Studies on the Punching Characteristics of Rigid Copper Clad Laminates

Transcription

1 Journal of National Kaohsiung University of Applied Sciences, Vol. 33 (2004), pp Studies on the Punching Characteristics of Rigid Copper Clad Laminates Quang-Cherng Hsu * Abstract In this paper, a measurement system for a punching test for small holes in rigid copper clad laminates is presented. Copper clad laminate is a main raw material for consumer electronics, information and telecommunications devices. The process parameters such as blank temperature, blank thickness and material specifications are closely related to the characteristics of the punching forces and displacements in copper clad laminates. The proposed system consists of small-scale force transducer, displacement transducer and data acquisition system, with which the punching forces can be measured in each punch steps. A small punch with outer diameter 2 mm is directly connected to the force transducer to measure the pushing and pulling forces during the hole-punching process. Bushings and guide pins are used inside the die configuration in order to ensure path straightness when the punch moves into the die cavity. The experimental results show that the increase of punch and die clearances decreases the punching forces. The same result is obtained with increase of blank temperatures. In the final part of the paper, a computer-aided analysis package DEFORM 2D is used to understand the deformation and crack initiation of the blank in the small hole-punching process. The analytical maximum punching force agrees with experimental result. The research results are a useful database when designing requirements of punch and ejecting force during manufacture. Keywords : Copper clad laminate, Printed circuit board, Punch test, Finite element analysis 1. Introduction Copper clad laminate (also known as printed circuit board) is a main raw material for consumer electronics, information and telecommunication devices. The punched or drilled small holes in rigid copper clad laminates can be used as supporting basements where many electric parts such as sockets for CPU, LED, resistance, etc. can be inserted. The quantity of drilled holes by high-speed drilling machines is higher than that of punched holes by mechanical presses, however, the productivity of the former is less than that of the latter (Dai, 1995). When holes are cut in the blank or the product being manufactured, the operation is called punching. The size and shape of the holes cut are almost unlimited as long as they are in the part. Holes may be punched in the scrap skeleton so that locating pilot pins may be used to position the sheet metal correctly in a progressive die (Eary, 1977). During punching, the workpiece is first elastically, then plastically, deformed by the action of the punch. Fracture starts at the regions of maximum stress concentration (sharp edges of Received December, 2003; revision received February, 2004; Accepted March, * Quang-Cherng Hsu: Associate Professor of the Department of Mechanical Engineering, National Kaohsiunt University of Applied Sciences.

2 32 Quang-Cherng Hsu punch and die) and propagates into the workpiece along regions of maximum shear stress. If the punch-die clearance is appropriate, the cracks interact in the center of the workpiece. If not, additional tearing is required (Meyers, 1984). The suitable punch and die clearance guidelines are given by (ASM, 1988) for low-carbon steel. Chen (1996) has evaluated the impact energy of copper clad laminates by using a falling weight impact method. The temperature of the specimen is the major process parameter in his research; however, this testing method is different to real punching processes. Aoki, etc. [1999] have studied the clearance effect of fine blanking between punch and die cavity by using the Fourier correlation method and found that the strain becomes large near the cutting edge when the clearance becomes small. In this paper, shearing theory is first studied. A punching die configuration for small holes is designed. The elastic deformation and stress concentration of the punch are studied and substantially proportional increase in punch force with punch displacement from O to A represents the predominately elastic phase of the process. From A to B, plastic shear deformation occurs and the manner in which the magnitude of the punch force changes with the punch displacement reflects the strain-hardening characteristics of the material. The maximum punching force is attained at B and from B to C although further strain-hardening of the material may occur, the punch force decreases in proportion to the decrease in the vertical area of shear. Beyond C, the initiation of cracks, no further plastic deformation occurs and the punch force decreases rapidly. From D to E there is frictional resistance between the blank and the hole, the punch and the hole and also between the blank and die. The loop from G to O represents the return of the punch to its initial position, and the negative ordinate indicates the magnitude of the stripping force which is required to remove the strip from the punch (Johnson, 1973). verified according to ANSYS (1994) simulation results. Two process parameters, such as die maximum punch force plastic deformation with strain hardening initiation of crack clearance and blank temperature are tested. In the PUNCH futher plastic deformation but shear area decreases final part of the paper, the hole-punching process is simulated by using the DEFORM 2D (2000) PRESSURE PLATE BLANK DIE PUNCH FORCE frictional resistance stripping of blank finite element package. CLEARANCE from punch PUNCH DISPLACEMENT 2. Theoretical methods 2.1 Shearing Theory A diagrammatic arrangement of a holepunching process and a typical hole-punching force and displacement diagram accomplished with its processing stage are shown in Fig.1. The (a) Fig. 1.(a)A diagrammatic arrangement of a holepunching process. (b)a typical hole-punching force vs. displacement diagram. 2.2 Finite Element Analysis Using the finite element method on a digital computer, it becomes possible to establish and (b)

