GOALI: Assessment of Solder Joint Quality in Microchip Packages Used in Printed Circuit Boards by means of Laser Generated Sound Waves

Georgia Tech Research Corporation, Atlanta GA

Investigators

Abstract

The design and manufacture of microchips are a multi-billion dollar business. There is hardly any electronic device that does not contain microchip. These microchips are interconnected to the printed circuit board with solder joints which are hidden between the chip and the board. Often times these solder joints have manufacturing defects which are difficult to inspect because they are hidden from the view. These defects must be detected early before the board is fully assembled. However, traditional inspection tools to detect these defects are very unsatisfactory. This Grant Opportunity for Academic Liaison with Industry (GOALI) award supports fundamental research to provide the knowledge to enable the development of a low cost and efficient inspection tool that is superior to the existing tools. This new tool has the potential to, (1) significantly increase the yields of chips, (2) reduce new product development cycle time, and (3) reduce the costs of devices that use chips. Commercialization of this tool will create employment and wealth, and generate tax revenues for governments. The research objective is to establish the relationship between the strength of the ultrasound generated by the laser array bundle and the number of fibers in the bundle, length of the fibers, and laser energy carried by each fiber. Another objective is to establish the effects of each solder defect (such as crack, open, head-in-pillow, and pad cratering) on the generated ultrasound (in terms of energy of the received time domain signals). In order to accomplish these objectives, a finite element analysis method will be developed and used to predict the strength of the ultrasound (longitudinal, shear, or Lamb waves) generated by the array when the number of fibers, fiber length, and energy density are varied. The model predicted results will be used to guide the experimental set-up of the array sub-system for generating ultrasound in the chip. The measurement system will include an array sub-system, pulse laser which feeds laser energy into the array bundle, and laser interferometer for picking up the vibration signals on the chip surface. The vibration signals picked up by the interferometer at different points on the test chip surface will be correlated to the reference signals which are obtained from a defect-free chip to determine the presence of solder or chip defects.