Copper Post WLP With Flange UBM For Stress Relief
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Solution Overview
Problem
Existing Wafer Level Packaging (WLP) configurations fail to provide a robust mechanical bond between copper posts and solder balls, leading to failures in temperature cycling, drop tests, and high current applications due to stress and expansion issues at the ball/post/epoxy interface.
Innovation Solution
A WLP design featuring a copper pillar with a recessed surface and an Under Ball Metal (UBM) layer that extends above the encapsulant surface, providing a three-dimensional metallic bond between the solder ball, UBM, and copper post, and optionally incorporating a nickel layer to reduce inter-metallic compound growth and enhance current carrying capability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a copper post is embedded below the encapsulant surface, then the encapsulant can surround and protect the copper post, but the sharp corner interface between encapsulant, solder ball, and copper post creates stress concentration leading to failure in temperature cycling and drop tests
Solution Approach 1:
A nickel under ball metal (UBM) layer is introduced as an intermediary between the copper post and solder ball. This UBM layer extends above the encapsulant surface to create a flush interface with the solder ball, eliminating the sharp corner stress concentration. The nickel UBM serves as a mediator that provides a smooth transition surface, preventing stress concentration while maintaining the protective encapsulation structure.
Solution Approach 2:
The UBM layer adds a dimensional element by extending radially outward from the copper post in a flange-like configuration. This radial extension creates a larger bonding surface area and distributes stresses more evenly across the interface, transforming the concentrated stress at a sharp corner into a distributed stress pattern across a broader area.
2Reliability
If the copper post surface is recessed below the encapsulant surface, then the encapsulant provides mechanical support, but the resulting interface topology causes expansion and contraction stresses during thermal cycling
Solution Approach 1:
The nickel UBM layer acts as a thermal expansion mediator between the copper post and solder ball. During thermal cycling, the UBM layer accommodates differential expansion between the copper post and solder ball, reducing thermal stress transmission. The UBM's intermediate position allows it to absorb expansion/contraction movements, stabilizing the interface during thermal cycles.
3Reliability
If the encapsulant directly contacts the solder ball at a sharp corner, then the structure is simple, but heat transfer from encapsulant to solder ball creates compression and tension stresses during thermo-mechanical testing
Solution Approach 1:
The nickel UBM layer serves as a thermal interface mediator between the encapsulant and solder ball. It provides a controlled thermal pathway that reduces abrupt heat transfer and associated thermal stresses. The UBM's presence creates a more gradual thermal gradient, minimizing compression and tension stresses during thermo-mechanical testing.
4Reliability
If a traditional copper post configuration is used, then the manufacturing process is simple, but the mechanical bond between copper post and solder ball is insufficient for high current applications
Solution Approach 1:
The electrical connection structure is transformed into a composite material system consisting of copper post, nickel UBM layer, and solder ball. This composite structure combines the advantages of different materials: copper for electrical conductivity, nickel for mechanical strength and bonding, and solder for joint formation. The composite nature of the structure provides superior mechanical bond strength and current carrying capability compared to pure copper configurations.
Solution Approach 2:
The UBM layer extends radially outward in a flange-like configuration, adding a dimensional element that increases the bonding surface area. This radial extension creates a larger interface for mechanical bonding between the copper post assembly and solder ball, significantly enhancing the mechanical bond strength required for high current applications.
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 enhanced WLP structure significantly improves reliability in temperature cycling and drop tests while supporting higher current applications by creating a stronger mechanical and electrical interface, reducing electro-migration, and increasing the number of cycles and tests the package can withstand.
Implementation Method 1
incorporating a nickel layer to reduce inter-metallic compound growth and enhance current carrying capability
Data Source
AI summary
A WLP device is provided with a flange shaped UBM or an embedded partial solder ball UBM on top of a copper post style circuit connection.