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OCTOBER 2018 I SMT007 MAGAZINE 71 ous traditional IC components such as BGAs, D-Paks and MLFs being directly mounted on the same board. This is driven by the need for lighting systems to go beyond general illumi - nation providing more complex features such as control, sensing and modulation. Large area voids can create issues in the electrical signal or generate cracks due to thermal cycling/fatigue thus need for a more robust, low voiding inter - connects are required at the Level 2 layer. Experimental Procedure – Design Case There are a variety of LED designs used in the industry ranging from design structure/size and package materials i.e. ceramic and plas- tic. The goal of this experiment is to observe the effect of voiding based on three varying structures with the key difference being the pad geometries. For example, a 2-pad design where the anode and cathode are symmetri- cal, a 3-pad high aspect ratio design where the central thermal pad is slightly larger in terms of total area and lastly, a 3-pad design where the central thermal pad is significantly larger in terms of total area when compare to the anode and cathode. Table 2 shows the total area and construction of the LED pads. ASSEMBLY MATERIALS & COMPONENTS Substrate The substrate used in this study is a custom designed aluminum core PCB. The particular details for this board are shown in Table 1. LED Components For this study, three commercially available high-power LEDs were selected with varying pad geometries. The parameters for anode, cathode, and thermal pad dimensions for the LEDs are listed in Table 2. All dimensions are in millimeters. The LEDs were chosen to emulate common packages selected in high and ultra- high-power assemblies. Solder Pastes A commercially available no-clean solder paste was used for this study known using a type 4 particle size SAC-based alloy (38–20 Particle size in μm (80% min. between)). PROSESS AND TEST METHOD Equipment Processing Details Solder paste printing was done using DEK Horizon 03iX printer with a 4 mil thick laser cut stainless steel stencil with a 1 to 1 ratio of aperture size to pad size. Stencil print- ing parameters used for all solder pastes are shown in Table 3. Reflow Soldering The soak reflow profile was used in this study shown in Figure 2 the temperatures are displayed in Table 4. Table 1: Test vehicle details. Table 3: Print conditions. Table 2: LED pad dimensions (mm).