Chip Resistors for High Temperature Applications
The special technological aspects of these components produced by Riedon, Inc. are the materials of the contact areas, which contains no Nickel, so they are nonmagnetic.
The contact areas are made by noble metal alloys based on Pt-Ag and Pt-Pd-Ag, available as thick film 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. Additionally these resistors and capacitors are also suited for contact processes using silver-based conductive glue with silver as the contact material, as there can arise no formation of unstable diffusion layers between tin and silver.
The main applications of these non-magnetic components are medical 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 cannot be applied for 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 thick film chip resistors with Pt-Ag or Pt-Pd-Ag 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 given mainly 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 contrast to thick film chip resistors the corresponding ceramic chip capacitors are not suited for high temperature applications, because the thin inner layers of conductive and isolating materials cannot withstand the expansion problems caused by high temperatures.
When using standard soldering techniques there exists a difference in the appearance of the solder-joint between such non-magnetic contacts and normally tinned contacts. Because Pt-Ag or Pd-Pt-Ag contacts do not have the same affinity to the tin of the solder-bath or printed reflow-tin as the galvanic tin-coated chip component, the soldering process does not result in an equal form of solder-meniscus. Nevertheless, the solder joints will have an equal mechanical strength as the tin-coated components.
The following product groups with the described properties are available from Riedon, Inc:
|CHR||Chip Resistors in the standard range||1Ω to 10MΩ||0402 to 4020|
|HVC||Chip Resistors in the high value range||10MΩ to 1TΩ||0402 to 4020|
|HVS||Precision Chip Resistors in the high value range||100KΩ to 10TΩ||0805 to 4020|
|CBR||Trimmable Chip Resistors||10Ω to 1TΩ||0402 to 1206|
The chip resistors in the high value range (HVC/HVS-series) are produced with these non-magnetic contacts for a technological reason. For the deposition of a tin-layer onto the resistor contacts it is necessary that the resistor-layer be 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
Giga-ohm range, this diffusion process changes the resistor in an unpredictable way and it cannot be applied in the high resistance range.
Besides the chip resistors there are some additional series of leaded resistors in the
Riedon Inc. program that have non-magnetic properties, as the copper-wire leads are soldered onto non-magnetic contact areas. These components are radial leaded, high value, and high voltage resistors in the resistance range up to 10 Tera-ohm and in voltages up to 30kV, and leaded temperature sensors in the same range as the chip-sensors. Due to the flexible production structure at Riedon Inc. it is possible to also produce custom specified resistor products with non-magnetic properties.