Types of Spindles
Dicing spindles are a popular type of spindle, specially designed for the purpose of dicing. Dicing spindles are available in various size and speed range depending upon the application.
What is dicing?
Dicing is the process of cutting anything into slices. In machining context, this concept arose in the late 50s and early 60s when there was a large scale need to slice silicon wafers. Originally, the process of dicing was very crude and the cut quality and accuracy was also very limited. Dicing spindles have come a long way since their origins in early 1960s.
Spindles in dicing: An evolution
Initially, the spindles used in dicing were not precision mechanical sub assemblies like those that are used in modern spindles. The spindles simply utilise standard ball bearings and belt drives to rotate a cutting wheel at high speed. In the late sixties, air bearings were used in place of ball bearings as they could improve cut quality as a result of which the air spindle gained popularity. It was in early 1970s, the air spindle with integral AC drive motor was first introduced and has become a forerunner of present day spindles. There were continual improvements in the 70s and 80s in the quality of cut and accuracy of silicon wafers. This led to a number of process refinements and led to the process broadly termed as a "precision sawing process". New developments like magnetic recording heads and other wafer substrates were generating a much wider range of dicing applications.
At the same time other wafer substrates and new developments like magnetic recording heads were generating a much wider range of dicing applications. Different wheel types and sizes were developed to meet the demand 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, there is the need for further spindle variants because of the trend towards larger diameter silicon wafers. Larger wafers lead to demand for higher productivity, which in turn lead to the latest designs of spindles which are faster and more powerful. They are more flexible in terms of the cutting conditions and have successfully extended the range of materials and applications to lead zirconate titanate printer heads, quartz surface acoustic wave filters, and positive temperature coefficient ceramic thermistors.
Style of dicing spindle and mounting arrangement
Spindle style and mounting arrangement is decided based upon the dicing machine configuration. There are a number of factors to take into consideration , such as follows:
Gantry mounted dicing spindle: The main factor which influences dicing spindle design is whether or not the machine utilises a gantry to span the working envelope. If it does then the spindle is fixed below the gantry by means of bolts or clamps positioned close to the spindle nose, as shown in the figure below.
Overhung dicing spindle: Without a gantry, the dicing spindle is cantilevered off a part of the machine structure so that it overhangs the working envelope. In such a case, the dicing spindle is mounted by using a flange or clamping arrangement at the back of the spindle and the length of the spindle is given by the size of the working zone. For example, dicing of a 300 mm diameter silicon wafer requires a dicing spindle of minimum length 330 mm. With the overhung spindle design the motor is placed at the back where it is clear of the working zone or placesd further forward where it must be within the spindle's main housing diameter. The overhung spindle with the motor at the rear can use larger diameter and hence more powerful motors.
Flange mounted dicing spindle: There is another option having the overhung spindle with forward mounted motor, which is known as flange mounted dicing spindle. This type of spindle has the advantage of an optimised shaft length and hence efficient high speed performance but must be fully water cooled to reduce axial thermal growth.
Speed and power of dicing machine tool spindle
Initially dicing spindles used AC drive motors which displayed a linearly increasing power-speed relationship which developed a useful operating torque over a narrow speed range approaching maximum spindle speed. The latest trend now is towards using DC brushless motors which can have constant power from a design speed up to maximum speed. They also have a higher power density than AC motors. The useful speed range of the newer dicing spindle is 10 000 rpm to 60 000 rpm which compares to 30 000 rpm to 40 000 rpm for the previous spindle. Thus the new spindle is not only more flexible in terms of the application range, it can handle but also the increased top speed gives it the potential for increasing productivity in applications like silicon wafer dicing.
Conclusion
Developments in dicing machine spindle technology in the last few years have made significant improvements in their main performance and productivity parameters. For air bearing dicing spindles drive powers have been increased by 20 %, the useful speed range has also increased by as much as 500%, radial stiffness and load capacity have increased by almost 50% and motion errors reduced by 50 %. These improvements in dicing spindles have not only enhance the productivity, cut accuracy and quality but also extends the range of applications they can be used for. They are available in a range of styles to suit different dicing machine configurations.
