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Mass Reflow Assembly 0201 of Components
December 31, 1969 |Estimated reading time: 16 minutes
By Jim Adriance and Jeff Schake
Combining flux type and reflow environment has the greatest effect on the number of defects produced. Boards with no-clean solder paste reflowed in an air atmosphere exhibit the best yields with the highest tolerance for attachment pad dimension variation.
The need to reduce the size and weight of electronic products continues as SMT advances. Size reductions of passive components, coupled with improved printed circuit board (PCB) technology, produces smaller, lighter and higher performing end products. Extensive research and development continues to reduce the size of active packages, as well.
Figure 1. The 0201 test vehicle.
Smaller passive components enable designers to use more compact PCBs to perform a given task. Using 0603 and 0402 components, for example, has been prevalent for numerous years these parts can be run in high-volume applications at very high yields. More recently, 0201 components have been implemented in high-density applications. These parts are approximately one-quarter the size of 0402 components and thus could reduce assembly process robustness and yield. This article describes the results of an ongoing study to determine the impact that specific assembly and board design parameters have on assembly yield of 0201-size components in a mass reflow setting.
Twenty-seven different attachment pad designs (three levels each for distance between pads, pad width and pad length) are used to determine the optimum attachment pad configuration, and five different stencil apertures are tested for each pad design. No-clean and water-soluble flux chemistries also are tested in both air and nitrogen reflow environments. Component-to-component spacing is tested at four levels at both 0° and 90° component orientations. Stencil thickness, stencil fabrication, pad metallurgy, soldermask type, stencil printer process settings, component placement system, thermal profile and reflow system are the project's major fixed parameters.
Materials and Equipment
A test board with both 0201 and 0402 components is designed for the experiment; Figure 1 displays the 0201 test vehicle.
The PCB is a single-sided panel measuring 7.5 x 12.5", and board thickness is a standard 0.062". Attachment pad metallurgy is bare copper covered by an organic solderability preservative; 0.5 oz copper is used for all traces and attachment pads.
Figure 2. Attachment pad dimensioning. A fully populated test vehicle contains more than 12,000 components.
Three different attachment pad widths, lengths and spacings between pads are tested for both the 0201 and 0402 components, giving 27 total attachment pad designs for both the 0201 and 0402 components. Each pad design is replicated 120 times within a single row with each row designated by a three-letter code based on the pad dimensions from Table 1.An example for an 0201 attachment pad design is ADG (pad width A = 0.012", pad length D = 0.008" and pad spacing G = 0.009"). Component-to-component spacings of 0.008, 0.012, 0.016 and 0.020" are tested. Thirty components of a given pad design are blocked to test component spacing. All traces are run out through the ends of the attachment pads, enabling component spacing testing to be conducted between the component side-to-side (not end-of-component to end-of-component). Hence, a fully populated test vehicle contains 12,960 components. Figure 2 shows the dimensioning legend for the 0201 attachment pads.
Stencils
All solder paste printing is conducted using 0.005"-thick, stainless steel, laser-cut stencils that are neither micro-etched nor surface-finish-plated. A thickness of 0.005" is selected as a compromise between a 0.004 and 0.006"-thick stencil the thinner version provides a better release for 0201 paste deposits. The 0.006"-thick stencil is rejected because of an unacceptable solder paste transfer that would result with 0201 components. The metal mask is then "center justified," i.e., mounted in a 29 x 29" stencil frame.
Two stencils are manufactured for the project. Stencil 1 is designed for the first (filter) experiment, and five different aperture openings are tested for each attachment pad design. Stencil 2's design is based on the results from stencil 1; only one stencil aperture size is used for a given pad design for stencil 2. Table 2 lists aperture size, distance between solder deposits (stencil apertures) and aperture position for stencil design 2. Figure 3 shows the three stencil aperture positions used relative to the center of the component.
Solder Paste
No-clean and water-soluble solder paste formulations are used during the project; both are 90 percent solids containing Type IV powder size. One no-clean and one water-soluble solder paste are selected to provide for the two most common flux chemistry types. Paste viscosity is approximately 900 Kcps.
The following process parameters are used for all stencil printing*:
- Print speed = 1.000" per second
- Squeegee type = metal blades
- Squeegee angle = 60°
- Squeegee pressure = 2.3 lb per inch
- Print gap = 0 (on contact)
- Separation speed = 0.020" per second.
All component placements are performed on a machine** equipped with the 0201 option, which includes nozzles, component camera lighting and feeders to handle these components. Two local fiducials ensure board alignment. All components are fed from tape-and-reel while automatic component pickup correction in the X and Y axes is constant.
