Lead Frame Plating Structure for Resin Adhesion
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Solution Overview
Problem
Conventional lead frames with silver plating suffer from poor adhesion to sealing resin due to inadequate surface roughening, which is not feasible for copper alloy substrates, and require thick precious metal plating layers to prevent copper diffusion, making them costly and inefficient for high-density packaging.
Innovation Solution
A lead frame with a copper-based substrate featuring laminated nickel, palladium, and gold layers, topped with a roughened silver plating layer having acicular projections, which enhances adhesion to sealing resin while maintaining a thin total plating thickness and reducing processing costs and time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If microetching is used to roughen the lead frame substrate surface to improve adhesion to sealing resin, then adhesion is improved, but impurity residue (smut) is generated making it unsuitable for copper alloy substrates
Solution Approach 1:
A nickel undercoat layer is introduced as an intermediary between the copper alloy lead frame substrate and the silver plating layer. This nickel layer serves as a mediator that enables surface roughening through microetching without generating harmful smut residue on the copper substrate, while still providing the physical anchor effect for improved resin adhesion.
Solution Approach 2:
The plating structure is segmented into multiple functional layers: a nickel undercoat layer for protection and controlled roughening, and a silver plating layer for adhesion and conductivity. This segmentation allows each layer to perform its specific function without interfering with the copper substrate.
2Reliability
If a thick plating layer made of precious metal or precious metal alloy is formed directly on the copper alloy substrate to prevent copper diffusion, then copper diffusion is prevented, but cost increases and plating thickness cannot be reduced for high-density packaging
Solution Approach 1:
The nickel undercoat layer acts as a diffusion barrier intermediary between the copper substrate and the silver plating layer. This allows the silver layer to be much thinner since it no longer needs to serve as the primary diffusion barrier, reducing both material cost and thickness for high-density packaging applications.
Solution Approach 2:
Different layers are assigned different functional qualities: the nickel undercoat layer provides diffusion protection and surface preparation, while the thin silver layer provides adhesion to sealing resin and electrical conductivity. This local specialization of functions optimizes both performance and cost.
3Quantity of substance
If a thin plating layer made of precious metal or precious metal alloy is used to reduce cost and enable high-density packaging, then cost is reduced and size is minimized, but adhesion to sealing resin cannot be improved
Solution Approach 1:
The nickel undercoat layer is formed in advance on the copper substrate before applying the silver plating layer. This preliminary action prepares the surface with appropriate roughness and chemical properties, enabling the subsequent thin silver layer to achieve excellent adhesion to sealing resin without requiring thick plating.
Solution Approach 2:
The surface parameters of the nickel undercoat layer are optimized to provide the right balance of roughness for adhesion and smoothness for subsequent plating. By controlling the microetching parameters and nickel layer properties, the thin silver layer achieves maximum adhesion potential.
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 significantly improves adhesion to sealing resin, minimizes plating thickness, and enhances productivity by forming a roughened silver plating layer with acicular projections, addressing the limitations of conventional techniques.
Implementation Method 1
the surface of the lead frame substrate is roughened to have concavities and convexities by microetching, to produce a physical anchor effect, thereby improving adhesion to sealing resin
Implementation Method 2
a plating layer made of nickel or an alloy containing nickel, which prevents copper diffusion
Data Source
AI summary
A lead frame includes a lead frame substrate made of a copper-based material, plating layers composed of nickel, palladium and gold layers laminated in this order on top faces and bottom faces of the lead frame substrate, and a roughened silver plating layer having acicular projections, provided as an outermost plating layer and covering faces of the lead frame substrate that form concavities or a through hole between the top faces and the bottom faces of the lead frame substrate. The roughened silver plating layer has a crystal structure in which the crystal direction <101> occupies a largest proportion among the crystal directions <001>, <111> and <101>. The lead frame can be manufactured with improved productivity owing to reduction in cost and operation time, and achieves remarkably high adhesion to sealing resin while keeping the total thickness of plating layers including the silver plating layer to be thin.