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Product category: Bearings
News Release from: Loadpoint | Subject: Dicing spindles
Edited by the Engineeringtalk Editorial Team on 30 April 2003

Dicing spindles advance productivity

Developments in dicing spindle technology within the last ten years have resulted in significant improvements in their main performance parameters.

Developments in dicing spindle technology within the last ten years have resulted in significant improvements in their main performance parameters For air bearing spindles, drive powers have been increased by 20%; the useful speed range widened by as much as 500%; radial stiffness and load capacity have increased by as much as 50% and motion errors reduced by 50%

These improvements not only enhance the productivity, cut accuracy and quality which can be achieved by modern dicing spindles but also extends the range of applications for which they can be used.

In this article we expand upon some of the more recent developments and explain their role in improving dicing spindle performance.

Dicing spindles have come a long way since their origins in the late 1950s and early 1960s when the need to slice silicon wafers on a large scale first arose.

Early spindles were crude affairs using standard ball bearings and belt drives to rotate a cutting wheel at high speed.

The limitations of these units, in terms of cut quality and accuracy, continued until the late 1960s when air bearings were first used to replace ball bearings.

These improved cut quality dramatically, establishing the technology as a benchmark that has continued to deliver optimum results across an ever-larger range of wafer sizes and material types.

The recent trend towards large wafers demands higher productivity and consequently the latest designs of spindle are faster and more powerful.

These latest machines are also even more flexible in terms of the cutting conditions they can achieve and have successfully extended the range of materials and applications to quartz surface acoustic wave filters, lead zirconate titanate printer heads and positive temperature coefficient ceramic thermistors.

In concert with these developments are the demands to cut hard materials and thicker components at high productivity rates.

These demands are best met using modern precision ball bearing spindles, which have the high rigidity and load capacity necessary to take heavy cuts and, hence, achieve the required productivity levels.

One of the key contributing factors to the high performance of the new generation of dicing spindles is the use of DC brushless motors.

These overcome the problems of early designs employing AC drive motors that developed useful operating torque only across a relatively narrow speed range.

The former are not only more flexible in terms of the range of applications they can handle but their increased top speed also gives the potential for increasing productivity in applications such as, for example, silicon wafer dicing.

In addition to offering higher speeds and powers, the latest generation of air bearing spindles is also stiffer and offers increased load capacity.

The former is important because the higher the radial stiffness of the spindle at the wheel position, the wider the range of cutting conditions that can be achieved.

In particular, the spindle's operating speed range can be extended downwards to cater for low speed heavy cuts.

The risk of imbalance leading to damage and possible breakage of the cutting wheel at speed when undertaking such cuts is minimised by doubling the number of journal bearings in the spindle from two to four.

Further improvements in the static radial stiffness have also resulted from improving the internal design of spindles, enabling larger bearing diameters to be used.

As part of this process, careful optimisation of the housing and its internal design have enabled shaft and journal bearing diameters to be increased by 25% compared with those used ten years ago.

This has improved the radial stiffness of the spindles by approximately 50%.

Complementing the high stiffness of modern dicing spindles is a level of running accuracy that enables peak to peak axial error motions of better than 0.5um to be achieved.

The ability of air bearings to provide this level of accuracy stems from the inherent advantages of the technology to control axial "wobble" of cutting wheels.

In addition to the fact that air bearings are manufactured to a high standard, the air film in the bearing attenuates the effects of surface imperfections on bearing motion by as much as a factor of 20.

Thus in practice, wheel "wobble" attributable to air bearings is negligible.

Radial running accuracy of the spindle is also important in respect of eccentricity or imbalance, both of which affect cut quality or accuracy.

The main source of imbalance is the cutting wheel, which continually wears, and the wheel spacers.

Even though the latter are balanced, small amounts of damage or wear accumulated over time either on the spacers or their location surfaces cause the balance of the spindle to drift.

The recent introduction of titanium spacers on hardened steel shafts is an answer to this problem, maintaining low out of balance for a far longer period than heavy steel or easily damaged aluminium equivalents.

Despite the above improvements, the performance of dicing spindles will be impaired if the spindle style and mounting arrangement is not matched to dicing machine configuration.

There are a number of variants here but the main factor which influences spindle design is whether or not the machine uses a gantry to span the working envelope.

If it does, then the spindle is slung below the gantry and attached to it by means of bolts or clamps positioned close to the spindle nose.

Without a gantry, the spindle must be cantilevered off a part of the machine structure or a slideway so that it overhangs the working envelope.

In this case the spindle is mounted via a flange or clamping arrangement at the rear of the spindle and spindle length is dictated by the size of the working zone.

For example, dicing a 300mm-diameter silicon wafer requires a spindle of minimum length 330mm.

With the overhung spindle design the motor may be placed at the rear where it is clear of the working zone, or positioned further forward where it must be contained within the spindle's main housing diameter.

The three basically different styles of spindle each have advantages.

The overhung spindle with the motor at the rear can use larger diameter and hence more powerful motors.

Thus, although motor power is not a limitation, the high length/diameter ratio of the spindle makes it prone to shaft bending modes of vibration, which in practice limits maximum spindle speed.

Spindle cooling is also more critical.

Any axial thermal growth which occurs in the spindle housing forward of the mounting flange adds to that of the section of the shaft between thrust bearing and cutting wheel.

Thus it is essential that this style of spindle is fully water cooled.

With the gantry-mounted spindle, housing diameter is constrained by cutting wheel diameter over its full length and the drive motor must fit within this envelope.

Motor power is thus limited, but spindle length can be optimised to achieve high rotational speeds.

Thermal growth of the spindle is only significant over the relatively short distance between cutting wheel and mounting position and is readily controlled by water cooling.

The overhung spindle with forward mounted motor has the advantage of an optimised shaft length and, hence, good high-speed performance, but it must be fully water cooled to minimise axial thermal growth.

The improvements in spindle load capacity and dynamic response described above reduce the risk of spindle damage due to overload, out of balance or wheel breakage.

In addition, bearing life has also been extended by improved sealing.

The hostile environment generated under cutting conditions demands a high integrity seal to prevent ingress of contamination or moisture into the front thrust bearing.

Field experience has shown that the addition of a flinger to a mechanical labyrinth seal provides an effective solution.

Spindles with electrical touch sense circuits require a brush to contact the rotating shaft.

The brush is the only wearing component on the spindle and as such must be replaced periodically.

Brush life has been maximised by the introduction of a self cleaning mechanism which prevents the build up of wear debris both local to the brush-shaft contact and within the motor cavity.

With its many years experience of air bearing and spindle design, Loadpoint has produced a new range of dicing spindles that integrate all the new development benefits highlighted in this article.

Spindles are designed to support wheel sizes between 55 and 110mm diameter and are available in a range of styles to suit different dicing machine configurations.

Furthermore, ball-bearing spindles are available for applications in which extremely heavy cutting conditions are encountered.

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