All solder paste reflow was performed in a forced-air convection oven containing eight heating and one cooling zones.*** Oxygen levels in the reflow zone are 50 ppm or less. Figure 4 is the thermal profile used to reflow all boards assembled during the project. Assembly defect inspection is conducted visually under a semiautomatic optical system with all defects manually recorded and verified.
Results
Two experiments are performed:
- The first, a filter experiment, is based on running four different processes in which no-clean and water-soluble solder pastes are run in both air and nitrogen reflow environments. Six fully populated boards are assembled for each of the four processes for a total of 311,040 components. Five stencil aperture sizes and positions are tested for each attachment pad size.
- The second experiment is based on running only three of the four processes. The water-soluble solder paste reflowed in nitrogen was dropped, as this combination typically is not used. Only one stencil aperture design is run per pad design (Table 2).
Figure 3. Stencil aperture position relative to attachment pad (component center positions shown).
The aperture design is selected based on assembly yield and quality from the first experiment. The largest spacings between attachment pads (I = 0.015") were dropped from this experiment, reducing the attachment pads from 27 to 18 different designs. Data from the first experiment show that the widest spacing produces more open solder joints than on attachment pads with smaller spacings. Fifty boards are assembled for each of the three processes for a total of 1,116,000 components.
Figure 5 shows the assembly yield from the three assembly processes. The no-clean solder paste reflowed in air produces the fewest assembly defects 66. The water-soluble solder paste reflowed in air produces the next lowest defect number at 1,499. And the no-clean solder paste process reflowed in nitrogen produces the greatest number of assembly defects 5,665.
Figure 5 shows that assembly defects increase when a nitrogen atmosphere reflow is used and when solder paste flux activity is increased (water-soluble solder paste).
Figure 4. Thermal profile used for all boards assembled for the project.
Figure 6 shows the assembly failure-mode distribution for each of the three processes. Tombstones (open solder joints) and solder bridging are the main assembly defects. Figure 6 shows that the water-soluble solder paste process reflowed in air produces the lowest percentage of solder bridges at 7.0 percent, followed by the no-clean solder paste process reflowed in nitrogen at 15.0 percent. The no-clean solder paste process reflowed in air produces the largest percentage of solder bridges at 21.0 percent.
Figure 7 shows the relationship between solder bridging and component-to-component spacing for the three assembly processes. It also shows that no solder bridging is recorded for any of the assembly processes at a spacing of 0.012" or larger, and that no-clean solder paste reflowed in air produces the fewest solder bridges 14.
The water-soluble solder paste reflowed in air produces the next largest number of solder bridges at 99. The no-clean paste reflowed in nitrogen produces the greatest number of solder bridges 866.
Figure 6. Assembly failure-mode distribution by assembly process. Tombstones (opens) and solder bridging were the two main assembly defects.
Twelve attachment pad designs out of 18 produce no solder bridges at the smallest spacing of 0.008" for no-clean solder paste reflowed in air. Ten attachment pad designs out of 18 also produce no solder bridges at the smallest spacing of 0.008" for water-soluble solder paste reflowed in air. And 6 pad designs out of 18 likewise produce no solder bridges at the smallest spacing of 0.008" for no-clean solder paste reflowed in nitrogen.
Attachment pad design AEG (L = 0.012", W = 0.012", S = 0.009"), which contains the widest distance between solder paste deposits (0.016"), produces the fewest solder beads. Solder beads are reduced when the distance between solder paste deposits is increased. The amount of solder paste displacement by the component during placement is reduced when the distance between deposits under the component is increased.
Component Orientation
An analysis of paired samples determines whether component orientation (0° and 90°) significantly influences assembly yield. 0° orientation is represented by both component terminations going through the oven at the same time (parallel to the source of heat). 90° orientation is represented by one component termination passing through ahead of the second termination. The hypotheses tested were:
- Null hypothesis, Z = 0: There is no statistically significant difference in the number of assembly defects between the 0° and 90° orientations.
- Alternate hypothesis, Z ¬ 0: There is a statistically significant difference in the number of assembly defects between the 0° and 90° orientations.
- The "t" test used: 1 t = (√n x u)/s.
The p-value for the no-clean paste process reflowed in air is 0.5765. Given the high value of p, the null hypothesis cannot be rejected. Hence, the no-clean solder paste process reflowed in air shows no significant difference in assembly yield when considering component orientation. The lower flux activity of the no-clean paste when reflowed in air does not increase the risk of tombstoning.
The p-value for the water-soluble paste process reflowed in air is 0.001959. Given the low value of p, the null hypothesis is again rejected. The increased flux activity in the water-soluble paste, when compared to the no-clean material, produces a significant increase in tombstoning for the components oriented at 90°.
Figure 5. 0201 assembly yield per process type. The no-clean solder paste reflowed in air produced the fewest assembly defects (66).
