Optimizing mass reflow assembly of 0201 components

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 reduced in size to enable designers to use less printed circuit board area for performing a given task. The use of 0603 and 0402 components have 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 particular applications. The 0201 component is approximately one-quarter the size of a 0402 component. The smaller size of the 0201 components, however, could potentially reduce assembly process robustness and yield.

The optimum attachment pad design was determined using a full factorial experimental design of 27 different attachment pad designs (3 levels each for distance between pads, pad width, and pad length). Five different stencil aperture designs were tested for each attachment pad design. No-clean and watersoluble 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 ninety degree component orientation. Stencil thickness, stencil fabrication, attachment pad metallurgy, solder mask type, screen printer process settings, thermal profile, reflow system, and component placement system were major parameters that were fixed during the research project.

Experiment materials & assembly equipment A test board with both 0201 and 0402 components was designed for the experimentation. Figure 1 is a photograph of the 0201 test vehicle. The printed circuit board was a single sided panel that measured 7.5” wide by 12.5” long. The board thickness was a standard 0.062”. Attachment pad metallurgy was bare Copper covered by Entek Plus (OSP).

Half ounce Copper was used for all traces and attachment pads. Taiyo PRS4000 was the solder mask used. Three different attachment pad widths, lengths, and spacing between pads were tested for both the 0201 and 0402 components in a full factorial design, giving a total of 27 different attachment pad designs for both the 0201 and 0402 components. Each pad design was replicated 120 times within a single row. Each row was designated a three letter code based on the attachment pad dimensions from Table 1.



An example for a 0201 attachment pad design would be ADG (pad width A = 0.012”, pad length D = 0.008”, and pad spacing G = 0.009”). Four different component to component spacing 0.008”, 0.012”, 0.016” and 0.020” were tested. Thirty components of a given attachment pad design were blocked together to test component spacing. All attachment pad traces were run out through the ends of the attachment pads, enabling only component spacing testing to be conducted between the components side to side (not end of component to end of component). The test vehicle was designed at both zero and ninety degree component orientation for all designs. A fully populated test vehicle contained 12,960 components. Table 1 lists the 02\01 attachment pad dimensions for all three levels. Figure 2 shows the dimensioning legend for the 02\01 attachment pads.



Two stencils were manufactured for the project. Stencil 1 was designed for the first experiment (filter). Five different stencil aperture openings were tested for each attachment pad design. Stencil 2 was designed based on the results from stencil 1. Only one stencil aperture size was used for a given attachment pad design for stencil 2. Table 2 contains the stencil aperture size and aperture position for stencil 2. Figure 3 shows the three different types of stencil aperture positions that were used relative to the center of the component.

Both no-clean and water-soluble solder paste formulations were used during the project. Both solder paste types were 90% solids and Type IV powder size. One noclean and one water-soluble solder paste were selected to provide for the two most common flux chemistry types. Two different solder paste vendors supplied the two solder paste types. The viscosity of the two pastes was approximately 900 KCPS.

A DEK 265 GSX screen printer was used for all solder paste printing. The following screen printer process parameters were used for all stencil printing:

a) Print speed = 1.0 inch\sec.
b) Squeegee type = metal blades (Transition Automation)
c) Squeegee angle = 60 degree
d) Squeegee pressure = 2.3 pounds\inch of squeegee
e) Print gap = 0 (on contact)
f) Separation speed = 0.02 inch\sec.



All component placement for this project was performed on a Universal 4796R HSP. The machine was equipped with the 0201 option, which includes nozzles, component camera lighting and feeders to handle 0201 components. All components were fed from tape and reel. Two local fiducials were used for board alignment. All solder paste reflow was performed in a Heller 1800W forced convection oven. The reflow system contained 8 heating and 1 cooling zone. Oxygen levels within the oven were less than 50ppm. Figure 4 is the thermal profile that was used to reflow all boards assembled during the project.

Assembly defect inspection was conducted by visual inspection under a semi-automatic optical system. All defects were manually recorded and visually verified.

Results

The project was conducted by performing two experiments. The first experiment, which was a filter experiment, was based on running four different processes. The four processes were no-clean and water-soluble solder pastes run in both air and Nitrogen reflow environments.

Six fully populated boards were assembled for each of the four processes for a total of 311,040 components. Five different stencil aperture sizes/aperture positions were tested for each attachment pad size.

The second and last experiment was based on running only three of the four processes. The water-soluble solder paste reflowed in Nitrogen was dropped. The combination of watersoluble flux chemistry and Nitrogen environment reflow is typically not used. Only one stencil aperture design was run per attachment pad design. Table 2 contains the stencil aperture designs. The stencil aperture design was selected based on assembly yield and assembly quality for experiment 1. All of the largest spacing between attachment pads (I = 0.015”) were dropped from this experiment. This reduced the total number of attachment pads from 27 down to 18 different designs. Data from experiment 1 showed that the widest spacing (I = 0.015”) produced more open solder joints than attachment pads with smaller spacing.




A total of fifty boards were assembled for each of the three processes for a total of 1,116,000 components. Results are shown from Figure 5 to 13.

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. The no-clean solder paste process reflowed in air 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) for 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 reflowedin Nitrogen.


Other observations from the experiments include: Use of low oxygen levels (under 50ppm) 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 increases solder wetting forces andreduces wetting times.

Component side to side spacing of 0.008” 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 lengthincrease the probability of solder bridging.






It was also observed that 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 (open solder joints) 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 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, however, found to be statistically significant for the water-soluble solder paste process reflowed in air as well as for the noclean solder paste process reflowed in Nitrogen. Increased flux activity of water-soluble solder pastes, compared to noclean solder paste and\or reduced oxygen content during reflow, was found to increase the wetting force of molten solder. Results also showed that components oriented at ninety degrees (one termination reaching the reflow zonebefore 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 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 0.009”. The results showed that no-clean solder paste process reflowed in air is a more robust process when compared to the other two processes. Also, fewer numbers of acceptable pad designs are available for the other two processes.





Attachment pad design CEG was determined to produce 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 0.003”. Therefore, it was concluded that increasing the 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.

Future research is planned to further investigate assembly placement accuracy and reflow parameter optimization. This will include testing component to component spacing under 0.008” to determine the absolute minimum spacing between components.

References
Montgomery, D.C., Design and Analysis of Experiments - 4th Edition, John Wiley & Sons, Inc., New York, NY, 1997, p. 51. Acknowledgements The authors would like to thank Shravan Jumani a Graduate Research Associate from Binghamton University for support of data analysis, Dii group for support of test board layout and manufacturing, IRI Alphametals for supplying the stencils; Kester and Alphametals for supplying solder paste.