Piezoelectric sensors stem from the natural effect in certain crystals that generate a voltage when the crystal is deformed. A piezoelectric sensor uses the piezoelectric effect to measure pressure, acceleration, strain or force by converting them to an electrical charge, as illustrated in Figure 1. Such sensors are used in medical, aerospace, nuclear instrumentation, and as a
pressure sensor in the touch pads of mobile phones. In the automotive industry, piezoelectric elements are used to monitor combustion when developing internal combustion engines. The sensors are either directly mounted into additional holes into the cylinder head or the spark/glow plug is equipped with a built in miniature piezoelectric sensor.
Two main groups of materials are used for piezoelectric sensors: piezoelectric ceramics and single crystal materials. The ceramic materials include PZT (lead zirconate ceramic) and new single crystal materials such as Lead Magnesium Niobate-Lead Titanate (PMN-PT).
The challenge when making sensors is to make an electrical connection to the crystal and also to bond the crystal to the sensor elements and housing. Many times these piezoelectric crystals are silver or gold coated on their attachment surfaces to which conventional solder joints can be made to make electrical and mechanical connection; however many times these solders introduce fluxes or require plating. Active solders such as S-Bond can, with their reactive elements, directly bond to the crystal surfaces without the need to initially coat the crystal surfaces. Also, S-Bond Technologies range of active solders, bond at temperatures from 115 – 400°C. These active solder alloys permit the bonding of the piezoelectric crystal well below the crystals’ Curie temperature, where these crystals lose their piezoelectric properties.
Figure 2 shows an S-Bond 220 joined PZT crystal, bonded to an aluminum housing. This bonded assembly is part of an ultrasonic actuator.
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