Preparation And Characterization Of High Performance LED Packaging Materials

Light emitting diodes (LEDs) are semiconductors which can convert electrical energy into light. They have wide applications in general illumination, display backlighting, signage and communications, due to their great superiorities such as low electric consumption, environment-friendly, high brightness, long lifetime, good reliability. Recently, continuing improvement in LED’s property and reliability are starting to challenge the packaging materials. As for the high-reliability and high-efficiency LEDs, the characteristics of high refractive index, high transmittance, excellent UV/thermal resistance, low coefficient of thermal expansion(CTE), low stress and water absorption are required.Silica fillers were incorporated into epoxy packaging materials to decrease the CTE and water sorption, which could avoid the occurrence of pop corn, decrease the hygro-mechanical stress and thermal stress in the packaging materials, and thus improve the reliability of LED. We studied the influence of fillers’concentrations and types on the properties and reliability of LED by the combination of the differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), dynamic mechanical analysis (DMA), thermomechanical analyzer (TMA), ultravioletvisible spectrophotometer (UV-vis), scanning acoustic microscopy (SAM) and scanning electron microscopy (SEM).We firstly studied the effect of filler contents on the properties of LED packaging materials.10 wt%～60 wt% silica fillers was added into the epoxy packaging materials, and the results showed that:the CTE values of the systems were effectively decreased with the addition of the fillers, and the more the concentrations of the fillers are, the more the CTE values decreased. The equilibrium water sorption content and diffusion coefficient in both water sorption and moisture sorption tests were also reduced with the increase in filler contents. TGA test showed that the thermal stability of the system was improved with the loading of the fillers. The activation energy of the curing reactions dropped down with the addition of the fillers. The fillers showed little difference in Tg values. The modulus of the systems were increased with the addition of the fiilers. The light transmittance of the systems during UV and thermal aging showed a declined tendency with the increase in the filler contents. The Shore D hardness was increased with the increasing filler contents.Then we studied the influence of filler types on the properties and reliability of packaging materials. Four kinds of inorganic silica fillers, namely, quartz, fused silica, cristobalite, and spherical silica, and one kind of organic filler, that is, spherical silicone powder, were incorporated into the epoxy packaging materials and then were used to encapsulate the real LED devices. A set of evaluation methods have been established to characterize the reliability of these LED devices. No delamination or internal cracking between packaging materials and lead frames has been found for the encapsulated high performance LED devices after the package saturation with moisture and subsequent exposure to high-temperature solder reflow and thermal cycling. The properties of each packaging materials were also tested, and the results showed that except the spherical silicone powder filled epoxy materials, all the other filled systems showed lower equilibrium water sorption content and smaller water diffusion coefficient in both water sorption and moisture sorption tests. The coefficient of thermal expansion (CTE) values were also decreased with the addition of fillers, and the systems filled with quartz, fused, and filled with spherical silica gave the best performance, which exhibited the reduced CTE values both below and above Tg. The results of TGA essentially showed no difference between filled and unfilled systems. The glass transition temperature changed little for all the filled systems, except the one incorporated with spherical silicone. The activation energy of the reactions dropped a little with the addition of the fillers. The modulus at room temperature was increased with the addition of fillers. The light transmittance of filled epoxy materials varied with fillers after UV and thermal aging.