Roughened Silver Plating on Lead Frame 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 can lead to peeling off under shock or heat, and require thick plating layers to prevent copper diffusion, making them unsuitable for high-density packaging.
Innovation Solution
A lead frame with a roughened silver plating layer having acicular projections as the outermost layer, formed on a copper-based substrate, 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 the surface of the lead frame substrate is roughened by microetching to improve adhesion to sealing resin, then adhesion is improved, but impurity residue called smut is generated when using copper alloy substrates
Solution Approach 1:
The invention changes the chemical parameters of the etching solution by adding organic additives (specifically carboxylic acid derivatives or their salts) to the conventional inorganic acid-based etching solution. This parameter change allows the etching process to create the necessary surface roughness for adhesion without generating smut impurity residue on copper alloy substrates.
Solution Approach 2:
The invention creates a composite etching system that combines inorganic acids (providing etching capability) with organic additives (preventing smut formation). This composite approach allows both functions to coexist: the inorganic acid component etches the surface to improve adhesion, while the organic component prevents smut generation during the etching process.
2Object-affected harmful factors
If the thickness of the silver plating layer is increased to prevent copper diffusion, then copper diffusion is prevented, but the lead frame size increases making it unsuitable for high-density packaging
Solution Approach 1:
The invention changes the surface morphology parameters of the silver plating layer by applying it to a pre-roughened substrate surface. This creates a physical anchor effect where the silver layer conforms to the surface irregularities, significantly improving adhesion strength. As a result, the required silver plating thickness can be reduced from several micrometers to just 0.01-0.5 μm while still preventing copper diffusion effectively.
Solution Approach 2:
The invention transitions from controlling diffusion resistance through thickness (one-dimensional approach) to controlling adhesion through surface morphology (three-dimensional approach). By creating surface roughness with concavities and convexities, the effective bonding area increases dramatically, allowing thin plating layers to achieve the same diffusion barrier performance as thick layers.
3Reliability
If a thick plating layer is used to ensure good bondability and prevent copper diffusion, then reliability is improved, but processing cost and time increase
Solution Approach 1:
The invention changes the surface area and surface energy parameters of the substrate by creating a roughened surface with concavities and convexities. This increases the effective bonding area and improves mechanical interlocking, allowing thin plating layers to achieve bondability comparable to or exceeding that of thick plating layers, thereby reducing material costs and processing time.
Solution Approach 2:
The invention replaces the reliance on mechanical thickness (massive plating layers) with a chemical and physical surface treatment approach. By using surface roughening combined with optimized etching chemistry, the system achieves equivalent or superior bonding performance with minimal material consumption and shorter processing cycles.
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 roughened silver plating layer with acicular projections significantly improves adhesion to sealing resin, reduces processing time and costs, and minimizes copper diffusion, enabling thinner plating layers suitable for high-density packaging.
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
it is necessary to make the thickness of the plating layer made of a precious metal or a precious metal alloy 2 μm or more to secure good bondability with metal wires used at the time of bonding with a semiconductor element. Therefore, when a plating layer made of a precious metal or a precious metal alloy such as silver or an alloy containing silver is formed directly on a lead frame substrate made of a copper alloy without an undercoat layer being provided, it is generally necessary to make the thickness of the plating layer made of a precious metal or a precious metal alloy 2 μm or more
Data Source
AI summary
A lead frame includes, as an outermost plating layer, a roughened silver plating layer having acicular projections and covering top faces and faces that form concavities or a through hole between the top faces and bottom faces of a lead frame substrate made of a copper-based material. 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.