LED용 고전도성 및 고방열 Silver Paste에 관한 연구

Abstract

LED requires increased optical power to replace existing lighting. More power supply is needed to increase optical power. As LED emits 85% power its electric energy by heat, the high power LED generates more heat. The heat radiation technology is mandatory for high LED as lifespan of LED is reduced its lifespan to 1/2 on every 10℃. General semiconductor elements radiates heat with heatsink but there are gaps between the contact surfaces as they are not completely flat. These gaps have air and the air has lowest heat conductivity. Therefore the semiconductor element and heatsink surfaces are filled with heat radiation paste to increase the effect. The traditional heat radiation paste contains lead (Pb) and widely used with soldering technology. Lead can be easily processed with cheap and low melting points, but it was assigned as harmful material internationally and alternative paste development is a necessity. Pb-free solder was developed but it needed higher melting point causing higher temperature processing with concern of damaging electronic parts. Conductive adhesive is another alternative. The conductive adhesive that is made as paste using silver with excellent heat conductivity and electro-conductivity is more eco-friendly and simpler processes with lower process temperature. And it does not require soldering, which made it easier to be used widely on hardwares and soldering unable hardwares. Generally, it is widely known as the metal has excellent heat conductivity but has better electro-conductivity. The reason for metal having better conductivity is that it has many free electrons and they work as mediums. The heat conductivity means the heat conducts from high to low and the electricity works the same. When heat and electricity are generated together, electricity won't conduct well on heating areas as the free electrons are conducting heat and cannot conduct electricity. Which means heat and resistance has close relationship. Electricity conductivity is as important as heat conductivity in conductive adhesive. Conductive adhesive work as an unique bridge connecting LED and circuit. Conductive adhesive not only takes a role of thermal emission but power supply. The heat from LED is released by free electrons but electricity conductivity is lowered as less free electrons to supply power. And when electric resistance gets higher, more heat will be generate. The objective of this paper is for high heat radiation and conductivity of silver paste used as conductive adhesive. Silver is a high conductivity material and frequently used as adhesive, but polymer resin is used as conductive adhesive for chip and circuit glue. Thus, the paste development was planned to increase silver density to reduce resistance and increase heat conductivity. Two types of polyester resins and two types of epoxy resins frequently used for silver paste comparison was the first test. The polyester resins had good conductivity, but had H and 2H hardness resulting no significant differences in conductivity and next test was performed with epoxy resins with 4H. Secondly, nano, sub-micro and micro silver powers were used as to minimize the pattern gaps with different mixture ratios. The result showed that the 20:65 ratio of sub-micro and micro paste had the best result. Next test was performed to find appropriate silver content of 20:65 from previous test. In this test, the hardness is weakened as resin got less with more silver and resistivity got increased with more resin and less silver. Of these contents, the 85% silver had best result. Fourth, gaps were found within the patterns when the printed patterns were observed with SEM. The reason was that the printed paste gets dried from the surface and solvents remained inside and they penetrate the surface or evaporate lately, causing the gaps. Thus, each solvent boiling points were test to induce fast evaporation and the result showed that ECA solvent had the best result. Fifth, equivalent tests were performed to improve conductivity and hardness of epoxy resin. The result showed that 2021P had excellent conductivity and YD-127 and 1009 had excellent hardness, thus, the sixth test was performed with 2021P and YD-127 for mixture ratio test. 1009 was mixed with 5:5 ratio with solvent and had the possibility of having solvent evaporation issues, thus, 2021P and YD-127 was mixed and 5:5 ratio had best result. Finally, heat conductivity samples were made with four final selected pastes for evaluation. The heat conductivity of new paste and existing paste were compared and the result showed that YD-127 and 2021P with 5:5 ration had best conductivity, hardness and heat conductivity.