An Implicit solver applies loads in steps and attempts to converge on a stable system before moving to the next step. For a step to be complete, the structure must be in equilibrium, with internal and external forces balanced. Errors in contacts, boundary conditions, mesh, or material definitions can disrupt equilibrium, preventing convergence. 

Explicit solvers use a different method. Instead of converging on a state of equilibrium, an explicit FEA solver calculates forces as they propagate through the structure using wave propagation. Explicit simulation evaluates transient responses, computing a stable time increment based on the smallest element, which is much shorter than a time step in an implicit simulation. Explicit FEA is typically used for shock events lasting milliseconds.