Production of highly uniform solder spheres using a digital integral control scheme

The uniform droplet spray (UDS) process produces solder spheres by controll ing the breakup of a continuous laminar jet into uniform droplets, which ar e then rapidly solidified in a liquid bath or an inert gas. Although the sp heres have a narrow size distribution (+/-9% from mean size), still greater size accuracy is required (+/-3% from mean size) for the use of these sphe res by the electronics industry, particularly for ball grid array (BOA) chi p interconnection technology. This article discusses the production of high ly size-accurate solder microspheres (75-1000 mu m in diameter) by the UDS process. An on-line droplet size control system was developed to achieve th e size distribution requirements. Droplet size control is accomplished by p erforming a real-time measurement of the droplet size, and then compensatin g for the difference between actual and target sizes by adjusting the break up frequency. The control system is effective irt controlling sphere size, enabling the UDS process to accurately determine and control solder sphere size within +/-2.5% of the target size. While this article focuses on the p roduction of large solder spheres (250-800 mu m), the control system can be applied to spheres of any size produced try the controlled breakup of a li quid jet.