Thermal Airflow Sensor Diaphragm Cavity Venting Design
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Solution Overview
Problem
The challenge is to maintain high detection accuracy in thermal airflow sensors, as the volume of air sealed in the cavity behind the thin film portion can vary with temperature and pressure fluctuations, leading to deformation and errors in airflow detection.
Innovation Solution
A thermal flow sensor design featuring a flow rate detection element with a diaphragm, a heating resistor, and resistance temperature detectors, where a support member with a communicating hole and ventilating holes in the sheet adhesive prevents the cavity from being sealed, allowing it to communicate with external air and control deformation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the cavity on the back of the thin film portion is sealed, then the structure is simplified and adhesion is improved, but the air volume inside the cavity varies with temperature and pressure causing thin film deformation and detection errors
Solution Approach 1:
The cavity back surface is segmented into a thin film portion and a non-thin film portion, allowing differential response to temperature changes. The thin film portion can deform to compensate for air volume changes while the non-thin film portion maintains structural stability, resolving the contradiction between maintaining sealed structure simplicity and preventing detection errors.
Solution Approach 2:
Different regions of the cavity back surface are given different properties: the thin film portion is designed with specific thickness and material characteristics to allow controlled deformation, while the non-thin film portion provides structural support. This local differentiation allows the sealed cavity to maintain both structural integrity and compensation capability.
2Reliability
If the cavity is not sealed on the back of the thin film portion, then detection accuracy is maintained, but adhesion between components is reduced and convex formations may occur
Solution Approach 1:
The adhesive application area is segmented into regions corresponding to the thin film portion and non-thin film portion of the cavity back surface. Adhesive is applied primarily to the non-thin film portion and peripheral areas, avoiding the thin film portion itself. This segmentation maintains strong adhesion while preventing convex formations that would occur if adhesive were applied over the entire back surface.
Solution Approach 2:
The non-thin film portion acts as an intermediary structure between the sealed cavity and the external environment. It provides a stable bonding surface for adhesive application while the thin film portion remains free to deform in response to pressure changes, thus maintaining both adhesion strength and detection accuracy.
3Productivity
If sheet adhesive is used to bond the flow rate detection element, then the bonding process is simplified and production efficiency is improved, but the entire back surface is covered with adhesive causing convex formations
Solution Approach 1:
The adhesive application is segmented to exclude the thin film portion area from direct adhesive coverage. The sheet adhesive is positioned and applied such that it bonds the lead frame to the cavity back surface only in regions where it will not create convex formations, maintaining both production efficiency and surface flatness.
Solution Approach 2:
The adhesive layer is applied with non-uniform distribution: thicker application in peripheral and non-thin film regions for strong bonding, and minimal or no application over the thin film portion to prevent convex formations. This local quality differentiation resolves the contradiction between efficient full-surface bonding and maintaining surface flatness.
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 enhances detection accuracy by preventing hermetic sealing and reducing errors caused by temperature fluctuations, ensuring reliable airflow measurement.
Implementation Method 1
a heating resistor provided on the diaphragm
Implementation Method 2
resistance temperature detectors installed upstream and downstream of the heating resistor
Data Source
AI summary
An object of the present invention is to provide a thermal airflow sensor with high detection accuracy. In achieving the above object, this invention provides a thermal flow sensor including: a flow rate detection element that has a diaphragm formed by processing a semiconductor substrate, a heating resistor provided on the diaphragm, and resistance temperature detectors installed upstream and downstream of the heating resistor; and a support member that adhesively holds the flow rate detection element with a sheet adhesive interposed therebetween. The support member includes a communicating hole of which one end has an opening to a cavity provided on the back side of the diaphragm. The sheet adhesive has a ventilating hole formed in an opening area of the communicating hole in the support member.