Thermal Cycling Aging Effect on the Shear Strength, Microstructure, Intermetallic Compounds (IMC) and Crack Initiation and Propagation of Reflow Soldered Sn-3.8Ag-0.7Cu and Wave Soldered Sn-3.5Ag Ceramic Chip Components

Abstract

Temperature cycling of electronic components was carried out at two different temperature profiles, the first ranging between -55degC and 100degC (TC1) and the second between 0degC and 100degC (TC2). Totally, 7000 cycles were run at TC1 and 14500cycles at TC2. The test board's top-side components were surface mounted using Sn-3.8Ag-0.7Cu solder alloy, and the bottom side surface mount devices (SMD) were wave soldered with Sn-3.5Ag alloy. The solder joint degradation was investigated as a function of cycle number by means of shear force measurements and cross-sectioning. The shear force drop was correlated to both crack initiation time and propagation rate, and microstructural changes. The effect of manufacturing process (reflow versus wave soldering) and component size (0805 versus 0603 components) on the shear force were also investigated. For both reflow and wave soldered components, the harsher the test environment the faster and largest the decrease in shear force. The shear force is higher for the 0805 components compared to the 0603. The effect of component size on the residual shear force is higher for the testing condition TC1. TC1 also seems to have a higher effect on the residual shear force compared to TC2. The main difference between wave soldered and reflow soldered components is that the shear force is in average higher for the wave soldered components compared to the reflow soldered. For the reflow soldered components using SAC, the microstructure coarsens as a function of temperature cycling, especially the Ag3Sn intermetallic particles. Furthermore, this alloy shows an increase in intermetallic compound (IMC) layer thickness (Cu-Ni-Sn), and its growth is controlled by a diffusion mechanism. The IMC growth coefficient is for the SAC system tested at TC1 0.0231 mum/h1/2 (0.00053 mum/h) and for TC2 0.0054 mum/h1/2 (2.9*10-5 mum/h). The microstructural changes during thermal cycling are a result of both static and strain-enhanced aging. For the wave soldered components the microstructure also became coarser, however, the IMC layer (Ni3Sn4) thickness did not change. The IMC layer growth does not affect the shear force for the test conditions applied in this work. The shear force decrease observed in the present work as a result of thermal cycling is a result of both microstructural coarsening and crack propagation inside the solder joint.