3 Studies on the Punching Characteristics of Rigid Copper Clad Laminates 33 solve the governing equations of complex problems in a very effective way. A most important formulation, which is widely used for the solution of practical problems, is the displacement-based finite element method (Bathe, 1982). The displacement- based finite element method can be regarded as an extension of the displacement method of analysis, which had been used for many years in the analysis of beam and truss structures. The most important steps of the complete analysis are the proper idealization of the actual problem, and the correct interpretation of the results. The ANSYS program has many finite element analysis capabilities, ranging from a simple, linear static analysis to a complex, nonlinear, transient dynamic analysis. A typical ANSYS analysis has three distinct steps: (1) build the model, (2) apply loads and obtain the solution, (3) review the results. ANSYS software was used to analyze the deformation and stress concentration of punch according to linear elastic theory. DEFORM 2D is another finite element based process simulation system designed to analyze various forming processes based on flow formulation and large plastic deformation theory. With a suitable damage model, such as Crokcroft and Latham s criterion, the cutting and shearing process can be simulated. 3. Experimental Procedures A knuckle-joint press is available in the department, therefore, the research work focuses on the design and manufacture of a small hole punching die and the fabrication of a force and displacement measuring module. In order to understand the deformation characteristics, die clearance and blank temperature have been varied by means of machining and heating. 3.1 Die Design There are many reasons resulting in punch failure or die breakage which are the major considerations of die design in the research: (1) The length of punch exceeds the buckling limit, however no further modification is applied. (2) The central lines of holes both in the punch supporting plate and the pressure plate are mis-aligned, therefore, bending moment in the punch will occur after assembly of the die set. (3) The central line of the punch does not coincide with the die cavity, therefore, the punch will directly hit the die edge. (4) The pressure plate automatically translates or deforms due to side force during punching. According to the previous points, an explosive view of the die configuration being designed is shown in Fig. 2. The design considerations of the die components are as follows: (1) Punch. A standard part is used for easy change or replacement in case of punch wear or failure. (2) Punch-fixing block. The punch is supported in the punch-fixing block that is connected with the load cell in order to measure pulling force and pushing force during the punching process. (3) Load cell-supporting block. The punch- supporting block can contain the load cell as a supporting basement. The second purpose is to fix guide

4 34 Quang-Cherng Hsu pins. (4) Guide pin. Two guide pins are used to ensure a suitable alignment of punch and die cavity during the punch process. (5) Stripper plate. A stripper plate is used to split the punch head from the specimen hole. (6) Die insert. In order to proceed with a punching test using different holes diameters, a die insert with varying hole-diameter is used. (7) Die block. The die block is a basement to support the insert, which is assembled into a die set. Load cell Load cell supporting block Punch fixing block (1) Punch Punch fixing block (2) Guide pin Stripper plate Die insert Die block Fig. 2.Explosive view of die configuration. 3.2 Verification of Die Design The diameter of the punch is small, however it will not buckle according to the calculation from Euler s equation of column theory (Shigley, 1983). In order to calculate the maximum deformation of the punch, an ANSYS finite element analysis package is used. The following are the input data for ANSYS analysis of small punches (SKD11): (1) Element Type: SOLID45; (2) Material Properties: Isotropic; (3) Young Modulus: 209,000 N/mm 2. The applied boundary conditions are as follows. (1) Pressure: specified at the front tip of the punch while the equivalent pressure 312 (N/mm 2 ), that is calculated according to maximum peak force 100 Kgf which is applied on the circular area of 2 mm diameter; (2) Displacement: specified at the back tip of the punch with zero displacement. According to ANSYS analysis, the maximum Von-Mises stress of N/mm 2 occurs at the inter-connecting corner of the punch stem which is less than the ultimate compressive strength of SKD11 (2058 N/mm 2 ). The maximum deformation of the punch is mm which is smaller than the thickness of the blank (1.56 mm). Based on the previous analysis, reasonable die design results are obtained. 3.3 Force and Displacement Measuring Module Measurement of the reaction force of the punch suffered from the specimen is a key point of the research. Fig. 3 shows that the pushing and pulling forces can be directly measured through the punch fixing block which is connected to the load cell. Through this design, although the punch size is small, it can be supported firmly.

