Core-Shell Solder Balls for Low-Temperature Large Package SMT
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Solution Overview
Problem
Current solder materials face challenges with high failure rates and reliability issues due to warpage during surface mount technology (SMT) for large die complex areas, particularly in high-performance computing products, and low temperature solders like tin-bismuth suffer from reduced current carrying capability and mechanical integrity.
Innovation Solution
A core-shell solder ball structure with a tin-silver-copper inner core and a tin-bismuth outer shell, optionally with a nickel barrier layer, allowing for lower reflow temperatures and reduced warpage, enhancing current carrying capability and mechanical integrity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If low temperature solder (LTS) materials are used to reduce reflow peak temperature, then warpage is reduced, but current carrying capability and reliability are reduced
Solution Approach 1:
The patent applies composite materials by creating a core-shell solder ball structure where the inner core is made of tin-silver-copper (SAC) alloy and the outer shell is made of tin-bismuth (SnBi) alloy. This composite structure combines the advantages of both materials: the SAC core provides high current carrying capability and mechanical strength, while the SnBi shell enables lower reflow temperature processing. The intermetallic layer at the interface ensures strong bonding between the two dissimilar materials, achieving both low-temperature soldering and high reliability.
2Temperature
If low temperature solder (LTS) materials are used to reduce reflow peak temperature, then warpage is reduced, but mechanical integrity is reduced
Solution Approach 1:
The core-shell structure with SAC core and SnBi shell provides both low-temperature processing capability and high mechanical integrity. The SAC core contributes superior mechanical strength and ductility, while the SnBi shell enables low-temperature bonding. The intermetallic layer formed at the interface between the two materials acts as a strong bonding zone, ensuring overall mechanical integrity of the solder joint.
Solution Approach 2:
Different regions of the solder ball are assigned different material properties optimized for their specific functions. The inner core region is designed with high-strength SAC alloy for mechanical support and current carrying, while the outer shell region uses low-melting-point SnBi alloy for temperature control during reflow. This spatial differentiation of material properties resolves the contradiction between low temperature processing and mechanical strength.
3Area of moving object
If die complex area increases for high-performance computing, then computing power is improved, but warpage during SMT process increases
Solution Approach 1:
The patent changes the thermal parameter of the solder material by using a core-shell structure with low-melting-point SnBi shell. This parameter change (lower reflow temperature) directly addresses the warpage issue in large die complex areas, as the reduced temperature minimizes thermal expansion and contraction stresses that cause warpage during SMT processing of large substrates.
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 core-shell solder balls enable reliable attachment of large form factor packages at lower temperatures, reducing substrate damage and improving reliability while maintaining mechanical and electrical integrity under high current loads.
Implementation Method 1
allowing for lower reflow temperatures
Implementation Method 2
a barrier layer on the inner core and between the inner core and the shell
Data Source
AI summary
Solder materials, solder balls, and solder features, and microelectronic devices and systems deploying the solders are discussed. A solder ball includes an inner core that is an alloy of tin, silver, and copper, which has a relatively high melting point. Surrounding the inner core is a shell or cladding of a tin-bismuth alloy having a lower melting point. An optional nickel coating is on the inner core and between the inner core and the shell or cladding. During surface mount, the lower melting point the tin-bismuth alloy is used as the reflow temperature.