Low-Temperature Solder Alloy and Flux for PET PCB Joining
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Solution Overview
Problem
Conventional solder alloys have high melting points, making them unsuitable for use with low-cost, heat-sensitive substrates like PET films, and their fluxes are not effective at lower temperatures, leading to reliability issues.
Innovation Solution
A solder alloy comprising 40-65 wt.% bismuth, 1-10 wt.% indium, and optional additives such as gallium, zinc, copper, and silver, which has a lower melting point and improved mechanical properties, allowing for reliable soldering on PET substrates, and a solder flux with a controlled molar ratio of organic acid to organic amine activators for effective low-temperature performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional solder alloys with high melting points (e.g., SAC at 218°C) are used, then reliable soldering can be achieved on high-temperature substrates like polyimide, but the substrates suffer from heat distortion and thermal breakdown when using low-cost PET films
Solution Approach 1:
The patent changes the compositional parameters of the solder alloy by incorporating bismuth (40-65 wt%), indium (1-10 wt%), and other elements to achieve a eutectic composition that melts at lower temperatures (below 150°C), enabling soldering of heat-sensitive PET substrates without thermal damage
Solution Approach 2:
The patent creates a composite solder alloy system combining multiple elements (Bi-In-Sn-Cu-Ag-Co-Ti-Ni) where each element contributes specific properties: bismuth provides low melting point, indium enhances ductility and lowers melting point, tin provides base matrix, and trace elements improve wetting and mechanical properties, achieving both low temperature processing and reliable joints
2Temperature
If conventional solder fluxes with aggressive activators are used to achieve low-temperature activity, then soldering at lower temperatures becomes possible, but the paste homogeneity is disturbed by reactions with solder alloys in storage and electrical reliability is adversely affected
Solution Approach 1:
The patent changes the chemical parameters of the flux by using milder activators (organic carboxylic acids with pKa 3-7 and organic amines) instead of conventional aggressive inorganic activators, enabling effective soldering at lower temperatures (below 150°C) without causing excessive reactions that would compromise electrical reliability
Solution Approach 2:
The patent applies different functional components in the flux with specific local roles: organic carboxylic acids for controlled activation at low temperature, organic amines for pH control and corrosion inhibition, and specific additives for wetting enhancement, creating a balanced flux system that achieves low-temperature soldering without compromising reliability
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 alloy achieves reliable soldering at lower temperatures, reducing reflow-induced warpage and improving mechanical stability on PET substrates, while the flux ensures high electrical reliability and stability at room temperature.
Implementation Method 1
The solder alloy is capable of being reflowed at a temperature of less than 150° C.
Implementation Method 2
the solder melts and wets the soldering surfaces on the boards as well as the components
Implementation Method 3
which laps against the bottom of the board to wet the metal surfaces to be joined
Data Source
AI summary
A solder alloy comprising: from 40 to 65 wt. % bismuth; from I to IO wt. % indium; at least one of: from 0.1 to 5 wt. % gallium, from 0.1 to 5 wt. % zinc, from 0.1 to 2 w. % copper, from 0.01 to 0.1 wt. % cobalt, from 0.1 to 2 wt. % silver, from 0.005 to 0.05 wt. % titanium, and from 0.01 to 1 wt. % nickel; optionally up to 1 wt. % of one or more of: vanadium, rare earth metals, neodymium, chromium, iron, aluminium, phosphorus, gold, tellurium, selenium, calcium, vanadium, molybdenum, platinum, magnesium, silicon, and manganese; and the balance tin together with any unavoidable impurities.