edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 1 30 of the edge of the tool face contacts the die. This makes the material removal rate a function of tool/pad compliance and the local slope of the die surface. This should be taken into account in the tool path and linear feed rate to reduce the effects of surface curvature on removal rate. Without this consideration, material will always be removed faster from low slope areas. This also means that the sample should be mounted so that the average plane of the die is as close as possible to the reference plane of the machine. How can the irregularities in the material removal process be corrected and to what accuracy? The addition of either in-situ or external thickness measurements can compensate for variations in material removal rates, but only to the characteristics of the correction algorithm. In the machines referenced in this article, a RKD Systems UltraPrep IV and OmegaPrep II, a nine-point thickness measurement is made either by the machine or externally. This data is used to linearly correct thickness with the reference points being the die center point and the edge measurements. The evaluation of the efficacy of this technique is very involved, but it should ensure a ± 2 micron result. At the area-of-interest level, thickness measurements may be made point by point with the surface profile on larger areas of interest. This ensures surface or thickness variations that are limited to the difference between actual curvature and the linear approximation. THE CONCEPT OF COMPLIANCE Everything solid is elastic. A steel I beam bends when force is applied to it. The forces on the cutter deflect the spindle of a 3000-pound milling machine. In this sense, compliance is the change in dimensions with applied force. In all cases, things deflect, move or compress, with applied force. In the case of lapping pads, the pad itself and the adhesive that attaches to the tool, is somewhat compressible. This makes the pad contact surface variable based on its compliance and the local die surface slope. Additionally, there is the compliance of the machine itself. The machine’s frame and other components have some compliance. Even a 3000 lb. Series II Bridgeport mill bends with the force on the tool. The force verses deflection may be small, as little as 0.003 in. per 100 lb of cutter force, but this works out to 1.68 microns per kg on a big machine. Smaller, bench top machines may have over 10 times this. The machine referenced here has a spindle compliance of 25 microns per kg. As typical downward forces are limited to 30 to 60 g, the spindle and frame will deflect less than 1.5 microns. This level still has an effect on the process results. Generally, without a compensating algorithm, the higher the slope of a surface, the lower the material removal rate and the less the die surface will meet the measured or desired profile. The relationship is based on the compliance of the lapping film or pad. The compliance of lapping film is in the range of 1 to 2 microns for a 50 g down force. A polishing pad could be over 20 times this. No algorithm can correct for wear at the edge of a lapping film disc or pad. A highly sloped surface will wear out the edges resulting in little material removal on the highly sloped areas and normal removal on the less sloped areas. The only way around this is frequent replacement of the pad or film. A FLAT TOOL AND A CURVED SURFACE The problem with a flat tool on a curved surface is geometric. As the surface slope changes so does the contact area between the pad and the die surface. Figure 3 shows the problem. A flat tool has limited contact with a curved surface. The problem here is determining where the center of the contact area is in relation to the tool center position. Some machines have algorithms that relate the center of the contact area to the center of the tool, but this all depends on the compliance of the pad attached to the lapping tool and the surface slope. These parameters may be characterized over some range of pads and tools, allowing for a reasonable compensation for the variables, but only if the pads and tools used are those that have been characterized. Deviating from the specified tools and pads will produce deviant results. The removal characteristics are also related to both tool rotational speed and tool face linear travel. Being allowed to alter these critical parameters can result in altered or inconsistent results. PROCESSES FOR THINNING AND POLISHING HIGHLY WARPED DIE (continued from page 27) Fig. 3 The center of the area contacted between the film and die changes with surface slope. At die center, nearly the entire film is in contact with the surface. As the tool moves away from the die center, the contact area reduces and the difference between the tool center and the center of the contact area increases.
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