Thermal Airflow Meter Crystal Orientation and Molding
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Solution Overview
Problem
Thermal type air flow sensors face challenges in achieving high measurement accuracy due to positional instability of the flow detecting unit and thermal stress caused by differences in linear expansion coefficients between the sensor assembly and housing, leading to inaccuracies in air flow rate detection.
Innovation Solution
A thermal type air flow sensor configuration where the sensor assembly, including a single-crystal Si integrated circuit with diffusion resistor bodies, is integrally molded with the housing using different resins, with the resistor bodies oriented parallel to the crystal orientation direction to minimize thermal stress and improve positional accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the sensor assembly is fixed to the housing using adhesive after separate manufacturing, then the manufacturing complexity is reduced and assembly is easier, but the positional accuracy of the flow detecting unit deteriorates due to gaps and adhesive state variations
Solution Approach 1:
The patent merges the sensor assembly and housing into a single integrally molded component, eliminating the separate assembly step and adhesive bonding process. This integration ensures precise positioning of the flow detecting unit within the sub passage without gaps or adhesive-related positional variations, directly resolving the contradiction between ease of manufacture and positioning accuracy.
2Manufacturing precision
If the sensor assembly is integrally molded with the housing, then the positional accuracy of the flow detecting unit is improved, but thermal stress increases due to linear expansion differences between materials
Solution Approach 1:
The patent changes the material parameter by using a resin with a linear expansion coefficient closely matching that of the sensor assembly (particularly the LSI and diffusion resistors). This parameter matching minimizes thermal stress during temperature changes while maintaining the integral molding structure, thus resolving the contradiction between positioning accuracy and thermal stress.
Solution Approach 2:
The patent employs composite material selection where the housing resin is specifically chosen to have thermal expansion properties compatible with the sensor assembly components. This composite approach allows the integrally molded structure to withstand thermal cycling without generating excessive stress, addressing both positioning accuracy and thermal stress concerns.
3Ease of manufacture
If the diffusion resistor bodies are oriented perpendicular to the crystal orientation direction, then the manufacturing process is simpler, but the output stability deteriorates due to thermal stress-induced resistance changes
Solution Approach 1:
The patent applies local quality optimization by specifically orienting the diffusion resistor bodies parallel to the <100> crystal orientation direction of the single-crystal Si substrate, while other structural elements may have different orientations. This localized directional arrangement minimizes thermal stress-induced resistance changes in the critical resistor elements, improving output stability without requiring all components to follow the same orientation rule.
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 configuration enhances the measurement accuracy of air flow rates by reducing the impact of thermal stress and maintaining a stable resistance output, thereby improving the overall performance of the thermal type air flow sensor.
Implementation Method 1
measures a state of a measurement target gas which flows through the sub passage by thermal transfer performed by the flow detecting unit between the measurement target gas and the flow detecting unit
Implementation Method 2
a thermosensitive flow rate detecting element is provided with a heating element for measuring a flow velocity or flow rate of a fluid based on heat transfer to the fluid from the heating element
Implementation Method 3
A gap between the hole in the sub passage and the flow detecting unit and a gap formed at a portion fitting to the housing of the sensor assembly are filled with an elastic adhesive and an elastic force of the adhesive acts as a counterbalance to a linear expansion difference therebetween
Data Source
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AI summary
In order to provide a thermal flow sensor having high measurement accuracy, the thermal type air flow sensor includes a flow detecting unit, a sub passage on which the flow detecting unit is disposed, and an LSI to which a signal obtained by the flow detecting unit is input and which outputs a signal to the outside. A side wall of the sub passage is disposed between the flow detecting unit and the LSI, or on the LSI. A diffusion resistor provided inside the LSI has its longitudinal direction in parallel with a <100> orientation of single-crystal Si.