Wiring Substrate Raised Nickel Layer for Solder Joint Reliability
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Solution Overview
Problem
Wiring substrates with solder bumps are prone to connection failures due to the mechanical brittleness of intermetallic compounds formed between the nickel layer and the solder, which are exacerbated by external stress and thinner nickel layers, leading to reduced bonding strength and potential solder ball detachment.
Innovation Solution
A wiring substrate design featuring a connection pad with a protective insulation layer having a protrusion portion and a metal layer with a raised peripheral portion, where the nickel layer is formed to be thicker in the peripheral area to reduce interdiffusion of tin and copper, thereby minimizing the thickness and brittleness of intermetallic compounds and enhancing bonding strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a nickel layer is used as a barrier layer between the connection pad and solder bump, then electrical connection is achieved, but intermetallic compounds form at the interface which are mechanically brittle and prone to damage
Solution Approach 1:
The patent applies local quality by forming a raised portion only in the peripheral area of the nickel layer, while the central area maintains a lower profile. This localized structural modification creates a gradient in intermetallic compound formation: the raised peripheral area produces thinner, more ductile IMCs that resist stress, while the central area allows sufficient IMC formation for strong bonding. This spatial differentiation resolves the contradiction between connection reliability and bonding strength.
Solution Approach 2:
The patent changes the geometric parameter of the nickel layer by introducing a raised portion with specific height (5-20 μm) and peripheral width (5-30 μm). This parameter modification controls the diffusion path length and stress distribution, thereby controlling the thickness and properties of intermetallic compounds. The raised peripheral area increases the nickel thickness locally, reducing IMC thickness and improving ductility, thus resolving the bonding strength issue.
2Strength
If the nickel layer thickness is increased to prevent interdiffusion, then bonding strength improves, but manufacturing complexity increases
Solution Approach 1:
Instead of uniformly increasing the nickel layer thickness throughout, the patent applies local quality by concentrating the additional thickness only in the peripheral raised portion. This localized approach achieves the desired bonding strength enhancement at the critical interface while avoiding the need to increase the entire nickel layer thickness, thereby reducing manufacturing complexity and material usage.
Solution Approach 2:
The patent segments the nickel layer into two distinct regions: a central flat area and a peripheral raised portion. This segmentation allows different thicknesses to be optimized for different functions - the central area provides electrical connection while the peripheral raised area provides mechanical reinforcement. The segmented structure can be manufactured using standard lithography and etching processes, avoiding excessive complexity.
3Strength
If external stress is applied to the wiring substrate, then mechanical strength is tested, but the brittle intermetallic compounds are damaged leading to connection failure
Solution Approach 1:
The patent addresses this contradiction by creating a local quality difference in the nickel layer structure. The raised peripheral portion specifically targets the stress-prone areas where intermetallic compounds most readily form and fail. By thickening the nickel locally at the periphery, the patent creates a stress-distributing structure that prevents the formation of thick brittle IMCs in high-stress regions, thereby maintaining connection reliability under external loading.
Solution Approach 2:
The raised peripheral portion of the nickel layer acts as a pre-formed cushioning structure that anticipates and mitigates stress concentration at the nickel-solder interface. This structural feature is built-in beforehand to prevent the formation of brittle intermetallic compounds under stress, thereby protecting the connection reliability before failure can occur during normal operation or thermal cycling.
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 solution effectively reduces the risk of intermetallic compound damage from external stress and maintains reliable solder bumps by increasing the thickness of the nickel layer in the peripheral area, ensuring stable electrical connections and preventing solder ball detachment.
Implementation Method 1
a nickel layer which is formed on the connection pad and has a raised portion in a peripheral area thereof, thereby tin and copper are restrained from being interdiffused
Data Source
AI summary
A wiring substrate includes: a connection pad having a first surface; a protective insulation layer formed on the first surface of the connection pad and having an opening portion therein, wherein a portion of the first surface of the connection pad is exposed from the opening portion; a metal layer having a lower surface facing the first surface of the connection pad and an upper surface opposite to the lower surface and formed on the first surface of the connection pad which is exposed from the opening portion, the metal layer including a raised portion that extends upward from the upper surface of the metal layer in a peripheral portion thereof; and a bump electrode formed on the upper surface of the metal layer.