Recent Developments In Dicing Spindles
F. P. Wardle T. Snow D. Gilham
Introduction
Dicing has its origins in the late fifties and early sixties when the need to slice silicon wafers on a large scale first arose. Like the machines of which they were a part, early dicing spindles were crude and limited in the cut quality and accuracy they could achieve. Unlike modern spindles they were not precision mechanical sub assemblies and simply utilised standard ball bearings and belt drives to rotate a cutting wheel at high speed. In the late sixties it was found that the use of air bearings in place of ball bearings could dramatically improve cut quality and from then on the air spindle has continued to gain popularity. The air spindle with integral AC drive motor was first introduced in the early seventies and may be regarded as the forerunner of present day spindles.
In the seventies and eighties the continual requirement for improvements in the cut quality and accuracy of silicon wafers led to a number of process refinements and resulted in what may be broadly termed as a ‘precision sawing process’. At the same time other wafer substrates and new developments such as magnetic recording heads were generating a much wider range of dicing applications. Different wheel types and sizes were developed to cater for the increased range of materials and this in turn required a family of spindles to ensure optimum cutting conditions. The single spindle dicing machine evolved to cater for these applications and became widely recognised as a flexible, high precision singulation process.
More recently the trend towards larger diameter silicon wafers has produced some radically new machine structures and with it the need for further spindle variants. Larger wafers also demand higher productivity and consequently the latest designs of spindle are faster and more powerful. These new machines are 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. Alongside these developments are the demands to cut hard materials and thicker components at high productivity rates. For these situations precision ball bearing spindles are an appropriate solution. They have the high rigidity and load capacity necessary to take heavy cuts and hence achieve required productivity levels. Dicing spindle development has not gone full circle however, modern ball bearing spindles have considerably better running accuracy, thermal growth, speed range and power levels than their early equivalents.
The main purpose of this article is to enlarge upon some of the more recent dicing spindle developments referred to above and explain how they have helped improve spindle performance.
Spindle Style and Mounting Arrangement
Spindle style and mounting arrangement is 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 utilises 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, fig 1. 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.
Fig 1.
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 of a 300 mm diameter silicon wafer requires a spindle of minimum length 330 mm.
Fig 2.
With the overhung spindle design the motor may be placed at the rear where it is clear of the working zone, fig 2, or positioned further forward where it must be contained within the spindle’s main housing diameter.
Fig 3.
The three basically different styles of spindle each have advantages. The overhung spindle with the motor at the rear, fig 2, 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 occurrs 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, fig 1, housing diameter is constrained by cutting wheel diameter over its full length and the drive motor must fit within this envelope. Thus motor power is 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, fig 3, has the advantage of an optimised shaft length and hence good high speed performance but must be fully water cooled to minimise axial thermal growth.
Speed and Power
Early dicing spindle designs used AC drive motors which exhibited a linearly increasing power – speed relationship and hence only developed a useful operating torque over a relatively narrow speed range approaching maximum spindle speed. The trend now is towards DC brushless motors which can have constant power from a design speed up to maximum speed. DC brushless motors also have a higher power density than AC motors and since for the spindle styles shown in figs 1 and 3 motor diameter is constrained by wheel diameter this translates to higher power output from a given spindle envelope. For example fig 4 compares torque and power – speed relationships for a 20 year old dicing spindle fitted with an AC motor and its modern equivalent fitted with a DC brushless motor.
Fig 4.
The useful speed range of the newer spindle is 10 000 rpm to 60 000 rpm which compares to 30 000 rpm to 40 000 rpm for the older spindle. Thus the former is not only more flexible in terms of the range of applications it can handle but the increased top speed gives it the potential for increasing productivity in applications such as, for example, silicon wafer dicing.
The above spindle is an air spindle designed to support the industry standard wheel diameter of 55 mm. Larger wheels generally require higher torques at lower rotational speeds and this is most easily achieved by maximising spindle diameter. Fig 5 compares the speed – torque characteristics of air bearing and heavy duty ball bearing spindles designed for 80 mm and 110 mm diameter wheels with that of the air spindle for 55 mm wheels.
Fig 5.
Spindle Stiffness and Load Capacity
Spindle stiffness is an important design parameter not just because it determines the deflection under cutting loads but also because it influences the spindle’s response to unbalance loading. The higher the static 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 down to cater for low speed heavy cuts. For imbalance response the main consideration is gross damage and breakage of a cutting wheel at speed and the consequential risk of spindle seizure. This is minimised by doubling the number of journal bearings to four as shown in fig 6.
Fig 6.
Spindle stiffness and radial load capacity is maximised with this bearing arrangement and the whirl response mode shape considerably improved. With just two journal bearings imbalance at the wheel position creates severe shaft bending deflections as shown in fig 6. These add to bearing deflections and effectively reduce the out of balance load the spindle can withstand without risk of seizure. For comparison, fig 6 also shows four bearings to offer better support of the shaft over its highly stressed central region and hence produce a much improved whirl mode shape.
Improvements in static radial stiffness come from a better internal design of the spindle enabling larger bearing diameters to be used in spindles of a given outside diameter. Spindles for 55 mm diameter wheels must allow approximately 5 mm wheel clearance below the spindle housing. A flat on the underside of the spindle 22.5 mm from the axis of rotation, fig 1, enables the required clearance to be achieved with the use of a somewhat larger diameter housing but it still limits shaft and bearing diameters. However careful optimisation of the housing and its internal design have enabled shaft and journal bearing diameters to be increased by 25 % compared to those used ten years ago. This has improved the radial stiffness of the spindles by approximetely 50%.
Running Accuracy
A principle advantage of air bearing spindles over rolling element bearing spindles is their inherently good running accuracy. Critically important in the dicing application is axial ‘wobble’ of the cutting wheel which is well known to affect cut quality on brittle materials. With an air spindle ‘wobble’ is controlled by the precision and in particular, the squareness, of the thrust bearing faces. 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 twenty. Thus in practice, wheel ‘wobble’ attributable to air bearings is negligible. Of far more importance is the accuracy of location sufaces on the shaft and wheel mounts. Modern manufacturing methods enable dicing spindles to achieve peak – peak axial error motions better than 0.5 um.
Radial running accuracy of the spindle is also important in respect of eccentricity or imbalance. This is often the largest source of vibration on the machine and it can manifest itself in terms of high audible noise and excitation of structural resonances which in turn affect cut quality or accuracy. The main source of imbalance is the cutting wheel, which continually wears, and the wheel spacers, which in the case of hubless wheels, are frequently removed and replaced to facilitate wheel changes. 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. Damage levels and wear rates can be minimised by optimising the shaft and spacer hardnesses but as out of balance forces also depend on the weight of the out of balance components material density also should be taken into consideration. The recent introduction of titanium spacers on hardened steel shafts is proving to be a good combination, maintaining low out of balance for a far longer period than heavy steel or easily damaged aluminium equivalents.
Reliability
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. Bearing life has also been enhanced by improving 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 labrynth 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.
Discussion
Developments in dicing spindle technology within the last ten years have made 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 they can be used for. The benefits are replicated in a family of spindles designed to support wheel sizes of between 55 mm and 110 mm diameter and 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.