Determining the physics of failure after a controlled experiment or after a field failure is essential in understanding products and their limitations. Finding a root cause is beneficial to decrease repeat failures and set the specification limits on products. Using failure analysis along with understanding the basic materials science behind the product is vital in resolving problems and customer satisfaction. This Physics of Failure section will include the “why's” of failure, including diffusion, metal migration, corrosion, phase diagrams and solder properties.