Destructive Physical Analysis (DPA) Testing of Magnetic Components

Magnetic components are passive devices that rely on an internal magnetic field to alter electrical current. Typical magnetic components are inductors, transformers, coils and ferrite chips. These components are available in many different designs and configurations.

Inductors are used to slow down surges in a current by adding resistance. Since they have low voltage regulation, the output voltage does not vary significantly with input voltage changes. In contrast, transformers either increase or decrease voltage levels while stabilizing the circuit’s voltage level. They have a high voltage regulation in which the output voltage varies significantly with changes in the input voltage. The internal construction of an inductor and transformer differ slightly. Inductors have a single coil of wire around the core, while transformers have two or more coils wound around a common core. Typical inductors and transformers are:

  • Common-mode choke
  • Low-pass filter
  • Toroid and choke
  • Flyback
  • Audio
  • Isolation
  • Power
  • Pulse

Coils are typically used in conjunction with transformers or inductors. They can transmit heat, electricity or sound. Coils are made of wire (typically copper) that winds around a toroidal core. They include:

  • Fixed Radio Frequency
  • Variable Radio Frequency

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    Ferrite Chips are noise-suppression components. The chip is made of coil patterns (electrodes) that are formed between the ferrite. The construction of a ferrite chip is basically the same as a multilayer type of chip inductor with the difference being the ferrite material.

    Non-destructive testing includes external visual examination, prohibited material inspection and X-Radiography. Hermeticity and terminal strength testing is also applicable depending on the package. Of these tests, X-Ray provides the most meaningful data as the internal construction can be inspected well. It is very common for extraneous material such as solder balls to become entrapped between wires or embedded within the packaging material. These small extraneous materials are quite difficult to locate during the destructive phases of testing, thus X-Ray can provide useful data without destroying the component itself. X-Ray is performed in accordance with Mil-Std-981. 

    Destructive testing consists of disassembly/decapsulation and cross-section. Given the wide range of packaging materials available; disassembly can sometimes be arduous. The disassembly process allows for inspection of the internal wire connections as well as the core. Cross-section inspection is performed as a means of inspecting the internal wire connections. It can also be performed to confirm X-Ray findings.

    Test Specifications:

    Magnetic components are covered within:

    • Mil-Std-1580 revision C, Requirement 15
    • Mil-Std-981C – Design, Manufacturing and Quality Standards for Custom Electromagnetic devices for Space Applications

    Associated DLA performance specifications:

    • Mil-Prf-27 – General Specification for transformers and inductors (audio, power, and high-power pulse)
    • Mil-Prf-15305 – General Specification for coils, fixed and variable, radio frequency
    • Mil-Prf-21038 – General Specification for transformers, pulse, low power
    • Mil-Prf-39010 – General Specification for coil, radio frequency, fixed, molded
    • Mil-Prf-83446 – General Specification for coils, radio frequency chip, fixed or variable

    Optical image of an inductor after decapsulation.

    Optical image of an inductor after decapsulation.

    X-Ray of a typical inductor.