Issue link: https://iconnect007.uberflip.com/i/989774
58 SMT007 MAGAZINE I JUNE 2018 issues on the component and reflow character- istics of solder paste make it difficult to achieve good voiding. Trials were conducted using a fully auto- mated in-line system. Use of such a world- class facility helped to maintain control and ensure consistent process repeatability among test vehicles. A production printer was used to print 5 mil paste layouts on both the solder paste only and solder paste and preform configurations under investigation. Paste was printed at a 65° squeegee angle at a rate of 30 mm/s and 80N pressure using a 5 mil stencil. Components and solder preforms were placed from tape and reel using a production compo- nent placement machine with twin-head and 2N force. Samples were then inspected for skew prior to reflow in a production belt driven reflow oven per the reflow profiles defined in Figure 3. Finally, upon reflow, every compo- nent was subjected to X-ray voiding analysis captur- ing its largest single void size and total void percent under the thermal pad. Results and Discussion Voiding analysis was performed on all test assemblies. Statistical soft- ware was used to compare voiding performance and processing factors to gener- ate a main effect plot to understand QFN voiding. The plot of the main effects on the voiding of the built assemblies are shown in Figure 6. Reviewing data, it was observed that no significant differ- ence was observed between the two surface finishes (ImmSn and ImmAg) evaluated in this study. A very small effect of the reflow profile type was observed when all data was analyzed. As expected, the lower voiding percentage was achieved with a high soak reflow profile compare to the straight ramp. Typically, higher soak profile allows more outgassing of volatiles during reflow, which could result in lower void- ing. Reflow in nitrogen atmosphere resulted in lower voiding. Higher voiding levels were observed on the larger QFN component with thermal pad 5.4 x 5.4 mm. The results for the small and medium sized LCS and QFN pack- ages were similar even though components' thermal pads were very different (3.6 x 3.6 mm and 2.1 x 2.1 mm). Assemblies without vias and with through hole vias produced the least amount of voiding overall; whereas, plugged vias showed higher voiding level. The biggest effect contributing to the reduction in voiding was the use of the solder paste and preform (PF+paste) configuration versus solder paste only (SPO) configuration. The solder paste only configuration was used as a benchmarking factor for the remain- der of the analysis. The average voiding for the contributing factors for SPO configuration Figure 5: Solder paste print design for QFN. Figure 6: The main effects on voiding (%) – overall plot.