Thermal substrates for efficient heat dissipation in LED packaging application

Abstract

In recent high power LED applications, metal core PCB (MCPCB) has replaced FR4 PCB in order to achieve greater heat dissipation ability. However, the low thermal conductivity of dielectric and other complicated structures in MCPCB has not supported to maintain junction temperature in safe level when LED is operated at higher power. The challenge is to identify and synthesize high thermal conductivity materials and thin dielectric layers that would favour the usage of MCPCB as a better heat dissipation elements for high power LEDs. This research is focused on developing such thermal substrates with enhanced thermal conductivity at a low cost for mass production. Variety of thin films especially metal oxides and also nitrides as dielectric material (with high thermal conductivities) have been researched rigorously, as these materials could be synthesized as thin films and also as thick films. As a result, the total thickness of thermal substrates could be reduced and hence lower thermal resistance is achievable. In this paper, two different kinds of materials such as ZnO (thick film) and B-AlN (thin film) have been synthesized by two different methods such as screen printing followed by co-precipitation method and chemical vapor deposition using bubbler technique respectively. As an outcome of this research, ZnO thick film was screen printed on Al substrates and cured at low temperatures (125°C) which is low as compared with current curing temperature (>350°C). The screen printed substrates was used as thermal substrates to replace the glassy dielectric material (current practice in industries) of low thermal conductivity. The performance of such thermal substrates were tested using commercial LEDs which has shown a low thermal resistance for pure ZnO thick films with a thickness of 25μm. As observed low thermal resistance with ZnO thick film, the junction temperature of the LED was reduced noticeably. ZnO thick film is also having good reflectivity and can be considered as reflective substrates in electronic packaging. Low curing temperature of the proposed ZnO dielectric paste will also lead to low cost fabrication and mass production of the product. In addition to the above, B-AlN thin film (400 nm) was also deposited by CVD method using gas bubbler on bare Al substrates to improve the performance of thermal substrates (prototype) and compared with commercial MCPCBs. Improved performance of LED was achieved with high value in lux for B-AlN thin films deposited thermal substrates, low thermal resistance and high difference in junction temperature (ATj = 13°C) in comparison with MCPCB. Overall, ZnO thick film and B-AlN thin film deposited Al substrates has been proposed as an alternative to replace commercially available thermal substrates, in place of MCPCBs and FR4s.