Study On Cutting Performance Of Drill For Supported Hole Of Printed Circuit Board

Printed circuit board (PCB) is the substrate board and key interconnect for integration and assembly of various electronic components. It is seen almost in every electronic device. PCB support holes are generally large holes whose diameter are greater than 2 mm. They are non-plated through hole, including mounting hole and toaling hole, which is used to fix the board. PCB is difficult-to-machine composite material which consists of resin, glass fiber and copper and so on and their material anisotropy is obvious. In the process of drilling PCB supporting holes, the problems of large drilling force, difficulty of chip evacuation and drill wear significantly reduce the efficiency of production.In this paper, the PCB drill for supporting holes was taken as the research object, using ordinary FR-4 boards and Halogen-free boards (HF) as workpiece materials. The process of holes formation was widely studied by a high speed photography camera; The process of chip formation and process of chip removal was analyzed. The influence of process parameters (spindle speed and feed rate) on the drilling forces, chip morphology and drill wear were researched and the cutting parameters were optimized; The cutting performance of different structure drills were studied; The general principles for improving cutting performance of drills were analyzed; Several new structure drills were designed and the optimized new structure drill achieved good cutting performance by experimental testings.The main conclusions of this paper are as follow:(1)The valuation for drill of PCB supported hole has some characteristics:drill chips and their removals, drilling force, tool life and wear,.(2)Drill chips and their removals:the aluminum entry board and copper foil is plastic metal material. When it is extruded and scraped by cutting edge and rake face of the drill, plastic shear deformation occurred The resin-glass fiber is removed in the form of brittle fracture. The chip roll to side because that the stress of inside and outside surface of the chip is different, the cutting velocity gradien of each point on the cutting edge of is large, and the rake angle of inside and outside edge varies considerably; The chip of resin-glass fiber can be categorized as spiral, fan, flat ribbon shap; The optimization of drill structure and cutting parameters can reduce the size of the chip and side roll of the chip, so as to improve the chip evacuation.(3)Drill frorce:in the process of drilling printed circuit board, the basic features of dilling thrust force and torque are closely related to the structure of the PCB material. Thrust force reaches to a peak when the drill contacts with the upper and bottom copper foil, and the first peak is larger than the second one. There are big fluctuations in the waveform of drilling torque. The thrust force decreases with the spindle speed and increases with the increasing feed rate. Spindle speed increases, the torque fluctuated in a certain range. Feed rate increases, the torque tend to increases. The thrust force and torque of drilling environmental HF board is larger than that of drilling ordinary FR-4 board. Drill structure is an important factor which affects thrust force and torque. Thinner chisel edge can reduce the drilling axial force significantly. Torque is much smaller than the thrust force, so thrust force should be used as the important evaluation indicator for the cutting performance of PCB drill and optimization of drilling parameters. Through orthogonal exprimental, the drilling parameters aiming to minimum cutting force were optimized, taking improving drilling efficiency into account. Multiple linear regression method was used to establish the empirical formula of the thrust force and torque.(4)Drill wear:drill wear form is mainly the rake face wear of the chisel edge e, flank wear of main cutting edge and conner wear of the outer turning point of the main cutting edge; Chip-break groove can improve the chip evacuation, take away most of the heat, and dcrease the cutting temperature, extended drill life; Drill wear increases with the spindle speed and decreases with the increasing feed rate. The drill wear of drilling environmental HF board is greater than that of drilling the ordinary FR-4 board. The spindle speed of drilling HF board should be lager than that of drilling common FR-4 board.(5)Optimum of drill:Both reduces chisel edge and opening chip-break groove can improve the cutting performance of drill bits significantly. The choice of number, size and shape of chip-break grooves should be reasonable. Based on Pro/E technology, a special model of PCB drill has been established, which has chip-groove and web thinning. Several new structure drills were designed and the optimized new structure drill achieved good cutting performance by experimental testings. The performance chip-breaking of double chip-breaker drill is improved significantly and drill life of double chip-breaker is longer than that of standard drill and single chip-breaker drill.