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Volume Assembly Qualification for 0201 Packages
December 31, 1969 |Estimated reading time: 8 minutes
A qualification vehicle is designed that includes studies of such process parameters as solder-paste printing, component pick-and-place, and solder reflow.
By Mei Wang, Dr. Dongkai Shangguan, M. T. Ong, Fredrik Mattsson, David Geiger and Sammy Yi
By means of a number of experiments on pad-design identification, pick-and-place machine evaluation and assembly process investigation, understandings for 0201 assembly can be summarized as follows.1,2
1. Two types of pad size (non-soldermask-defined) are recommended in previous work.
2. The pick-and-place machines evaluated are able to place 0201-size components side-by-side, as close as 0.004" and at acceptable defect levels.
3. Placement height vs. solder paste location is critical to achieve low defect rates. Thus, any placement machine having the highest ability to control placement height will be recommended.
4. For reflow, low soak time and low peak temperatures will help reduce the number of components skewing.
Based on the experience, a Qualification Vehicle (QV) is designed and more experiments performed to further understand the 0201 assembly process. These experiments include assembly with solder paste and components from various vendors.
Figure 1. The 0201 qualification vehicle. A double-sided panel, four boards with a cell phone form factor are designed into it.
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Qualification Vehicle Design
As shown in Figure 1, the 0201 QV is a double-sided panel with mirror images. The outside dimension is 5 x 7" with 0.030" thickness. Four boards with a cell phone form factor are designed into this panel. For 0201s, two types of pad design (Pad U and Pad H) also are used; boards A and B are designed with Pad U, boards C and D with Pad H. In addition, Boards A and C will test 0201-to-0201 spacings (0.006, 0.008 and 0.010"), while Boards B and D will test the spacing between 0201 and other components. The same spacings (0.006, 0.008 and 0.010") will be tested for between 0201 and 0402, while between 0201 and chip scale and SO8 packages, an 0.008" spacing is designed. Finally, with a 0.005"-thick stencil, the Area Ratio (AR) is 0.60 for Pad U and 0.74 for Pad H. (AR is the ratio of the aperture opening area to that of the aperture wall.)
On the top side of the QV there are total 5,728 components, including 5,092 locations for 0201s, 624 locations for 0402s, eight locations for SO8s and four locations for CSPs. The 0201 resistors from two vendors and 0201 capacitors from two suppliers are distributed equally in the pick-and-place program.
Experimental Work
No-clean solder pastes from three vendors are selected for this experiment. All are particle Size 3 with a mesh size of 25 to 45 µm and with metal contents from 90 to 90.25 percent (by weight). An electroform stencil of 0.005" thickness and 1:1 ratio (between the aperture size and pad size) is used.
A standard assembly line consists of a solder paste printer, a turret pick-and-place machine, and a nine-zone reflow oven. During the experiment, solder paste height, area and volume for each board are measured by using an automatic measurement machine, and the process control (Cpk) for solder paste volume also is calculated. Manual inspection is performed after printing, pick-and-place, and reflow. At least 35 panels are assembled for each solder paste, yielding a total of more than 555,000 0201s and 68,000 0402s in the experiment.
Results and Discussion
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Post-printing Data Analysis. As can be seen from Table 1, for all types of solder paste, on both Pad U and Pad H, the average height is about 0.0055" with 0.005"-thick stencil, and the average solder paste volume transfer ratio is 65 to 70 percent for Pad U and 84 to 89 percent for Pad H. The higher the AR, the higher the Cp of solder paste volume. During visual inspection, no bridging or missing solder paste is observed in any of the printed boards in this experiment, indicating that the printing process is under control during the entire experiment. No misalignment above 20 percent of pad width is observed in either X or Y direction.
Figure 2. A location with a missing component. The part actually had been placed but did not remain fixed.
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Post Pick-and-Place Inspection reveals that missed components is the primary defect. A typical appearance of a location with a missing component is presented in Figure 2, showing that the component had been placed onto the solder paste but did not remain in place.
Post-reflow Analysis on Pad U. Figure 3 shows that when using solder paste from vendor A, five types of defects are observed, such as component missing, billboarding, bridging, tombstoning and off-pad, with component missing and tombstoning being the main defects. Compared with the results from solder paste vendors B and C, more defects of missing components and tombstoning are observed with solder paste A. (In a previous solder paste study, the experiment data had revealed that a higher tack force was observed with solder paste B than with solder paste A. The tack test procedure was based on IPC-TM-650 and the tack force was measured up to eight hours.) Accordingly, the reason for missing components may be due to the relatively low tackiness for solder paste A than that for paste B.
Figure 3. Defects charted on Pad U using solder paste from vendor A exhibits five types of defects with missing component and tombstoning being the primary faults.
