Destructive Physical Analysis (DPA) Testing for Thermistors

A thermistor acts like a resistor but it is thermally sensitive. In fact, the name thermistor is derived from the term THERMally sensitive ResISTOR! These devices are used for temperature measurement and control due to their accuracy, long-term stability and cost effectiveness.

There are two types of thermistors; Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC). The resistance of an NTC thermistor decreases, when the device body temperature increases. The opposite occurs when using a PTC thermistor, the resistance increases when the device’s body temperature increases. In terms of application, NTC thermistors are typically used for temperature measurement while PTC thermistors are primarily used for circuit protection.

A thermistor is typically constructed using a mixture of metal oxides and binder material that are fused together to create a ceramic-like material. To protect the device from external environments, chemical attack and corrosion, thermistors are usually encapsulated with epoxy or glass. Thermistors come in many different shapes and forms; the most common forms are disk, bead, rod and chip packages.

DPA testing of thermistors includes non-destructive and destructive testing. Typical non-destructive tests are external visual and prohibited material inspection. Although, X-Radiographic inspection is not a requirement; it is a good non-destructive inspection technique. The destructive portion of testing consists of decapsulation and/or cross-section followed by optical inspection using magnification.

There are several failure modes of a thermistor. The most common is an open circuit. Excessive heat, improper handling and thermal damage are typical examples that can cause mechanical separation between the lead material and the element; resulting in an open circuit. Another common failure mode is aging; this causes inaccurate temperature measurements due to resistance value drift, which in turn causes the thermistor to provide incorrect thermal compensation.

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    DPA Test Specification:

    Thermistors are covered within Mil-Std-1580 revision C, Requirement 20.

    Associated DLA performance specifications::

    • Mil-Prf-23648 – General Specification for resistor, thermal (thermistor) insulated
    • Mil-Prf-32192 – General Specification for resistor, chip, thermal (thermistor)
    Cross-sectional view of secondary copper bond with acceptable effective bond area

    Optical image of a leaded thermistor

    Optical image of a cross-sectioned thermistor

    Optical image of a thermistor with the encapsulant removed

    Oblique image of a typical thermistor with the encapsulant removed