Portable electronic devices such as digital cameras, mobile phones, and PDAs use printed circuit boards (PCBs). Due to increased demands for convenience and multi-functionality, these devices are designed with a focus on miniaturization to accommodate a higher density and smaller dimensions of integrated circuit (IC) packages. These design constraints require smaller solder joints with finer pitch, contributing to the vulnerability of interconnections at the board level. Exposure to harsh dynamic loading environments during transportation and customer use are a critical issue for PCBs. PSD analysis simulates the random excitations with unknown loading encountered in these harsh conditions.

The modal superposition method efficiently solves a large linear dynamic system by transforming it into a set of uncoupled equations using the normal modes of the system. The first step in the modal superposition method is to obtain the eigenfrequencies and eigenmodes of the system through modal analysis. The downstream modal transient, modal harmonic, and spectral analyses are then performed.

In the modal analysis, only a subset of the lower frequencies is usually extracted, truncating the higher frequency modes. As a result, the accuracy of the modal subspace based solution is not guaranteed, though accuracy can be improved using residual vectors. The residual vectors are calculated and normalized to the modes extracted, and can then be used in all the downstream analyses (modal transient, modal harmonic, and spectral analyses).

The efficiency of the modal superposition expansion pass has been improved using a direct combination approach of stress/strain modes. You can activate the expansion in the modal analysis by applying the element results expansion option.