Low Voiding Reliable Solder Interconnects for LED Packages on Metal Core PCBs
BY ELLEN TORMEY, PH.D., RAHUL RAUT, WESTIN BENT, RANJIT PANDHER, PH.D., BAWA SINGH, PH.D. AND RAVI BHATKAL, PH.D., MACDERMID ALPHA ELECTRONICS SOLUTIONS
SSL Assemblies need to meet high reliability requirements such as Energy Star Category A which dictates a B50/L70 lifetime of 35,000 hours for commercial and outdoor residential lighting. Solder joints with low void content are critical for long term performance and reliability. Two types of MCPCB substrates, 4 different solder pastes and one type of LED ceramic package were evaluated in this study to develop a low voiding assembly process. Results of the study and recommendations for achieving low voiding are presented.
Applications for light emitting diodes (LEDs) are increasing dramatically in the lighting sector. The benefits of LEDs over competing technologies include versatility and long- term reliability. Package and luminaire design are critical considerations in ensuring that performance and reliability targets are met for commercial applications.
Customer expectations for LED based luminaires (Solid State Lighting) are very high due to the relatively high cost of such luminaires. For commercial and outdoor residential lighting, 70% lumen maintenance after 35,000 hours and a
(3) year warranty is required for LED packages, modules and arrays to meet Energy Star Category A criteria. For high reliability, long lifetime and color maintenance of LED lights, it is critical to have excellent assembly interconnect reliability; i.e., package to insulated metal-core substrate solder joints with low voiding for low thermal resistance and hence good heat dissipation.
Assembly materials & components
Materials and components were chosen based on commercially available LED packages, solder pastes and MCPCB substrates.
High power LED package: A high power InGaN-based LED package (Ref.1) was used in this study. It is a compact package that can be surface mounted and can provide high lumen output and superior thermal performance. An image and cross-section of the LED package are shown in Figures 1 and 2, respectively1.
MCPCBs and Dielectric: The LED package is a surface mount component and can be assembled on a typical FR4 board or on an MCPCB (Metal Core Printed Circuit Board). MCPCBs are also referred as Metal Clad PCBs. An MCPCB has a thin, thermally conductive dielectric layer bonded to an aluminum or copper substrate for good heat dissipation.
Each of the board materials has its own benefits and limitations. For example, a FR4 board with open or filled and capped vias is a low cost solution for a regular LED assembly. MCPCBs offer more rigidity than typical FR4 boards along with improved thermal performance, enabling heat conduction from LED packages into the metal core board material. A partial image of the MCPCB used in this study is shown in Figure 3; a 6×6 array of 36
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