Rubidium Fluoride (RbF)–Assisted Lead-Free Sn-Ag-Cu-Bi-Sb-Ni Solder Alloy Preparation for Electronics Joining

1) Overview and Technical Value

This workflow produces a lead-free tin-based multi-element solder alloy (Sn-Ag-Cu-Bi-Sb-Ni) designed for stable melting behavior and improved joint reliability. The core technical lever is introducing rubidium fluoride (RbF) together with sodium bromide (NaBr) during a sealed, high-temperature melt. Under sealed conditions, the halide additives generate a reactive halide-rich atmosphere that improves element compatibility during alloying, helping deliver a lower and more consistent melting point and higher tensile strength compared with non-matching halide systems.

Note: Processing temperatures listed below follow the provided specification (1480–1520 °C). Use appropriate refractory/crucible materials, sealed high-temperature furnace capability, and halide off-gas handling.

2) Detailed Experimental Procedure

Target mass ratio (by weight)

Component	Mass ratio range	Purpose in the workflow
Sn	80–85	Base solder matrix
Ag	4–5	Strengthening; solder performance tuning
Cu	4–5	Intermetallic control; joint reinforcement
Bi	0.9–1.1	Melting point reduction; microstructure tuning
Sb	0.9–1.1	Strength/stability improvement
Ni	0.9–1.1	Intermetallic modification; mechanical stability
RbF	10–12	Halide-vapor assisted alloying; compatibility and melting stability
NaBr	10–12	Works with RbF to form the process atmosphere; assists fusion and later degassing

Step-by-step process

1. Powder preparation
   • Ball-mill Sn, Ag, Cu, Bi, Sb, Ni into powders.
   • Sieve to control particle size: ≤ 1 mm.
   • Blend powders thoroughly in a mixing vessel: 120–150 rpm for 30–45 min to obtain a uniform mixed metal powder.
2. Ethanol cleaning (optional but recommended)
   • Soak the mixed metal powder in anhydrous ethanol for 10–15 min.
   • Stir during soaking to enhance cleaning and reduce entrained impurities.
   • Filter and dry/remove solvent before charging into the furnace.
3. Sealed halide-vapor assisted melting and alloying
   • Charge the cleaned mixed metal powder into the melting furnace/crucible.
   • Add RbF and NaBr according to the selected ratio.
   • Seal the furnace (close feed/discharge ports) and heat to 1480–1520 °C.
   • Hold at temperature for 60–90 min under sealed conditions to promote alloying in the halide-rich atmosphere.
4. Controlled depressurization and degassing hold
   • Cancel the sealed state (open the furnace to non-sealed condition) and allow natural pressure release.
   • Maintain 1480–1520 °C for an additional 15–20 min to let halide gases exit the melt.
   • Stir during this stage (typical guidance: 10–15 rpm) to improve gas removal and melt homogeneity.
   • Obtain a uniform alloy melt.
5. Casting and cooling
   • Pour the alloy melt into a mold.
   • Cool to room temperature at a controlled rate of 10–20 °C/min to reduce cracking risk and stabilize properties.
   • Demold to obtain the Sn-Ag-Cu-Bi-Sb-Ni alloy ingot (or proceed to downstream wire production as needed).

3) Comparison vs. Conventional/Alternative Approaches

Traditional Sn-based solder systems often relied on Pb to reduce melting point and simplify processing, but lead-free compliance requires alternative routes to achieve low, stable melting behavior and strong joints. In this process, the RbF + NaBr sealed-atmosphere stage is the differentiator versus approaches that use other salts or omit the matched halide atmosphere.

Item	RbF + NaBr sealed-atmosphere alloying	Alternative salt combinations (examples)
Alloying compatibility	Enhanced mixing/compatibility of Sn/Ag/Cu/Bi/Sb/Ni during melting	Reduced compatibility when the matched halide atmosphere is disrupted
Melting point stability	Lower melting point with reduced fluctuation	Higher melting point and weaker stability trend
Mechanical strength	Higher tensile strength trend (about 51–52 MPa)	Lower tensile strength trend (about 44–45 MPa)
Impurity/gas management	Dedicated unsealing + high-temp hold helps drive halide gases out of the melt	Less effective atmosphere design and/or residual impacts possible

Measured results summary (DSC melting point + tensile test)

Sample	Salt system during melting	Melting point (°C)	Tensile strength (MPa)
Example 1	RbF + NaBr	188.6	51.3
Example 2	RbF + NaBr	187.4	51.9
Example 3	RbF + NaBr	188.1	51.7
Comparative 1	NaCl + NaBr	193.5	44.5
Comparative 2	RbF + Na2SO4	194.3	44.9
Comparative 3	NaCl + Na2SO4	194.8	43.8

4) Why Rubidium Fluoride (RbF) Is Superior in This Application

In this lead-free solder alloying route, RbF is not a minor additive; it is a functional raw material that enables the process atmosphere and drives the property gains. Based on the described mechanism and comparative results, the key advantages of using RbF are:

• Halide-vapor assisted alloying: Under sealed heating, RbF (with NaBr) contributes to a halide-rich atmosphere that promotes fusion and improves compatibility among Sn, Ag, Cu, Bi, Sb, and Ni, supporting a more uniform alloy.
• More stable low-melting behavior: The RbF-enabled atmosphere correlates with a lower melting point range (~187–189 °C) compared with alternative salt systems (~193–195 °C), which is valuable for solder processing windows and repeatability.
• Higher joint reliability potential via strength increase: The RbF + NaBr route shows higher tensile strength (~51–52 MPa), which can translate to improved crack resistance at solder joints under mechanical/thermal stress.
• Cleaner final alloy through designed degassing: The workflow explicitly switches from sealed to unsealed and holds at temperature with stirring to drive halide gases out, reducing the likelihood that residual gas-phase species degrade alloy performance.
• Process robustness for engineering scale-up: With defined ratios, temperature windows, hold times, and cooling rates, RbF becomes a controllable “process atmosphere reagent,” helping engineers tune melting stability and mechanical outcomes without relying on lead.

The mentioned synthesis method references patent document number CN202410665662.4