Parametric shifts in devices: role of packaging variables and some novel solutions

The resistance-shift phenomena typically encountered in precision analog devices such as operational amplifiers which use the BiFET structure is discussed and resolved. A test chip consisting of fourteen implant resistors acting as piezoelectric strain gauges and temperature-sensing diodes for accurate recording of chip temperature was designed and fabricated. The incremental stressing effect of each packaging operation performed in sequence was measured and characterized using the chip. It was found that the chip-attachment operation results in small residual tensile stress at the chip surface corresponding to positive resistance shifts in the P-type regions of the chip. This stress state prevails in most ceramic packages; however; in plastic packages, severe compressive tractions resulting from the shrinkage of the plastic change the resistance shifts to large negative values for P-type resistors. This clearly limits the merits of low-stress die attach in plastic packages. The merits of some commonly used stress-reduction schemes such as compliant die attach, low-stress encapsulents, and die coatings were assessed. Some novel solutions to package stress, such as thermoplastic die attachment, five-sided coating, and silicon pad, are demonstrated.<
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