For predicting the operating cycles to fatigue, the effect of temperature cycling is evaluated. Nonlinear material properties of the solder joint are employed to evaluate the rate of non-elastic stress accumulation. The Darveaux model is used to overcome the problem of stress singularities in material interfaces. Using these procedures, the fatigue life of a design can be evaluated, relative to other designs.
The challenges of modeling the BGA arise from a number of conditions:
- The small length scale of the most vulnerable part (solder) relative to the rest of the component
- The higher & ever increasing # of solder bumps/joints
- The material interfaces & resulting stress singularities
- The nonlinear nature of the solder material (creep, plasticity, Anand’s commonly used)
- Because many of these effects are interdependent, predicting the reliability of solder joints in devices is a complex problem. ANSYS Multiphysics has emerged as the industry and academic standard for modeling these issues.
Critical ball plastic work density evolution through first cycle.
Electromigration is a difficult coupled physics problem involving structural, electric, thermal and diffusive terms. Ansys Multiphysics has had the capabilities to solve this challenging problem. In addition to developing a training course on this topic, Ozen Engineering also has streamlined the process with an ACT Extension.
Drop tests are a JEDEC (JESD22-B111) requirement that can be evaluated in silico to save physical testing time and money. Here the overall assembly is modeled first and a detailed submodel of the solder joints is subsequently created and analyzed. This is one approach to the length scale challenges of the solder joints relative to the entire device, yielding a very efficient simulation.