Cambridge Ultrasonics

Vector Impedance Testing

Vector impedance testing is used for quality assurance of some electronic components and assemblies, particularly ultrasonic transducers but also passive components.

The variation of electrical impedance (magnitude and phase or related measures) of a passive electronic component is a reliable and sensitive indicator of its properties. It is particularly useful for ultrasonic transducers.

Cambridge Ultrasonics can perform batch testing of nominally identical components and group together components with similar properties. With some calculation and using our experience, we are often able to identify groups of faulty components or groups of poor quality and groups of good quality.

Components can be returned individually bagged with individual plots of impedance against frequency or bagged in groups. Optionally, a report with conclusions and a spreadsheet with tables of important values can also be provided.

Key features:

• Frequency range:         20 Hz to 120 MHz
• DC bias:                         0 V to +40 V DC
• Component:                  SMD, axial lead, terminal
• Accuracy:                        0.01% typically

An example of the usefulness of vector impedance testing is instructive. It concerns piezoelectric components used to build power ultrasonic transducers. The client was buying assemblies from a supplier but delivery was too long (about 16 weeks) and the failure rate in service was too high (about 20% in the first year of warranty). The client wanted to investigate the feasibility of manufacturing the power transducers in-house and contacted Cambridge Ultrasonics for help. We devised a manufacturing process, using best practise, which used vector impedance measurements of the piezoelectric components. We found that about 5% of the components had negligible piezoelectric properties and 10% were in a grouping that indicated poor piezoelectric properties. We used near clean-room methods to assemble the components with vacuum processing of adhesives. We assembled a few sample transducers and gave them to the client, with vector impedance plots of the individual piezoelectric components and the assembled transducers. The client reviewed the parts, was delighted with them and then started to introduce the assembly method into its production.

Another example is a start-up company that wanted to process biological material using ultrasound and wanted to make its own equipment for processing. At first, Cambridge Ultrasonics was used for a brainstorming audit of the several processes and made recommendations for achieving uniformity and achieving good quality that involved impedance testing. When prototype processing sections were being made the client returned several piezoelectric elements for impedance QA screening, which showed good quality in nearly all of the components. The piezoelectric elements were returned to the client for partial assembly then return to Cambridge Ultrasonics for re-testing. At this stage a major problem in the previous stage of manufacture was discovered using impedance testing and the client was alerted to the problem. Changes were made to the manufacturing process to correct for the error.

Impedance testing is also useful in electronic design to answer the question: when is a capacitor an inductor? Electrolytic capacitors become inductors at frequencies as low as 100 Hz. Multi-layer ceramic capacitors become inductors at about 1 MHz. Some single-layer ceramic capacitors remain capacitors up to 100 MHz, but values are rather small at about 1 nF. Vector impedance testing is useful in selecting capacitors used for decoupling. After all, you should never use an inductor for supply decoupling.