Design and fabrication of a new multi-active-layer transducer with a single-copper-layer FPCB

A diagnostic ultrasound transducer which operates at low frequency and has a small pitch has a high resultant element electrical impedance, aggravating the impedance mismatch which typically occurs between an ultrasound array and cable. A transducer constructed from multiple layers of active material that are electrically in parallel is one way to improve this impedance mismatch. Such a multi-active-layer transducer is most commonly composed of odd numbers of piezoelectric layers to simplify the electrical connections, so a three-layered structure is the simplest choice. But then the design thickness of each of the three PZT layers for a 3.5MHz transducer would be less than 100µm, which is too thin to fabricate easily and too risky to handle. In this paper, a two-active-layer structure is presented that reduces the breakage risk while achieving a lower impedance and thereby improving the transducer performance. A single-copper-layer flexible printed circuit board (FPCB) is developed to electrically connect the two PZT layers. The two-active-layer transducer presented is a phased array with a 3.5MHz center frequency, 150µm pitch, and 128 elements. The transducer is designed using a finite-element analysis program (PZflex) and then fabricated in accordance with the optimal design. The two-active-layer array performance is compared with a single-active-layer transducer that is similarly optimized.