Whole-field board strain and displacement characterization during drop impact using a single camera DIC technique

Abstract

A single-camera technique has been developed to evaluate inplane strains and out-of-plane displacement experienced by a printed circuit board assembly under drop impact. The technique essentially employs a high-speed camera to capture images of the deflecting board surface that had been spray painted to produce fine speckles. A code developed by the Experimental Mechanics Laboratory, NUS, was then used to process the series of images during the period of board flexure by speckle correlation to obtain board strains and deflection data. The results show very good correlation with those obtained from strain gauge technique. Furthermore, as the investigation focuses on dynamic impact, the capturing of localized strain fields close to features such as edges of microelectronic packages and screw/constraint locations, as well as, the overall view of the general board warpage/flexure during different loading conditions makes it a superior method compared to the strain gauge technique that provides only localized data. The single-camera DIC technique takes away the hassle of the traditional two-camera setup and reduces cost of inventory especially for very expensive instruments such as high-speed cameras and lenses. The single-camera DIC technique is a non-contact measurement technique that is easy to setup and implement compared to the strain gauge technique that is a contact method. Mounting of multiple gauges on a flexible specimen such as a PCB may also artificially stiffen the specimen which results in inaccurate data. Furthermore, the DIC method which relies on sprayed particles has spatial resolution that is determined by the smallest speckle that can be observed by the camera, whereas for hard-to-reach places or locations with limited flat surfaces, the use of strain gauges may not be feasible. The singlecamera DIC technique not only provides applications to qualitative investigation of board or package deformation but also in material characterization (where specimens can be tested in tension or shear and the material properties and responses evaluated together with full-field profiles of their cross-sections); and crack studies (to evaluate full-field strain/stress concentration regions close to notches or other geometrical discontinuities).