Light emitting diodes reliability review

Heat and fluid flow in high-power LED packaging and applications

New interconnect materials are always necessary as a result of evolving packaging technologies and increasing performance and environmental demands on electronic systems. Polymer-based conductive-adhesive materials have become widely used in many electronic packaging interconnect applications. Among all the conductive-adhesive materials, the anisotropic conductive adhesives (ACA) (or anisotropic conductive adhesive films, ACF) have gained popularity as a potential replacement for solder interconnects. The interest in using ACA instead of solder comes partly from the fact that the use of ACA for the direct interconnection of flipped silicon chips to printed circuits (flip chip packaging) offers numerous advantages such as reduced thickness, improved environmental compatibility, lowered assembly process temperature, increased metallization options, reduced cost, and decreased equipment needs. In this review, a summary of our understanding of the electrical, physical, thermal, chemical, environmental, and cost behaviors of ACA in conjunction with various packaging applications is elaborated. First, the formulation and curing kinetics of ACA materials, as well as the conduction mechanisms of ACA joints, are introduced; second, the influencing factors, including the boding process (boding temperature, boding pressure, curing conditions, reflow and misalignment processes, etc), the environmental factors (temperature, humidity, impact load, etc), and the properties of the components (the properties of the ACA, substrates, conductive particles, the bump height, etc), on the reliability of ACA joining technology are presented. Finally, future research areas and remaining issues are pointed out. The purpose is simply to pinpoint the most important papers that have played significant role for the advancement of the ACA bonding technology.

A review of the influencing factors on anisotropic conductive adhesives joining technology in electrical applications

This project seeks to extend the lifetime of organic light-emitting devices (OLEDs) by blocking the transmission of ambient oxygen and moisture into the delicate emissive region of the device. Our approach is to encapsulate the organic device with hybrid thin-film stacks of both organic and inorganic materials. It is projected that, for commercial applications, such an encapsulation scheme will need to reduce the water vapor transmission to 1 x10 -6 g/m 2 -day and the oxygen transmission to 1 x10 -5 cm 3 (STP)/m 2 -day. We have developed a low-temperature (100oC) PECVD (ltPECVD) process for both amorphous silicon nitride and amorphous silicon oxide layers which yield optically transparent layers (>85% transmission) with low stress (-1.6 x10 9

Thin-Film Encapsulation of Organic Light-Emitting Devices

Review paper: Flip chip bonding with anisotropic conductive film (ACF) and nonconductive adhesive (NCA)

A conformal phosphor coating can realize a phosphor layer with uniform thickness, which could enhance the angular color uniformity (ACU) of light-emitting diode (LED) packaging. In this study, a novel freeform lens was designed for simultaneous realization of LED uniform illumination and conformal phosphor coating. The detailed algorithm of the design method, which involves an extended light source and double refractions, was presented. The packaging configuration of the LED modules and the modeling of the light-conversion process were also presented. Monte Carlo ray-tracing simulations were conducted to validate the design method by comparisons with a conventional freeform lens. It is demonstrated that for the LED module with the present freeform lens, the illumination uniformity and ACU was 0.89 and 0.9283, respectively. The present freeform lens can realize equivalent illumination uniformity, but the angular color uniformity can be enhanced by 282.3% when compared with the conventional freeform lens.

Design of a novel freeform lens for LED uniform illumination and conformal phosphor coating

Unless their reliability is no longer a concern, light-emitting diodes (LEDs) will be unable to be used for broader applications. One important factor in the operating environment of LEDs is moisture, which is always present if the packaging is not hermetically sealed. In order to investigate the effects of moisture on the reliability of LEDs, several high-power white LEDs were subjected to extremely moist conditions at different temperature points in this study. Different light-output regression rates were measured. Digital microscopy was used to observe moisture diffusion on the LED module. The results demonstrate that the light output of LEDs decreases as the environmental moisture changes. Moisture diffuses into the interfaces of the packaging material, which not only decreases light output but may also disable the LED module due to electronic failure.

Effects of Moist Environments on LED Module Reliability

Dynamic strength of anisotropic conductive joints in flip chip on glass and flip chip on flex packages

Gold wire bonding is an important packaging process of lighting emitting diode (LED). In this work, we studied the effect of gold wire bonding on the angular uniformity of correlated color temperature (CCT) in white LEDs whose phosphor layers were coated by freely dispersed coating process. Experimental study indicated that different gold wire bonding impacts the geometry of phosphor layer, and it results in different fluctuation trends of angular CCT at different spatial planes in one LED sample. It also results in various fluctuating amplitudes of angular CCT distributions at the same spatial plane for samples with different wire bonding angles. The gold wire bonding process has important impact on angular uniformity of CCT in LED package.

Effect of gold wire bonding process on angular correlated color temperature uniformity of white light-emitting diode

The effects of the outer EMC on the thermal deformation of an automotive electronic control unit (ECU) are studied using moire interferometry and FEA modeling. Two sets of ECU specimens molded with and without outer EMC are subjected to passive and active thermal conditions, respectively. Two orthogonal in-plane displacement fields on the cross section of specimens are documented at various temperatures using moire interferometry. The results of passive condition case are used to verify the complex FEA modeling of the units, and the initial material properties are subsequently calibrated. The validity of FEA modeling after calibration is corroborated by the results obtained from the active thermal condition.

Bulong Wu

Papers

Design of a novel freeform lens for LED uniform illumination and conformal phosphor coating

Effects of Moist Environments on LED Module Reliability

Dynamic strength of anisotropic conductive joints in flip chip on glass and flip chip on flex packages

Effect of gold wire bonding process on angular correlated color temperature uniformity of white light-emitting diode

Thermal deformation analysis of automotive electronic control units subjected to passive and active thermal conditions