Cornerstone Sensors’ NTC thermistor manufacturing process begins with the mixing of powders such as the metal oxides of manganese, nickel, cobalt, copper, and/or iron. Different combinations of the oxides are used to create the desired R/T characteristics and R25 values.
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| Thermistor metal oxide powders | |
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Cornerstone Sensors’ chip thermistors are manufactured by an advanced ceramics processing technique called tape casting, a method that was derived from the ceramic chip capacitor and ceramic substrate industries.
The first step of tape casting involves the combination of the metal oxides with a binder to form a slurry. The ratio of the metal oxides in the slurry determines the R/T characteristic of the final product. Next, the slurry is poured into a fixture which allows a very tightly controlled thickness of material to be cast onto a belt or moveable carrier. The cast material is allowed to dry into a flexible green ceramic tape, which is cut into smaller sections and then sintered at high temperatures to form ceramic wafers 0.01 in to 0.03 in (0.25 mm to 0.80 mm) thick. Sintering is the process where the metal oxide grains bond to form a solid ceramic material. After sintering, a conductive material is applied to opposite sides of a wafer to provide electrical contact with the thermistor ceramic.
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| [Click to enlarge] Thermistor manufacturing process |
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Cornerstone Sensors offers four types of metal contacts: thick film silver, thick film gold, a patented, high temperature solder leach resistant gold termination, and a patented, high temperature solder leach resistant gold contact with pre-deposited solder.
After metalization, the wafer is diced into individual thermistor elements commonly referred to as “chips.” Typical thermistor chip sizes range from 0.04 in × 0.04 in (1 mm × 1 mm) to 0.10 in × 0.10 in (2.5 mm × 2.5 mm) in square or rectangular shapes. Chip sizes as small as 0.016 in x 0.016 in (0.4 mm × 0.4 mm) are available.
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| NTC thermistor chips | |
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After dicing, the NTC thermistor chips are sorted to a specified resistance value and tolerance.
If the final application is for a surface mount application, then the leadless thermistor chips are ready for final packaging. Leadless thermistors are suitable for surface mount technology using die bonding and wire bonding assembly techniques. Cornerstone Sensors offers different contacts depending on the method used to mount the thermistor chip.
If not used for a surface mount application, then lead wires are attached to the thermistor die, typically by soldering. Depending on the final electrical specification, the thermistor may follow one of two pathways in the final assembly process. If the requirement is for a broad tolerance part, it may be sent directly to the coating and final test operations. However, if the requirement is for a tight tolerance part, it may undergo a precise resistance trim operation.
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| NTC thermistors | |
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After the soldering or electrical resistance trimming operation, the NTC thermistor chip is then coated with epoxy, phenolic resin, or other appropriate encapsulation for environmental and mechanical protection.
After the coating material has been applied, the NTC thermistor goes through a final electrical resistance test.
Throughout the entire NTC thermistor manufacturing process, Cornerstone Sensors monitors the critical parameters at each step to ensure that temperature sensors of the highest quality are produced. Cornerstone Sensors thermistors are manufactured and tested utilizing high-precision, high-accuracy temperature measurement systems that are traceable to NIST.
Cornerstone Sensors does offer custom NTC thermistors built to non-standard R25 values if the application demands a special resistance value at a particular temperature point.