In recent years, light-emitting diode (LED) technology has been rapidly gaining market share in the general lighting market around the world. To meet the demands of general lighting applications, the output power of LEDs has substantially increased. However, this also results in more heat being generated, which may be detrimental to the performance and the reliability of LEDs. Flip-chip packaging has been identified as a first-level interconnect method which offers both improved light extraction and better thermal management than conventional wire-bonded LEDs. Bump interconnects together with underfill encapsulation have been recently explored as a packaging method for flip-chip LEDs (FCLED).

Existing studies which concern bump interconnects and highly thermally conductive unde...[ Read more ]

In recent years, light-emitting diode (LED) technology has been rapidly gaining market share in the general lighting market around the world. To meet the demands of general lighting applications, the output power of LEDs has substantially increased. However, this also results in more heat being generated, which may be detrimental to the performance and the reliability of LEDs. Flip-chip packaging has been identified as a first-level interconnect method which offers both improved light extraction and better thermal management than conventional wire-bonded LEDs. Bump interconnects together with underfill encapsulation have been recently explored as a packaging method for flip-chip LEDs (FCLED).

Existing studies which concern bump interconnects and highly thermally conductive underfill claim that increasing the thermal conductivity of the underfill material can be an effective method to reduce the thermal resistance of FCLEDs. However, in the capillary underfill dispensing process, an underfill fillet which covers the side of the FCLED chip is unavoidable. This effect has not yet been considered in the literature. To address this concern, the present study strived to provide insight into side-wall light absorption by underfill fillets, and to demonstrate a means by which this issue can be remedied.

In the current research, a thermally conductive underfill material was fabricated. This was accomplished by loading an epoxy material with highly thermally conductive fillers. The fabricated underfill materials were then characterized thermally and tested for moisture resistance. A volume-optimized underfill dispensing method was then implemented, whereby a void-free underfill encapsulation was achieved for sample preparation.

Subsequently, the issue of side-wall light emission absorption was investigated. Reduction of radiant power as a result of opaque underfill fillets was studied, followed by another demonstration which showed filler particles in epoxy did absorb the emitted light.

The impacts of increasing underfill thermal conductivity on the thermal resistance of flip-chip light emitting diode (FCLED) packages with different interconnect layouts were then investigated. Experimental and simulation results indicated that highly thermally conductive underfill was able to reduce the thermal resistance of FCLEDs. However, simulations also revealed that proper interconnect design could provide a better means of reducing thermal resistance than simply increasing the thermal conductivity of the underfill, and that smaller interconnect distances might negate the benefits of increasing underfill thermal conductivity.

At last, an investigation on how the interconnect structure affects the effectiveness of thermally conductivity was performed. Interconnect gap lengths and the effectiveness of underfill thermal conductivity were shown to be related and key points were identified which allowed for unfilled, transparent epoxy to be used to the same effect as highly thermally conductive underfill, thus reducing the side-wall blockage issue.