Dynamic Characteristics Of Micro-drilling On Printed Circuit Boards

Micro-holes are important parts of the printed circuit boards(PCBs). With the fast development of plating and packaging technologies, high density interconnection(HDI) PCBs has become popular. The mechanical drilling of PCBs is facing significant challenges due to the apertures of drilled holes of PCBs are becoming smaller and smaller, which means an increased demand on the life span of micro-drills and the quality of drilled holes. In the field of HDI PCBs drilling, increasing the life span of micro-drills, advancing the quality of drilled holes and improving micro-drilling behaviors have drawn much attention.Focusing on the fracture problem of PCB drills, the aim of this paper is to reduce the micro-drilling machining vibration and improve the micro-holes quality. Point of departure from dynamics, the cutting mechanics, mechanical vibration, material science and multi-body dynamics theory are adopted, and non-contact laser vibration measurement technology and dynamic vibration measurement technology are used. Massive drilling experiments and dynamic simulation experiments are set to study dynamic characteristics of PCB micro-drills and micro-drilling machine during process. Then, a detailed analysis on dynamic characteristics of the micro-drilling system and CPK is presented. In according to the drilling parameters, the micro-drilling system stability is studied. The research results enrich the drilling theory of PCB micro-drills, provide guidance to the design and application of PCB micro-drills, instruct reasonable design and choice of drilling machine for PCB micro-holes, and optimize parameters of PCB micro-drilling. It is theoretically valuable and useful in improving the micro-holes quality..The main achievements of the research are as follows:In the study of instant dynamics characteristics of micro-drills, Introducing coupling node to propose a new method for exerting load on micro-drills. At the beginning of micro-drilling, the instant displacement of the chisel edge is relatively big, the vibration is significant too and the instant stress is the biggest. During the process of drilling, the instant displacement of X and Y directions are relatively big, and the vibration is obvious while that of Z direction is relatively small and the vibration is small as well. At the end of drilling, the instant displacement and vibration of the main cutting edge reduce, and the instant stress value decreases accordingly. During the process of drilling, the equivalent stress value of the main cutting edge is the biggest, and the wear is big too. The largest width of wear appears on the brim of the main cutting edge. It is pointed out that the root of the spiral grooves of micro-drills is the relatively weak part. The main reason for micro-drills fracture is the torque overloaded.Thus, the rigidity of the root of the spiral grooves should be enhanced.In the study of dynamical characteristics of micro-drills, a full parameterized model of PCB micro-drills is created for researching dynamical characteristics and influence factors of dynamical characteristics. It is found that the mode shapes of vibration of PCB micro-drills with the spiral blade quasi symmetrical structure are respectively around the X axis or Y axis in pairs, and all are bent mode shapes of vibration, especially the vibration mode shapes of 5th order and 6th order are bent and twisting. It is pointed out that the axial stiffness of micro-drill is bigger than the radial stiffness. The natural frequencies of the micro-drills in all stages reduces as the overhanging length of the micro-drill increases. The best overhanging length of the D=0.1mm micro-drill is 20 mm and 23 mm which are consistent with the values applied in the factory. No obvious rules of the influence of transition lengths on the natural frequencies in all stages of micro-drills are observed. The natural frequencies in all stages of micro-drills are related to the diameter of micro-drills as the web thickness changing. For D=0.1 mm micro-drill, the natural frequencies of all stages does not change obviously with the web thickness of micro-drill changing. For D=0.2 mm and D=0.3 mm micro-drills, the natural frequencies of all stages increases as the web thickness of micro-drill increasing.The change of the chamfer angle has a modest impact on structure parameters of micro-drills, optimizing the chamfer angle of micro-drills have no influence on the dynamical characteristics.In the analysis of multi-body dynamics for PCB drilling machine, a multi-body dynamical model of PCB drilling machine is created. Through analyzing the dynamical characteristics of the PCB drilling machine, a curve of logarithm and amplitude-frequency obtained to show the direction displacement and acceleration of X, Y, Z as the PCB drilling machine operating. Thus it is found that the beam is the weak part of the drilling machine. It is proposed to strengthen the rigidity of the beam and optimize the principles of designing the shape design of cross section of the beam so as to improve the machining stability of the drilling machine. Finally, it is proposed that the drilling acceleration frequency of the PCB drilling machine responses to the mathematical model, providing theoretical basis for further optimizing designs of PCB drilling machines.In the study of dynamical processing and machining stability of PCB micro-drilling, a dynamical model of PCB micro-drilling of is built, and a mathematical regression model is built to show that drilling vibration acceleration and drilling torque varying with spindle speed and feed speed. Drilling vibration acceleration augments as spindle speed and feed speed increasing. However, it reduces fast after reaching a certain value. The trend of drilling torque as spindle speed changing is similar. However, drilling torque and feed speed have a linear relationship, drilling torque increases as feed speed increasing. The typical vibration curve of PCB micro-drills indicates that the drilling process is unsteady and the peak value are obvious when drilling the first and the second layers of copper foil. With the conduct of drilling process, drilling vibration acceleration decreased. Higher spindle speed, faster the vibration damping, and the less time needed to achieve stable drilling. The peak value of pressure foot increases with the feed speed increasing. There is no obvious change of vibration as the feed speed changing during micro-drilling. The change of vibration has little to do with return speed during micro-drilling.In the study of surface quality of PCB micro-holes, there is an obvious relationship between CPK and vibration characteristics of spindle, micro-drill diameter as well as the wear of micro-drill.The vibration acceleration of micro-drill with weak rigid and small diameter is large. Axial force of micro-drills with strong rigid and large diameter is large in drilling, so the vibration acceleration is large too. The minimum vibration acceleration of micro-drills which owns medium diameter, and its CPK is bigger. The less the drilled holes are, the higher the average vibration acceleration and the lower CPK. As the drilling holes increases, the drilling machine achieves a stable status and then the average vibration acceleration reduces while the CPK increases. However, as the drilling holes increases, the wear of the micro-drill and the vibration acceleration increase while the CPK decreases.