Drilling Printed Circuit Boards
In recent years, the production of printed circuit boards (PCBs) has not only increased in quantity but also been improved in quality. The high packaging density of PCB is required to be improved and miniaturized. For instance, the downsizing of televisions and telephones has made it possible to change these from stationary to portable devices. Thus, high-quality micro-machining is necessary for PCB.
The basic materials forming a board are an epoxy glass mix, made by impregnating rolls of woven glass cloth with resin, and sheets of copper foil which are used to make the electrical connections . A typical board may have multiple alternate layers of epoxy glass and copper foil, manufactured by squeezing in a large hydraulic press. The board will not become a circuit until connections are made between the different copper layers and when the discrete electrical components are soldered or welded in place. Thus, through hole drilling is one of the fundamental processes required in the manufacture to produce an electrical connection between the circuits on the board.
Drilling is a particularly complicated machining process, and it becomes much more complicated when the work piece is PCB. PCB is composite materials with anisotropy. Tool trends to wear and break. The accuracy of hole location, the presence of entry and exit burrs, smear of resin on the sides of holes, delimitation and so on may affect quality of boards. Only a small one of these defects may cause great losses. In order to overcome these problems, both the drilling process and PCB structure design have been researched by many scholars. But the investigations into the drilling processes of PCB are not systematic. The present review article address the report about tool materials and geometrics, cutting force, cutting temperature, radial run-out and damages occurred in drilling processes. And as a conclusion, some of these critical issues are proposed to meet the challenges in analysis and optimization for PCB drilling.
Tool materials and geometrics
Tool materials. High speed steel (HSS) and tungsten carbide (WC) are mostly used as tool materials for the drilling of composites, while polycrystalline diamond (PCD) is less frequently tested . Carbide tools yield good results in terms of tool wear and tool life during the machining of GFRP
Carbide drills are manufactured using powdered metallurgy technology . Micro-fine (less than one micrometer) tungsten-carbide and cobalt powders are formed into blank and sintered. In this process the blanks are subjected to extremely high pressures and temperatures. The sintering temperature is high enough to melt or flow the cobalt in the powdered mix which acts as the binder for the tungsten-carbide base material. Bit geometry is formed on automated grinding machines. The cobalt content of small diameter drills, less than 0.50 mm, is generally increased from 6 percent to 12 percent for improving toughness. In smaller diameters, micro-fine powders with particle sizes less than one micron are used to improve wear resistance.
Tool geometrics. Tool geometrics are a relevant aspect to be considered in drilling PCB, particularly when the quality of the machined hole is critical. Fig. 1 is a drawing of a typical solid carbide bit used in printed circuit board drilling. Various aspects of bit geometry are noted in the figure. Several features of the geometrics are unique to printed circuit board bit design. PCB drills generally have a point angle of approximately 130°. The helix angle is generally 30° to 35 °. Two relief angles are commonly used on the back side of the lip or cutting edge. The diameter of the bit is usually relieved behind the margin. The diameters of most bits are back tapered to reduce side wall friction during drilling. A spade design is commonly used for larger diameters to reduce side wall friction. The diameter of the bit is sharply reduced a short distance from the tip forming a spade head. Most drills taper out from the base of the flute end to a shank approximately 3.18 mm in diameter.
Drill bit terminology
The diagram of the micro-drill and its characteristics are shown in Fig. 2 . The cutting plane (also called first facet or lip relief plane) is consisted of four edges, of which the cutting lip and chisel edge are two important cutting edges on the first facet. The centering point is the first contact point of a micro-drill with the material surface, and cutting lips are the major cutting edges for material removal. Chisel edges are comprised of two intersecting planes that define two primary cutting edges of the micro-drill. They remove the material by extrusion and cut at a highly negative rake angle.
