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The special technological aspect of these components produced by SRT Resistor Technology GmbH and SRT Microcéramique Sarl is the material of the contact areas, which contains no Nickel, so they are non-magnetic. These contact areas are made by noble metal alloys based on PtAg and PtPdAg, available as thickfilm pastes, which are applied by roller-coating or dipping processes onto the wrap-around areas of the chip components and fired in the range of 850ºC.
By appropriate choice of materials and the coating and firing process, for these components the standard SMD soldering processes can be used. Additional these resistors and capacitors are also suited for contact processes using silver-based conductive glue silver as contact material, as there can arise no formation of unstable diffusion layers between tin and silver.
The main application of these non-magnetic components is in medicine electronics in the field of magnetic-resonance (MRI) and computer tomography (CT), where electronic circuits are located in high magnetic fields. The contact-technique of conductive gluing is used mainly in applications, where soldering techniques can not be applied by technical reasons like the usage of temperature-sensitive components which would be affected by high soldering temperatures, or the usage of unpackaged semiconductors on the same board.
An additional advantage of thickfilm chip resistors with PtAg- or PtPdAg-contacts is suitability for applications at higher temperatures than the normal maximum temperature of 155ºC. These resistors contain no organic material and no tin- or tin-lead-layers, and the firing temperatures for the resistive and conductive layers are in the temperature range of 850ºC. Due to these reasons such resistors will have no substantial change of their properties and stability up to the range of 300ºC. The limit for the highest application temperature is mainly given by the solder or material for the connection to the PCB or ceramic carrier, as the temperature difference between the highest application temperature and the melding or deterioration point of the contact material should be at least in the range of 30ºC.
In contrary to thickfilm chip resistors the corresponding ceramic chip capacitors are not suited for high temperature applications, because the thin inner layers of conductive and isolating materials can not withstand the expansion problems caused by high temperatures.
By using standard soldering techniques there are existing a difference in the appearance of the solder-joint between such non-magnetic contacts and normally tinned contacts. Because PtAg- or PdPtAg-contacts have not the same affinity to the tin of the solder-bath or printed reflow-tin as the galvanic tin-coating of the chip component, there will not arise by the soldering process an equal form of solder-meniscus. Nevertheless, the solder joints have an equal mechanical strength like at the tin-coated components.
Actual the following product groups with the described properties are available at SRT Resistor Technology GmbH:
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Series
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Description
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Value range
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Sizes
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CHR
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Chip Resistors in the standard range
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1R to 10M
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0402 to
1206
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CHS
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Chip Resistors in the high value range
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10M to 1T
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0402 to
4020
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CHM
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Precision Chip Resistors in the high value range
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100k to 10T
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0805 to 4020
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CRB
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Trimmable Chip Resistors
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1R to 10M
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0603 to
1216
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TPS
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PTC-Temperature Sensors
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1R to 100k
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0402 to
1210
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TNS
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NTC-Temperature Sensors
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100R to
100M
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0402 to
1210
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MLCC
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Ceramic Chip Capacitors
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0,47pF to
3,3µF
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0402 to
2225
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The chip resistors in the high value range (CHS/CHM-series) are
produced with these non-magnetic contacts by a technological reason.
For deposition of a tin-layer onto the resistor contacts it is
necessary, that the resistor layer is protected by a glass-layer to
avoid the deposition of the tin onto the resistor surface.
Unfortunately, this glass-layer is creating diffusion-processes into
the resistor-layer, and at high values mainly in the gigaohm-range,
this diffusion process change the resistor in an unpredictable way, so
it can not be applied in the high resistance range.
Besides Chip Resistors also some series of leaded resistors in the
SRT-program have non-magnetic properties, as the copper wire-leads are
soldered onto non-magnetic contact areas. This components are radial
leaded High Value and High Voltage Resistors in the resistance range up
to 10 Teraohm and in the voltage up to 30kV, and leaded temperature
sensors in the same range as the chip-sensors. Due to the flexible
production structure a SRT it is possible to produce also custom
specified resistor products with non-magnetic properties.
Chip resistors and leaded resistors in different sizes with non magnetic features are suitable for applications in strong magnetic fields
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