Characterization of Solder Defects in Package-on-Package with AXI Systems for Inspection Quality Improvement (Part 2)

Estimated reading time: 7 minutes

The three pins with arrows are indicated are good solder pins on Figure 4. Because there is circle at the top-left area ball on the 2DX image, it shows that pin has good joint with pad of the top layer of POP.

Figures 5 and 6 are images of the pin #V28 on top layer POP from AXI 5 and the 2DX, respectively. Hence, the volume ratio (Vld/Vud =1.116) for pin #V28, so that AXI 5 calls the pin as good solder joint due to its measurement data, is smaller than the threshold (Vld/Vud =1.25). However, its pCT image shows that pin is not perfect solder joint as shown in Figure 5.

In Figure 6, two yellow arrows indicate defective pins (V28, W28) on the 2DX image, one green arrow points to good pin (Y28) as it has a visible circular feature at the top-left area of the ball on 2DX image.

Table 2 summarizes test results of AXI 3, AXI 4, and AXI 5 with top layer of POP. The AXI 5 has defect escaped 3.7% with false call PPM as 18,518, which is significantly better compared to the AXI 3, AXI4 systems. We are working to further reduce false call PPM by the Algorithm optimization and additional profile testing.

Table 2: AXI 5 Volume ratio Threshold Setting versus Testing Results.

Obviously, the algorithm threshold setting affects its testing results. Collecting more data in real production environment will certainly enhance such engineering decision of threshold setting. In Table 2, we tabulated AXI 5 Volume Ratio threshold settings versus its testing results. It is obvious to see the setting is 1.25 is good number for this case. Table 3 is the summary of AXI 5 with other two AXIs test results previously.

Table 3: Test Summary of AXI 3, AXI 4, and AXI 5 with Top Layer of POP

Figure 7 shows AXI 5 CT images for nine pins: the four pins at the top are good, while the bottom three pins are defective pins. The middle two pins are defective pins, too, however pin # E29 is a false call, while W28 is real call per 2DX in the Table 4.

Conclusions

As an extended study of earlier reported projects, we have conducted inspection of solder defects on the top layer of mounted POP devices by an AXI system capable of high speed CT imaging, AXI 5.

• AXI 5 produced better testing results with top layer of POP shown in Table 3. The interesting facts are that pCT pass/fail calls generated by AXI 5 are independent of operator judgments, and they can be automatically saved to a server to be reviewed by the engineers.
• The attribute Gage RR results improvement are in progress.
• This project is still ongoing to develop a process for AXI with pCT algorithm and parameter optimization.
• For POP applications, 3D imaging offers less ambiguous decisions for solder bridges, while 2D or 2.5D images sometimes encounter with confusion from shadows of neighbors.
• This study on POP with AXI 5 is just a starter, while more packages like connectors are ongoing to utilize the machine’s 3D imaging capability in wider range of applications³.
• AXI with CT may be considered as the optimal X-ray inspection direction for PCBAs due to clear image and AUTO testing results output.

References

1] David Geiger, Zhen (Jane) Feng, Ph. D., Weifeng Liu, Ph.D., Hung Le, Tho Vu, Anwar Mohammed, Murad Kurwa and Evstatin Krastev Ph.D., "Optimizing X-ray Inspection with Package on Package", Proceedings of SMTAI, September, 2014.

[2] Alejandro Castellanos, Zhen (Jane) Feng, Ph. D., David Geiger, Murad Kurwa, "Head in Pillow X-ray Inspection", SMT, May 2014.

[3] David Geiger, et al, "Optimization of X-ray Inspection for Solder Charge and Press-Fit Connectors", to be published in SMTAI 2015.

Acknowledgements

The authors would like to thank Flextronics X-ray team who were involved in supporting this study, especially for Tho Vu, Hung Le, Phuong Chau, Jiyang Zhang; AXI3, AXI4, AXI5, AXI6 and 2DX support teams.

If you missed

Part 1, you can check it out here.

Editor's Note: This paper has been published in the technical proceedings of IPC APEX EXPO 2015.

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