Micro-scaled surface profile measurement on packages by digital projection moiré

With the rapid trend for miniaturization of electronic products, new optical techniques are increasingly needed for measuring the surface profile or deflection/warpage in the micro-scale for electronic packages mounted on printed circuit boards. In this paper, we reduced the grating spacing of the digital projection moiré (DPM) by directing the grating pattern into a stereo zoom microscope and performed surface profile measurement under this microscope. In this method, the reference grating is generated digitally having sinusoidal intensity. Another digitally generated grating is projected by the means of a digital light processing (DLP) projector and a set of carefully arranged optical lens into the microscope to form the object grating. The pitch of micro-scaled object gratings can be adjusted by the reduction ratio of the microscope. As a result, this micro-scaled digital projection moiré method produces micro object gratings on the order of 10-um pitch and is suitable for surface profile measurement in a square dimension on the order of 100-um. The method of linear mismatch is utilized to obtain more fringes in each measurement and the guideline to achieve the optimal degree of linear mismatch between the reference and the object gratings is proposed. In addition, the phase shifting technique is employed to extract the data between the recorded fringes. Verification of the method is demonstrated by measuring an inclined plane of a micro prism. The deviation between the measured data and the given values was found to be less than 5%, which demonstrates the validity of the developed method.