encapsulated aluminum substrate.
DLC has many excellent material characteristics: high thermal conductivity, thermal uniformity and high material strength.
Therefore, replacing the traditional metal-based printed circuit board (MCPCB) with DLC, the epoxy resin insulation layer is expected to greatly improve the thermal conductivity of MCPCB, but its actual use effect remains to be tested by the market.
A better aluminum substrate is created directly on the aluminum plate, and then printed on the circuit.
The biggest advantage of this method is that it has strong binding force and the thermal conductivity is as high as 2.1w /(m·K).
However, the manufacturing process of this aluminum substrate is complex and costly. Moreover, the thermal expansion coefficient of metal aluminum is greatly different from that of the chip material. Large stress is often generated in the thermal cycle when the device works, which may eventually lead to failure.
Silicon package substrate: the yield rate is less than 60%
Silicon substrate is faced with challenges in the preparation of insulation layer, metal layer and conductive hole, with a yield of less than 60%.
In recent years, silicon material has been introduced into the LED industry.
The thermal conductivity and thermal expansion of silicon substrate show that silicon is a kind of encapsulating material matching LED.
The thermal conductivity of silicon is 140W/m·K. When applied to LED packaging, the thermal resistance is only 0.66k /W.
Moreover, silicon-based materials have been widely used in semiconductor manufacturing process and related packaging field, and related equipment and materials have been quite mature.
Therefore, if silicon is made into LED encapsulation substrate, it is easy to form mass production.
However, LED silicon substrate packaging still has many technical problems.
For example, in terms of materials, silicon is prone to fracture and has problems with the strength of the mechanism.
In terms of structure, although silicon is a good heat conductor, it has poor insulation and must be treated with oxidative insulation.
In addition, the metal layer should be prepared by splashing and electroplating, and the conductive hole should be corroded.
Generally speaking, the preparation of insulation layer, metal layer and conductive hole is facing challenges and the yield is not high.
Although some Taiwan enterprises have developed LED silicon substrate and produced in quantity, the yield rate is not more than 60%.
Ceramic package substrate: improve heat dissipation efficiency to meet high power LED requirements
Combined with ceramic matrix with high thermal conductivity, DPC significantly improves the heat dissipation efficiency and is the most suitable product for high-power and small-size LED development.
The ceramic heat dissipation base plate has new heat conducting material and new internal structure, which makes up for the defects of aluminum metal base plate, thus improving the overall heat dissipation effect of base plate.
Although BeO has high thermal conductivity, its linear expansion coefficient differs greatly from silicon (Si) in ceramic materials that can be used as heat dissipation substrate at present. Moreover, it is toxic when manufactured, which limits its application.
BN has good comprehensive performance, but as a substrate material, it has no outstanding advantages and is expensive.
Silicon carbide (SiC) has high strength and high thermal conductivity, but its resistance and insulation resistance are low. Bonding instability after metallization will cause changes in thermal conductivity and dielectric constant, so it should not be used as insulating packaging substrate material.
Although Al2O3 ceramic substrates are the most productive and widely used ceramic substrates at present, the thermal expansion coefficient of Al2O3 ceramic substrates is relatively higher than Si single crystals, resulting in that Al2O3 ceramic substrates are not suitable for use in high-frequency, high-power and ultra-large scale integrated circuits.
The A1N crystal has high thermal conductivity and is considered as an ideal material for new generation semiconductor substrate and packaging.
AlN ceramic material has been widely studied and developed since the 1990s. It is widely considered as a promising electronic ceramic packaging material.
The heat dissipation efficiency of AlN ceramic substrate is 7 times more than that of Al2O3 substrate, and the heat dissipation efficiency of AlN substrate applied to high-power LED is remarkable, thus greatly improving the service life of LED.
The disadvantage of AlN substrate is that even if there is a very thin oxide layer on the surface, the thermal conductivity will be greatly affected.
At present, the mass production of AlN is still immature. Compared with the Al2O3 substrate currently in common use, the cost of AlN substrate is about 3 to 5 times that of Al2O3 substrate.
However, if energy production in the future, the cost of AlN base plate can be rapidly reduced, and then the AlN base plate with strong heat dissipation efficiency will have the opportunity to replace the Al2O3 base plate.
At present, ceramic substrate applied in LED packaging can be divided into HTCC, LTCC, DBC and DPC4 according to the preparation technology.
HTCC is also known as high-temperature co-firing multi-layer ceramics. Its main material is tungsten, molybdenum, manganese and other metals with high melting point but poor conductivity, which are expensive to make and seldom used now.
LTCC, also known as low temperature co-firing multilayer ceramic substrate, has a thermal conductivity of about 2W/(m·K) ~ 3W/(m·K), which is not much superior to existing aluminum substrate.
In addition, the LTCC used thick film printing technology to finish the line making, which made the line surface rough and inaccurate.
Furthermore, the shrinkage ratio of the laminated sintering process of multilayered ceramics is also a problem, which limits the process resolution and greatly challenges the promotion and application of LTCC ceramic substrate.
The direct copper clad ceramic plate (DBC) developed based on the encapsulation technology is also a ceramic substrate with excellent thermal conductivity.
The DBC substrate did not use binder in the preparation process, so the thermal conductivity is good, the strength is high, the insulation is strong, and the thermal expansion coefficient is matched with the semiconductor materials such as Si.
However, the ceramic substrate has low reaction capacity and poor wettability with metal materials, so it is difficult to carry out metallization, and it is not easy to solve the problem of micro pores between Al2O3 and copper plates. As a result, the mass production and yield of this product are greatly challenged, and it is still the focus of research by researchers at home and abroad.
DPC ceramic substrate, also known as direct copper plating ceramic plate, has the characteristics of high line accuracy and high surface smoothness. It is very suitable for LED clad crystal/eutectic process. With the combination of ceramic matrix with high thermal conductivity, it significantly improves the heat dissipation efficiency.