Investigation on effects of solder paste voids on thermal and optical performance of white high-power surface-mounted device LEDs

Abstract

Purpose The presence of voids in the solder layer has been considered as one of the main issues causing reliability problems in optoelectronic devices. Voids can be created due to trapped gas, clean-up agent residues (fluxes), poor wettability at interface or shortcoming of the reflow process. The voids hinder the heat conduction path and subsequently, the thermal resistance will increase. The purpose of this paper is to investigate the influence of lead-free water-washable Sn96.5Ag3.0Cu0.5 (SAC305) solder paste (SP) voids on the thermal and optical performance of white high-power (HP) surface-mounted device (SMD) light-emitting diode (LED). Design/methodology/approach Five LEDs are mounted on five SinkPAD substrates by using the SP. The SMT stencil printing is used to control the thickness of the SP and reflow oven for the soldering process. The fraction of voids in the SP layer is calculated using the X-ray machine software. The thermal parameters of the LEDs with different voids fraction and configuration are measured using a thermal transient tester (T3Ster) system. In addition, the optical characterizations of the LEDs are determined by the thermal and radiometric characterization of power LEDs (TeraLED) and the electroluminescence by using the spectrometer. Findings The results showed that the thermal performance and temperature distribution are improved for the LED with lower voids fraction and good filling state of soldering. In addition, luminous flux, efficacy and color shift of the LEDs with different fraction and configurations of voids on the SP layer are compared and discussed. It is found that the color shift of LED1 of low voids fraction and higher thickness are less than other LEDs. Originality/value The paper provides valuable information about the effect of water-washable SAC305 SP voids fraction and filling state of solder on the thermal and optical performance of ThinGaN HP SMD LED. A comprehensive overview of the outcomes is not available in the literature. It was shown experimentally that the voids fraction, height and configuration of the SP layer could strongly influence the heat dissipation efficiency and thermal resistance. This study can help in heat diffusion investigation and failure analysis of HP SMD LEDs.