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As one type of the chip attachment materials, the solder layer inside the light-emitting diode (LED) packages can not only bond the chips to the substrate tightly, but also play the role of thermal interface material (TIM). Therefore, it makes a great influence on the thermal performance LED packages, especially in high density chip-array LED packaging. The solder layer has become one of the dominant research interests both in academics and industries. In this paper, numerical simulations were conducted to analyze the thermal behaviors of solder layer. The interactions between exterior heat dissipation condition and interior solder layer were studied. Simulations results show that improving the exterior heat dissipation condition is not always useful, a threshold may exist. Selecting different solders for die bonding can lead to different thermal characteristics, and decreasing the thickness of the solder layer is not always effective for lowering the junction temperature. Moreover, voids in the solder layer may not only lead to increase of the junction temperature, but also result in the addition of the temperature difference. It is concluded that thermal solutions to the high power LED module should take into account both the solder layer and exterior heat dissipation condition.