Dynamic digital fringe projection technique for measuring the warpage of unpainted PBGA packages and boards

Abstract

A crucial factor for ensuring the reliability of chip packages is the measurement and control of warpage. The warpage of chip packages, printed wiring boards (PWBs), and PWB assemblies (PWBAs) are typically measured by moire techniques (i.e., shadow moire, laser fringe projection, and digital fringe projection), which have noncontact, full-field, and high-resolution capabilities. The application of moire techniques typically involves spraying a mist of reflective paint on the surface of a sample to ensure the uniformity of surface reflectance and to obtain better fringe image contrast and better sinusoidal waveforms in the fringe patterns for the measurement process. However, since painted samples may no longer be re-used, the process of spray painting is not suited for inline inspection; a new technique for measuring warpage without painting is required. In this research, a dynamic digital fringe projection (DDFP) technique for measuring the warpage of unpainted plastic ball grid array (PBGA) packages and boards was developed. The DDFP technique dynamically determines proper fringe intensity distributions for PBGA packages and PWBs and uses them to produce a better fringe image contrast and better sinusoidal waveforms without painting. Experimental results showed that the DDFP technique successfully measures the warpage of PBGA packages and PWBs in unpainted PWBAs accurately.