Identification of PCB and solder-joint defects through X-ray inspection

Abstract

Voiding in solder joints is a very complex subject to explore and analyse. During this project voiding was split into four categories: total void ratio with respect to the pad area, void count, maximum void ratio with respect to the pad area and the degree of champagne voiding. The exploration of voiding was conducted by experimenting with the soldering process. The soldering process is very complex with many varying parameters. For ease of interpretation the soldering process was split into four sections: Printed Circuit Board (PCB) design, dispensing of the solder paste, placement of the component and the soldering. All four sections were explored individually through familiarisation with the software and hardware available at the Electronics Systems Engineering Department Laboratory. The PCB design involves integration with all the machines available such that a design that can accommodate all the machines is achieved. The dispensing of the solder paste was done automatically using a programmable dispensing machine. This gives better accuracy and precision for both the dispensed amount and the dispensing position. The solder paste used was kept constant throughout the whole project. For the placement of the components a semi-automatic machine was used, providing constant pressure during the placement of the components. This further helped in reducing any factors which may hinder the final results. For the soldering of the PCB two types of ovens were considered: a reflow oven and a vapour phase oven. The reflow oven was chosen as it gives the user greater control over the soldering profile. An X-ray machine was used during this experiment to explore the voiding inside the solder joints. X-ray imaging is one of the best ways to explore this phenomenon as it is a non-destructive way to view the voids inside the solder joints. Each component was explored manually and individually as the machine does not support automatic analysis for Surface Mount Resistors (SMRs). From this study it was concluded that the factor which is mostly significant in reducing the voiding parameters is the component size. However both the theoretical fill and the oven peak temperature were also significant in some aspects of voiding.