Fig. 2 Schematic diagram of a micro-drill
Cemented carbide drills which are generally shaped by grinding are commonly applied in PCB drilling. At present, drills with diameters down to 0.1 mm and less are commercially available but their reliability in the computer numerically controlled (CNC) machines used for PCB drilling decreases significantly as the size reduces from 0.25 to 0.1 mm . Kosmowski, Kang, Ehmann and Chyan, etc. have done systematic researches on micro-drill. Kosmowski indicated that the reduction in diameter increases drill flexibility and reduces the strength of the bit. Small diameter bits are used in dense packaging formats where accuracy is critical. As the drill diameter is reduced, it approaches the size of inhomogeneities in the stack causing significant bit wander in deep holes. Kang established a machematical models for the planar micro-drill point and developed its grinding method. Chyan designed a curved helical micro-drill point and established its mathematical models and grinding systems in computer. It was found that curved helical micro-drill was much better than planar micro-drill in grinding and cutting characteristic. Zhou established a mathematical model of the helical flute of micro-drill and developed a CAD system of helical flute (0.4 mm in diameter). Xiang proposed a regrinding method for micro-drill (diameter: 0.3~0.5 mm).
Methods of drilling.
Drilling is one of the basic, most frequently performed, machining processes. Several methods are used for producing micro-holes including conventional punches and dies, electrical discharge machining (EDM), vibration drilling, laser machining, etc.
Electrical discharge machining can be used for graphite-epoxy laminates since the graphite fibers are electrically conductive. Noting that high currents and high temperatures are produced which can cause severe melting of the composite surface, thermal expansion of the graphite fibers in the lateral direction and debonding between fibers and the matrix. The production rate of an electrical discharge machine is slow .
Vibration drilling is a branch of vibration cutting, which is fundamentally different from conventional drilling. The conventional drilling is a continuous cutting process, while the vibration-assisted drilling is a pulsed intermittent cutting process by piezoelectric crystal oscillator. Vibration thrust and torque are smaller than conventional values by 20-30% under the same drilling condition . Although vibration drilling can avoid burr and improve cutting stability, there are still a lot of problems to conquer.
Laser machining is a process that no contact between the tool and the workpiece, and it eliminates the delamination. Laser drilling is being developed as a solution for smaller hole sizes, but there are also problems associated with the process. A PCB is made up of three materials ——copper, glass fiber and resin, which have very different material and optical properties. This makes it difficult for a laser beam with particular characteristics to cut cleanly and efficiently through the board. The hole produced by a laser in a standard PCB is sometimes tapered. Also, very small holes produced by laser have been limited to a depth equivalent to the diameter. The main use of lasers is seen to be for small blind and buried vias.
In order to produce through-holes in MPCBs, the main area of interest in the present research, only mechanical drilling is favorable as hole depth increases, because of the need for high accuracy, low temperature and deformation forces in order to produce high quality holes on a glass-epoxy composite board known as the FR-4 board .
Thrust force and Torque In a drilling process, the drill and workpiece have a relative motion along with the axis of the hole so that the surplus material is removed. To generate and maintain the relative motion, a thrust force and torque are applied to the drill.
Boston and Gilbert examined the forces on a drill, end-load and torque, depend on cutting conditions. Then, many authors have subsequently investigated how forces depend on drill geometry. A general conclusion is that end-load largely depends on the chisel edge, while most of the torque contribution comes from the cutting action of the two main cutting edges. Cheong indicated that when drilling holes with a large aspect ratio, the cutting forces acting on the drill bit increase as the drill penetrates the workpiece. The chip produced during cutting is the primary cause of this increase, since it increases friction between the flute and the cut surface of the workpiece. Hinds indicated that drill forces, comprising an end-load and a torque, depend upon the process conditions and the composition of the material being cut. Force trends associated with wear, glass content, chip-load and aspect ratio are presented together with an indication of the contribution from the copper layers. Yang built a real-time monitoring system of micro-hole drilling force based on neural network．
Bit temperature. When drilling, temperature usually exceeds the glass transition temperature of adhesive matrix in the laminate base material. Drilling temperature usually increases with wear of the bit. Gross defects such as land tear out are common at high drilling temperature. Lands are small copper rings in conductive planes that surround plated holes and provide the connection between the holes and circuit traces that radiate from the holes.