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Figure 3 also indicates that there were many more missing component defects from locations with 0201 capacitors than with 0201 resistors, probably due to the height and weight difference between these two parts. As shown in Figure 4, the solder paste height before placing a component is about 0.0055". The average height is about 0.012" for a 0201 capacitor and about 0.009 to 0.010" for a 0201 resistor. The average weight is about 0.00028 g for a 0201 capacitor and about 0.00014 g for a 0201 resistor. During the pick-and-place process, when a 0201 resistor is placed into the solder paste about two-thirds of its body will sink into the solder paste while only about half of the 0201 capacitor body will settle into the material. With air blow or board movement, a 0201 capacitor would be more likely to fall out of its correct location in the solder paste, causing the defect of missing component.
Figure 4. A 0201 component location with solder paste. Because of less "sink-in" on placement, a 0201 capacitor is more likely to fall out of its location that its resistor counterpart.
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With respect to the bridging defect, more than 98 percent come from locations with 0.006" spacing with solder paste from vendor B, while 100 percent of bridging defects come from locations with 0.006" spacing with solder paste from A and C, most likely due to more solder paste being deposited when printing with solder paste B.
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More defects concerning 0201 capacitors (missing) are observed with components from vendor C than from vendor A.
Figure 5. Solder paste chart from vendor A, Pad H shows a similar trend to Pad U, i.e., more missing component defects are observed using solder paste from vendor A.
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Post-reflow Analysis on Pad H ( Figure 5) shows a similar trend to Pad U. More defects of missing components are observed with solder paste from vendor A, and most from 0201 capacitors rather than 0201 resistors (especially from component vendor C). More defects of bridging were found with solder paste from vendor B than the other vendors.
Post-reflow Analysis on Pad U vs. Pad H. For all type of defects, fewer are observed on Pad H as compared with Pad U. Reason: the pad size (and therefore the aperture size) for Pad H is larger than that of Pad U.
Solder Paste Comparison. As shown in Table 2, regardless of component types, the results from assembly with solder paste B exhibit the lowest defect level, with solder paste A having the highest defect level, probably due to the different tackiness.
Based on previous experience, after a printer idles for more than five minutes, solder paste will exhibit a release issue. Solution: it will need to print a couple of boards before achieving a consistent print. Solder paste C has less of that problem than displayed by solder paste B.
Component Type. As shown in Table 3, for different component types, 0201 resistors display fewer defects than 0201 capacitors. The latter from component vendor C have twice as many defects as those from component vendor A.
To understand the relationship between the defect level and 0201 capacitors from different vendors, another experiment is performed: Using a different assembly machine, five types of capacitors from four 0201 component vendors (Comp A, C, E and F) are tested. Two types are tested from Comp C, labeled as "Comp C(1)" and "Comp C(2)." The only difference is that Comp C(2) has a smaller spacing between it and the pocket. Twenty-three 0201 test panels are used in this study. Five types of capacitors are equally distributed in panels A and C and a total of 82,800 capacitors are assembled, with 16,560 components for each type of capacitor.
Figure 6. Chart shows the number of defects during the pickup process vs. component type, or 0201 capacitors from different suppliers.
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The pickup errors for each type are presented in Table 4 and Figure 6. As can be seen, there are mainly three types of defects: mispick, component pickup in vertical position and camera-recognition error. No component drop is detected for any type of component tested. Results: Comp A shows the best pick-up. The pocket-size difference between Comp C(1) and C(2) shows some improvement on the pick-up process. However, the improvement is not very significant.
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Figure 7. The number of defects after reflow vs. component type, or 0201 capacitors from different suppliers. See Tables 5 and 6.
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All boards are inspected post-reflow and five types of defects are observed (Table 5 and Figure 7). The defect level from component F and C(1) is lower than 100 DPMOs. The final comparison (Table 6) is made by combining the results from post pick-up and post-reflow.
Summary
In this work, a QV is designed for 0201 packages including more than 5,000 locations for 0201s and 1,400 locations for 0402 components, with the pad-to-pad spacing for 0201 components ranging from 0.010, 0.008 to 0.006". No-clean solder pastes and 0201 components from several different vendors are evaluated. Detailed studies are carried out on important process parameters, including solder paste printing, component pick-and-place and reflow. Based on this study, 0201 capacitors show a higher defect level than that of 0201 resistors, and solder pastes with higher tackiness exhibit a lower defect level for 0201 assembly.
Mei Wang, Dr. Dongkai Shangguan, M. T. Ong, Fredrik Mattsson, David Geiger, and Sammy Yi may be contacted at Flextronics, 2090 Fortune Dr., San Jose, CA 95131; (408) 576-7000; Web site: www.flextronics.com.
ACKNOWLEDGEMENT
The authors recognize the assistance of their colleagues at Flextronics worldwide as well as engineers and managers from several suppliers for their help and support during the course of this project. This article was originally published at SMTA International.
REFERENCES
1. M. Wang, D. Shangguan, D. Geiger, F. Mattsson, S. Yi, "PCB Design Optimization of 0201 Packages for Assembly Processes," IMAPS 2002, Dallas.
2. M. Wang, D. Geiger, K. Nakajima, D. Shangguang, C. C. Ho, S. Yi, "Investigation of Printing Issues and Stencil Design for 0201 Package," SMTA 2001, Chicago.