Multi-step sintering of metal paste for semiconductor wire bonding
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Solution Overview
Problem
Conventional sintering processes for metal pastes result in significant porosity of sintered metal materials, leading to weak wire bond connections and instability in wire-bonded semiconductor devices, characterized by high resistance contacts and low pull strength.
Innovation Solution
A multi-step sintering process involving a heat up step in a reducing gas atmosphere followed by vacuum sintering at temperatures of at least 200°C, which reduces surface oxides and allows metal nanoparticles to combine, increasing grain size and density of the sintered metal material.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional single-step sintering in N2 or inert gas is used, then the process is simple and fast, but the sintered metal material remains significantly porous leading to weak wire bond connections
Solution Approach 1:
The sintering process is divided into multiple sequential steps: (1) heating in N2 or inert gas to remove binder, (2) heating in reducing gas atmosphere to reduce surface oxides, and (3) vacuum sintering to densify the metal material. This segmentation allows each step to address specific issues (binder removal, oxide reduction, porosity elimination) that cannot be effectively addressed in a single step, thereby improving wire bond connection strength.
Solution Approach 2:
Before the final vacuum sintering step, the patent performs preliminary actions of binder removal and oxide reduction in controlled atmospheres. The reducing gas treatment (e.g., H2 atmosphere) preliminarily reduces surface oxides on metal particles, creating a cleaner surface that facilitates better sintering and bonding in the subsequent vacuum step, leading to stronger wire bond connections.
2Manufacturing precision
If reducing gas treatment is applied to reduce surface oxides, then oxide reduction is achieved, but additional process steps and time are required
Solution Approach 1:
The patent merges the oxide reduction step with the sintering process by using a reducing gas atmosphere (such as H2 or forming gas) during the heating/sintering cycle. This combination allows oxide reduction to occur concurrently with or immediately before sintering without requiring a completely separate reduction step, thereby achieving surface oxide reduction while minimizing additional process time.
Solution Approach 2:
The patent changes the atmospheric parameters during sintering by introducing reducing gases (e.g., H2, forming gas) at specific temperature ranges. By controlling the gas composition and temperature parameters, the process achieves effective oxide reduction during the sintering cycle itself, optimizing both oxide reduction quality and process efficiency without requiring excessive additional time.
3Strength
If vacuum sintering is used to reduce porosity, then density increases improving bond strength, but equipment complexity and processing time increase
Solution Approach 1:
The sintering process is segmented into atmospheric heating/binder removal followed by vacuum sintering. This segmentation allows the complex vacuum sintering step to be focused specifically on the densification phase after binder removal is complete, making the overall process more manageable and enabling the use of vacuum equipment to achieve the necessary density for strong wire bonds.
Solution Approach 2:
Before vacuum sintering, preliminary binder removal is performed in an atmospheric environment. This preliminary action eliminates the organic binder that would otherwise interfere with vacuum sintering and metal particle densification, allowing the vacuum sintering step to focus solely on removing porosity and achieving high density for improved bond strength.
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 approach significantly reduces porosity and enhances bond strength, resulting in improved wire bond performance, higher pull strength, and increased shear strength, thereby stabilizing the wire bond connections.
Implementation Method 1
heat treating the metal paste in a non-oxidizing atmosphere, such in N2 or an inert gas at a peak temperature of about 150° C. to 300° C. to remove the binder
Implementation Method 2
a multi-step sintering process involving a heat up step in a reducing gas atmosphere followed by vacuum sintering at temperatures of at least 200°C, which reduces surface oxides and allows metal nanoparticles to combine, increasing grain size and density of the sintered metal material
Implementation Method 3
heat treating the metal paste in a non-oxidizing atmosphere, such in N2 or an inert gas at a peak temperature of about 150° C. to 300° C. to remove the binder
Data Source
AI summary
A method of assembling semiconductor devices includes applying a metal paste including a plurality of metal particles having an average size less than 50 nanometers and a binder material onto a metal terminal of a package substrate. The metal paste is processed including a heat up step in a reducing gas atmosphere and then a vacuum sintering step at a temperature of at least 200° C. for forming a sintered metal coating. A semiconductor die is attached onto a die attach area of the package substrate. A bond wire is then connected between a bond pad on the semiconductor die and the sintered metal coating on the metal terminal.