Wiring Substrate Pad Embedding for Thickness Reduction
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Solution Overview
Problem
Conventional wiring substrates face challenges in reducing thickness while maintaining connection reliability due to the need for thick solder resist layers to protect surface plating, which can lead to degradation of nickel layer thickness and barrier effects, causing copper spread during bonding processes.
Innovation Solution
A wiring substrate design featuring a recessed wiring layer with a solder resist layer that buries the step part formed by the insulating and wiring pattern surfaces, embedding the pad's first metal layer within the insulating layer, and using a thin second metal layer to cover the projecting part, allowing for reduced thickness without compromising connection reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thick solder resist layer is provided to protect the surface plating layer, then the surface plating layer is protected from damage, but the wiring substrate thickness cannot be reduced
Solution Approach 1:
The pad structure transitions from a purely surface-level configuration to a three-dimensional embedded structure. The pad is positioned within a recess of the insulating layer, allowing the solder resist layer to cover both the pad and the recess without increasing overall substrate thickness. This dimensional reconfiguration enables protection functionality while maintaining thin profile.
Solution Approach 2:
The pad structure is nested within the insulating layer recess, with the solder resist layer subsequently covering this nested structure. This nesting approach allows multiple functional layers (pad, insulating material, solder resist) to be compactly arranged vertically without increasing the horizontal footprint or overall thickness of the substrate.
2Length of stationary object
If the nickel layer thickness is reduced to achieve thickness reduction, then the wiring substrate thickness is reduced, but connection reliability and barrier effect are degraded
Solution Approach 1:
The nickel layer thickness is differentiated between the pad region and other wiring regions. The pad area maintains sufficient nickel thickness for reliable bonding and copper barrier, while other areas can use thinner nickel layers for overall thickness reduction. This localized quality variation resolves the contradiction between thinning and maintaining reliability.
Solution Approach 2:
The nickel layer thickness parameter is optimized differently for different functional regions. The pad region uses a thicker nickel layer to maintain wettability and barrier properties, while the overall substrate thickness is reduced through thinner insulating layers and optimized layer configurations in non-critical areas.
3Length of stationary object
If the nickel layer thickness is significantly reduced, then the barrier effect becomes insufficient, but copper spreads to the solder
Solution Approach 1:
The pad is positioned within a recess of the insulating layer before solder application. This preliminary structural arrangement prevents solder from directly contacting and potentially spreading along the nickel-copper interface, providing mechanical containment that complements the barrier function of the nickel layer even when thinned.
Solution Approach 2:
The pad structure moves from a surface-level configuration to an embedded configuration within the insulating layer recess. This dimensional change creates a physical containment structure that prevents lateral copper spread to solder while allowing the nickel layer to be thinner than in conventional surface-mounted pad designs.
Data Source
AI summary
A wiring substrate includes an insulating layer, a wiring layer, a via wiring, and a solder resist layer. The wiring layer includes a pad body that constitutes a part of a pad and a wiring pattern including an upper surface. The pad includes the pad body, a first metal layer formed on an upper surface of the pad body and including an embedded part embedded in the insulating layer and a projecting part including upper and side surfaces and projecting from the upper surface of the insulating layer, and a second metal layer including an upper surface and covering the upper and side surfaces of the projecting part. The upper surface of the pad body and the upper surface of the wiring pattern are on the same plane. The upper surface of the second metal layer is positioned lower than the upper surface of the solder resist layer.