MEMS Sensor Packaging with Compliant Die Pad for Thermal Stress
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Solution Overview
Problem
MEMS sensors in harsh environments face strain due to thermal expansion mismatch between support structures and sensors, leading to inaccurate measurements, and existing solutions fail to provide a hermetic seal, causing leaks and safety issues.
Innovation Solution
A sensor package design featuring a die pad and support structure made of materials with low coefficients of thermal expansion, allowing for stress minimization and hermetic sealing through a die pad with a base and support structure configuration that isolates the sensor from thermal stresses and uses a eutectic alloy for a hermetic seal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thermally compliant die attach made of silicone elastomer is used to reduce strain between MEMS sensor and support structure, then thermal expansion mismatch strain is reduced, but hermetic sealing capability deteriorates allowing gas or liquid leaks
Solution Approach 1:
The die attach structure is divided into two distinct parts: a compliant layer (silicone elastomer) that absorbs thermal expansion strain, and a rigid outer portion that provides hermetic sealing. This segmentation allows each part to fulfill its specific function without compromising the other.
Solution Approach 2:
The die attach uses a composite structure combining silicone elastomer (for thermal compliance) with a rigid sealing material (for hermetic seal). This composite approach integrates the beneficial properties of both materials to simultaneously reduce strain and prevent leaks.
2Strength
If a rigid support structure is used to provide structural stability, then mechanical strength is improved, but thermal expansion mismatch strain increases causing measurement errors
Solution Approach 1:
The compliant die attach layer acts as an intermediary between the rigid support structure and the MEMS sensor. It mediates the thermal expansion forces, absorbing the mismatch strain while allowing the rigid structure to maintain its structural stability.
Solution Approach 2:
The die attach material's mechanical parameters (compliance, elasticity) are specifically chosen to match the thermal expansion characteristics of the MEMS sensor, creating a parameter bridge between the rigid support structure and the sensitive sensor element.
3Reliability
If hermetic sealing is implemented to prevent leaks in harsh environments, then sealing reliability is improved, but device complexity increases
Solution Approach 1:
The hermetic sealing function is merged with the mechanical support structure by integrating the sealing portion directly into the die attach assembly. This eliminates the need for separate sealing components, reducing overall device complexity while maintaining sealing reliability.
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
The solution effectively minimizes thermal stresses on MEMS sensors, ensuring accurate measurements and preventing leaks, thus enhancing the reliability and safety of sensor systems in harsh environments.
Implementation Method 1
Support structures have a high thermal expansion mismatch between the support material and the MEMS sensor. This mismatch may cause strain, unrelated to pressure, which results in unintended results and errors in the sensor measurements.
Implementation Method 2
The outer portion is adapted to hermetically seal to a conduit of a header. The die pad may include a trench between outer edges of the outer and inner portions.
Data Source
Figure 1
Figure 2A~2B
Figure 3A~3B
AI summary
A sensor device and a method of forming comprises a die pad receives a sensor device, such as a MEMS device. The MEMS device has a first coefficient of thermal expansion (CTE). The die pad is made of a material having a second CTE compliant with the first CTE. The die pad includes a base and a support structure with a CTE compliant with the first and second CTE. The die pad has a support structure that protrudes from a base. The support structure has a height and wall thickness which minimize forces felt by the die pad and MEMS device when the base undergoes thermal expansion or contraction forces from a header.