The need to reduce the size and weight of electronic products is continuing as surface mount technology matures further. Size reduction in both active and passive components, coupled with improved printed circuit board technology, is producing smaller, lighter weight and higher performing end products.
Extensive research and development continues to reduce the size of active packages. Passive components have also been reduced in size to enable designers to use smaller printed circuit boards to perform a given task.
The use of 0603 and 0402 components has been prevalent for a number of years. These component sizes can be run in high volume applications at very high yields. More recently, 0201 components have been implemented in high density applications. The 0201 component is approximately one-quarter the size of an 0402 component and this could reduce the assembly process robustness and yield. This paper presents the results of an ongoing study designed to determine the impact that specific assembly and board design parameters have on assembly yield of these components in a mass reflow scenario. A full factorial experimental design of 27 different attachment pad designs (three levels each for distance between pads, pad width, and pad length) were used to determine the optimum attachment pad design. Five different stencil aperture designs were tested for each attachment pad design. No-clean and water-soluble flux chemistries were tested in both air and nitrogen reflow environments. Component to component spacing was tested at four different levels, at both zero and 90° component orientations.
Stencil thickness, stencil fabrication, attachment pad metallurgy, solder mask type, screen printer process settings, component placement system, thermal profile and reflow system were major parameters that were fixed during the research project.
Market drivers and attributes for 0201 components
Continued miniaturisation of consumer electronics is shrinking component size from 1210 and 1206 in the 1980s down to 0402 and 0201 in the late 1990s. The main driver is the demand for higher performance in smaller packages at the lowest cost. 0201 components are 75% smaller than 0402s in both volume and weight. 0201 components also consume 66% less board area than 0402s. These components can produce significant reductions in size, weight and volume for handheld and portable consumer electronic products. Figure 1 shows the size comparison of 1206, 0805, 0603, 0402 and 0201 components. At high frequencies 0201 capacitors perform better than 0402 for low equivalent series resistance (ESR) and low impedance. A reduction in the dielectric layer thickness and an increase in the layer count allow the 0201 capacitance ranges to be in line with 0402 devices. The capacitance range of 0201 capacitors covers approximately 80% of high frequency module demands.
Conclusions
Of the three assembly processes tested, the no-clean solder paste process reflowed in air produced the fewest number of assembly defects for both tombstones (open solder joints) and solder bridges. This process also produced the most attachment pad designs that were free from assembly defects. Furthermore, this assembly process type was found to be the least sensitive (of the three considered in this study) in influencing the number of solder joint defects across a variety of pad designs.
The water-soluble solder paste process reflowed in air produced the next fewest number of assembly defects, followed by the no-clean solder paste process that was reflowed in nitrogen.
The use of low oxygen levels (under 50 ppm) and more active solder paste flux chemistry decreases assembly yield and assembly robustness. Longer thermal reflow profiles may reduce the number of assembly defects for the water-soluble solder paste reflowed in air and for the no-clean solder paste process reflowed in nitrogen. Higher oxygen content during reflow for the nitrogen reflow process would most likely also reduce assembly defects. The use of nitrogen generally increases solder wetting forces and reduces wetting times. Component side to side spacing of 8 mils was achievable for all three processes without producing solder bridges.
The use of nitrogen during reflow and water-soluble solder paste increases the number of solder bridges. Small attachment pad sizes also tend to solder bridge more readily than larger attachment pad sizes. Combinations of either the smallest attachment pad width or smallest attachment pad length increase the probability of solder bridging. Research is currently under way to test component to component spacing under 8 mils to determine the absolute minimum spacing between components for a given assembly process.
Solder beads can be reduced or eliminated by reducing the amount of solder paste that is printed under the component terminations. It should be noted that the number of tombstones 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 10 mils to 12 mils. 'Home plate' or 'v-notch' stencil designs were not tested because of the small attachment pad sizes for 0201 components.
Component orientation was determined to be insignificant for the no-clean solder paste process that is reflowed in air. Component orientation was statistically significant for the water-soluble solder paste process reflowed in air as well as for the no-clean solder paste process reflowed in nitrogen. Increased flux activity of water-soluble solder pastes, compared to the no-clean solder paste and/or reduced oxygen content during reflow, increases the wetting force and wetting 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.
The following discussion is based on the pad designs shown in Table 1.
Seven attachment pad designs out of the 18 tested for the no-clean solder paste process reflowed in air produced no assembly defects. Attachment pad design BEG was selected as the top choice based on attachment pad size, solder joint quality, and ease of solder paste printing. The BEG design also uses the smallest distance between attachment pads. The wider distance between attachment pads for design CEH was the reason this design ranked second. The preferred attachment pad designs from the other two processes also contained the smaller distance between attachment pads of 9 mils.
The no-clean solder paste process reflowed in air is a more robust process when compared to the other two processes. Fewer numbers of acceptable pad designs are available for the other two processes. Attachment pad design CEG produced the best assembly yield for both the water-soluble solder paste process reflowed in air and the no-clean solder paste process reflowed in nitrogen. The only difference in the design of BEG and CEG is the pad width difference of 3 mil. Increasing attachment pad width and decreasing the distance between attachment pads reduces the amount of component placement accuracy needed and increases the robustness of the placement process. Attachment pad design BEG ranked third for assembly yield for both the water-soluble solder paste process reflowed in air and the no-clean solder paste process reflowed in nitrogen.
Unacceptable assembly yield results were produced from the no-clean solder paste process reflowed in nitrogen for all attachment pad designs. Unacceptable assembly yield results were also produced from the water-soluble solder paste process reflowed in air for all attachment pad designs when both component orientations are considered.
Figure 2 shows a photograph of an 0201 assembled on attachment pad design BEG. The photograph shows the soldering results from the optimised assembly process.
Figure 3 shows the recommended attachment pad design for solder paste mass reflow of 0201 components. The current recommendation is an attachment pad spacing of 9 mil, an attachment pad length of 12 mil and an attachment pad width of 15 mil to 18 mil, dependent upon flux type and solder paste reflow atmosphere. The 18 mil wide attachment pad should be used when nitrogen atmosphere reflow is performed with low oxygen content (under 50 ppm). This pad width should also be used when using solder paste flux binders that are very active and/or have fast wetting times.
The narrower 15 mil wide attachment pad should be used for air reflow and when solder paste flux activity is lower and wetting times shorter. Figure 4 shows the cross section of an 0201 capacitor mounted on attachment pad design BEG. The photograph from the optimised assembly process shows proper solder volume and solder wetting angles. The solder fillets wet 90 to 100% up the face of the component terminations. The photograph also shows that the solder mask between the attachment pads is holding the component off of the attachment pads. The solder mask was measured at approximately 15 mil to 17 mil thick. The solder mask is approximately 1 mil taller than the attachment pads.
Further research is currently under way to test printed circuit boards with no solder mask under the component as well as a thinner (0,7 mil to 1 mil) solder mask thickness which is more typical. The significance of any of the solder mask lifting the component off of the attachment pads will then be determined. Research is also currently under way to further investigate assembly placement accuracy, solder paste flux chemistry, smaller (under 8 mil) component to component spacing and reflow parameter optimisation.
References
1. Prasad, R., 'Surface Mount Technology: Principals and Practice', Van Nostrand Reinhold, New York,1997.
2. Montgomery, D.C., Design and Analysis of Experiments - 4th Edition, John Wiley & Sons, Inc., New York, NY, 1997, p. 51.
The purpose of this article was to present some of the more important observations and conclusions from these experiments. Visit www.dataweek.co.za/+dw2364_p to access the full white paper.
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