By John Boucher, President, Thermocarbon, Inc., Casselberry, Florida
Reprinted from Microelectronic Manufacturing and Testing May & June, 1986
We highly recommend reading the following series of articles before making blade or equipment selection decisions. First, it is necessary for the reader to understand diamond grinding/dicing technology on a general scale before making specific application requirements choices. The proper background will enhance the selection process, and minimize the chance of using the specific recommendations out of context.*
A pragmatic approach is to deal with diamond dicing technology as a total system made up of interacting variable components. Each component, whether it be the wafer dicing blade, part of a machine, or an operating parameter, must be given due consideration in order to optimize an effective cutting process.
Many of the areas covered will parallel existing engineering subjects, and will not be repeated in this article. However, appropriate dicing/grinding technology will be pointed out as to how they apply, and why they are necessary for implementing an effective diamond grinding system. Our approach is simple, to at least bring the reader to a higher level of awareness.
Creating the efficient sawing system - an analytical approach
Today, there is no doubt that diamond sawing is a viable method for the die separation of microelectronic substrates. Diamond grinding/dicing technologies allow for cutting completely through a wide variety of substrate materials without the need for the secondary operations associated with previous scribe and break techniques.
This may lead one to assume that, today, there are no problems associated with die separation and that the discussion should be closed. In fact, one is tempted to think that any cutting task could be successfully achieved by simply acquiring any available dicing machine/blade combination. However, only the combination of diamond dicing blades, flanges, other equipment, and substrates will enable users to produce parts with virtually no loss of the wafer material being processed, and no edge damage to the finished parts. However, reality dictates that much careful planning and control over these variables is necessary in order to create the required efficient sawing system.
Material type, depth of cut, desired throughput, feed rates, spindle speed, cooling nozzle design, mounting, kerf, dicing blade exposure, diamond particle size, available power, and wafer dicing blade and flange design, are but a partial list of the variable components affecting the sawing operation. It is not possible to cover all of the variables and their effects within this text. However, they can be simplified by understanding the three basic fundamentals for dicing and diamond grinding technology and applying the same guidelines for the proper selection of the process components.
The fundamental requirements of rigidity, power, and cooling must be considered for each system component selection. It must also be understood that each component of the wafer dicing process cannot create sawing efficiency alone, but rather that all of the components that become an interactive system must be compatible. In addition, if just one component is wrong, it could render all other properly selected components ineffective.
Such situations often mislead end-users when making diamond blade selection and/or setting the system operating parameters. The result is an attempt to treat a symptom rather than the cause of a problem. This approach can preclude ever achieving a truly economical and efficient sawing system. Experience has shown that major sawing problems are often solved by the user changing a coolant nozzle, the method of mounting, or educating the relatively simple means such as about diamond dicing blade and dicing flange care. In the majority of cases, simple solutions such as these have increased throughput and efficiency by as much as tenfold.
Dicing machines power as a primary requirement for successful dicing operations
Dicing saw manufacturers recently have begun to address power requirements for the sawing of very hard and brittle materials. Earlier machines were designed specifically for the dicing of silicon materials with relatively low horsepower and fixed spindle speeds (30,000 RPM).
However, caution must still be exercised when purchasing newer models, even though they carry the higher power ratings on their spindle motor drives. This is because variable spindle speeds are required to match cutting requirements for various hard materials, and most machines deliver maximum horsepower at only the top end of the variable range.
Dicing saws that are set to deliver maximum power at 30,000 RPM, may lose substantial power when operated at lower spindle speeds. The benefits of lowering the peripheral speed of the diamond blade for a given application (softening the cutting action) can be hindered due to the resultant lack of actual power delivered at the cutting interface. The user often is forced to continue cutting at higher than recommended peripheral speeds to avoid stalling or overworking the spindle power train. The end results are either less than optimum sawing efficiency, or a complete failure in processing the dicing material.
It is not accurate to describe present dicing machines or their earlier conventional grinder style counterparts as poorly designed because each piece of equipment was designed and manufactured for a specific purpose. The end-user must determine the work to be done prior to machine acquisition. The volume of dicing material to be removed, type of material, diamond size, operating speed, and throughput are prime considerations in determining optimum power requirements.
It usually is possible to compensate with the diamond blade’s internal matrix in order to reduce the power requirements for doing specific work. Lower density compositions or larger abrasives 3 are the typical approaches. Normally the price for such compensation is reduced wafer dicing blade life, throughput, and cut quality.
Heavy work loads or a sawing system’s inability to deliver adequate power to the blade/material interface demands that the cutting blade be very forgiving (free-cutting).
The importance of rigidity for high-precision cuts
Whether dicing thin silicon materials at inch-per-second feed rates or cutting into heavy cross-sections of ceramic-based materials, system rigidity plays a major role in sawing efficiency. It is most important to note that rigidity not only pertains to the equipment being used but also to the diamond wafer dicing blade and workpiece mounting methods, as well as to the operating parameters; hence the reason for discussing system rigidity with the machine being but a component of the total system.
Although many options are available on most dicing and slicing saws, these features do not necessarily enhance the basic qualities of a machine for successful cutting. The potential user should first consider the basic construction and movement requirements of the machine as criteria for selection. When these items are properly qualified for doing the work specified, then (and only then) should selection be based on the available controls for operation.
The machine base foundation should be designed and constructed to prevent excessive flexing under load or the transmission of vibration to the integral movements of the machine. Materials such as cast iron, normally used in floor model machines, offer good stiffness and dampening properties, while cast aluminum, frequently used in desk top dicing machines, must be adequately ribbed to provide appropriate stiffness. In extremely high precision applications, some base castings are being constructed with epoxy cements or in combination with cast iron material, in order to provide improved vibration dampening effects and stability. With various materials being presently used in saw foundation design, a basic knowledge of material strengths and properties helps in proper base selection for specific purposes.
A machine mounted on a solid base and incorporating “V” type or cross roller bearing design configurations will normally provide a stable and smooth operation through X, Y, and Z axial movements. Some machine manufacturers are presently experimenting with ceramic air slides to provide stability under changing environmental conditions for extremely close tolerance applications.
The stepping motor drivers are most commonly used to provide smooth, automatic axial operation. Hydraulic systems are frequently used on the “X” drive in place of a stepping motor driver but may present “sticking” problems when operating at slow feed rates. Regardless of the method used to provide axial movement, it is necessary for the machine to deliver smooth, accurate travel at variable feed rates and workload conditions. Again, determine the work to be accomplished and the tolerances desired prior to equipment purchase.
A rigidly mounted spindle with virtually no end play or vibration is mandatory for dicing and diamond grinding. Additionally, to maintain the perpendicularity of the spindle to the “X” slide is essential for the diamond dicing blade to run true throughout subsequent operations. The choice of air versus ball bearing spindle is mainly a factor of the amount of work to be accomplished. Precision ball bearing spindles are typically used for heavy cuts and multi-blade dicing operations. Both types of spindles are capable of producing fine finishes and tolerances within their intended applications. Air bearing spindles are used predominantly in the desk top dicing saw due to their exceptionally smooth operation at high working speeds. The life duration of the “frictionless” air spindle is extremely dependent upon the user’s care in operation.
Continue. Part II: Wafer dicing System and Substrate Interaction Considerations