Leadframe Oxidation for Encapsulant Adhesion and Wire Bonding
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Solution Overview
Problem
Existing methods for enhancing encapsulant adhesion to leadframes, particularly those with silver plating, are inadequate due to impurities and organo-metallic coatings that degrade bond strength between the leadframe and bonding wires, and fail to address thermal stress issues introduced by lead-free solders at higher temperatures.
Innovation Solution
A method involving oxidation treatment to form copper oxide on the leadframe surface, followed by a complexing agent dip to enhance purity, and acid cleaning to remove contaminants, creating a unique fibrillar structure for superior encapsulant adhesion while maintaining clean silver plating for strong wire bonding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the leadframe surface is roughened by oxidizing treatment to enhance encapsulant adhesion, then the adhesion strength between encapsulant and leadframe is improved, but impurities and organo-metallic coatings are formed on the silver plating surface that degrade bond strength with bonding wires
Solution Approach 1:
The patent applies different surface treatments to different regions of the leadframe: the encapsulation area receives oxidizing treatment to create copper oxide for enhanced encapsulant adhesion, while the bonding pad areas are protected or selectively treated to maintain clean silver plating surfaces for optimal wire bonding. This spatial differentiation of surface properties resolves the contradiction between needing rough surfaces for adhesion and clean surfaces for bonding.
Solution Approach 2:
The leadframe surface is divided into functionally distinct zones: encapsulation regions that are oxidized for adhesion enhancement, and bonding regions that are kept clean or differently treated for wire bonding. This segmentation allows each region to have the surface characteristics optimal for its specific function, eliminating the trade-off between adhesion and bonding quality.
2Reliability
If higher temperatures are used during soldering with lead-free solders, then soldering process reliability is improved, but thermal stress inside the package increases and aggravates the risk of package failure
Solution Approach 1:
The patent performs surface treatment and adhesion enhancement procedures before the soldering process and encapsulation. By pre-conditioning the leadframe surface with proper oxidation and cleaning, the encapsulant forms strong bonds in advance, preparing the structure to withstand the thermal stress that will occur during subsequent lead-free soldering and packaging operations.
Solution Approach 2:
The oxidized copper oxide layer acts as a cushioning interface that absorbs and distributes thermal stress during subsequent high-temperature soldering processes. This pre-formed adhesion layer protects the encapsulant-leadframe interface from thermal degradation, allowing the package to withstand lead-free soldering temperatures without failure.
3Strength
If chemical etchants containing sulfuric acid and hydrogen peroxide are used to roughen the leadframe surface, then encapsulant adhesion is improved, but organo-metallic coatings are formed that may degrade bond strength between leadframe and bonding wire
Solution Approach 1:
The patent extracts or removes the harmful organo-metallic coatings and impurities from the leadframe surface after the oxidizing treatment. This is achieved through selective cleaning steps that remove contaminants from the silver plating surfaces while preserving the beneficial copper oxide layer on encapsulation areas, thereby maintaining surface purity without sacrificing adhesion enhancement.
Solution Approach 2:
The patent converts the harmful effect of chemical etchants that create organo-metallic coatings into a beneficial process by followed-up with specific cleaning treatments. The initial roughening provides adhesion enhancement, and the subsequent cleaning removes harmful residues, transforming a potentially harmful chemical treatment into a beneficial two-step surface preparation process.
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 method achieves enhanced adhesion of the encapsulant to the leadframe and maintains excellent bondability with bonding wires, improving thermal stability and reducing the risk of 'popcorn failure' in semiconductor packages.
Implementation Method 1
oxidizing the leadframe in an oxidation treatment bath to form copper oxide on the surface of the leadframe
Implementation Method 2
dipping the leadframe in a complexing or chelating agent to enhance the purity of the copper oxide formed
Implementation Method 3
cleaning the leadframe with an acid to remove contaminants remaining on the leadframe
Data Source
AI summary
A method is provided for treating a leadframe comprising copper or copper alloy to enhance adhesion of molding compound to it. The leadframe is oxidized in an oxidation treatment bath to form copper oxide on the surface of the leadframe. It is then dipped in a complexing or chelating agent to enhance the purity of the copper oxide formed. Thereafter, the leadframe is cleaned with an acid to remove any contaminants remaining on the leadframe.