5 Studies on the Punching Characteristics of Rigid Copper Clad Laminates 35 Another advantage is that the punch can be changed easily. The signal of the load cell was recorded through AD/DA interface and hard disk of computer. A block diagram of the measuring system is shown in Fig. 4. Fig. 3.Pushing and pulling forces can be measured through a load cell. Monitor PC Mouse Amplifier AD/DA Interface Adapter Force transducer Displacement transducer Power Fig. 4.Block diagram of measurement system. 4. Results And Discussion 4.1 Experimental Results In order to realize the accuracy of die assembly, especially the concentricity of the punch and die insert, a punching test without a specimen is conducted. Fig. 5(a) shows the punch force and displacement being measured against the cycle time; (b) shows the X-Y plot of force and displacement. The small resultant forces (within Kgf) show good concentricity of punch and die insert. However, the vibrating forces show that the stripper plate supported by the load cell fixing block through a spring and bolts as well as the vibration of the forming machine have some dynamic influences on the load cell. The real punching test with specimen (copper clad laminate with blank thickness 1.56 mm) was performed also. Figure 6 shows the punching force and displacement diagram. (1) Peak a is the contact force between the stripper plate and the die insert. (2) Peak b is the fracture force of specimen. (3) Peak c is the friction force between the blank and die cavity. (4) Peak d is a strange peak force which may result from the blank accumulating into the die cavity and needing more pushing force to cover the friction force due to more contact area, as shown in Fig. 7. In order to realize the reason for force peak d (in Fig. 6), the die cavity is machined by draft angle of 5 degrees, and another punching test is performed which no longer shows the force peak d exists as shown in Fig. 8. In order to understand the effects of process parameters such as the clearance of punch and die cavity, and the temperature of the specimen, repetitive tests are conducted. Figure 9 shows force and displacement diagram with large clearance (more than 5% punch diameter). The peak forces at c and d no longer exist, and the maximum punching force decreases. The maximum punching force also decreases when the blank temperature increases as

6 36 Quang-Cherng Hsu shown in Fig. 10. Fig. 6.(a)Punching force and displacement diagram against cycle time (24 C and tight clearance between punch and die cavity). (b)diagram of punching force vs. displacement. (a) Punch Specimen Scrap Die insert (b) Die supporting plate Fig. 7.Scrap accumulated in die cavity due to original die design without draft angle. Fig. 5. (a)punch force and displacement being measured against the cycle time without specimen. (b)diagram of force and displacement at punching test without specimen. (a) b c d Fig. 8.Force and displacement against cycle time with adequate draft angle of 5 degrees in die cavity (no longer force peak d exists). a 150 Load (Kg) (b) a b c d kg ; mm 100 (a) b 50 a c ms Displacement (mm)

