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SMT007-Oct2018

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74 SMT007 MAGAZINE I OCTOBER 2018 Discussion From the experimental results, it is evident that solder pad design influences the level of voiding for a given package. The selected pack- ages, which have the largest physical difference of pad area and asymmetry in design, create the highest levels of voiding. This is directly related to the total volume of solder deposit for a particular LED package. Table 5 depicts the pad variance for a given design. It is known that having different solder volumes on the same board makes it very diffi- cult to adjust reflow profile setting that balances the activation and evaporation component. A smaller volume consumes the activator faster than a larger volume, which also requires a much longer soak profile to remove the dilu- ents/solvents in the flux system. Experimental Procedure — Process Case [1] Adjusting the reflow profile is a very common starting point in order to optimize the level of voiding. In the first set of experiments, multi- ple solder paste formulations were subjected to various reflow profiles. The profile varia- tions included preheat soaks, and time above liquidus. All reflow profiles were created using a seven-zone convection oven using no nitro- gen. An in-house developed test vehicle, based on a 0.062 thick FR4 laminate, finished with OSP/Copper pads was used. The test vehicle includes BGA, D-Pak, and MLF devices. Test Method Voiding was measured with a Nikon Metrix XT V160 2-D x-ray machine. In each case, alter- ing the profile had an effect on the observed level of voiding. Varying the Preheat Profile Two reflow profiles with different preheat settings. The first is known as a straight ramp profile, where the test vehicle's surface temperature increases at a near linear rate of +1.5°C per second up to a peak temperature if 245°C. The second profile has a slightly faster ramp up (+1.56°C/second) to approximately 160°C, then the assembly is allowed to "soak" at temperatures between 160°C and 184°C for approximately 60 seconds, followed by a moderate (0.96°C/second) ramp up to a peak temperature of 240°C. RESULTS AND DISCUSSION Results Figure 5 shows the effect the two reflow profiles on a BGA 256 device using one solder paste. Both the solder paste alloy and the BGA spheres were made with SAC 305 alloy. Discussion The soak profile resulted in over 30% of the 265 BGA I-Os having no voids. With the straight ramp profile, fewer than 10% had zero voids. Table 5: LED design dimensions and difference in thermal vs. electrical pads.

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