Creep of thermally aged SnAgCu-solder joints
ABSTRACT The creep behaviour of Sn96.5Ag3.5- and Sn95.5Ag3.8Cu0.7-solder was studied specifically for its dependence on technological and environmental factors. The technological factors considered were typical cooling rates and pad metallizations for solder joints in electronic packaging. The environmental factors included microstructural changes as a result of thermal aging of solder joints. Creep experiments were conducted on three types of specimens—flip–chip joints, PCB solder joints and bulk specimens. flip–chip specimens were altered through the selection of various under bump metallizations (Cu vs. NiAu), cooling rates (40 K/min vs. 120 K/min), and thermal storage (24 h, 168 h, and 1176 h at 125 °C). PCB solder joints were studied by using a copper pin soldered into a thru-hole connection on a printed circuit board having a NiAu metallization. Bulk specimens contained the pure alloys. The creep behaviour of the SnAg and SnAgCu solders varied in dependence of specimen type, pad metallization and aging condition. Constitutive models for SnAg and SnAgCu solders as they depend on the reviewed factors are provided.
Conference Proceeding: The influence of aging on the stress-strain and creep behavior of SAC solder alloys[show abstract] [hide abstract]
ABSTRACT: The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. In our prior work on aging effects, we have demonstrated that the observed material behavior variations of Sn-Ag-Cu (SAC) lead free solders during room temperature aging (25°C) and elevated temperature aging (125°C) were unexpectedly large and universally detrimental to reliability. Such effects for lead free solder materials are especially important for the harsh applications environments present in high performance computing and in automotive, aerospace, and defense applications. However, there has been little work in the literature, and the work that has been done has concentrated on the degradation of solder ball shear strength (e.g. Dage Shear Tester). Current finite element models for solder joint reliability during thermal cycling accelerated life testing are based on traditional solder constitutive and failure models that do not evolve with material aging. Thus, there will be significant errors in the calculations with the new lead free SAC alloys that illustrate dramatic aging phenomena. In the current work, we have extended our previous studies to include a full test matrix of aging temperatures and solder alloys. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on four different SAC alloys (SAC105, SAC205, SAC305, SAC405) that were aged for various durations (0-6 months) at room temperature (25°C), and several elevated temperatures (50, 75, 100, and 125°C). Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. Variations of the mechanical and creep properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) were observed and modeled as a function of aging time and aging temperature. The chosen sel- - ection of SAC alloys has allowed us to explore the effects of silver content on aging behavior (we have examined SACN05 withN= 1%, 2%, 3%, and 4% silver; with all alloys containing 0.5% copper). In order to reduce the aging induced degradation of the material behavior of the SAC alloys, we are testing several doped SAC alloys in our ongoing work. These materials include SAC0307-X, SAC105-X, and SAC305-X; where the standard SAC alloys have been modified by the addition of small percentages of one or more additional elements (X). Using dopants (e.g. Bi,In, Ni, La, Mg, Mn, Ce, Co, Ti, Zn etc.) has become widespread to enhance shock/drop reliability, and we have extended this approach to examine the ability of dopants to reduce the effects of aging and extend thermal cycling reliability.Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), 2010 12th IEEE Intersociety Conference on; 07/2010