The role of thermal properties of PCB substrates in heat dissipation of LED back light bars

With special features of high color rendering index, environmental friendliness, low power consumption, long lifetime, small form factor, and short response time, the light emitting diode (LED) has gradually been replacing the conventional cold cathode fluorescent lamp (CCFL) as a back lighting source in TV and computer monitors industry. However, the high junction temperature (T_j) in the LED chips would be detrimental for its service life and reliability. Thereby, the thermal management of the LED back light bars is an important issue. The goal of this study is to experimentally and numerically study the effect of PCB substrates on the thermal performance of the LED back light bars, shown in Fig. 1. In these light bars, the commercial LED packages with a size of 5.7 mm × 3 mm × 1 mm are mounted on various 6.3 mm-wide PCB strip substrates with a pitch of 8.50 mm. Four types of the PCB substrates with different layer up and thermal properties are evaluated in term of T_j and thermal resistance experimentally by junction temperature tester, shown in Fig. 2, and numerically by CFdesign simulation (one of commercial computational fluid dynamic codes). The effect of PCB substrates on T_j and thermal resistance of the LED light bars will be demonstrated and its mechanism will be investigated and interpreted based on experimental and numerical simulation results. In addition, the effective thermal conductivity of the PCB substrates will be proposed for easy calculation, evaluation and comparisons.