7 Studies on the Punching Characteristics of Rigid Copper Clad Laminates 37 Kg) Load ( (b) a b c Displacement (mm) Fig. 9.Force and displacement diagram with large clearance between punch and die cavity. (a)force and displacement diagram against cycle time. (b)diagram of force vs. displacement. constant because there is no rate effect in this analysis work. Figure 14 shows the flow net of punched hole simulated by DEFORM 2D indicating the sharp deformation along the corners of punch and die. Figure 15 represents the crack initiating from the corner of the die and propagating to the corner of the punch. The fracture model adapted in the analysis is normalized Cockcroft and Latham fracture model whose value is set to The load-stroke of hole-punching process at room temperature simulated by DEFORM 2D is shown in Fig. 16 which shows good agreement of the maximum punching load between analytical and experimental results at room temperature. This value is typically important for engineer to decide the suitable press capacity. Fig. 10.Maximum punching force decreases when the blank temperature increases. 4.2 Analytical Results The die configuration for punching process simulation is depicted in Fig. 11 including punch, pressure plate, blank and die. Due to axisymmetric deformation of the hole-punching process, only one-half cross section is analyzed. In order to obtain fine meshes for improving accuracy of finite element analysis near the corner of the punch and die, the mesh density windows are set also as Fig. 11 shown. Figure 12 shows the irregular meshing for hole-punching analysis. Figure 13 shows the flow stress diagram of copper clad laminate which can be decided from tensile test. The Columbus friction factors between all objects are The punch velocity is just assumed Fig. 11.Die configuration and mesh density windows for DEFORM 2D analysis of hole-punching process. Fig. 12.Irregular meshing for hole-punching analysis because the fine meshes are needed near the corners of punch and die.

8 38 Quang-Cherng Hsu (b) Fig. 13.Flow stress of copper clad laminate at room temperature. Fig. 15. (a)crack initiation predicted by normalized Crokcroft and Latham fracture model; (b)crack propagation. Fig. 14.Flow net of punched hole simulated by DEFORM 2D. Fig.16. Load and stroke of hole-punching process of copper clad laminate simulated by DEFORM 2D. (a) 5. CONCLUSIONS To design a small punch system, the concentricity of the punch and the die cavity is the key point to consider. That is, when the diameter of punch decreases the punch will probably buckle or fracture due to an ill assembled die set. This can be avoided by designing suitable guide pins and bushing. From practical testing, the draft angle of the die cavity affects the peak force during punching. With suitable draft angle only one peak force (the fracture force) will exist. The experimental

9 Studies on the Punching Characteristics of Rigid Copper Clad Laminates 39 results also show that increase of punch or die clearance decreases the punch force. The same consequence is obtained with increasing blank temperature. By means of ANSYS finite element analysis, the maximum stress and displacement in small punches can be obtained. This information can be used as a useful die design guide due to the difficult calculation for irregular punches. By means of DEFORM 2D, the flow net for punched holes, the load-stroke of the punching process, and crack initiation and propagation in blanks are obtained. The analytical maximum punching force agrees with the experimental one. The research results are a useful database in die and punch design of printed circuit boards. 7. Dai, I.-C., Press Working and Die Design, XinRu Book Co., Taiwan (1995) (in Chinese). 8. Eary, D. F., and Reed, E. A. Techniques of Pressworking Sheet Metal An Engineering Approach to Die Design, Prentice-Hall, New Jersey (1977). 9. Johnson, W., and Mellor, P. B. Engineering Plasticity, Van Nostrand Reinhold, London (1973). 10. Meyers, M. A., and Chawla, K. K., Mechanical Metallurgy Principals and Applications, Prentice-Hall, New Jersey (1984). 11. Shigley, J. E., and Mitchell, L. D., Mechanical Engineering Design, McGraw-Hill (1983). References 1. Aoki, I., and Takohashi, T., Visioplasticity analysis of fine manufacturing of Fourier phase correlation method, Advanced Technology of Plasticity, Proceeding of the 6 th ICTP, III, pp (1999). 2. Anon, ANSYS Basic Analysis Procedures Guide, Release 5.5, ANSYS Inc. (1994). 3. Anon, DEFORM 2D Version 7.0 User s Manual, Scientific Forming Technologies Co. (2000). 4. ASM International ed., Metals Handbook, Ninth Edition, Vol. 14 Forming and Forging (1988). 5. Bathe, K.-J., Finite Element Procedures in Engineering Analysis, Prentice-Hall, New Jersey (1982). 6. Chen, M. F., Characteristics of CEM-1 copper clad laminate punching test, Chemical Engineering Technology, Vol. 5, pp (1996) (in Chinese).

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