Moisture resistance is critical for some harsh applications
amide/imide adhesive as a replacement to the conventional epoxy adhesive.
The screen-printable poly-amide/imide adhesive used in the VPG Foil Resistors was specially optimised to: Deliver the strongest adhesion to the nickel- chromium (NiCr) interface
Essentially improve heat dissipation Uniquely adjust the moisture sorption mechanism
All of the above have been achieved by the adjustment of the chemical composition of the adhesive material, for type, and the amount and mechanical dimension of the Boron Nitride filler, as well as optimizing the thickness and curing profile of the applied adhesion layer.
The cold-rolled nickel-chrome foil is adhered to a ceramic substrate. The foil is then photoetched with very fine lines and precise spacing, using the latest mask-less photolithography, to set the resistance value. The nickel-chrome foil in Z1 technology offers an improvement in the chemical impurities as well as the thermal and conductive behaviour of the resistor. The stronger stabilization process used in Z1 technology also supports extra outgassing of latent moisture. The addition of a polyimide over-coating to the chip’s active element provides a uniform protective layer free from pinholes. This protects the active foil element of the resistor from chemical attack and moisture ingress, as well as adding to the thermal stability of the resistor.
Testing the result The performance of the Bulk Metal® Foil
resistors with Z1 foil technology is tested across a number of methods which include a highly accelerated temperature and humidity stress test (HAST), as well as an 85/85 relative humidity test for 1,000 hours. The moisture test to MIL-STD-202 is for 10 days at 65ºC and 93% relative humidity, and the steady-state
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humidity test to MIL-STD-202 is for 56 days at 40ºC and 90% relative humidity. The resistors also undergo a pressure cooker test pressure for 100 hours at 121ºC and 99% relative humidity, or without 1 V bias.
These tests have shown that the failure rate caused by moisture or humidity is at least halved for resistors based on Z1 technology.
Higher stability, power and temperature
A number of applications can use the Z1 resistor technology which has been optimised to combine high moisture resistance, stability and reliability with the option of high power or high temperature.
For tight circuits, the latest resistor with Z1 technology uses a flip-chip with Stand-Off design in an 0402 case which gives a 40% space saving compared to a wraparound package and delivers 100mW of power with load life stability of 0.01%.
One example of the advantages of the improved Z1 technology is the FRSM0805 which has a rated power of 300mW, for values up to 30KΩ. The previous version provided 200mW with maximum resistance value of 10kΩ. A screen and test flow which is compliant to EEE-INST-002 and MIL-PRF-55342 is combined with an ultra-high precision specification in a wraparound chip resistor, and another Z1 resistor is rated for high-temperature applications up to 250°C.
The recent innovations in Z1 technology add stronger stabilization, with advanced adhesive and coating materials, together with improvements to the internal construction of the resistor. The result is that Z1 resistors have been developed to deliver increased resistance to moisture in addition to higher stability and reliability for a long lifetime when used in harsh environments.
www.charcroft.com Components in Electronics July/August 2023 21
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