Stability of thin-film resistors embedded in printed circuit boards

Long-term stability of embedded passives is very important issue. To determine this matter an aging process can be used, where test samples are exposed to environmental conditions. Objective of this research was analyzing reliability of thin-film resistors based on Ohmega-Ply^® technology. Resistive material was a nickel-phosphorus (Ni-P) alloy (with sheet resistance 25 Ω/sq or 100 Ω/sq) on FR-4 substrate. A number of variables were considered in this study (sheet resistance, geometry of resistor, type of cladding, laser trimming, pulse stress and environmental conditions). Two different accelerated ageing processes - Ex-Situ (samples exposed to elevated temperature and/or relative humidity but resistance measurements made at room temperature) and In-Situ (resistance of test samples performed directly at the ageing conditions) were carried out to perform long-term behavior analysis. The results revealed a significant influence of thickness of resistor, type of cladding and temperature on stability of investigated structures. Signification of other factors was negligible.