Lead Frame Concavity for Microchip Stress Mitigation
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Solution Overview
Problem
Conventional plastic packages for microchips experience stress concentrations due to differences in thermal expansion coefficients between the die and the lead frame, leading to delamination and catastrophic wirebond failures, especially during temperature changes and vibration forces.
Innovation Solution
A lead frame with a concavity on its top surface is used to mitigate these stresses, where the die is directly attached to the lead frame, and the mold material encapsulates the lead frame, reducing the likelihood of separation by distributing thermal expansion mismatch stresses effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the die is directly mounted to the lead frame, then the electrical connection is improved, but stress concentration at the interface between plastic/mold compound and lead frame increases
Solution Approach 1:
The patent introduces an intermediary layer (adhesive or stress relief layer) between the die and lead frame to mediate the stress transfer. This layer absorbs or redistributes the thermal expansion mismatch stresses, preventing direct stress concentration at the plastic/lead frame interface while maintaining electrical connectivity through the die attachment.
Solution Approach 2:
The patent modifies the mechanical parameters of the lead frame by creating a recess or depression in its surface. This geometric parameter change allows the mold compound to flow into and fill the recess, creating a gradual transition zone that reduces stress concentration at the interface while maintaining secure die attachment.
2Reliability
If conventional plastic packages are used to protect microchips, then environmental protection is improved, but delamination occurs due to thermal expansion mismatch
Solution Approach 1:
The patent changes the geometric parameters of the lead frame by incorporating a recess or depression. This allows the mold compound to form a interlocking structure with the lead frame, increasing the surface area of adhesion and creating a mechanical key that prevents delamination caused by thermal expansion mismatch during temperature cycling.
Solution Approach 2:
The patent creates a composite structure where the mold compound, lead frame with recess, and die form an integrated assembly. The recess-filled mold compound acts as a composite material system that combines the protective properties of the plastic package with stress-mitigating features, preventing delamination while maintaining environmental protection.
3Strength
If the mold material encapsulates the lead frame, then the structural integrity is improved, but vibration forces cause the mold material to pull away from lead frame at the edge
Solution Approach 1:
The patent introduces a curved or recessed surface feature in the lead frame that allows the mold compound to form a mechanically interlocking structure. This geometric feature creates a gradual transition zone that distributes vibration forces more evenly, preventing the mold material from pulling away at sharp edges while maintaining overall structural integrity.
4Temperature
If temperature changes occur, then the thermal expansion mismatch causes damaging stresses, but no design feature mitigates this stress
Solution Approach 1:
The patent modifies the physical parameters of the lead frame by creating a recess or depression in its surface. This geometric parameter change allows the mold compound to form a stress-distributing structure that accommodates thermal expansion mismatch, reducing thermal stresses generated during temperature cycling while maintaining secure component attachment.
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 concavity design significantly reduces the transmission of thermal expansion mismatch stresses, preventing delamination and wirebond failures, ensuring the reliability of microchip packages by maintaining the integrity of the mold material and electrical connections.
Implementation Method 1
the differential between the coefficient of thermal expansion of the die and that of the lead frame is significant. Accordingly, temperature changes can cause damaging stresses between the lead frame and plastic/mold material of the package.
Implementation Method 2
an adhesive for directly contacting the die with the portion of the lead frame
Data Source
AI summary
A packaged microchip has a lead frame with a die directly contacting at least a single, contiguous portion of the lead frame. The portion of the lead frame has a top surface forming a concavity and contacting the die. The packaged microchip also has mold material substantially encapsulating part of the top surface of the portion of the lead frame.