Semiconductor Package Air Gap for Thermal Stress Relief
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Solution Overview
Problem
The coefficient of thermal expansion mismatch between semiconductor dies and package substrates in Ball Grid Array (BGA) packaging leads to high stress at solder balls, particularly at die edges, causing mechanical failure of interconnects.
Innovation Solution
An air gap is formed between the semiconductor die and the package substrate, either between their bottom surfaces or within the substrate, using a thermally decomposable material that decomposes during reflow to create a vent and subsequently an air gap, decoupling the die from the substrate and reducing stress on interconnects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a semiconductor die is mechanically attached to a BGA package substrate using die attach adhesive, then the die is securely mounted and electrically connected, but high stress regions form at the solder balls below the die edges due to CTE mismatch
Solution Approach 1:
The patent segments the die attach adhesive into two functional zones: a first region under the die edges that provides mechanical support, and a second region that forms an air gap to decouple the die from the substrate. This segmentation allows the adhesive to simultaneously provide mechanical attachment while creating stress-relief zones, resolving the contradiction between secure mounting and interconnect reliability.
Solution Approach 2:
The air gap acts as an intermediary element between the semiconductor die and the BGA package substrate. By introducing this intermediate space, the patent decouples the mechanical constraint from the electrical connection path, allowing the solder balls to accommodate thermal expansion differences without transmitting high stress to the interconnects, thus maintaining both attachment strength and interconnect reliability.
2Stability of the object's composition
If the die is fully supported by die attach adhesive across the entire bottom surface, then mechanical stability is maximized, but stress concentration occurs at the solder balls directly below the die edges
Solution Approach 1:
The patent applies local quality by making the die attach adhesive's support function location-dependent. The adhesive provides full support under the die center while creating an air gap under the die edges. This localized differentiation in adhesive presence allows the die to remain mechanically stable while the edge regions are decoupled from substrate constraints, eliminating stress concentration at the solder balls below the edges.
3Reliability
If an air gap is formed between the die and substrate, then stress on interconnects is reduced, but the die may become less mechanically stable
Solution Approach 1:
The segmented adhesive structure provides mechanical stability in the first region while creating the stress-relief air gap in the second region. This segmentation ensures that the die remains mechanically stable through the supported edge regions while the air gap portion reduces stress on the interconnects, resolving the contradiction between stability and reliability.
Solution Approach 2:
The air gap serves as a mechanical intermediary that selectively decouples only the portions of the die that would otherwise transmit stress to the interconnects. The remaining adhesive-supported regions continue to provide mechanical stability, allowing the system to achieve both interconnect reliability and overall mechanical stability simultaneously.
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 air gap reduces stress on solder balls, enhancing the reliability of electrical connections between the BGA package and a printed circuit board by mitigating thermal expansion mismatch effects.
Implementation Method 1
using a thermally decomposable material that decomposes during reflow to create a vent and subsequently an air gap
Data Source
AI summary
A package structure includes a package substrate having a top surface and a bottom surface. A semiconductor die having a top surface and a bottom surface. The semiconductor die is mounted to the package substrate. The bottom surface of the semiconductor die is adjacent to the top surface of the package substrate. An air gap is between the bottom surface of the package substrate and the bottom surface of semiconductor die.