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To find out more, see our Privacy and Cookies policy. Close this notification. Measurement Science and Technology. Download Article PDF. Share this article. Article information. Piezoelectric and piezo-resistive sensors have been printed on flexible polyester, then bonded to smartcard blanks. The tactile interaction of a person with these sensors has been investigated.

It was found that whilst piezo-resistive films offer good dynamic and static sensing properties, the relative complexity of their measurement circuitry over that of piezoelectrics, favours the use of piezoelectric films in the smartcard arena. The outline of a novel biometric discrimination method is presented, for which an equal false-acceptance and false-rejection error rate of 2. AB - The potential of polymer thick-film sensors are assessed for use as biometric sensors on smartcards.

Polymer thick-film sensors: Possibilities for smartcard biometrics Neil J. Abstract The potential of polymer thick-film sensors are assessed for use as biometric sensors on smartcards. Grattan, S. Fingerprint Biometrics. Polymer films. These have good bond strength, good solder leach resistance and are relatively easy to solder. The conductivity of the gold alloys tends to be inferior to that of the other type of thick film conductor, but they can be used with both ultrasonic aluminium and thermosonic gold wire bonds.

The earliest resistors were made from materials such as carbon, silver and iron oxide and were found to suffer from poor long-term stability, unpredictability of fired resistivity, and unacceptably high temperature coefficients. Modern thick film resistor systems are mainly based on ruthenium dioxide RuO 2 and have much-improved characteristics. Within the resistive paste, the conductive phase comprises submicron particles, and these are mixed with larger glass particles several microns in diameter. Various additives are added to the formulation to improve the stability and electrical properties of the fired film.

The nature of these additives is proprietary knowledge of the paste manufacturer. At elevated temperatures the stability is still impressive, with most resistance values changing by only 0. If accurate values of resistance are required, it is therefore necessary to trim the resistor. Resistor values can only be increased by trimming, so the printed value is always designed to be less than that of the post-trimmed value.

There is an occasional requirement to fabricate thick film capacitors, although the wide availability of surface-mounted capacitors has largely precluded their need.


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It is also difficult to ensure that post-fired values of thick film capacitors closely match those for which they were designed. An expensive trimming process is needed to ensure that the correct valves are obtained. Overglazes are mainly used to protect resistors from overspray during trimming and also the circuit from environmental attack during operation.

OEM pressure sensor

The use of insulated stainless steel substrates was mentioned earlier. There is an inherent mismatch between the thermal coefficient of expansion of the metal substrate and that of the insulation layer, which limits the type of steels that can be used for this purpose. For conductors, the most commonly used active phases are silver, copper and nickel.

Owing to their poor stability at high temperatures, polymer thick film conductors cannot be soldered and alternative forms of attachment must be adopted. Carbon is typically used as the active material in polymer thick film resistors. The performance of these resistors is inferior to that of their cermet counterparts and they are therefore seldom used in critical applications.

The dielectric pastes are similar in nature to conductors and resistors except that the conducting phase is omitted from the formulation.

Some manufacturers add minerals to improve the electrical and mechanical properties of the dielectric films. The process for fabricating polymer thick films is similar to that used with cermet materials. This can lead to improved film stability and increased shrinkage. Planar screen-printed capacitors are rarely used in thick film hybrid circuits owing to their poor stability and high production costs.

They typically comprise at least three layers two electrodes and one dielectric layer , and trimming is often required to achieve the target value of capacitance.

Hyb technological capacities:

Chip inductors are also available typical range: 0. Other forms of IC include flip chips, which are essentially naked chips with raised connection contacts bumps made of solder, gold or aluminium. These are mounted by turning over the chip flipping and bonding directly to the substrate. Beam lead chips also examples of naked dice with either gold or aluminium leads protruding from the edge. The leads beams are an integral part of the chip metallization process. In this manner, it is possible to achieve good yields on high-density circuit populations.

The two most popular techniques used are trimming by laser or by an air-abrasive jet. Laser trimming, however, is more amenable to large-scale component adjustment. The diameter of the jet nozzle is around 0. During trimming, the particles in the jet stream remove material from the component. In the case of resistor trimming, the value can only increase because the material is being removed. Resistor values cannot be reduced by the trimming process. The thick film is vaporized with high-energy laser beam pulses. Resistors are the most common component requiring trimming.

Polymer thick-film sensors: Possibilities for smartcard biometrics

Sometimes conductor tracks need to be adjusted in special circumstances. In rare circumstances, thick film capacitors can be trimmed by removing an area of one of the plates, although the post-trimmed stability is poor.


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  • The combination of pressure and ultrasonic vibration causes the materials to bond together at the interface of the wire and the bond pad. This method is amenable to multilevel and multidirectional bonding and is therefore the preferred method, allowing bonding of up to wires per minute.

    Applications

    Surface-mounted components are generally much smaller than their through-hole counterparts. Wave soldering also requires the components to be fixed in position prior to the soldering process. Wave soldering machines were originally used for soldering through-hole components onto printed circuit boards, but they can also be used effectively with surface-mounted devices.

    Reflow soldering is the preferred method of attaching surface-mounted devices. The flux within the cream is sufficiently tacky to hold the component in place so that handling of the substrate is possible. After all of the components have been positioned on the circuit, the solder cream is dried and then reflowed. This process takes place by belt reflow, vapor phase or infrared belt system.

    The vapor condenses onto the substrate and raises the temperature uniformly to that of the liquid below. Infrared belt reflow systems are similar to those used for drying thick film materials.

    Biomedical sensor using thick film technology for transcutaneous oxygen measurement.

    Conformal coatings are often used to protect the circuit from environmental attack. In the former case, the substrate is heated and immersed into the powder. The temperature is then increased so that the coating dries. The hermetic seal is made by brazing, welding or glass sealing techniques. This form of packaging is often very expensive and is therefore only used in special application areas. The final stage of the process is to test the circuit to see that its performance matches the design specification.

    Electrical testing can be difficult if the circuit has been coated or hermetically sealed, as physical access to components may be restricted. It is therefore usual to ensure that key test points are brought out to an external pin on the package. Advances in the field of sensor development are greatly affected by the technologies that are used for their fabrication. The use of thick film processes as an enabling technology for modern-day sensors continues to expand. As we have already seen, the ability to produce miniaturized circuits is clearly one area in which thick film technology excels.


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    • Commercial thick film platinum conductors can be trimmed and used as calibrated temperature sensors. Most standard pastes, however, have not been specifically developed for sensor applications and do not necessarily have optimum sensing properties. Piezoelectric materials exhibit the property of producing an electric charge when subjected to an applied mechanical force.

      They also deform in response to an externally applied electric field. Certain crystals such as quartz and Rochelle salt are naturally occurring piezoelectrics, whilst others, like the ceramic materials barium titanate, lead zirconate titanate PZT and the polymer material polyvinylidene fluoride PVDF , are ferroelectric. Ferroelectric materials are those that exhibit spontaneous polarization upon the application of an applied electric field. This means that ferroelectrics must be poled polarized prior to use in order to obtain piezoelectric behavior.

      Positive temperature coefficient PTC thermistors are also available, although they are generally not as stable or repeatable as NTCs and are therefore used as simple thermal detectors rather than calibrated devices. Another type of temperature sensor can be made by joining together two dissimilar metals or semiconductors.