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Process Development of the Stencil Printing Process
December 31, 1969 |Estimated reading time: 10 minutes
The need for more reliable, lighter, and smaller products has increased the use of flip chips (FCs), chip-scale packages (CSPs), microBGAs, and 0201s in applications where reliability is a main concern. For fine-pitch packages, solder paste volume and consistency are critical to joint reliability. This article focuses on the characterization and optimization of the stencil printing process for ultra-fine-pitch packages.
By Rita Mohanty, Ph.D., Speedline Technologies & Daryl Santos, Ph.D., Binghamton University
For fine-pitch packages, solder paste volume and consistency are critical to solder joint reliability. The process becomes more challenging when the combination of paste rheology and stencil geometry causes inadequate or inconsistent solder paste transfer. For flip chips and CSPs, standoff from the board is an important parameter for predicting long-term solder joint reliability, with pad size and solder volume being the primary factors that influence the standoff.1,2 Therefore, it is important to maximize the volume of solder paste deposited and increase package reliability.
This article details a study focusing on the characterization and optimization of the stencil printing process for ultra-fine-pitch packages to understand the process. Designed experiments are performed to determine the optimum level of aperture taper, wall finish, and which of three major stencil manufacturing techniques (chemical etching, laser cutting, and electroforming) work best with small aperture printing. While these elements are widely considered important, there is no well-known data that show the interaction between the aperture taper and the wall-surface finish. This article gives results of a study conducted to determine the effects of stencil design elements on paste transfer efficiency for small apertures.
Test Vehicle
Test vehicles used for this study were 10" × 13" × 0.062" bare copper boards with four tooling holes used as fiducials. Since X/Y coordinate accuracy of print deposits was not considered in this work, bare copper boards are more than adequate for experimentation purposes. In addition to being economical, they also provide optimum reference points for height measurements, aid visual inspection, have reduced gasketing problems, and are easier to clean and reuse.
Figure 1. Layout of apertures in a cell.
Stencil design. Many CSP pitches range from 7.87 to 19.68 mils, with pad sizes ranging from 8 to 16 mils.3,4 A stencil thickness of 5 mils is standard for CSPs, microBGAs, 0201s, and other SMT components.5 Reducing stencil thickness to 4 mils would increase transfer efficiency; however, it would also decrease the volume of paste deposited, which ultimately affects solder joint reliability. Therefore, a stencil thickness of 5 mils was chosen for the study.
To determine the best combination of taper and electropolish, test stencils were designed with the following geometries: circle, square, home plate, rectangle (5:1) and oblong (5:1). However, only circular apertures were chosen for this study. Low, medium, and high tapers were chosen; and four levels of electropolish - no polish, low polish, medium polish, and high polish -were used. The test matrix consisted of 12 cells with different combinations of taper and electropolish levels. Aperture sizes used were 12, 10, 8, 6, and 4 mils. Each cell consisted of 14 circles per size. The total number of apertures on the stencil, considering all sizes and shapes, was 9,408. The total number of apertures considered in this study (circles only) was 840 (Figure 1). Aperture sizes and shapes cover most stencil printing scenarios for FCs, CSPs, microBGAs, and 0201 applications. To compare the different stencil manufacturing techniques in a single print stroke, three stencils were made with a combination of manufacturing methods. Figure 2 shows a layout of all three stencils used in the study.
Figure 2. Test stencil layout: A, B, & C.
Design features. These combinations of different stencil manufacturing techniques (12 combinations on each of the three stencils) will help compare manufacturing methods under a single stroke, and largely eliminates the source of variations obtained if using a different stencil for each manufacturing technique. Had a different stencil been used for each combination, 36 stencils, instead of three, would have been required, potentially introducing tremendous variability.
Solder paste. A commercially available Type III, no-clean, 63Sn/37Pb solder paste was used throughout the study. The paste had 90% metal loading and a stencil life greater than 8 hrs at 50% relative humidity (RH), 74˚F. The primary reason for choosing Type III over Types IV or V was because it is widely used in assembly processes due to cost advantages and available data. Response variables chosen for the study were volume of paste deposited, transfer efficiency, and ratio of standard deviation to volume.
Optimization Study
A central composite design (CCD) with two factors was used to optimize print speed and pressure. The design was blocked by stroke direction to eliminate its influence on print performance. Optimization was carried out for 6- to 12-mils circular apertures and used Stencil A because it represented an industry standard stencil manufacturing method. Optimization was performed for the apertures in Cell F, with medium taper and low polish. This cell represents typical taper and electropolish settings used in the industry. Fixed and variable factors associated with the design are:
Validation StudyThe main goal of the validation study was to verify optimized parameters obtained using the CCD. To perform the validation study, 30 boards were printed in each print direction. Figure 3 shows the comparison of the predicted vs. actual standard deviation to paste volume. The graph shows agreement between predicted and actual transfer efficiencies and standard deviation for all geometries, confirming the validation of optimized print parameters.
Comparison Study
The main objective of the comparison study6 was to determine an optimum level of taper and degree of electropolish for the three different manufacturing techniques, and compare the print performance of three different stencil fabrication methods for small apertures ranging from 4 to 12 mils. The aperture dimension measurements for electroformed (Stencil B) and chem-etch (Stencil C) stencils might not reveal the true dimensions of the apertures due to the blooming effect and irregular shapes of the apertures. To compensate for this, paste deposit volume on the board was taken as one of the primary response factors, and all data were analyzed with respect to the volume. To compare the effect of taper and electropolish, 30 boards were printed in each stroke direction, and analyses were blocked according to stroke direction to eliminate its influence. The procedure used for the study is shown in Figure 4. Comparison tests were performed using optimized parameters. The same parameters were used for Stencils B and C.
