To investigate the effect of these factors on yield, R. Wayne Johnson, Ph.D., and a team of researchers at Auburn University (Auburn, AL) designed a test board specifically for 01005s. The team then performed experiments to optimize the printing, placement and reflow processes. The results of the study were presented in February at the APEX show in Anaheim, CA.

The pads on the test board were various sizes and shapes. Most were rectangular; others were shaped like home plate on a baseball diamond.

The team also designed the board to test other variables, including:

• laser-drilled vias (no vias, one via or two vias-one in each pad).
• spacing between resistors.
• resistor orientation (0, 45 and 90 degrees).
• solder mask misalignment.

Paste Parameters

For paste printing, the team investigated four variables: stencil thickness, squeegee pressure, squeegee speed and stencil-board separation speed. For each set of experiments, the team printed three boards using a fully automatic printer and a metal squeegee blade.

The team used a lead-free solder paste with a metal content of 88.5 percent and a mesh size of 400. The paste was 96.5 percent tin, 3 percent silver and 0.5 percent copper. Earlier studies conducted by researchers at Indium Corp. of America (Utica, NY) have found that tombstones are much more likely to occur when using lead-free solder paste with a silver content of 3.5 percent or more. The Indium researchers found that increasing the silver content in solder paste reduces wetting speed at the onset of reflow.

To print the paste, the Auburn researchers tested two electroformed stencils, one 3 mils thick and the other 4 mils thick. The aperture shape was the same as the pad shape, and the ratio of aperture size to pad size was 100 percent.

Stencil thickness was the only variable that had an obvious effect on the transfer efficiency of the paste, says Johnson, who is a professor of electrical engineering and director of Auburn's Laboratory for Electronics Assembly and Packaging. The 3-mil thick stencil produced a better transfer efficiency than the 4-mil thick stencil. Separation speed had a slight effect on transfer efficiency. Quicker separation contributed to higher transfer efficiency. Printing speed had a minor effect on transfer efficiency, while squeegee pressure had an insignificant effect.

In the end, the researchers found that the optimal printing parameters were:

• stencil thickness, 3 mils.
• printing speed, 2 ips.
• squeegee pressure, 14 pounds.
• separation speed, 0.01 ips.

To ensure that solder paste doesn't stay in the stencil apertures after printing, assemblers should also pay attention to the aperture area ratio, adds Joe Belmonte, project manager with Speedline Technologies Advanced Process Group (Franklin, MA). The aperture area ratio is the ratio of the area of the aperture opening to the area of the aperture walls. For reliable printing of very small deposits, this ratio must be 0.6 or greater.

Placement Accuracy

Like 0201s, 01005s are supplied on regular 8-millimeter tape, with 2-millemeter spacing between pockets. On the tape, the components aren't always in the correct orientation for placement. The components may be upside down or standing on edge.

Edge-standing components can damage the nozzle tip during the pick-and-place operation. In addition, the placement machine may deposit edge-standing components as is. The width and thickness of these tiny components are so similar that the placement machine's vision system may be unable to tell the difference. For the same reason, the vision system may not detect upside down components, since they have the same length and width as correctly oriented components.

The nozzle on the pick-and-place head must be specifically designed to handle 01005s. Because the component and the nozzle tip opening are so small, vacuum sensing cannot be used to verify that a part has been picked up.

Maintenance of these small, precise tips is important, says Johnson. When the nozzle is worn, the surface of the tip becomes shiny, and the vision system may occasionally recognize the shiny tip as an 01005. To avoid missing parts, tip wear should be monitored closely.

To avoid distorting tiny paste deposits, 01005s must be placed with very light pressure. As a result, it's important for the solder paste to have good tack force. A paste with a tack force of 0.98 newton is sufficient to maintain the position of the components throughout the assembly process.

The number and type of board fiducials also affect placement accuracy. The Auburn team compared placement accuracy using four different boards: one with two round, global fiducials; one with three round, global fiducials; one with two round, local fiducials; and one with three round, local fiducials. The researchers found that using three local fiducials produced the best placement accuracy. Local fiducials better accommodate area variations within the PCB caused during fabrication, such as board stretch, overetching and underetching, Johnson says.

Like other surface-mount devices, 01005s will self-align during reflow. In a paper presented at the SMTA International Conference in Rosemont, IL, in September 2004, Fredrik Mattson and a team of engineers from Flextronics (Singapore) reported that the placement of 01005s could be off by as much as 45 microns and still be well-aligned after reflow. Similar results were reported by Chrys Shea and colleagues from Cookson Electronics Assembly Materials (Jersey City, NJ) at SMTA International in September 2005. In addition, they found that 01005s will self-align despite theta offsets as large as 25 degrees. Nevertheless, the researchers advise assemblers to keep the theta offset at 10 degrees or less.

Reflow and Inspection

For lead-free soldering of 01005s, a nitrogen atmosphere is better than air. Solder paste deposits for 01005s are so small that most of the solder alloy is exposed to the reflow atmosphere. This can lead to surface oxidation in an air environment.

The Auburn researchers tested two lead-free reflow profiles for 01005s: a quick-ramp profile and a soak profile. Both profiles resulted in some voiding, primarily in the fillet, but also under the resistor terminals.

After reflow, the researchers inspected all the boards. Defects included bridging, tombstones, edge-standing parts, missing parts and upside-down parts. In all, the researchers detected 2,296 defects out of a total of 28,920 resistor placements, for a defect rate of 7.94 percent. Bridging and tombstones were the most common defects, accounting for 68 percent of the total.

More than 90 percent of the bridging defects occurred among the 01005s separated by a space of 4 mils. The remaining bridging defects were found among 01005s separated by 5 mils. No bridges occurred among chips separated by 6 mils or more. Pads shaped like home plate were more likely to show bridging than rectangular pads.

Tombstones were most often linked to pads with vias, especially if only one of the two pads had a via. Although the orientation of the resistors had no affect on the number of tombstones that occurred, the tombstone rate increased significantly as the pad size increased. A pad 7.2 mils wide and 7.8 mils long produced the least amount of tombstones. Solder mask misalignment increased the rate of tombstones, and the quick-ramp profile produced fewer tombstones than the soak profile.