The p-value for the no-clean solder-paste process reflowed in nitrogen is 0.000002. Given the very low value of p, the null hypothesis is rejected. Nitrogen use increases the number of tombstoned components in the 90° orientation. The majority of open solder joints are on the component termination that is reflowed second (trailing termination). Nitrogen use increases the surface tension of the molten solder and produces open solder joints at a significantly higher rate for components oriented at 90° vs. those at 0°.
Pad Design Defects
Figure 8 shows the assembly defects by attachment pad design for no-clean solder paste reflowed in air. Seven attachment pad designs (BDH, BEG, BFG, BFH, CDH, CEH and CFH) out of 18 produce no assembly defects. Based on paste printing degree of difficulty, solder joint shape quality and attachment pad size, designs BEG and CEH are preferred. The smallest pads require a smaller aperture design that tend to clog faster than a larger stencil aperture. Stencils designed at 0.004" thick will reduce 0201 stencil clogging, but other surface mount devices that require more solder may result in marginal or insufficient solder volume. Solder joint fillet shape on the smallest attachment pad designs did not produce the desired concave appearance. The largest pad designs are good in solder paste release from the stencil aperture and also produce acceptable solder joint fillet shapes, but naturally require more board space.
Figure 7. Solder bridging defects by component-to-component spacing and assembly process type. No bridging was recorded for any component spaced at 0.012" or wider.
Figure 9 shows the assembly defects by attachment pad design for water-soluble solder paste reflowed in air, which produces defects on all attachment pad combinations when considering both component orientations. Attachment pad CEG produces the fewest assembly defects. Attachment pad CDH produces no defects in the 0° orientation but did result in a relatively high number of assembly defects for components in the 90° orientation. Attachment pad CEG produces good solder joint shapes and does not take up as much PCB space as the larger pad designs. Also, solder paste clogging in the stencil aperture is not a problem for the CEG attachment pad design.
Figure 8. Assembly defects by attachment pad design for no-clean solder-paste reflowed in air. Seven of 18 pad designs were defect-free.
Figure 10 shows the assembly defects by attachment pad design for no-clean solder paste reflowed in nitrogen, which produces defects on all attachment pad combinations when considering both component orientations. Attachment pad CEG produces the fewest assembly defects. Attachment pad design CEG also exhibits good solder joint shape and, naturally, requires less board space than larger designs. Solder paste clogging in the stencil aperture does not represent a problem for the CEG pad design.
Pad Dimension Factor
Figure 11 shows the number of solder joint defects tracked by attachment pad width and assembly process type. These data are generated based on the optimal pad designs with respect to each assembly process type, holding the corresponding attachment pad length and spacing parameters constant, and varying their width across all experimental levels.
Figure 9. Assembly defects by attachment pad design for the water-soluble solder paste reflowed in air. Here, defects were produced on all pad combinations when both component orientations were considered.
Generally, for the three assembly process types, yield improves as attachment pad width increases. Similarly, among all assembly process types, the defect levels are more sensitive to pad widths between 0.012 and 0.015". For both the water-soluble process reflowed in air and the no-clean process reflowed in nitrogen, the minimum number of solder joint defects are achieved at the highest level (0.018") of attachment pad width. This trend changes slightly for the no-clean process reflowed in air, where the best yield actually is produced at the intermediate level (0.015") attachment pad width.
However, because of the limited number of defects found across the boards built by this assembly process type, the difference in the defect levels between attachment pad widths of 0.015 and 0.018" is found not to be statistically significant.
Figure 10. Assembly defects by attachment pad design for no-clean solder paste reflowed in nitrogen.
Yield produced by the no-clean process reflowed in air is the least sensitive to attachment pad width, while the no-clean process reflowed in nitrogen is the assembly process type most sensitive to width variation.
Figure 12 displays the number of solder joint defects occurring as a function of attachment pad length and assembly process type. These data are generated based on the optimal pad designs with respect to each assembly process type, holding the corresponding pad width and spacing parameters constant, and varying pad length across all experimental levels.
Figure 11. Assembly defects by attachment pad width and assembly process type. Generally, for the three process types, yield improves as pad width increases.
The plotted results suggest that the optimal attachment pad length is the intermediate level of 0.012" for all three assembly process types. Generally, the largest impact on yield is shown to occur between the low and intermediate pad length levels of 0.008 and 0.012". The no-clean process reflowed in nitrogen, having more substantial dependence on attachment pad length than any other assembly process type, clearly is the most sensitive process. No defects are observed on any of the boards assembled with a no-clean process and air reflow for both the intermediate and high-level attachment pad lengths of 0.012 and 0.016".
Figure 12. Assembly defects by attachment pad length and assembly process type. Results suggest that optimal pad length is at 0.012" for all process types.
