A 0.003˝ stencil provided better paste transfer for miniature passives.

Ed.: Part 1 (in July) summarized the study. Here, the findings are revealed.

Presenting the detailed results and analysis for the current study is beyond the scope of this column. Hence, results from selected areas, representing the board design, are presented here. The test samples contained miniature components such as 01005s and 0201s, and larger components with varied aperture sizes, such as 0402s, QFP 160s and TSOP32s. As the primary goal of the printing experiment was to understand the relationship between board finish, stencil technologies and stencil thickness, only the pad-to-aperture ratio of 1:1 data is presented.

Data analysis. Variability plots for QFP160 and 0201 using the laser-cut stencil DOE showed that with the larger aperture size, absolute volume deposited increases steadily, as expected. We also observed the sample-to-sample measurement is quite consistent. This is a good indication that repeatability within each run order is consistent. Variability plots are particularly useful in determining different source of variations affecting the DOE. They also are useful in determining qualitative results for the DOE. Results for the electroform nickel stencil were very similar to those of the laser-cut stainless steel type.

Print DOE analysis. The stainless steel laser-cut and electroform stencil displayed similar behavior. Pad finish was shown to have the strongest effect on paste transfer efficiency. Next to pad finish, interaction between pad finish and stencil thickness appears to be significant. Stencil thickness is not the factor for QFP160 and TSOP32 as it is for 0402, 0201 and 01005. For miniature components, the thinner stencil appears to have better transfer efficiency than does the thicker stencil, due to a higher area ratio. As expected, absolute paste volume as the response shows stencil thickness has stronger effect. A thicker stencil delivers a higher volume of paste regardless of component type. For both stencil technologies, SPI and visual inspection showed no significant bridging for the components reported here.

Print DOE summary. It can be summarized from the above results, in regard to stencil thickness, that the thinner stencil (0.003˝) provided better transfer efficiency, while the thicker stencil (0.004˝) delivered a higher volume of paste. This is significant for miniature components, because a 0.003˝ stencil can provide adequate volume of paste with better transfer efficiency. Larger volumes of paste may not be desirable due to higher opportunity for assembly defects such as tombstoning and paste squeeze-out (bridging).

DOE analysis showed that regardless of the component type and stencil technologies, OSP finish provided higher transfer efficiency than ENIG. Further investigation is underway to understand the effect of pad finish on transfer efficiency.

In summary, the current study showed that all factors considered in this study (stencil thickness, stencil technologies, and pad finish) show significant effect on the printing process. In addition, complex interactions exist between these factors that need further study.

Board assembly study. Based on the results from the printing DOE, the board assembly study was designed as follows:

Variable factors:

• Stencil technology (laser cut stainless steel and electroform)
• Pad finish (OSP and ENIG)
• Reflow environment (nitrogen and air)

Fixed factors:

• Stencil thickness (0.003˝)
• Paste, Type 4 Alpha OM-350
• Optimum printing parameters (based on stencil technologies)
• Response factors:
• Visual inspection based on IPC-A-610D
• X-ray inspection
• Assembly defects (tombstoning, solderballing, missing components and bridging).

A fractional factorial DOE was conducted for this phase of the study to understand the effect of various land patterns and aperture shapes on component assembly. Three boards were printed per run order using the optimum print parameters with one squeegee stroke direction (rear to front) only. Components were placed on the boards and reflowed according to treatment combination. Reflowed boards were visually inspected to meet IPC-A-610D for solder paste characterization, and then x-ray inspection was performed. Table 4 shows the standard order design table.

Summarized analysis. Reflowed boards were inspected to ensure they met IPC-A-610D. Visual inspection clearly showed the overprinted pads (pads with more than a 100% pad-to-aperture ratio) did not cause bridging after reflow. In other words, the larger pads with excess paste showed good “pull back” characteristics. Visual and x-ray analysis showed that only passive components displayed defects, such as solderballing and tombstoning. Other packages such as QFP and TSOP showed no visual defects. All components showed voiding after reflow. Further investigation is necessary to fully understand the implication of voiding.

DOE analysis showed reflow atmosphere had the strongest effect on assembly defects. Board finish, which was a significant factor for paste transfer efficiency (from the print experiment), did not appear to be significant for board assembly.

Multiple DOEs were performed to understand the effect of various design and process factors on the overall assembly of boards with mixed-size components. Various statistical analyses were performed to determine the effect of stencil technologies, stencil thickness and pad finish on paste transfer efficiency. Results showed the 0.003˝ stencil provided better paste transfer for miniature components, while the larger components were not greatly affected by stencil thickness. In regard to the stencil technologies, laser-cut stainless steel and electroformed performed comparably.

Stronger than the stencil thickness effect was the pad finish. OSP pad finish showed higher paste transfer regardless of component size and stencil technologies. One of the explanations for this phenomenon could be the interaction between pad surface finish and paste chemistry. ANOVA analysis also showed a complex interaction exists between stencil and pad finish.

Based on the printing study, only a 0.003˝-thick stencil was included in the reflow DOE. Since the aperture design incorporated overprinting of larger components, a 0.003˝ stencil was believed better suited to study the effect of overprinting. The results suggest that nitrogen definitely improves the reflow characteristics of the solder paste with miniature components. Larger components had minimum-to-no effect in regard to the reflow environment. Unlike the printing results, pad finish had no significant effect on assembly yield. In regard to the effect of overprinting, results showed good pull back of the paste, causing no bridging. This is clearly encouraging. Visual inspection showed the fillet formation around the larger components appears adequate.

It is encouraging to see the 0.003˝ stencil with creative stencil design (overprinting the larger components) meets IPC-A-610D visual inspection criteria. This is a step in the right direction when dealing with mixed-size components. Additional work is underway to determine if the solder joints truly meet mechanical and long-term reliability requirements. 

Rita Mohanty, Ph.D., is director advanced development at Speedline Technologies (speedlinetech.com); rmohanty@speedlinetech.com.