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It should be noted that the equipment used in these experiments was subject to manufacturers’ regular calibration procedures with no special tuning. GR&R studies were not performed on the printer or placement machine prior to the execution of the tests. Actual print alignment and placement accuracy are assumed to be within the manufacturer’s specifications and exhibit typical positional drift from programmed positions within these specifications.

Two placement offset cases were studied: nominal (0,0) and +0.1 mm (4 mil) in both X and Y directions concurrently (+0.1, +0.1). Placement offsets were not subdivided into individual axis shifts (0, +0.1) and (+0.1,0), but print alignment offsets were performed with all four combinations. The combination of paste offset in X with placement offset had a tremendous effect on defects, but the combination of paste offset in Y with placement offset did not. The offset in X was in the positive direction, which is also the leading edge of the device as it enters the reflow zone. The effect witnessed here is likely similar to the effect of component orientation. Note that offsets were not tested in opposing directions, i.e. paste in negative X and placement in positive X and Y or vice versa, but it is assumed that it would hurt the tombstoning rates rather than help them.

The remainder of variables that exhibited large influences on tombstoning were singular or main effects, and not as dramatic as the combined effect of the paste and placement offsets. The second most influential factor was component type. Capacitors exhibited a defect rate of 15,996 ppm (14,316 ppm excluding solder balls), while resistors exhibited a defect rate of 7,650 ppm (1,374 ppm excluding solder balls) (Figure 11).

Capacitors consistently showed higher defect rates in all categories except solder balls. The main differences between capacitors and resistors are their heights and termination geometries. Capacitors are 25% taller than resistors, and have a higher center of gravity, which can make them more likely to yield to wetting forces that cause tombstones.

Resistors showed three times the solder ball rate of capacitors. Regardless of whether certain solder balls are considered defects, the fact that resistors showed three times the incidence rate indicates they are more susceptible to coalescence issues and solder balling defect modes, likely because of their three-sided termination design, which has less surface area to draw molten solder than similarly sized capacitors (Figure 12).

The third most common effect was that of orientation. Components oriented at 0°, or parallel to the direction of travel through the oven, exhibited a defect rate of 15,046 ppm (11,699 ppm excluding solder balls), while components oriented at 90°, or perpendicular to the direction of travel, exhibited a defect rate of 8600 ppm (3,991 ppm excluding solder balls). The results are shown in Figure 13.

The components oriented perpendicular to the direction of travel showed much fewer defects than those oriented parallel to the direction of travel, presumably due to thermal gradients across the component. When a component is oriented at 90°, both terminations enter reflow at approximately the same time. When a component is oriented at 0°, the leading termination enters reflow before the trailing termination.

The paste offset in the positive X direction was 0.1 mm (0.004"). Similar paste offsets in the Y direction did not show a significant impact on defect rates. This behavior may be similar to, or exacerbate the effect of, component orientation on the time differential reaching reflow temperatures.

The fourth largest effect was that of reflow atmosphere. Overall, air atmospheres were associated with defect rates of 18,581 ppm (11,172 ppm excluding solder balls), while the inert environments were associated with 5,065 ppm (4,518 ppm excluding solder balls). The results, shown in Figure 14, are split. Nitrogen environments produced more tombstone defects, but less positional and wetting defects. The nitrogen environment also minimized solder balls formation when compared to air, indicating improved coalescence. Overall, nitrogen environments produced fewer defects than air, but the impact on total defect reduction was not as large as the one provided by print and placement controls.

Solder Paste Type Effect

Figure 15 shows defect rates for each solder paste type. Of the three paste types, the no-clean SnPb paste produced the fewest overall defects, a total of 4,768 ppm (3,679 excluding solder balls). The no-clean Pb-free paste more than doubled that defect rate, with 12,637 ppm (4,697 excluding solder balls). Solder balls were the most common issue for this paste, accounting for almost two-thirds of the recorded defects. Excluding the solder ball count, the defect rates for no-clean SnPb and Pb-free were not extremely different. The water-soluble Pb-free paste had the highest defect rate at 18,154 ppm (15,459 excluding solder balls). When the key factors of print and placement offsets are removed, the defect rate drops dramatically for all paste types (Figure 16).

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