Finite element modeling of printed circuit board for structural analysis

Printed circuit boards having unfavorable deflection have been considered for a better mechanical design. During reflow process, pcb has experienced a range of the temperature between ambient temperature and peak temperature in furnace. Out of plane deflection is generated due to the property mismatch of material composition. Combination degree of board and copper plating will be researched for further investigation of trouble cause and design improvement. The soldering effect and mounting components weight are ignored considering input data complexity. And a set of real pcb has been exposed in reflow process and the deflection data for comparison are collected. In this study, setting up the analysis procedure and a finite element model for board deflection was focused. INTRODUCTION The demand for structurally reliable pcb has grown as electronic products are asked more functions, less weight and smaller sizes. It gives certain limitations and critical requirements. The unfavorable pcb deflection can ask extra effort for pcb design. Multimedia portable electronic products such as personal digital assistance and cellular phone, etc. are asking more and more. And finally new breakthrough i s needed. Before looking for the improvement, a source of problem and theoretical approach for main causes are sought. Several studies have been carried out to show a behavior of the bonded materials due to a mismatch of thermal and mechanical properties. The first solution, based on beam theory, was given by Timoshenko (1925). The problem of edge effects was first addressed by Aleck (1 949) The more recent solutions include elasticity solution of Kuo( 1989) and Lee(1991). The finite element solutions of this problem are discussed by Lau(1989). Pao( 199 1 ) tries to develop an analytical approach of multilayered stacks. His approach, which is based on an extension of Suhir's bimetal thermostat model(1986, 1988) provides a system of coupled linear second order diffeirential equations used in solving for the interfacial stresses. The purpose of this paper is to develop a proper finite element model describing a pcb strudtural instability and its boundary conditions. And the real examples from one of electronic industries (LG Electronics Inc.) are introduced. 0.4" thickness 2 sided surface mounted pcb has been experienced large out-of plane deflection in audio assembly. A mechanical grip like cross bar is used to keep the board straight This coinpulsion can 0-7803-3865-0197 $8.00 01997 IEEE 42 PEP'97 create unexpected stresses on pcb. Pcbs mounting thin QFP or thin SOP face double trouble due to a board deflection and package popcorning in CD-ROM. This pcb has a specified deflection limit in Fig.l for a pick and place machine and after reflow. Over this limit ~ . . . . _ _ _ . . . . , . . . . . . . . . . ' . . . . ' a