ACTIVE THERMOGRAPHY APPLICATION FOR SOLDER THICKNESS MEASUREMENT IN SURFACE MOUNTED DEVICE TECHNOLOGY

In this paper we discuss an active thermography application measuring solder (Sn/Pb) thickness on printed circuit boards (PCBs). We use an i nfrared thermographic camera to measure the transient thermal response on the PCB during back side irradiation of the latter with an infrare d light source. Different solder thickness yields a different thermal behaviour on the solder surface. This because of the difference in par ameters such as the capacity and the conductivity. Using basic paramet er extraction procedures, we were able to measure solder thickness. Th e resolution of the method depends on the properties of the camera. Wi th the available camera (sensitivity 0.2K, 16 gray scales) this resolu tion is estimated to be 20 mu m. Of course other measurement methods e xist. However, the method used by us allows fast measurements during t he production cycle. Also, numerical simulations for a complex layered structure (epoxy-solder) have been done in order to obtain the temper ature decay after irradiation with the infrared source, and its depend ency upon the solder thickness. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.