Lead-Free Solder Paste with High Melting Particles for Thermal Interface
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Solution Overview
Problem
Conventional solder pastes used for thermal via filling and thermal interface materials suffer from volume reduction during reflow soldering, leading to inefficient heat transfer and potential collapse of solder joints, and require solderable metallization surfaces, which increase manufacturing costs and time.
Innovation Solution
A solder paste composition with a solder powder and high melting temperature particles, such as copper or copper-alloys, in a specific volume ratio, along with a flux that forms an intermetallic compound to maintain the solder joint's integrity and prevent volume loss during reflow, allowing for efficient thermal via filling and eliminating the need for solderable metallization surfaces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional solder paste is used for thermal via filling, then the solder joint can be formed, but the solder paste undergoes volume reduction during reflow soldering, leading to inefficient heat transfer and potential collapse of solder joints
Solution Approach 1:
The patent uses a composite material consisting of solder powder particles (9-15 microns) and metal powder particles (15-25 microns) with different melting temperatures. The solder powder melts during reflow to form intermetallic compounds that bond the metal particles together, creating a rigid structure that maintains volume and prevents collapse, while the composite structure provides efficient thermal conduction pathways.
Solution Approach 2:
The patent changes the particle size parameters and composition ratios of the solder paste. Specifically, it uses solder powder particles of 9-15 microns and metal powder particles of 15-25 microns, with the metal powder comprising 30-70 wt% of the total powder weight. These parameter optimizations ensure proper melting behavior and structural integrity during reflow soldering.
2Reliability
If solderable metallization surfaces are required for thermal interface material, then good thermal contact can be achieved, but manufacturing cost and time increase
Solution Approach 1:
The patent extracts the requirement for solderable metallization surfaces from the thermal interface material system. The solder paste composition itself provides the necessary thermal conduction and bonding capabilities without requiring the substrate surfaces to be pre-metallized, thereby eliminating an additional manufacturing step while maintaining reliable thermal contact.
Solution Approach 2:
The solder paste serves multiple functions simultaneously: it provides thermal conduction, structural bonding, and surface filling. The composite powder structure enables the paste to function as both a thermal interface material and a bonding agent, eliminating the need for separate metallization processes and simplifying the manufacturing workflow.
3Reliability
If thermal grease is used as thermal interface material, then good thermal performance is achieved upon installation, but the grease degrades over time and is pumped out due to thermal expansion differences, resulting in higher thermal resistance
Solution Approach 1:
The patent utilizes controlled phase transitions of the solder powder during reflow soldering. The solder powder melts at a specific temperature range, wets the metal particles, and forms intermetallic compounds upon cooling. This phase transition creates a rigid, stable structure that prevents pump-out and maintains thermal performance over time, unlike grease-based materials that remain soft and susceptible to degradation.
Solution Approach 2:
The patent specifies that both solder powder and metal powder particles should have spherical shapes. The spherical morphology improves flow characteristics during paste application, ensures uniform distribution in the thermal via, and facilitates better contact between particles, enhancing both thermal conduction and structural stability while preventing degradation over time.
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 solder paste maintains its shape and provides effective thermal conductivity, reducing thermal resistance and preventing pump-out, even at subsequent reflow temperatures, while eliminating the need for metallized surfaces, thus enhancing the reliability and efficiency of thermal management in electronic assemblies.
Implementation Method 1
the solder powder melts and wets to the copper or copper-alloy powder to form an intermetallic compound
Implementation Method 2
the solder powder melts and wets to the copper or copper-alloy powder
Implementation Method 3
The powering up and down of the devices causes a relative motion between the die and the heat-spreader due to their different coefficients of thermal expansion
Implementation Method 4
the solder composite provides a thermal interface between the heat generating device and the heat transferring device
Data Source
AI summary
Some implementations of the disclosure are directed to a thermal interface material. In some implementations, a method comprises: applying a solder paste between a surface of a heat generating device and a surface of a heat transferring device to form an assembly; and reflow soldering the assembly to form a solder composite, wherein the solder composite provides a thermal interface between the heat generating device and the heat transferring device, wherein the solder paste comprises: a solder powder; particles having a higher melting temperature than a soldering temperature of the solder paste, wherein the solder paste has a volume ratio of solder powder to high melting temperature particles between 5:1 and 1:1.5; and flux.