Resistors are passive electronic devices that impede the flow of electric current in a circuit. Resistors are used in virtually all electronic systems. Their purpose in a circuit varies. They can be used to divide a voltage or to limit a current. There are a variety of resistors ranging from conventional ceramic elements, embedded resistors inside of a printed circuit board, thin film resistors, or thick film resistors. Figure 1 shows a typical thick film resistor construction. Figure 2 shows thick film power resistor package that was investigated at CALCE Test Services and Failure Analysis (TSFA) Lab. Thick film devices are “single or multilayer structures fabricated by screen printing a layer of a specially formulated paste on a substrate that can be made of ceramic, polymeric, or metallic material.”[2]. The paste thickness generally ranges from a few microns to mils [1]. The material used in the construction of a thick film power resistor include a ceramic substrate, a resistive layer, a protective glass layer, heat sink, attach material to bond the substrate to the package heat sink, and mold compound to package the entire assembly. Figure 3 shows different layers present in a typical thick film power resistor. Thick film resistors are widely used in consumer and industrial products such as timers, motor controls and a broad range of high performance electronic equipment. Thick film resistors are commonly used in hybrid circuits, for current sensing, power resistor or power conversion [5]. They are used as damping resistors for pull-ups or pull-down resistors for digital circuits [6]. Additionally, high-speed switching, snubbers, and RF systems use thick film resistors [5].

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