Thin-Nickel ENEPIG Surface Finish for Reliable Embedded Die Attachments
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Solution Overview
Problem
Embedding circuit components within a package substrate is challenging due to thickness mismatches, leading to issues like tilting or shifting, and existing surface finishes without nickel in the electroless nickel-electroless palladium-immersion gold (ENEPIG) process result in solder joint reliability concerns and formation of brittle inter-metallic compounds.
Innovation Solution
Implementing a process flow that uses ENEPIG with a thin nickel layer or alternative surface finishes like immersion gold-electroless palladium-immersion gold (IGEPIG), indium, or cobalt-iron layers to address solder joint reliability and inter-metallic compound formation issues, while reducing the need for additional lithography steps and maintaining cost-effectiveness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a traditional ENEPIG surface finish with standard nickel layer thickness is used, then solder joint reliability is improved, but the formation of brittle inter-metallic compounds increases and manufacturing complexity increases due to additional lithography steps
Solution Approach 1:
The patent extracts and removes the nickel layer from the traditional ENEPIG surface finish, creating a new ENIG (Electroless Nickel-Electroless Palladium-Immersion Gold) or EPG (Electroless Palladium-Immersion Gold) process. This extraction eliminates the need for additional lithography steps required for thick nickel deposition while maintaining solder joint reliability through optimized thinner nickel layers (50-200 nm) or alternative metal compositions.
Solution Approach 2:
The patent changes the thickness parameter of the nickel layer from traditional thick deposits (micrometers) to thin films (50-200 nm), and alternatively changes the material composition by using palladium or cobalt-iron layers. These parameter changes reduce the formation of brittle inter-metallic compounds and eliminate the need for additional lithography steps while maintaining or improving solder joint reliability.
2Object-generated harmful factors
If the nickel layer thickness is reduced to minimize inter-metallic compound formation, then brittleness is reduced, but solder joint reliability may deteriorate
Solution Approach 1:
The patent uses composite material structures by combining multiple thin metal layers (nickel-palladium-gold in ENEPIG, or palladium-gold in EPG, or cobalt-iron-gold alternatives). This composite structure provides both mechanical strength for solder joint reliability and controlled inter-metallic compound formation at each interface, preventing excessive brittleness while maintaining reliability.
Solution Approach 2:
The patent introduces palladium as an intermediary layer between nickel and gold, and between the metal stack and solder joint. This intermediary layer controls the formation of inter-metallic compounds, preventing direct reaction between nickel and solder while maintaining joint reliability. The palladium layer acts as a diffusion barrier and mechanical buffer.
3Ease of manufacture
If alternative surface finishes like IGEPIG or indium layers are used, then cost-effectiveness is improved and manufacturing steps are reduced, but solder joint reliability may be compromised
Solution Approach 1:
The patent changes the material composition parameter by using indium layers or cobalt-iron alloys as alternatives to traditional nickel-palladium-gold stacks. These alternative materials provide comparable or superior solder joint reliability through different mechanisms (indium's low melting point and wetting properties, cobalt-iron's mechanical strength) while reducing manufacturing steps and costs by eliminating electroless nickel deposition and associated lithography requirements.
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 proposed solutions enhance solder joint reliability, reduce the formation of brittle inter-metallic compounds, and improve wettability, providing better electrical connections and cost efficiency in integrated circuit packages.
Implementation Method 1
a first surface finish layer formed on the conductive contact, the first surface finish layer comprising nickel
Implementation Method 2
a second surface finish layer formed on the first surface finish layer, the second surface finish layer comprising palladium
Implementation Method 3
a third surface finish layer formed on the second surface finish layer, the third surface finish layer comprising gold
Data Source
AI summary
In embodiments herein, a surface finish (SF) is formed on conductive contacts of a package substrate for connection to an embedded interconnect bridge circuitry die. In some embodiments, the SF may be electroless nickel-electroless palladium-immersion gold (ENEPIG). In other embodiments, the SF may be immersion gold-electroless palladium-immersion gold (IGEPIG). In other embodiments, the SF may include a layer of electrolytic palladium-gold on a layer of indium or on a layer of cobalt-iron.