Void Inspection in Lead-Free Solder Bumps on Ball Grid Array (BGA) Packages Using Laser Ultrasound Technique

Abstract

Solder joint voids are usually formed by the entrapped gas bubbles during the reflow process, and are common in all surface mount applications. It is a controversial issue on the reliability of the solder joint, however the consensus is that voiding is acceptable at low contents, while excessive voiding affects mechanical properties, and decreases strength, ductility and fatigue life of the interconnections. X-ray is the most widely used technique to evaluate the voids, including the size and occurrence frequency. In this paper, a laser ultrasound and interferometer inspection system is used to inspect the voids in lead-free solder bumps in ball grid array (BGA) packages. This system uses a pulsed Nd:YAG laser to induce ultrasound in the chip packages in the thermoelastic regime; and laser interferometer is used to measure the transient out-of-plane displacement response of the package surface to the laser irradiation. The quality of solder bumps is evaluated by analyzing the transient responses. In this work, voids were intentionally created by adding the volatile flux during the assembly process. By controlling the volume of flux dip, three different levels of voiding were proposed: void-free, relatively low and relatively high. The presence of voids in the solder bumps was first verified using 2-D X-ray techniques. Meanwhile, the built-in image-processing software in X-ray tool measured the void fraction to quantify the level of voiding. Then the laser ultrasound inspection system was used to evaluate the voids in these samples. By comparing the vibration responses from voided samples and void-free samples, it was found that the laser ultrasound inspection system is capable to differentiate samples with relatively high voiding from void-free samples while the relatively low voiding was below the resolution of the inspection system. Lastly, a further comparison between the void-free and voided solder bumps was carried out by the destructive cross-section technique. The comparisons between these three solder bump evaluation methods will be presented in this paper.