Figure 3. Predicted vs. actual standard deviation/volume.
Statistical comparisons. To determine the optimal combination of taper and electropolish, a test compared the performance mean, or average behavior, of the cells on a stencil. To perform multiple comparison tests of means for different stencil manufacturing conditions, a Tukey’s multiple comparison test was used. In performing this test, data from one cell are compared as a group to data in another cell. If during this comparison one cell is shown to be significantly different from another, then the analysis can stop and a better performing cell can be selected. On the other hand, if one group appears to be inseparable from another group, then a fall-back analysis is performed. Referring to the earlier example, an important design aspect is that multiple cells (manufacturing conditions) exist on the same stencil, all deposits on that stencil are made during the same stroke. This allows us to have paired data. Therefore, if Tukey’s method, which that does not automatically pair data, fails to show a significant difference in the cells’ performance, we can follow that analysis with a paired-t test. One comparison test administered Tukey’s method to determine the volume of paste deposited in 12-mil circles on Stencil A. The cell with the maximum volume of paste is determined from the data. For example, Cell L has the highest mean and; therefore, is the best performer. However, it is necessary to determine if Cell L is statistically the best. The statistically best and worst cells are determined using confidence intervals for each cell. The mean transfer efficiency over 30 boards is compared for each cell.
Cells are compared with other cells, and a confidence interval is generated for each comparison. From this analysis, Cell L is the best-performing cell, and no further analysis needs to be performed because all CIs pertaining to Cell L do not contain zero. Cells that include zero in their confidence interval are not statistically different, as a group-to-group comparison. When considering the comparison of Cell A to Cell B, the confidence interval goes from negative (-30.52) to positive (0.25), and includes zero; therefore, they are not statistically different according to Tukey’s test. A paired-t test is needed to determine if the cells are statistically different considering that the data can, in fact, be paired for analysis. Cell A is compared to other cells, and a paired-t test is performed for combinations whose confidence interval contains zero to determine the best cell(s).
Paired-t Test
In the paired-t test, every board for a cell is compared to the board for another cell, and the difference between the two cells is calculated. If the confidence interval for the combination again contains zero, and the p-value is greater than 0.05, then the cells are not statistically different. Table 1 shows the paired-t test to determine if Cell B and Cell A have statistically different performance for print deposits for 12-mil circles after data are paired.
Figure 4. Experimental procedure compares stencil manufacturing conditions.
The CI and p-value indicate that Cells A and B are statistically different, after pairing their data; the mean volume deposited for 12-mil circles of Cell B is greater than Cell A. Similar analyses are performed for transfer efficiency and standard deviation vs. volume and aperture size can be found elsewhere.6
Conclusion
Multiple comparison tests based on Tukey’s test and the paired-t test were performed to statistically determine the best- and worst-performing cells for different stencil fabrication methods. From the analysis it was found that, for a laser-cut aperture, a high tapered aperture with high electropolish improved stencil performance. However, the effect of electropolish is unclear due to reduced stencil thickness for high electropolished apertures. An increase in the level of taper caused an increase in the volume of paste deposited. This trend was noted for all apertures studied.
For electroformed apertures, a medium tapered cell performed best compared to other electroformed apertures. This trend was observed for all apertures. The effect of taper was more dominant in 10- and 12-mil circles. In the case of the chemical-etching process, over-etched aperture cells performed better than other apertures. However, the better performance of the over-etched apertures can be attributed to the larger board-side diameter.
Among the three stencil fabrication techniques, laser-cutting performed better than both electroformed and chemical-etching processes for 6- to 10-mil apertures. For 12-mil circles, both laser-cutting and electroformed processes showed comparable performance. Future and ongoing studies are planned, and will include:
- Stencils with different levels of electropolish (maintaining the same thickness) to determine the effect of electropolish on print performance of small apertures.
- All experiments in this study were performed on a bare copper board. Future experiments will use an actual PCB (with pads, circuitry, and surface finish).
- Future experiments will include different paste types, such as Types IV and V.
- Ongoing studies will use lead-free SAC pastes.
ACKNOWLEDGMENTS
This article is based on the MS thesis of Srinivasa Aravamudhan.6 A version of this article was originally presented at APEX 2007. For a complete list of figures, contact the authors.
REFERENCES
- Yee, S., “Optimization of Design and Process Parameters for CSP Solder Joints,” APEX 2000, pp. 55-58.
- Pnaik, M., “Process Development for Fine-feature Stencil Printing,” MA Thesis, State University of New York at Binghamton, 2002.
- Paridge, J., et al., “Paste Printing and Characterization for Chip-scale Package Assemblies,” SMTAI 1998, pp. 405-416.
- Solberg, V., et al., “Developing a Repeatable SMT Assembly Process for Chip-scale Packaging,” APEX 2000.
- Burr, D., “Printing Guidelines for BGA and CSP Assemblies,” Surface Mount International Conference, Calif., 1998, pp. 417-424.
- Aravamudhan, S., “Process Development and Characterization of Stencil Printing Process for Small Stencil Apertures,” MA Thesis, State University of New York at Binghamton, 2003.
Rita Mohanty, Ph.D., director of advanced development, Speedline Technologies, may be contacted via e-mail: rmohanty@speedlinetech.com. Daryl Santos, Ph.D., Binghamton University, may be contacted via e-mail: santos@binghamton.edu.