Silver-Oxide Nickel-Silver Leadframe for Pb-Free Package Adhesion

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Solution Overview

Problem

The challenges in semiconductor package construction include shrinking package dimensions, which reduce adhesion surfaces, and the need for Pb-free solders that increase reflow temperatures, making it difficult to maintain mold compound adhesion to leadframes, especially in small devices like QFN and SON. Additionally, the high costs of metals used in pre-plated leadframe finishes, such as palladium, pose economic concerns.

Innovation Solution

A pre-plated leadframe with a copper base metal, a nickel-plated layer, a silver-plated layer, and an adhesion promotion coating of silver oxide is used, where the nickel layer protects the copper from oxidation and the silver from diffusion, and the silver oxide enhances adhesion to the mold compound, eliminating the need for palladium and gold layers.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a conventional multi-layer plating (Ni-Pd-Au) is used on copper leadframe, then wire-bonding reliability and corrosion resistance are improved, but manufacturing cost increases significantly

Engineering Contradiction:

Improvewire-bonding reliabilityVSAvoidmanufacturing cost

Core Design Contradiction:

ReliabilityVSQuantity of substance

Solution Approach 1:

The patent removes the Pd and Au layers from the conventional Ni-Pd-Au plating structure, retaining only the essential Ni layer on copper leadframe. This extraction eliminates expensive materials while preserving the core functionality of corrosion resistance and wire-bonding compatibility through the Ni layer alone.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent replaces expensive Pd and Au plating layers with a simpler, more economical Ni layer that provides sufficient protection for the copper base metal. This substitution uses cheaper materials that achieve the required performance without the need for multiple expensive layers.

Inventive Principle:

Principle #27Cheap short-living objects (Disposable)

2Adaptability or versatility

If Pb-free solder is used to meet environmental requirements, then environmental compliance is improved, but reflow temperature increases causing mold compound adhesion failure

Engineering Contradiction:

Improveenvironmental complianceVSAvoidmold compound adhesion

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The patent applies a tin-based solder resist coating to the leadframe surfaces before the molding process. This preliminary coating creates a protective barrier that prevents mold compound degradation during high-temperature Pb-free reflow soldering, thereby maintaining adhesion reliability while enabling environmental compliance.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The tin-based coating serves as a protective cushion layer that absorbs and mitigates the harmful effects of high reflow temperatures on the mold compound adhesion. This beforehand protection allows the package to withstand Pb-free soldering temperatures without adhesion failure.

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

3Volume of moving object

If package dimensions are reduced to meet miniaturization requirements, then device size is improved, but adhesion surface area decreases leading to weaker bonding

Engineering Contradiction:

Improvepackage sizeVSAvoidadhesion strength

Core Design Contradiction:

Volume of moving objectVSReliability

Solution Approach 1:

The patent changes the chemical composition and surface properties of the leadframe coating by applying a tin-based solder resist layer. This parameter change enhances the surface energy and chemical reactivity of the leadframe, improving adhesion strength per unit area and compensating for the reduced total adhesion surface area in miniaturized packages.

Inventive Principle:

Principle #35Parameter changes

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 reduces processing time and costs, improves wire-bonding reliability, and maintains adhesion to the mold compound, even at higher reflow temperatures, while minimizing nickel usage and avoiding delamination issues.

Implementation Method 1

the nickel layer protects the copper from oxidation

Methodology Applied

Scientific EffectOxidation: Oxidation

Implementation Method 2

the nickel layer protects the copper from oxidation and the silver from diffusion

Methodology Applied

Scientific EffectDiffusion: Diffusion

Implementation Method 3

the silver oxide enhances adhesion to the mold compound

Methodology Applied

Scientific EffectAdhesion: Adhesive

Data Source

PatentUS11848258B2Semiconductor package with nickel-silver pre-plated leadframe

Publication Date: 2023.12.19 TEXAS INSTRUMENTS INC

AI summary

A semiconductor package includes a pad and leads, the pad and leads including a base metal predominantly including copper, a first plated metal layer predominantly including nickel in contact with the base metal, and a second plated metal layer predominantly including silver in contact with the first plated metal layer. The first plated metal layer has a first plated metal layer thickness of 0.1 to 5 microns, and the second plated metal layer has a second plated metal layer thickness of 0.2 to 5 microns. The semiconductor package further includes an adhesion promotion coating predominantly including silver oxide in contact with the second plated metal layer opposite the first plated metal layer, a semiconductor die mounted on the pad, a wire bond extending between the semiconductor die and a lead of the leads, and a mold compound covering the semiconductor die and the wire bond.