Finite element analysis of thermal stresses in laser packaging

A finite -element method (FEM) analysis is performed on the calculation of residual stresses during spot -welding for Au- coated Invar materials. Numerical results show that the high residual tensile stresses of the phosphorus rich segregation layer generated by rapid solidification shrinkage is the possible cause for crack formation. This indicates that the FEM calculations may provide one of the effective methods for predicting the crack formation in laser -welded Au- coated materials Keywords: Crack, thermal stress, laser packaging, finite -element method 1. INTRODUCTION To enhance the solderability for chip and wire bonding, optoelectronic materials of Invar' or Kovar2 of very low coefficient of thermal expansion (CTE) are often coated with a Au thin film. There have been well documented in the area of thin film coating that an inadequate thickness of Au coating on optoelectronic materials in laser welding process can cause undesirable reactions such as crack defects in the welded joints34. Prior to Au plating on optoelectronic material, a Ni underlayer coating is often applied to improve adhesion. The Ni underlayer can be formed by P -free electroplating or P- containing electroless plating, denoted hereafter as Ni and Ni(P), respectively. However, the chemical reducing agent NaH2P02 is required in the electroless Ni(P) plating process, which introduces additional phosphorus (P) element in the Ni(P) underlayer. Recently comprehensive measurements of laser -welded Au -, Ni -, Ni(P), and Au/Ni- coated Invar have shown that the existence of the P element content in the Ni(P) underlayer instead of the Au element in the Au plating layer play a major role in determining the crack formation in laser welded Au- coated optoelectronic materials8. The purpose of this work is to study the solidification crack formation mechanism in laser -welded Au- coated optoelectronic materials due to P- containing underlayer by using finite- element method (FEM). This work has led to important result that the FEM provides an effective method for predicting the crack formation in laser -welded Au- coated optoelectronic materials. 2. LASER WELDING SYSTEM AND PACKAGE CONSTRUCTION 2.1 Laser Welding System: Fig. 1 (a) shows the experimental setup of the laser welding system. The system consisted of a pulsed Nd: YAG laser and a dual -beam fiber optic beam delivery. Two laser beams delivered from the Nd:YAG laser to the workpiece were accurately adjusted with the same energy and with the incident angles of (45° ± 1 0)9. The laser energy required to create the welds was delivered simultaneously through two fibers placed 180° apart. The simultaneous and equal energy delivery is designed to reduce the post -weld -shift (PWS) in the two components because the solidification- shrinkage of both welds can compensate each other, resulting in minimized displacement shifts9. 2.2 Package Construction: A top view of dual -in -line package (DIP) indicating the pigtail fiber to the laser chip is also shown in Fig. 1 (a). The DIP construction consisted of a 1.3 j.tm laser, the Invar housing materials, a thermoelectric cooler, W.H. Cheng (correspondence): E -mail: whcheng@eo.nsysu.edu.tw; Telephone: (886) 7 -525 -2000 ext. 4450; Fax: (886) 7 -525 -4499 Part of the SPIE Conference on Optoelectronic Materials and Devices Taipei, Taiwan July 1998 SPIE Vol. 3419 0277 -786X/98/$10.00 93 Finite-element analysis of thermal stresses in laser packaging Maw-Tyan Sheen#, Cheng-Huang Chen*, Jao-Hwa Kuang', and Wood-Hi Cheng* #Mh•c1 Engineering Department and *Jjj ofElectro—Optical Engineering, National Sun Yatsen University, Kaohsiung, Taiwan 804 Huang-Lon Chang, Szu-Chun Wang, Chungyung Wang, Chy-Ming Wang, and Jy-Wang Liaw Chunghwa Telecom Laboratories, 12, Lane 55 1, Min-Tsu Rd, Sec. 3, Yang-Mei, Taoyuan, Taiwan 326 ABSTRACT A finite-element method (FEM) analysis is performed on the calculation of residual stresses during spot-welding for Au- coated Invar materials. Numerical results show that the high residual tensile stresses of the phosphorus rich segregation layer generated by rapid solidification shrinkage is the possible cause for crack formation. This indicates that the FEM calculations may provide one of the effective methods for predicting the crack formation in laser-welded Au-coated materials