Solder Preforms for Thermal Stress Reduction in Electronic Assembly
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Solution Overview
Problem
Lead-free solder alloys, such as SAC and tin-bismuth alloys, face challenges with high processing temperatures and loss of ductility at high strain rates, which complicates the assembly of electronic components on printed circuit boards.
Innovation Solution
A method involving the application of solder paste to a substrate, placement of low or high temperature preforms within the solder paste deposits, and processing at specific reflow temperatures to create low temperature solder joints, allowing for a second solder process at a lower temperature without disturbing existing joints, using compositions like Sn42Bi58 and SAC305 alloys.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If lead-free solder alloys such as SAC are used, then reliability of solder joints is improved, but processing temperature increases
Solution Approach 1:
The soldering process is segmented into two distinct stages: first, a low-temperature solder paste (tin-bismuth alloy) is applied and reflowed at approximately 138°C to create initial solder joints; second, a high-temperature preform (SAC alloy) is placed and reflowed at approximately 217°C to create the final reliable solder joints. This segmentation allows each stage to use optimized materials for its specific temperature range.
Solution Approach 2:
The low-temperature tin-bismuth solder paste is applied in advance as a preliminary action to create initial solder joints at lower temperature. This preliminary solder layer serves as a foundation that facilitates subsequent placement and bonding of the high-temperature SAC preform, enabling a two-stage reflow process that combines the benefits of both alloy systems.
2Strength
If high temperature solder paste is used, then solder joint strength is improved, but temperature sensitive components may be damaged
Solution Approach 1:
The soldering process is divided into two temperature stages: a first reflow at low temperature (approximately 138°C) using tin-bismuth solder paste, and a second reflow at high temperature (approximately 217°C) using SAC preform. This segmentation allows temperature-sensitive components to be protected during the initial stage while still achieving strong joints in the final stage.
Solution Approach 2:
The low-temperature tin-bismuth solder paste is applied as a preliminary action before the high-temperature SAC preform is introduced. This preliminary low-temperature bonding step creates an initial joint structure that can accommodate subsequent high-temperature processing without damaging sensitive components, as the critical bonding has already occurred at lower temperature.
3Ease of manufacture
If a single high temperature reflow process is used, then manufacturing simplicity is maintained, but rework capability is reduced
Solution Approach 1:
The solder joint structure is segmented into two distinct material layers: a low-temperature tin-bismuth solder paste layer and a high-temperature SAC preform layer. This material segmentation creates functional differentiation where the lower-temperature alloy provides rework capability while the higher-temperature alloy provides joint strength, enabling both manufacturing efficiency and repairability.
Solution Approach 2:
The invention changes the temperature parameter characteristics of the solder joint by using two different melting points: the tin-bismuth alloy melts at approximately 138°C while the SAC alloy melts at approximately 217°C. This parameter differentiation allows selective reflow and rework at lower temperatures without damaging the overall joint structure, improving rework capability while maintaining manufacturing simplicity.
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
Enables efficient assembly of electronic components with reduced thermal stress, allowing for the attachment of RF shields and temperature-sensitive components without disturbing existing solder joints, while optimizing thermal cycling endurance and drop shock resistance through alloy tuning.
Implementation Method 1
placing a high temperature preform in the solder paste deposit; and processing the electronic substrate at a reflow temperature that is appropriate for the low temperature solder paste to create a low temperature solder joint resulting from a dissolution of the high temperature solder preform into the low temperature solder paste deposit
Data Source
AI summary
A method of assembling components, such as electronic components, onto a substrate, such as an electronic substrate, includes applying solder paste to an electronic substrate to form a solder paste deposit, placing a low temperature preform in the solder paste deposit, processing the electronic substrate at a reflow temperature of the solder paste to create a low temperature solder joint, and processing the low temperature solder joint at a reflow temperature that is lower than the reflow temperature of the solder paste. Other methods of assembling components and solder joint compositions are further disclosed.