Sensor Packaging Cavity Structure to Prevent Moulding Stress
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Solution Overview
Problem
Existing sensor packaging techniques require multiple wire bonding steps, apply stress to the sensor element due to high stiffness and thermal expansion of the mould compound, and introduce errors and drift in output signals, while also being costly and susceptible to harsh media exposure.
Innovation Solution
A method involving a lead frame that delineates two notional volumes, with a cavity reservation portion around the sensor element, allowing for a single moulding step to encapsulate the sensor and integrated circuit, while preventing mould compound contact with the sensor element.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the sensor element is encapsulated in mould compound, then the sensor device is protected and manufacturing is simplified, but the high stiffness and thermal expansion of the mould compound apply stress to the sensor element causing signal deterioration and drift
Solution Approach 1:
The packaging is segmented into two distinct volumes: a first volume containing the sensor element where mould compound is excluded, and a second volume containing the integrated circuit that is fully encapsulated. This segmentation allows the sensor element to remain stress-free while still providing protection and manufacturing simplicity through encapsulation of other components.
Solution Approach 2:
The sensor element is extracted from the mould compound encapsulation environment. By defining a cavity reservation portion that excludes mould compound from the first volume, the sensor element is protected from the harmful effects of mould compound stress and thermal expansion, while other components remain encapsulated for protection.
2Reliability
If multiple wire bonding steps are performed before and after moulding, then the sensor can be properly connected, but the process complexity and cost increase
Solution Approach 1:
All wire bonding and die attach operations are performed preliminarily before the moulding process. The lead frame is configured with bonding pads and electrical contacts in advance, allowing all necessary electrical connections to be established before encapsulation, thereby eliminating the need for subsequent wire bonding steps after moulding.
Solution Approach 2:
The manufacturing process merges the encapsulation step with the final protection stage, performing all electrical interconnections before moulding. This combines multiple discrete steps (wire bonding, die attach, encapsulation) into a more streamlined sequence where the moulding process serves as the final protective step rather than requiring additional bonding operations afterward.
3Reliability
If gel is applied to protect bond wires, then the sensor is protected, but the gel applies disturbing force on the sensing membrane causing error signals
Solution Approach 1:
The harmful gel medium is extracted from the first volume containing the sensor element. By configuring the lead frame to define a cavity reservation portion that excludes mould compound and eliminating gel application, the sensor membrane is freed from disturbing forces while bond wires in the second volume remain protected through alternative means or the encapsulation environment.
Solution Approach 2:
The invention eliminates the need for gel protection entirely by redesigning the packaging structure. Instead of using gel as a protective medium that causes measurement errors, the lead frame cavity reservation portion provides structural protection for bond wires without introducing harmful forces on the sensing membrane, effectively replacing a harmful protective method with a neutral structural solution.
4Reliability
If all metals in the cavity are made noble for chemical protection, then the sensor is protected from harsh media, but the processing becomes complicated and expensive
Solution Approach 1:
The sensor element and its metal components are extracted from the harsh chemical environment by excluding mould compound from the first volume. This eliminates the need for noble metal plating on all cavity metals, as the sensor element is isolated from harsh media exposure while the lead frame structure provides the necessary chemical resistance in areas where metals are exposed.
Data Source
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AI summary
A method of manufacturing a sensor device comprising: configuring a moulding support structure and a packaging mould so as to provide predetermined pathways to accommodate a moulding compound, the moulding support structure defining a first notional volume adjacent a second notional volume. An elongate sensor element and the moulding support structure are configured so that the moulding support structure fixedly carries the elongate sensor element and the elongate sensor element resides substantially in the first notional volume and extends towards the second notional volume, the elongate sensor element having an electrical contact electrically coupled to another electrical contact disposed within the second notional volume. The moulding support structure carrying (102) the elongate sensor element is disposed within the packaging mould (106). The moulding compound is then introduced (110) into the packaging mould during a predetermined period of time (112) so that the moulding compound fills the predetermined pathways, thereby filling the second notional volume and surrounding the elongate sensor element within the second notional volume without contacting the elongate sensor element.