A Study On Fixed Hole Drilling Of Printed Circuit Board

As the electronics develops, the PCB is widely used. Very frequently, drilling is a preliminary operation to PCB production The common PCB drilling can be divided into two types:one is the micro hole drilling (D≤0.3mm). Micro hole is an important part of the PCB. The other type is the fixed hole process. The fixed hole diameter is usually over 2mm. Comparing with microhole, the position hole process is the big diameter process. There is unsmooth chip removal, drill wear, serious burr while using common drill to do the job. With the increasing rotation speed, the problem is much more serious. Although the number of position holes is less than the microholes, the drill consuming is still larger due to the more material removal of bigger hole.In order to address the needs of high speed drilling of PCB fixed holes in practice, systematic researches were conducted on chip formation mechanisms of high speed drilling of PCB, tool wear mechanisms, surface finish, cutting forces and optimization of cutting parameters by numerical simulation analysis, micro-material analysis. High-speed photography, infrared temperature and optimization of drill geometry parameters. Chip formation, cutting forces and drill wear principles were studied. The relationship amony cutting conditions and chips, chip formation, surface finish, drill wear was established. Based on thermo-mechanical coupling theory, model can be applied to printed circuit board (PCB) which consists of thin copper foil. Finally, based on applications of basic theory about drilling process and characteristics, interaction between drill structure and chip removal was analyzed. Basic principle and method of drill improvement were put forward, which proved well production. All these researches will improve PCB drilling process, holes formation technique and drill production, which are great valuable.The key conclusions are as follows:1. The basic characteristics of PCB chip formation are combined with the Cu, Al, and fiber chips, which show different ship, size and types. As form of chip, fiber chips are rather long but very soft, medium length of copper chips, aluminum chips the length of the shortest, and was curled-type. Cu chips have types of long and short strip, and C-type. Glass fiber/resin chips have types of small cone-shaped spiral fan, longer cone-shaped spiral fan, small fan, and continuous chips. Different size and ship are produced through the helix groove of the drill when cutting. Great deformation first by the tension cracks and then enlarges the cracks to break down, which become various crack ships (fan, C-type). Drilling conditions, drill geometry and wear have great effects on them.2. Every material of PCB shows its unique drilling force property. The force reaches largest in the first force wave when drill the upper copper foil. It decreases in glass fiber/resin. The intertwined fibers cause great changes of drilling torques. The direction of chip movement shows large effect on the force. The same direction of feed and chip removal causes much larger force than opposite direction. The thrust depends more on the chip removal.3. For cutting conditions, feed has the greatest effect on the cutting force. It increases linearly with the increase of feed rate increases. The diameter of drills does not show the significant effect on the cutting force (D:2-3 mm). For the geometry of the drill, the web has greater effect on the trust, but not on the torque. Wet thinning can reduce the thrust. The point angle shows less influence on the force for its changing the direction of chips movement and hardly on the torque.4. The characteristics of force waves vary with different types of PCB. Evey material has its own best drill parameters. For the halogen-free plate (HF board), helix of 15°cause largest thrust, but other boards show the smallest. When point angle is 150°,all the three materials show the smallest force. Empirical formula of drilling thrust for FR-4 and HF has been obtained.5. The temperature between drill and workpiece is about 200℃. The maximum cutting temperature for copper foil of PCB is about 198.5℃, which is obviously larger than that of resin. With the increase of the feed rate, the temperature goes down. Most drilling heat is taken away by chips. The larger the web thickness and helix angle, the higher the drilling temperature. At the same time, it decreases with the point angle decrease. The influence of cutting speed on the temperature is greater than the feed rate.6. Based on thermo-mechanical coupling, the mechanism of the creation of holes of copper foil is verified by the finite element models. The SEM results of chip formation, cutting forces and temperature and experimental of PCB results are consistent.7. Carbide drill wear during high-speed PCB drilling has three stages, the initial wear, normal wear and severe wear. Carbide drill wear mechanism mainly includes abrasive wear, adhesive wear, chipping and micro-extrusion. Flank wear is the stable form of abrasion wear of the main, in the normal wear stage. Different parts of the drill wear are different. The drill point and the main edge of cutter exit abrasive and adhesive wear. Extruded plastic deformation occurs in the web. With the increase in the number of holes, tool flank wear a linear increase. The appropriate cutting parameters must be selected in order to reduce tool wear, improving tool life.8. Burr in drilling PCB is from the copper foil. Feed rate and the burr height are positively correlated in some degree. Small feed can improve the exit burr. Web has the greater effect on the burr generation, than the point angle, the least one is the helix angle. Less drill wear can also improve it. The fixed device and back-up board proves useful for it.9. A design method based on lower cutting forces and better chips removal was proposed. For the PCB fixed hole high speed drilling, chip groove and web thinning of the drill is the right choice. To decrease the cutting force and cutter wear, different kinds of drills are tried on. The single chip diving groove and web thinning drill show better results of chip break, the force decreased 20-30% from the thinning web. The new type drill shows the good performance in chip removing, avoiding of winding in the drill, which improves the quality of the drilling and drill life.