Abstract
The effect of the microstructure of solder joints on the thermal properties of power LEDs is investigated. Solder joints were prepared with different solder pastes, namely 99Sn0.3Ag0.7Cu (as reference solder) and reinforced 99Sn0.3Ag0.7Cu–TiO2 (composite solder). TiO2 ceramic was used at 1 wt.% and with two different primary particle sizes, which were 20 nm (nano) and 200 nm (submicron). The thermal resistance, the electric thermal resistance, and the luminous efficiency of the power LED assemblies were measured. Furthermore, the microstructure of the different solder joints was analyzed on the basis of cross-sections using scanning electron and optical microscopy. It was found that the addition of submicron TiO2 decreased the thermal and electric thermal resistances of the light sources by 20% and 16%, respectively, and it slightly increased the luminous efficiency. Microstructural evaluations showed that the TiO2 particles were incorporated at the Sn grain boundaries and at the interface of the intermetallic layer and the solder bulk. This caused considerable refinement of the Sn grain structure. The precipitated TiO2 particles at the bottom of the solder joint changed the thermodynamics of Cu6Sn5 formation and enhanced the spalling of intermetallic grain to solder bulk, which resulted in a general decrease in the thickness of the intermetallic layer. These phenomena improved the heat paths in the composite solder joints, and resulted in better thermal and electrical properties of power LED assemblies. However, the TiO2 nanoparticles could also cause considerable local IMC (Intermetallic Compounds) growth, which could inhibit thermal and electrical improvements.
Highlights
Solid-state light sources become prevailing devices in the lighting technique [1,2,3]
3 A) was increased to 5 A with the forced cooling system. It resulted in a higher junction temperature of the LEDs over the ambient temperature, and it allowed for more accurate measurement of Rth and Rthe
The measurement error of the thermal resistance is diminished due to the use of a high forward current, which resulted in a high temperature inside the LED [36]
Summary
Solid-state light sources become prevailing devices in the lighting technique [1,2,3]. An essential component of them is the power LED (Light Emitting Diode) [1,2,3,4], in which the conversion of electrical energy into light takes place. During this conversion, some energy is changed into heat and increases the internal temperature of the diodes as a self-heating phenomenon [4,5,6,7,8]. The increase in internal temperature decreases the emitted luminous flux and worsens the luminous efficiency (ηE ) of solid-state light sources [8,9]. At very high temperatures (e.g., 120 ◦ C), the ηE can decrease several times [10].
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