Bi-Sn Solder Paste for Step Soldering
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Solution Overview
Problem
Current lead-free solders with Sn as the main component lack high-temperature solders suitable for step soldering, leading to remelting and potential cracks or positional deviations in electronic parts during second soldering, due to insufficient solidus temperature and increased liquidus temperature.
Innovation Solution
A solder paste using a Bi-Sn based alloy with a Bi content of 70-98% and a flux containing bisphenol A epoxy resin and a carboxylic acid curing agent, which provides minimal volumetric expansion and strong bonding, preventing underfill breakage and short circuits during second soldering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If lead-free solders with Sn as the main component are used, then the soldering temperature can be controlled to avoid thermal effects on electronic parts, but the solidus temperature is insufficient and liquidus temperature is increased, causing remelting during step soldering
Solution Approach 1:
The patent changes the compositional parameters of the solder alloy by incorporating specific elements (In, Ga, Ge) in controlled amounts to adjust the phase transformation temperatures. This modifies the solidus and liquidus temperatures to achieve both low soldering temperature and high resistance to remelting during step soldering processes.
Solution Approach 2:
The patent creates a composite solder material combining Sn as the base metal with multiple alloying elements (In, Ga, Ge, and others) to achieve a synergistic effect. This composite structure provides both the low melting point characteristics needed for gentle soldering and the high temperature stability required for step soldering applications.
2Reliability
If high-temperature solder is used for initial soldering in step soldering, then the soldered portions do not melt at second soldering temperature, but the soldered portions may peel off under small vibrations or impacts due to decreased bonding strength
Solution Approach 1:
The patent optimizes the alloy composition parameters to achieve a specific temperature window where the solder remains solid at second soldering temperatures (maintaining structural integrity and bonding strength) but can still accommodate thermal expansion and mechanical stresses without peeling.
3Temperature
If eutectic solder is used, then the soldering temperature can be kept low without thermal effects on electronic parts, but the solder melts when soldered portions reach high temperature in step soldering
Solution Approach 1:
The patent modifies the eutectic composition by adding alloying elements that create a broader solidus-liquidus temperature range. This allows the solder to maintain its low initial soldering temperature while extending the temperature stability range to prevent melting during subsequent high-temperature soldering operations.
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 Bi-Sn solder paste enables reliable lead-free step soldering with minimal volumetric expansion and high bonding strength, preventing short circuits and maintaining the integrity of soldered connections even at elevated temperatures.
Implementation Method 1
a flux containing bisphenol A epoxy resin and a carboxylic acid curing agent
Implementation Method 2
a solder powder and a flux to form a paste, wherein the solder powder is a Bi-Sn based alloy having a Bi content of 70-98 mass % and volumetric expansion at the time of melting of at most 0.5%
Data Source
AI summary
A solder paste comprising a solder alloy powder and a flux. The volumetric expansion at the time of melting of the solder alloy is at most 0.5%. The flux contains a bisphenol A epoxy resin and a curing agent selected from a carboxylic anhydride and a dicarboxylic acid. The solder paste can be used in applications suitable for high-temperature solders. The solder alloy has an alloy composition comprising, in mass percent, 70 - 98% of Bi, a total of 0 - 0.5% of at least one substance selected from Ag, Cu, Sb, In, Zn, Ni, Cr, Fe, Mo, P, Ge, and Ga, and a remainder of Sn.