Composite Wiring Substrate Joint Material Design
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Solution Overview
Problem
Existing composite wiring substrates often experience cracks near the joint material between the build-up substrate and the interposer, which can lead to thermal stress and instability during semiconductor chip mounting.
Innovation Solution
A composite wiring substrate design featuring a joint material with a first portion of intermetallic copper-tin alloy and a second portion of tin-bismuth alloy, where the tin-bismuth portion has a higher Bi concentration than the eutectic composition, is used to join the build-up substrate and interposer, providing thermal stability and toughness to prevent cracks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a conventional joint material is used to join the build-up substrate and interposer, then the joining strength is sufficient, but cracks occur near the joint material due to thermal stress during semiconductor chip mounting
Solution Approach 1:
The joint material is divided into two distinct portions: a first portion made of intermetallic alloy (Cu-Sn) that contacts the connection terminals and provides strong bonding, and a second portion made of tin-bismuth alloy with high Bi concentration (≥30 mass%) that fills the remaining space and provides crack resistance. This segmentation allows each portion to perform its specific function optimally without compromising the other.
Solution Approach 2:
Different regions of the joint material are assigned different material compositions tailored to their specific functional requirements. The first portion near the terminals uses intermetallic alloy for maximum bonding strength, while the second portion in the annular region uses high-Bi tin-bismuth alloy for superior toughness and crack resistance, creating a spatially varying material property distribution.
2Strength
If the joint material is made entirely of intermetallic alloy for strong bonding, then joining strength is maximized, but thermal stress causes cracks during mounting
Solution Approach 1:
The joint material is designed as a composite structure combining two different alloy systems: intermetallic Cu-Sn alloy for bonding strength and high-Bi tin-bismuth alloy for thermal stress resistance. This composite approach leverages the complementary properties of both materials to achieve both strong bonding and crack resistance under thermal loading.
Solution Approach 2:
The composition parameter (Bi concentration) is strategically varied within the joint material. The second portion contains Bi at ≥30 mass%, significantly higher than conventional eutectic composition (38.7 mass% Sn-61.3 mass% Bi), which lowers the melting point and enhances ductility, allowing the material to accommodate thermal expansion differences and reduce stress concentration during temperature cycling.
3Reliability
If the joint material contains high Bi concentration alloy, then toughness and crack resistance improve, but the alloy separates from the connection terminal
Solution Approach 1:
The joint material is segmented into two distinct portions with clear spatial separation: the first portion of intermetallic alloy maintains intimate contact with both connection terminals ensuring electrical and mechanical stability, while the second portion of high-Bi alloy is positioned in the annular region where it provides crack resistance without directly contacting the terminals, thus avoiding separation issues.
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 solution effectively controls cracks near the joint material, maintains thermal stability, and reduces thermal stress by allowing the tin-bismuth portion to deform under stress, ensuring reliable mounting of semiconductor chips.
Implementation Method 1
a first portion formed of an intermetallic alloy of copper and tin, and contacting each of the first connection terminal and the second connection terminal
Implementation Method 2
The second portion contains the bismuth at a higher concentration than in the eutectic composition of a tin-bismuth alloy, and is separated from the second connection terminal
Data Source
AI summary
A composite wiring substrate includes a first wiring substrate including a first connection terminal, a second wiring substrate including a second connection terminal facing the first connection terminal, and a joint material joining the first connection terminal and the second connection terminal. The first outline of the first connection terminal is inside the second outline of the second connection terminal in a plan view. The joint material includes a first portion formed of an intermetallic alloy of copper and tin, and contacting each of the first connection terminal and the second connection terminal, and a second portion formed of an alloy of tin and bismuth, and including a portion between the first outline and the second outline in the plan view. The second portion contains the bismuth at a higher concentration than in the eutectic composition of a tin-bismuth alloy, and is separated from the second connection terminal.