Metal Foil Resistor Free-Standing Package
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Solution Overview
Problem
Conventional metal foil resistors face challenges in reducing and stabilizing the temperature coefficient of resistance (TCR) due to differences in thermal expansion coefficients between the metal foil and the substrate, requiring strict control of various design factors, which is difficult to achieve and affects TCR stability over a broad temperature range.
Innovation Solution
The metal foil resistive element is encapsulated in a package that allows it to expand and contract freely without stress, using a metal foil with a low TCR achieved through alloy composition, rolling process, and etching, and the package is designed to prevent external stress transmission, reducing the number of control factors and facilitating attachment to a heat sink.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the metal foil is bonded to the substrate with adhesive in a sealed package, then the resistor structure is stable and protected, but differential thermal expansion causes stress and strain on the metal foil, increasing TCR and reducing stability
Solution Approach 1:
The invention extracts the metal foil resistive element from the constrained substrate-bonded structure and places it in a free state within the package cavity. The foil is suspended without adhesive bonding to substrates, eliminating the source of differential thermal expansion stress. This extraction allows the foil to expand and contract freely with temperature changes without inducing strain, directly resolving the TCR stability problem.
Solution Approach 2:
The invention segments the traditional integrated substrate-foil-adhesive structure into separate components: the metal foil resistive element is isolated and suspended independently within the package cavity, separated from the package walls by insulating spacers. This segmentation prevents stress transmission from the package substrate to the foil while maintaining structural support and electrical insulation.
2Manufacturing precision
If multiple control factors (materials, thicknesses, structures) are strictly set to reduce TCR, then TCR reduction is achieved, but the design becomes remarkably difficult and TCR stability is seriously affected by temporal changes
Solution Approach 1:
By extracting the metal foil from the adhesive-bonded substrate configuration and placing it in a free suspended state, the invention eliminates the need to control multiple parameters such as adhesive thickness, substrate material properties, and bonding stress. The TCR is now determined solely by the metal foil's intrinsic properties and its stress-free state, dramatically simplifying the design and manufacturing process while improving long-term stability.
3Reliability
If the package is sealed airtight with the metal foil bonded to substrate, then protection is provided, but external stress applied to the package is transmitted to the metal foil, affecting resistor characteristics
Solution Approach 1:
The invention introduces insulating spacers as intermediary elements between the metal foil resistive element and the package walls. These spacers create a physical buffer that prevents direct stress transmission from the package substrate to the foil while maintaining electrical insulation and positional stability. The foil remains in a free state, isolated from external mechanical stresses applied to the package.
4Stability of the object's composition
If the metal foil is bonded to substrate with adhesive, then the structure is stable, but thermal expansion differences cause strain and distortion of the metal foil
Solution Approach 1:
The invention extracts the metal foil from the adhesive-bonded configuration and suspends it in free space within the package cavity using insulating spacers. This eliminates the thermal expansion constraint imposed by the substrate and adhesive, allowing the foil to expand and contract freely with temperature changes without developing internal stress or distortion, while the package structure remains stable and protective.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design significantly reduces and stabilizes the TCR, increases design freedom, and prevents adverse effects on the resistor characteristics when attached to a heat sink, enhancing thermal management and stability.
Implementation Method 1
the metal foil resistive element is contained in the package in the insulated state so as to be expandable and contractible in the extending direction (planar direction) of the metal foil... the metal foil itself can freely expand and contract in its extending direction since any stresses are not induced and not applied to the metal foil
Implementation Method 2
the package is designed to prevent external stress transmission to the metal foil resistive element
Data Source
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AI summary
The metal foil resistor having a metal foil resistive element 20 composed of a metal foil whereupon a resistance circuit pattern is formed. The metal foil resistor comprises: a package 10 which contains the metal foil resistive element 20 in an electrically insulated state so that the resistive element can be expandable and contractible in a spreading direction of the metal foil; and a relay terminal 26 which is held in the package 10 in the electrically insulated state and is connected to an electrode 20a of the metal foil resistive element 20. A temperature coefficient of resistance can be reduced and stabilized. Control factors can be reduced to increase degrees in freedom in designing. Further, an external stress applied to a package is prevented from transmitting to the metal foil resistive element, and therefore the package can be easily attached to a discretionary heat sink.