Wiring Board Inner Electrode Adhesion via Composite Metal Layers

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Solution Overview

Problem

Wiring boards with inner electrodes formed by thin film deposition on the inner surfaces of cutouts have lower adhesion, leading to potential detachment during soldering due to thermal expansion stress differences between the wiring board and module substrate.

Innovation Solution

A wiring board with an insulating substrate featuring cutouts and inner electrodes composed of multiple metal layers, including a nickel, chromium, or titanium intermediate layer and a gold outermost layer, exposed at the outer edge portion to prevent solder application and stress transfer.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If inner electrodes are formed by thin film deposition on the inner surfaces of cutouts to achieve high circuit density, then circuit density is improved, but adhesion of the inner electrodes deteriorates

Engineering Contradiction:

Improvecircuit densityVSAvoidadhesion of inner electrodes

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The inner electrode is constructed as a composite structure with multiple metal layers (first metal layer, second metal layer, and third metal layer) having different properties. The first metal layer provides adhesion to the insulating substrate, the second metal layer provides electrical conductivity and structural integrity, and the third metal layer provides solderability. This composite structure resolves the contradiction by combining materials with different functions to achieve both high circuit density and reliable adhesion.

Inventive Principle:

Principle #40Composite materials

2Reliability

If solder is applied onto the outer edge portions of the inner electrodes during joining, then electrical connection is achieved, but thermal expansion stress is transferred to the inner electrodes causing them to come off the insulating substrate

Engineering Contradiction:

Improveelectrical connectionVSAvoidbond strength of inner electrode

Core Design Contradiction:

ReliabilityVSStrength

Solution Approach 1:

The outer edge portions of the inner electrode are designed with a specific structure where the first metal layer is exposed, creating a local quality difference. This exposed first metal layer has different mechanical and thermal properties compared to the central portions covered by the second and third metal layers. This local structural variation allows the outer edges to better accommodate thermal expansion stress while maintaining electrical connection through the solderable third metal layer in the central regions.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The multi-layer composite structure of the inner electrode provides differential response to thermal stress. The first metal layer (exposed at edges) has properties that accommodate thermal expansion, while the second and third metal layers provide electrical conductivity and solderability. This composite design allows the electrode to maintain both electrical connection and mechanical strength under thermal cycling conditions.

Inventive Principle:

Principle #40Composite materials

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

This configuration enhances the adhesion and reliability of the inner electrodes, ensuring a stable electrical connection over time by preventing solder application on the outer edge portions and mitigating thermal expansion-induced stress.

Implementation Method 1

inner electrodes formed by thin film deposition on the inner surfaces of cutouts can have lower adhesion than wiring conductors arranged on the surface of the insulating substrate

Methodology Applied

Scientific EffectAdhesion: Adhesive

Implementation Method 2

Wiring boards nowadays have higher circuit densities and use thin film deposition to form wiring conductors and other parts on the surface of an insulating substrate

Methodology Applied

Scientific EffectThin film deposition: Physical Vapour Deposition

Implementation Method 3

In joining the inner electrodes of the wiring board to the connection pads of the module substrate by soldering

Methodology Applied

Scientific EffectSoldering: Soldering

Data Source

PatentUS10157753B2Wiring board, electronic device, and electronic module

Publication Date: 2018.12.18 KYOCERA CORP

AI summary

A wiring board (1) includes an insulating substrate (11) having a cutout (12) opened in a main surface and a side surface of the insulating substrate (11), and an inner electrode (13) formed on an inner surface of the cutout (12). The inner electrode (13) includes a plurality of metal layers. The inner electrode (13) includes, as an intermediate layer, at least one metal layer (17b) selected from the group consisting of a nickel layer, a chromium layer, a platinum layer, and a titanium layer, and includes a gold layer as an outermost layer (17a). The metal layer (17b) is exposed at an outer edge portion of the inner electrode (13).