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After placing the components on the board, the assembly was sent through reflow. Overall, the soldering with the Type 5 solder paste was acceptable. However, some issues were observed. With the larger pad with a component with terminations on the bottom side only, the part tended to float on top of the solder. Figure 11 shows the results after reflow.
Figure 11: Components floating on the solder.
These results did not match the results seen in other studies, and it has been found that the pad sizes can cause this because there is an excess of solder that forms a larger bump on the pad (semicircle). The component with bottom-side termination can only solder in that area, so the solder bump raises the component; depending on the location of the component, it will be raised up and on the side of the solder bump. If there are three or more sides covered with solderable surface, the part will flatten out. Further studies have also shown with a pad size closer to the terminal size, the part will not raise as much, and there will not be as rounded of a solder joint that would push the part to become tilted.
Another defect seen was some shorting on the 100-µm spacing due to the design of the traces connecting the pads together. The solder would wick down those traces since there was no solder mask covering these areas for the 100-µm spacing area. The tilting of the component also caused issues with AOI and AXI not being able to detect the parts and determine defects. A separate study covers more on those evaluations.
Further Work
Additional work is in progress, and a new test vehicle has been designed to include m0201 components. Initial printing and placement have been done, and additional trials are scheduled to be done. The pad designs on this SiP test vehicle are designed to the same size as the terminations on the m03015 and m0201 components. The spacings on this board have been reduced as well. The board contains component spacing of 100, 75, and 50 µm, which would be needed for SiP products.
Figure 12 shows some of the initial results of m0201 at 50-µm spacing. The print process still needs to be dialed in further because the printing is inconsistent with many high-volume spikes, which also caused the bridging in Figure 12, and the insufficient and no solder results. More trials are underway with different types of solder pastes to enable better printability. The placement equipment was able to pick and place even smaller components with little issues seen. New software and nozzles were required for the m0201 component on the equipment that was being used.
Figure 12: m0201 after reflow at 50-µm spacing.
Conclusion
This article demonstrates that smaller components can be done in an SMT process with standard equipment with the latest cameras, software, and nozzles with high yield. Printing was shown to be possible with a high Cpk. Pick-and-place rates were at a high level, and a minimum number of defects were seen. The pad size in this study was slightly larger, so there is room for more reduction to enhance miniaturization. This will be seen on the ongoing work with m0201 and m03015 for SIP products.
Acknowledgments
The authors would like to thank everyone involved with this project from inside and outside of our company.
Further Reading
1. N. Heilmann. "03015 Information." ASM Productronica, 2013.
2. D. Geiger, A. Mohammed, M. Kurwa, AEG, & Flextronics International Inc. "Overview Miniaturization on Large Form Factor PCBA." IPC APEX EXPO 2016.
Editor's Note: This article was presented at the IPC APEX EXPO 2018 Technical Conference.
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