Air-Abrasive Functional Trimming of Thick Film Circuits

High tolerance chip capacitors are at best expensive and at the worst unobtainable. When asked to custom build a small signal circuit containing close tolerance RC networks it became obvious that functional resistor trimming was unavoidable if suitable quantities were to be produced at the right price. The product which is shown in Figure is constructed by conventional thick film techniques. It uses a l"x 2" alumina substrate and attached components, capacitors, transistors and double diodes. The following points may be of interest:1) To enable thick film compatible components to be used, extensive use is made of cross-over techniques. The increased cost due to these cross-overs is small compared to the reduction in assembly costs which these devices allow. 2) The majority of the resistors are trimmed prior to the attachment of the discrete components, using an air abrasive system. The high tolerance, high stability resistors required (top left hand corner of Figure 1) are glazed prior to trimming to increase their stability, to give protection from the encapsulating epoxy, and to warn the trimmer operator that extra care is required.