Wiring Board Pad with Thick Copper Layer for Adhesion
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Solution Overview
Problem
Wiring boards with a two-layer (Au/Ni) structure pad face adhesion issues due to Ni oxide formation, leading to decreased reliability and tensile strength when external connection terminals are bonded, particularly as the thickness of the Cu layer becomes thin, causing potential peeling problems.
Innovation Solution
A three-layer structure pad is introduced, with a Cu layer interposed between the Ni layer and the via, where the Cu layer is made thicker to enhance adhesion and tensile strength by increasing its surface area contact with the resin layer, and its material is harder to oxidize than the Ni layer, thereby reducing oxide formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a two-layer (Au/Ni) structure pad is used, then the pad structure is simple and manufacturing is easy, but Ni oxide forms on the Ni layer surface causing decreased adhesion between pad and via
Solution Approach 1:
A Cu layer is introduced as an intermediary between the Ni layer and the via. This Cu layer serves as a mediator that prevents direct contact between the oxidized Ni layer and the via, thereby eliminating the adhesion problem caused by Ni oxide while maintaining the protective function of the Ni layer.
Solution Approach 2:
The pad structure is segmented into three distinct layers (Au, Ni, Cu) with each layer performing a specific function. The Au layer provides corrosion resistance, the Ni layer prevents Cu diffusion, and the Cu layer provides oxidation resistance at the via interface, thereby solving the adhesion problem through functional segmentation.
2Length of stationary object
If the Cu layer thickness is reduced to make the wiring board thinner, then the board thickness decreases, but the tensile strength and adhesion of the pad decrease causing peeling issues
Solution Approach 1:
The invention changes the thickness parameter of the Cu layer to be relatively thick (5-20 μm) compared to conventional designs. This parameter change increases the surface area of the Cu layer that contacts the resin layer, thereby enhancing adhesion and tensile strength while still allowing the overall board thickness to be controlled.
Solution Approach 2:
The pad structure uses a composite of three metal layers (Au, Ni, Cu) with different properties. The Cu layer in this composite provides both oxidation resistance and enhanced mechanical adhesion through its thicker design, creating a multi-functional composite structure that simultaneously addresses electrical, chemical, and mechanical 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 thicker Cu layer improves adhesion between the pad and via, increases tensile strength, and reduces the likelihood of peeling, enhancing the reliability of mounting semiconductor elements and wiring boards.
Implementation Method 1
a third metal layer provided between the second metal layer and the via and formed of a material harder to be oxidized than that of the second metal layer
Implementation Method 2
forming a pad by stacking first metal layer, a second metal layer and a third metal layer by a plating method on the support base member
Data Source
AI summary
A wiring board includes a pad exposed from an opening portion of an outermost insulating layer. The pad includes: a first metal layer, a surface of which is exposed from the wiring board; a second metal layer provided on the first metal layer and formed of a material effective in preventing a metal contained in a via inside the board from diffusing into the first metal layer; and a third metal layer provided between the second metal layer and the via, and formed of a material harder to be oxidized than that of the second metal layer. The thickness of the third metal layer is relatively thick, and is preferably selected to be three times or greater than a thickness of the second metal layer. A side surface of the third metal layer and a surface of the third metal layer to which the via is to be connected are roughed.