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20 SMT Magazine • May 2015 mINIaTURIzaTION WITH THE HELP OF REDUCED COmPONENT-TO-COmPONENT SPaCING continues Feature CSPs. In true production the cleaning frequen- cy could potentially be reduced but this would be product dependent. The solder paste volume criteria were set to 40–120% of the theoretical value. All the Cp and Cpk calculations used statis- tical software. True Cp and Cpk using this soft- ware are Pp and Ppk since this is the overall cal- culated capability (Figure 6 and Table 1). Using a 3 mil (75 µm) stencil gives a much higher Pp and Ppk compared to a 4 mil (100 µm) stencil (Figure 6) and this is expected due to an area ratio (AR) of 0.5 with a 4 mil (100 µm) stencil and 0.66 with a 3 mil (75 µm) stencil. All solder paste deposits with a 3 mil (75 µm) stencil were within specification showing 0 DPMO print failures. The solder paste printing with a 4 mil (100 µm) stencil showed a DPMO of 1382 on 01005 but these PCBs were still as- sembled and the solder joints still met IPC 610E standard with regards to solder joint quality. Pick-and-Place Details All the assemblies were done on standard fine pitch surface mount machines equipped with an accuracy of 40 µm at 3 Sigma. The flux dipping unit used was the linear dipping unit which comes with replaceable dipping plates. No assembly-related defects were detected dur- ing for the 0.3 mm pitch CSP or 0.4/0.4 mm pitch PoP components once the correct process parameters were set. Reflow A standard lead-free reflow profile was used. The reflow was done in air and nitrogen, with 65 seconds above 217°C and 245°C peak tem- perature. Our previous studies on 01005 showed that a fast ramp of 1.0°C/s and above between 180–217°C gives better wetting when reflowed in air but the same basic process was used both for the air and nitrogen reflow in this study. Graping/incomplete wetting is a clear sign of too slow a ramp between 180–217°C (Figure 7). Rework Process Rework becomes more challenging with re- duced component-to-component spacing, but if the process is set up correctly the amount of rework should be at a minimum. A standard re- work process was used for all components on the PCB. For the passive components manual rework was done with help of fine tip tweezers and a hot air blower. For the CSP and PoP a BGA repair machine was used. Results The DPMO values (Figure 8) show a very clear breakpoint between 100–125 µm compo- nent-to-component spacing and it is also shown that the stencil thickness has a major impact at the smaller component-to-component spacing figure 7: 01005 reflowed in air with a ramp be- low 1.0°c/s, between 180–217°c. figure 8: DpMo at different component-to- component spacing with 3 mil and 4 mil stencil thickness.