Study On Water Glass Based Thermally Conduvtive Adhesive For LED Packaging

Epoxy resin and silicone are always used as the base of the thermally conductive adhesives for light emitting diodes packaging (LED). The aim of this paper is to study an inorganic thermally conductive adhesive, the base of which is sodium silicate solution and the filler is hexagonal sheet boron nitride (BN). The module and mass fraction of the water glass used is 3.3 and 40wt%, respectively. The sizes of the BN chosen here are 2μm, 5μm and 15μm. The thermal conductivity of the adhesives consisted of water glass and BN is measured here using steady-state heat flow method. To contrast the character of BN, aluminum nitride (AlN) as the filler is also studied. The dispersion conditions of BN and AlN with different sizes are researched using SEM. And compositions of water glass filled by BN or AlN are characterized using XRD and FTIR. The effect of BN content and size on the adhesive strength is analysed, as well as MgO, through the measurements of the tensile shear strength of the adhesives. The best curing temperature of the adhesive is obtained by comparing the adhesive strength under different cured temperature. Still, the dielectric strength of this kind of adhesive is measured and its influencing factors are analysed.Ammonia is released when BN or AlN mixed with water glass solution. The results of XRD and FTIR show no chemical bonds of the hydrolyzed products. BN can be mixed well with the water glass while the AlN particles precipitate easily because their high density. With the increase of BN content and the decrease of the size, the thermal conductivity of the adhesive increases. When the 2μm BN content is 40% and 50% of the total dry mass, the thermal conductivity of the adhesive is 2.41W/m·K and 3.52W/m·K, respectively. When the mass ratio of 10μmAlN particles and water glass is 5:3, the thermal conductivity is 2.74W/m·K. When heated during the range of temperature of 25℃～150℃, the highest tensile shear strength of the water glass solution can be obtained at 100℃, as 2.58MPa. For the water glass based adhesives having the same BN content, the tensile shear strength decreases when the BN size is large. When adding some MgO particles into the adhesive,the tensile shear strength can be improved greatly. The experimental results show that this kind of adhesive has low dielectric strength, which is mainly dependent on the dielectric properties of the water glass.