Semiconductor Joining Structure with Graded Crystal Grain Sizes
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Solution Overview
Problem
The existing joining structures between metal members and metal porous bodies experience strain concentration and crack initiation due to differences in crystal grain size, leading to reliability issues under repeated strain applications.
Innovation Solution
A joining structure is developed with a laminated body configuration where the average crystal grain size of the outer layer is smaller than the inner layer, and the metal porous body's grain size is smaller than the outer layer's, ensuring continuous yield strain variation and reduced strain concentration, thereby preventing crack initiation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If metal particles are used for joining, then high thermal conductivity and high electric conductivity are achieved, but strain concentration occurs at the interface due to large difference in crystal grain size between paste connection part and copper metal member
Solution Approach 1:
The patent applies local quality by creating distinct crystal grain size zones at different locations within the metal member. The surface layer has a first crystal grain size while the inner layer has a second crystal grain size, with the ratio controlled between 0.5-2.0. This local variation in crystal structure allows the surface to better match the metal porous body and reduce strain concentration at the joining interface.
Solution Approach 2:
The patent changes the crystal grain size parameter of the metal member by controlling the ratio between surface layer and inner layer crystal grains. By adjusting this ratio to fall within 0.5-2.0, the yield strain characteristics are modified to prevent strain concentration, thereby improving joining reliability without sacrificing conductivity.
2Reliability
If Ag content in paste is increased to achieve high thermal conductivity and high electric conductivity, then conductivity improves, but resin ratio decreases leading to increased rigidity and decreased strain absorbing capability
Solution Approach 1:
The patent addresses this contradiction by applying local quality to the metal member's crystal structure. Instead of modifying the paste composition, it creates a gradient in crystal grain size within the metal member itself, allowing the surface layer to provide strain compatibility while the bulk material maintains structural integrity and conductivity.
3Strength
If noble metal plating is applied on electrode to improve joining strength with Ag paste, then adhesiveness improves, but thermal stress on semiconductor chip increases due to difference in thermal expansion
Solution Approach 1:
The patent applies the taking out principle by removing the noble metal plating layer from the electrode structure. Instead of using Ag paste with noble metal plating, the invention uses a metal porous body directly joined to the metal member with controlled crystal grain sizes, eliminating the thermal expansion mismatch problem caused by noble metal plating while maintaining joining 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 configuration effectively suppresses strain concentration and crack initiation, achieving reliable and durable joining by allowing for greater strain relaxation and maintaining structural integrity under thermal cycling.
Implementation Method 1
yield strain continuously varies in a thickness direction of a laminated body, a strain concentration is suppressed, and thus a crack initiation point is less likely to occur when strain is applied repeatedly
Data Source
Figure 1~2
Figure 3A~3C
Figure 4A~4B
AI summary
Provided is a joining structure that prevents occurrence of a crack at a joining portion between a metal member and a metal porous body, and can achieve a highly reliable joining. The joining structure of the present invention has a metal member 1 and a metal porous body 2 formed on the metal member 1. The metal member 1 includes an outer layer 1b and an inner layer 1c, the outer layer 1b including one of main surfaces 1a and being formed on the metal porous body 2 side, the inner layer 1c being formed at a position farther from the metal porous body 2 than the outer layer 1b in a thickness direction, the outer layer 1b has an average crystal grain size ds that is smaller than an average crystal grain size di of the inner layer 1c, and the metal porous body 2 has an average crystal grain size dp that is smaller than or equal to the average crystal grain size ds of the outer layer 1b.