Fatigue Crack Growth Behavior of Small Sn-Bi-Ag Solder Joints
- PDF / 874,879 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 52 Downloads / 217 Views
Tao Liu, M.A. Korhonen, S. Ting, D. Kim and C.-Y. Li Department of Materials Science and Engineering, Cornell University Bard Hall, Ithaca, NY 14853
Abstract Reliability of solder interconnects is normally characterized by extrapolation of bulk solder reliability data, such as crack growth measurements of solder materials in the bulk form. In the present study, experimental evidence will be presented to show that crack growth, which is the most commonly observed failure mode in small solder joints strongly depends on the size and shape of the joint cross-section. Furthermore, the crack growth rate can be appropriately correlated to the size of the intense plastic yielding zone at the crack tip using a plastic decohesion argument. Introduction Thermal fatigue behavior of solder interconnects is a very important reliability issue in first and second level packaging. A host of literature has been published on deformation and fatigue studies of solder alloys. In most studies, however, bulk specimens of solder alloys were used to characterize the behavior of the finite geometry real solder joints used in microelectronic packaging, which may be only some tens of microns in crosssection, as e.g. flip-chip solder joints. In only a few references can deformation and fatigue data for small, actual size solder joints be found [1,2]. As a result of recently rekindled interest in flip-chip bonding technology with varied solder compositions, accurate characterization and quantification of fatigue properties of small solder joints are required. In this paper we discuss fatigue crack propagation in solder joints and how it is affected by the size of its cross-section. Experiment The solder tested in this study was a lead-free 57Bi/41Sn/2Ag alloy used to join pairs of Cr/Cu/Au metallized contact pads patterned onto silicon. Large (0.4mm diameter) and small (0.075mm diameter) commercially acquired solder spheres were used to make solder joints between contact pads of 0.625mm and 0.05mm diameter, respectively. The smaller solder joints were made by placing a single solder sphere directly between a pair of contact pads followed by a reflow. Solder joints made in this fashion featured a characteristic barrel shape with heights of -0.066mm. The larger solder joints were made by first reflowing solder spheres onto individual contact pads and then joining two of the resulting solder bumps together and conducting a second reflow. Two distinct joint geometries were achieved by varying the joint height: a barrel-shaped joint of 0.254mm
177
Mat. Res. Soc. Symp. Proc. Vol. 390 @1995 Materials Research Society
height, and an hourglass-shaped joint 0.457mm in height with a minimum cross-sectional diameter of -0.450mm. In the actual tests, the solder joints were mounted on a total displacement controlled micro-mechanical tester using cyanoacrylate adhesive (super glue), and room temperature, isothermal mechanical tests were performed. The total shear strains and shear strain rates for each joint geometry are listed in Table- 1. Shear displac
Data Loading...