Figure 13 shows the relationship between solder joint defects, pad spacing and assembly process type. These data also are generated based on the optimal attachment pad designs with respect to each assembly process type, holding the corresponding pad width and length parameters constant, and varying pad spacing across both experimental levels. The three assembly process types all give similar defect trends, with more solder joint failures occurring at the larger (0.012") attachment pad spacing. The combination of no-clean process and nitrogen reflow environment is the assembly process type most likely to affect yield with changes in the attachment pad spacing.
Conclusion
- Of the three assembly processes tested, the no-clean solder-paste process reflowed in air produces the fewest number of assembly defects (tombstones and solder bridges). The no-clean process in air also produces the most attachment pad designs free from assembly defects. Furthermore, this assembly type is found to be the least sensitive of the three in influencing the number of solder joint defects across various pad designs.
- The water-soluble solder paste process reflowed in air produces the next fewest number of assembly defects, followed by the no-clean solder paste process reflowed in nitrogen. Low oxygen level use (under 50 ppm) and more active solder paste flux chemistry decreases assembly yield and robustness. Longer thermal reflow profiles may reduce the number of assembly defects for the water-soluble paste reflowed in air and the no-clean paste reflowed in nitrogen. Higher oxygen content during reflow for the nitrogen process would most likely also reduce assembly defects. Nitrogen use generally increases solder wetting forces and reduces wetting times.
- Component side-to-side spacing of 0.008" is achievable for all three processes without producing solder bridges. Nitrogen use during reflow of water-soluble solder paste increases the number of solder bridges, however. Small attachment pad sizes also bridge more readily than larger pad sizes. Combinations of either the smallest pad width or smallest pad length increase the probability of solder bridging. Research is under way to test component-to-component spacing under 0.008" to determine the absolute minimum spacing for a given assembly process.
- Solder beads can be reduced or eliminated by decreasing the amount of solder paste printed under the component terminations. It should be noted that the number of tombstoned components increases as the distance between solder paste deposits increases. When designing the stencil, the distance between stencil apertures should be held to a maximum of 0.010 to 0.012". "Home plate" or "v-notch" stencil designs were not tested because of the small size of the attachment pads.
- Component orientation is insignificant for the no-clean solder paste process reflowed in air, but statistically significant for the water-soluble paste process reflowed in air, as well as for the no-clean process reflowed in nitrogen. Increased flux activity of water-soluble solder pastes, compared to the no-clean variety or reduced oxygen content during reflow, increases the wetting force or speed of molten solder. Components oriented at 90° (one termination reaching the reflow zone before the other) are more likely to tombstone when higher wetting forces and reduced wetting times are experienced.
- Seven attachment pad designs out of the 18 tested for the no-clean solder paste process reflowed in air produce no assembly defects. Attachment pad design BEG is selected as the top choice based on pad size, solder joint quality and paste printing ease. The BEG design also uses the smallest distance between attachment pads (the wider distance between pads for design CEH is the reason this design ranked second). The preferred attachment pad designs from the other two processes also contained a smaller distance (0.009") between attachment pads.
Figure 13. Assembly defects by attachment pad spacing and assembly process type. For all process types, more solder joint defects occur at the larger pad spacing.
Attachment pad design CEG produces the best assembly yield for both the water-soluble solder paste process reflowed in air and the no-clean process reflowed in nitrogen. The only difference in the design of BEG and CEG is the pad-width variance of 0.003". Increasing pad width and decreasing distance between pads reduces the amount of component placement accuracy needed, as well as increases placement process robustness. Attachment pad design BEG ranks third for assembly yield for both the water-soluble solder paste process reflowed in air and the no-clean process reflowed in nitrogen. Unacceptable assembly yield results are produced from the no-clean process reflowed in nitrogen for all attachment pad designs. Unacceptable assembly yield results are produced from the water-soluble paste process reflowed in air for all pad designs when both component orientations are considered.
This article is adapted from a presentation originally given at APEX 2000.
*DEK 265 GSX.** Universal 4796R HSP.*** Heller 1800W.
ACKNOWLEDGEMENTS
The authors thank Shravan Jumani of Binghamton University for support of data analysis, Dii Group for support of test board layout and manufacturing, IRI Alpha Metals for supplying the stencils, and Kester and Alpha Metals for furnishing the solder pastes.
REFERENCES
- R. Prasad, Surface Mount Technology: Principals and Practice, Van Nostrand Reinhold, New York, 1997.
- D.C. Montgomery, Design and Analysis of Experiments 4th Edition, John Wiley & Sons, Inc., New York, 1997, p. 51.
JIM ADRIANCE and JEFF SCHAKE may be contacted at Universal Instruments Corp., P.O. Box 825, Binghamton, NY 13902-0825; (607) 779-5298; Fax: (607) 779-4646; E-mail: adriance@uic.com.