Prior researches have shown that drilling significantly affects the quality and reliability of plated hole connections. Many defects in plated holes are related to the thermally sensitive nature of the lamina used in modern circuit boards. Di , Jain and Chen considered that problems occur if temperatures in the work material are allowed to rise to too high a level. The difference in the coefficients of linear expansion between the resin and fibers gives rise to residual stresses which makes it more difficult to attain high dimensional accuracy, with drilled holes showing a smaller diameter than the drill used. Bolton Robert William developed a suitable on-line measurement system with an infrared pyrometer, documented sensor responses and evaluated the sensitivity of temperature while drilling printed circuit board stacks.
Radial run-out at the drill tip. Ultrahigh rotational spindles have been developed for higher precision and productivity drilling with smaller micro-drills. PCB manufacturers can use commercial spindles rotating as high as 3×105 min−1 or more. However, high rotational speed might cause a radial run-out at the drill tip.
Hidehito Watanabe observed the drilling behavior at contact with a work surface using a high-speed video camera (drill diameter is 0.1 mm, rotational speed is 3×105 min−1) and proposed a centering model of micro-drills with some run-out. He concluded that: (1) Orbital revolving drills with the radial run-out substantially move toward the centripetal direction, just after starting contact with a work surface. (2) The entry sheet effectively intensifies the centripetal action. (3) The radial run-out is insensitive to drill wear as well as hole quality, because of the centripetal action. A radial run-out at the drill tip might deteriorate hole quality, such as hole location accuracy or surface roughness on a hole wall.
Centering model of microdrill for PCB
Damage of drilling:
Tool wear. The high packaging density of PCBs requires smaller size drills to be produced. But the smaller size drills are inherently more prone to wear severely and its life and reliability are decreasing sharply. Shorter life and tool change more frequently have become a bottleneck of PCB industries.
A worn-out micro-drill damages the quality of the surface finished and the dimensions of the drilled hole. Tool wear not only reduces the part geometry accuracy directly but also increases the cutting forces drastically. While a drill begins to wear, the cutting forces increases and the temperature of the drill raises, which speeds up the physical and chemical reactions associated with drill wear, and causes rapid deterioration of the drill quality .
Ross conducted a Taguchi multi-factorial experiment to identify the influential factors on drill life . Treanor and Hinds considered that it is possible to maintain good drill condition and consequently a longer drill life if correct values are used for the influential parameters. In particular, drills survive better when cutting the minimum number of layers of copper with thinner foil, the use of an entry material is set aside, the holes have a lower aspect ratio and with minimal penetration into the back-up material. Hinds  concluded that the copper content used in a board should be held to a minimum by using the thinnest copper foil possible and having a small number of layers. The aspect ratio to be drilled should be kept to a minimum in order to reduce drill wear. The maximum chip-load that should be used is dependent on the drill diameter. Su  developed an automated flank wear measurement scheme using vision-aided system for a micro-drill and proved the feasibility of measuring frank wear with the hole-drilling tests on a 10-layered PCB. Teti  reviewed the effectiveness of the tool materials for various machining processes of composites and indicated that tool wear mechanisms are primarily related to the physical and mechanical characteristics of the different fiber matrix systems. Glass fibers show a strongly abrasive behavior because they are extremely abrasive by nature.
