FOIL RESISTORS

Foil resistors are similar in characteristics as metal films. Their main advantages are better stability than metal films and lower TCRs. They have excellent frequency response, low TCR, good stability, and very accurate. They are manufactured by rolling the same wire materials as used in precision wirewound resistors to make thin strips of foil. This foil is then bonded to a ceramic substrate and etched to produce the value required. They can be trimmed further by abrasive processes, chemical machining or heat treating to achieve the desired tolerance. Their main disadvantage is the maximum value is less than Metal Film Resistors. The accuracy is about the same as metal film resistors, the TCR and stability approaches Precision Wirewounds but somewhat less because the rolling process and the packaging process produce stresses in the foil. The resistive materials used in Precision Wirewound Resistors is very sensitive to stresses which result in instability and higher TCRs. Any stresses on these material will result in a change in the resistance value and TCR, the greater the stress, the larger the change. This type can be used as strain gauges, strain being measured as a change in the resistance. When used as a strain gauge, the foil is bonded to a flexible substrate that can be mounted on a part where the stress is to be measured.