Lead Free Solder

Chapter 1 provides an introduction on the use and application of lead- free solders in the semiconductor packaging and electronics manufacturing industry. The importance of solder joint reliability testing and assessments is described within a framework of a DFR methodology. This DFR methodology provides a systematic approach to understanding the mechanics of deformation of solder materials, characterizing the mechanical properties, constitutive models for creep and viscoplastic analysis, fatigue life prediction models, which are then employed in a finite element modeling and simulation analysis to assess the solder joint reliability performance.

Chapter 2 reviews the fundamental theory on mechanics of solder materials. As solder materials are subject to high operating temperatures relative to their melting point, the thermo-mechanical deformation response of the solder is dependent on both temperature and strain-rate conditions. Hence, the theory on mechanics of solder materials will focus on elastic-plastic-creep and viscoplastic models for describing the thermo-mechanical deformation response of lead-free solder materials operating over a wide range of temperatures (−40°C to +125°C) and strain rates (0.0001–1,000 s⁻¹).

Chapter 3 reports on experimental tests to characterize the mechanical properties for the apparent elastic modulus, yield stress, and ultimate tensile strength. These mechanical properties are highly dependent on test temperature and test strain rate. For high strain-rate test conditions, a Split-Hopkinson pressure bar (SHPB) test method was employed for measuring strain-rate influence on the yield stress of lead-free solder material. Comparisons of these mechanical properties were made for Sn–Ag–Cu, Sn–Cu, and Sn–Pb solder alloys. Strain rate and temperature-dependent mechanics of material models were curve-fitted for the range of temperatures (−40°C to +125°C) and strain-rates (0.0001–1,000 s⁻¹) tested. Creep tests results are presented in steady state creep models. A rate dependent viscoplastic deformation model by Anand, was fitted to the test data from the creep test and tensile test of Sn–Ag–Cu, Sn–Cu, and Sn–Pb solder alloys.

Chapter 4 deals with low cycle fatigue tests and derivation of fatigue life prediction models curve-fitted to the fatigue test data for Sn–Ag–Cu and Sn–Cu solder alloys. The fatigue tests were conducted under low cycle fatigue test conditions over a wide range of isothermal temperatures (−40°C to +125°C) and frequencies. Both the strain-range and energy-based low cycle fatigue models were used. The fatigue life prediction model constants are presented in tables and closed-form equations for easy use to predict the solder joint fatigue life when the inelastic strain range or inelastic strain energy density fatigue driving force parameter is obtained from an FEA result.

Chapter 5 reports on the application of FEA modeling and simulation techniques used for design of package assembly design for reliability. FEA modeling techniques have been developed for three-dimensional models, sub-modeling, and global–local modeling techniques. The global–local 3D modeling techniques like sub-modeling and Global–Local-Beam (GLB) methods, were applied to model solder joint reliability behavior for various test cases of thermal cycling, vibration, and impact drop tests. Implementation of the nonlinear mechanics of materials models have been integrated to the ANSYS finite element analysis program.

Chapter 6 reports on solder joint reliability test case studies for thermal cycling tests. Thermal cycling test and FEA of lead-free solder PCB assemblies for BGA and FCOB test specimens were evaluated to determine the Weibull cumulative failure distribution. Finite element modeling and simulation of this reliability test was employed to predict the solder joint reliability performance. Experimental study was conducted on lead-free 95.5Sn–3.8Ag–0.7Cu soldered assemblies provided by Solectron Technology [8]. The test variables include different packages (PBGA, PQFP) and different PCB board surface finishes on copper pads (Cu-OSP, ENIG, and Im-Ag). FEA modeling of the PBGA assembly with 95.5Sn–3.8Ag–0.7Cu solder joints and fatigue analysis was applied to predict the solder joint mean-time-to-failure life cycles. Failure analysis investigation on intermetallic compound or IMC layer growth subject to isothermal aging, thermal cycling, and thermal shock aging experiments were conducted to correlate IMC layer growth properties in lead-free 95.5Sn–3.8Ag–0.7Cu solder joint specimens. Highly Accelerated Life Test (HALT) approaches were also developed for lead-free 95.5Sn–3.8Ag–0.7Cu soldered PCB assemblies.

Chapter 7 focuses on vibration and drop impact test investigation. Experimental testing and FEA of the vibration mode and frequency for clamped–clamped PCB assemblies were compared. Constant amplitude vibration fatigue tests were conducted for the FCOB assemblies and test data were developed for 3G, 5G, and 10G respectively. Variable amplitude vibration fatigue tests for an increasing block loading of 3G-to-5G-to-10G repeated loading was conducted to develop cumulative damage index (CDI) vibration fatigue analysis methods using board-level fatigue data and solder material-level fatigue data. The Global–Local Sub-modeling technique developed was used in a quasi-static vibration fatigue analysis method for predicting the vibration fatigue life of the block loading test results. Impact drop testing is increasingly employed by many electronic product manufacturers to evaluate the product reliability to accidental or repeated drop events. Impact drop test and solder joint reliability investigations for Pb-based and Pb-free soldered assemblies were investigated. Explicit dynamic FEA modeling and simulation of the board-level drop test were used to predict the transient vibration deformation and acceleration from the drop test result. Dynamic stress strain analysis of the solder joints reveals fairly high plastic strain range generated and fatigue life prediction confirms the low cycle fatigue failure mechanism.

Chapter 8 deals with Board-Level Drop Test and reports on the effects of thermal cycling aging on board level drop reliability for lead-free SnAgCu (SAC) in fine pitch ball grid array (FPBGA) packages. The drop life was observed before and after thermal cycling aging. Comparison of SAC/ENIG packages to SAC/OSP packages was investigated. Thermal cycling (TC) aging subject to −40°C to +125°C causes concurrent degradation of the solder joints and result in IMC growth and thermal fatigue damage. The IMC growth, as well as the void/crack formation subject to thermal cycling aging will affect the long-term solder joint reliability performance. It can also affect the impact shock reliability as thicker IMC layer causes brittle fracture failure.