Manufacturing is the cornerstone of many industrial activities and significantly contributes toward the economic growth of a nation. Generally, the higher the level of manufacturing activity in a country, the better is the standard of living of its citizens. Manufacturing is the process of making large quantities of products by effectively utilizing the raw materials. It is a multidisciplinary design activity simply involving the synergistic integration of production and mechatronics engineering. The products vary greatly from application to application and are prepared through various processes. It encompasses the design and production of goods and systems, using various production principles, methodologies and techniques. The concept is hierarchical in nature in the sense that it inherits a cascade behaviour in which the manufactured product itself can be used to make other products or items. The manufacturing process may produce discrete or continuous products. In general, discrete products mean individual parts or pieces such as nails, gears, steel balls, beverage cans, and engine blocks, for example. Conversely, examples of continuous products are spools of wires, hoses, metal sheets, plastic sheets, tubes, and pipes. Continuous products may be cut into individual pieces and become discrete parts. The scope of manufacturing technology includes the following broad topics:
• Precision engineering and ultra-precision engineering
• Micromanufacturing (Microelectronics and MEMS)
The technical field of precision engineering has expanded over the past 25 years. In 1933, the Precision Engineering Society was established in Japan and soon thereafter the activities were accelerated due to new impetus from Europe. The first issue of the journal Precision Engineering appeared in 1979 and the first academic program began in 1982 (Source: American Society of Precision Engineering (ASPE)). According to ASPE, "...precision engineering is dedicated to the continual pursuit of the next decimal place." Precision engineering includes design methodology, uncertainty analysis, metrology, calibration, error compensation, controls, actuators and sensors design. A more complete list is given below (www.aspe.net).
• Dimensional metrology and surface metrology
• Instrument/machine design
• Materials and materials processing
• Precision optics
• Scanning microscopes
• Semiconductor processing
Frequently used terms within the domain of precision and ultra-precision engineering are precision processes, scaling, accuracy, resolution and repeatability. The precision process is a concept of design, fabrication, and testing where variations in product parameters are caused by logical scientific occurrences. Identification of these logical phenomena and strategically controlling them is very fundamental to precision manufacturing. Scaling is a parameter that defines the ratio attributes with respect to the prototype model. It is also considered as a fundamental attribute for predicting the behaviour of structures and systems for analysis and synthesis of miniaturised systems. Accuracy defines the quality of nearness to the true value. In the context of machine or production systems, accuracy is the ability to move to a desired position. As an example, if the actual value is 1.123 units and it is recorded as 1.1 units, we are precise to the first decimal place but inaccurate by 0.023 units. Resolution is the fineness of position precision that is attainable by a motion system. The smallest increment that is produced by a servo system is the resolution. There are two types of resolutions, electrical and mechanical. With regard to mechanical resolution, it is defined as the smallest increment that can be controlled by a motion system, i.e., the minimum actual mechanical increment. One can note that mechanical resolution is significantly coarser than that due to the involvement of friction, stiction, deflections, and so on. Repeatability is the variation in measurements obtained when one person takes multiple measurements using the same instruments and techniques. Repeatability is typically specified as the expected deviation, i.e., a repeatability of 1 part in 10,000 or 1:10,000, for example.
Micromilling and microdrilling are two important processes of precision engineering. The micromilling process is considered versatile and facilitates creating three-dimensional miniaturised structures. The process is characterised by milling tools that are usually in the order of hundreds of micrometers in diameter. These tools are designed by the use of focused-ion beam machining process and are used in a specially designed, high-precision milling machine. The focused-ion beam machining process uses a sharp tungsten needle wetted with gallium metal. The tip of the needle is subjected to a 5-10 kV (sometimes higher) so as to enable the field ionization effect on the gallium. The gallium ions are then accelerated by the use of another energy source and focused into a spot of sub-micrometer order.
The kinetic energy acquired by the ions makes it possible to eject the atoms from the workpiece. This is referred to as a sputtering process. The sputtering yield varies inversely with the strength of the chemical bond in the materials. Either the movement of ions or the workpiece, depending upon the environmental conditions, can be controlled to obtain a wide variety of three-dimensional shapes and structures. It should be pointed out that the machining forces present in micromilling with tools of the order of micrometer diameters are dominated by contact pressure and friction between the tool cutting edges and the workpiece. As a rough calculation, one can note that in the focused-ion beam machining process, for a spot size of 0.45 |im with 2.5 nA of current, the required current density would be approximately 1.65 A/cm2. The micromilling process is applied for making micromolds and masks to aid in the development of microcomponents. Typically, a high milling rate of 0.65 |im3/nAs, corresponding to an average yield of 6.5 atoms/ion, can be obtained at 45 keV, 30° incidence, and 45 scans.
Microdrilling is characterised by the drilling of ultrafine holes. Drilling in the micro ranges, using the special microdrills, requires a precision microdrilling instrument. The end of the microdrill is called the chisel edge, which is indeed removed material cutting at a negative rake angle. Microdrills are made of either micrograin tungsten carbide or cobalt steel. Some coarse microdrilling machines are available that drill holes from the size of 0.03 mm in diameter to 0.50 mm in diameter, with increments of 0.01 mm. However, the present demand is for drills capable of drilling in the order of micrometers. An example of this is a sub-microdrilling technique utilising the phenomenon of ultrafast pulse laser interference. In this regard, for microdrilling and other delicate laser processing applications, Holo-Or Ltd. has released an optical element that creates an output spot in the form of a top hat circle with a diameter of 350 ^m. The element accepts a collimated Gaussian incident beam with a diameter of 12 mm from a 10.6-^m CO2 laser. Smooth 300 nm holes were successfully drilled on a 1000-A-thick gold film using the interfered laser beam, as compared to micrometer holes ablated using the conventional non-interfered laser beam. The most important parameters considered in microdrilling are: accuracy, sensitivity, quality and affordability. Some of the applications of microdrilling are given below:
• Air bearings and bushings
• Electronic components
• Microwave components
• Optical components
The major problem of conventional laser microdrilling is that the process has a short focal depth. It is known that this method typically achieves aspect ratios up to 100 in thick material, such as for a 15-^m hole in 1.5-mm-thick foil, for instance. This problem can be overcome by utilizing a Bessel beam. Deep high-
aspect ratio drilling is achieved due to the reason that the Bessel beam is non-diffracting and in practice they do not spread out. In the case of deep high-aspect ratio laser drilling, a pseudo-Bessel beam is generated using a pulsed laser. Some of the examples of microdrilling applications using a laser system developed by ATLASER di Andrea Tappi are presented in Table 1.1. The application of lasers to micromanufacturing has several advantages: noncontact processing, the capability of remote processing, automation, no tool wear and the possibility of machining hard and brittle materials.
Table 1.1. ATLASER di Andrea Tappi microdrilling system performance parameters (Courtesy: ATLASER di Andrea Tappi)
Thickness: 0.54 mm Hole diameter: 25 ^m Hole pitch: 50 ^m Process time: 0.65 s
Silicon Carbide Wafer Thickness: 0.64 mm Through Hole: 130x500 ^m In width: 130 ^m Out width: 110 ^m
Aluminum Nitride Thickness: 425 ^m In side width: 300 ^m Out side width: 290 ^m Drilling time: 33 s
Cu-FR4 sandwich Thickness: 0.5mm Hole dimension: 200 ^m Process time: 3.3 s
Stainless Steel Sheet Thickness: 120 ^m Hole diameter: 9 ^m Hole pitch: 50 ^m Matrix Process time: 0.15 s
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