Tool breakage. Small diameter drills used in drilling PCB appear to be bad rigidity and prone to break. Hinds used finite element methods to analyze the stresses occurring in micro-drills. Gong developed a computer program for the calculation of the variations of the critical speeds and of the buckling loads, and analyzed the effects of drill geometry, boundary condition, rotational speed, and cross-sectional area on the buckling loads and the critical speeds. Yang developed the on-line monitoring to prevent micro-drills from breaking and to increase micro-drill availability. Yan built a 3D drill model by simulating the grinding process of flute and flank by using UG. The various ratios of the web thickness to the diameter, the flute length and the helix are calculated by ANSYS, in order to analysis the torsional, twisty and pressural rigidity. Fuestablished a 3D model of a micro-drill bit via CAD software and this enables the construction of an accurate drill bit mode. Hidehito Watanabe indicated that rotational speed is significantly effective on breakage life.
Surface integrity. The drilled hole quality is a key factor determining plating quality and the quality of the finished MPCB . The quality of holes can be judged in terms of accuracy of hole location, the presence of entry and exit burrs, smear of resin on the sides of holes and delimitation, all of which are features which may affect the integrity of the connections made by plating the drilled holes
PCB quality index
Coombs presented a comprehensive list of quality issues and their probable causes and solutions. For example, the smear problem can derive from a poor back-up or entry material, an uncured laminate, a worn drill bit or from having too high a surface speed at the drill. Burrs occur again with poor back-up or entry materials, when feeds are too high or when there are circumstances that result in loose stacking of boards. Su proposed that in PCB manufacturing, a worn-out micro-drill damages the quality of the surface finished and the dimensions of the drilled hole. Aoyama demonstrated that the reduction of fiber bundle thickness is effective to decrease the internal damage of drilled hole. Du experimentally discovered that the factors influencing the hole roughness might be different when the drill diameter range from 0.3 mm to 1.0 mm by drilling 6 layer PCBs. Watanabe considered that radial run-out at the drill tip, which might deteriorate hole quality, such as hole location accuracy or surface roughness on a hole wall. Some researchers proposed vibration drilling to avoid burr and improve cutting stability.
The following conclusions can be drawn with regard to the drilling of printed circuit boards:
- Cemented carbide drills which are generally shaped by grinding are commonly applied in PCB drilling. As the diameter of drills is reduced, the drill flexibility increases and the strength of the bit decreases. Thus, researches have been done to establish the drill model and grinding system. But these researches mostly relate to general metal machining. Drills specially used in drilling PCB need to be improved.
- Although there are lots of methods of drilling such as electrical discharge machining (EDM), vibration drilling, laser machining and so on, mechanical drilling is available as hole depth increases, because of the need for high accuracy low temperatures and deformation forces in order to produce high quality holes in PCB.
- It was indicated that thrust force and torque depends on the cutting condition, drill geometry, the composition of the material being cut. The chip produced during cutting is the primary cause of this increase, since it increases friction between the flute and the cut surface of the workpiece. When drilling holes with a large aspect ratio, the cutting forces acting on the drill bit increase as the drill penetrates the workpiece. Copper layers also contribute to the force increase.
- As the drill wear, the drilling temperature increases. When the temperature rise to too a high level, defects occur to reduce dimensional accuracy and drill holes show a smaller diameter than the drill used.
- Although ultrahigh rotational speed drilling can developed higher precision and productivity using smaller micro-drills, it might cause a radial run-out at the drill tip. A radial run-out at the drill tip might deteriorate hole quality, such as hole location accuracy or surface roughness on a hole wall.
- Tool wear mechanisms are primarily related to the physical and mechanical characteristics of the different fiber matrix systems. Reducing the number of layers of copper with thinner foil and the aspect ratio to be drilled can decrease drill wear.
- The quality of holes can be judged in terms of accuracy of hole location, the presence of entry and exit burrs, smear of resin on the sides of holes and delamination, all of which are features which may affect the integrity of the connections made by plating the drilled holes. Drill wear, drilling methods, radial run-out at the drill bit, materials of entry or back-up and fiber bundle thickness are all associated with the hole quality.
This project was supported by the Key Program of NSFC-Guangdong Joint Fund, China (Grant No.U0734007).
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