Ultrasonic soldering is a flux-free soldering method that is considered more environmentally friendly than conventional soldering methods. The surface oxide layer is removed from the soldering surface using vibration and cavitation phenomena instead of chemicals.
Ultrasonic soldering technology differs from ultrasonic plastic welding in that heat is generated by vibration to melt the part being joined. The principle of using ultrasonic soldering is the same as the process of ultrasonic cleaning, where vibrational energy causes cavitation in a water bath or cleaning solvent. The part immersed in the liquid medium is cleaned from the surface by the cavitation bubbles’ intense erosive action.
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Ultrasonic Solder Process
In the ultrasonic soldering process, heat from a separate energy source melts the solder before vibration is applied. The molten solder is then used as an acoustic transfer medium for the ultrasonic vibrations. When high-frequency vibration energy is applied to the molten solder, controlled acoustic cavitation is induced at the tip of the soldering tool to destroy and disperse surface oxides. The cavitation microbubbles rupture, cleaning all surfaces, wetting the liquid solder and bonding the pure metal.
Vibration also ensures that the solder joint is void-free, with the vibration energy forcing the liquid solder to penetrate the substrate’s gaps and micro-pores. It helps seal the part and increases the surface area to which the solder can bond. Ultrasonic vibration also squeezes air bubbles out of the liquid solder, so this method makes the solder joint suitable for applications where a high vacuum is required for sealing.
Ultrasonic soldering allows the joining of different materials and can be used for difficult materials to solder by conventional methods. Since no flux is required, users can save time and costs in cleaning flux residue while reducing corrosion and increasing the soldered joint’s durability.
The ultrasonic soldering method can be easily performed manually with handheld ultrasonic soldering iron equipment or grafted onto ultrasonic soldering machines and assembly lines.
Glass, ceramic, stainless steel, aluminum soldering
During years of extensive research into glass-to-metal bonding, Japanese engineers developed a special solder alloy called CERASOLZER (solder wire). This active solder alloy is specially formulated to work with ultrasonic soldering methods. It has a unique bonding ability that can replace the commonly used silver baking, indium brazing, molybdenum-manganese and resin bonding methods.CERASOLZER Chemical Bonding (Glass Substrates) In addition to direct metal-to-metal bonding, Cerasolzer also forms a strong chemical bond with the substrate to which it is soldered. The alloy consists of the same primary components, just like a standard solder alloy (lead/tin). It contains small amounts of zinc, titanium, silicon, aluminum, beryllium, rare earth, etc., which have a strong chemical affinity for oxygen.
During the soldering process, these additional elements combine with the surrounding oxygen to form an oxide that is chemically bonded to various materials, including glass, ceramics, aluminum, stainless steel, conductive oxides and many other substrates previously considered non-weldable. The resulting oxide binds to the weld substrate and forms a powerful chemical bond (RO) at the interface.
Therefore, if oxygen is virtually eliminated by replacing the air around the bonding equipment with an inert gas (e.g. nitrogen), the adhesion of Cerasolzer will be lost. The results show that a suitable critical oxygen concentration for bonding is around 2%. The melting temperature of Cerasolzer alloys is between 155 ~ 297°C, and the welding method is flux-free due to ultrasonic vibrations. Our ultrasonic welding method should not be used because if the flux is used, it will break the oxygen bond and destroy the whole welding process.
Ultrasonic soldering iron applications
- Glass jewellery manufacturing
- Optical glasses coating/metallization
- Making electrodes on glass and ceramic plates
- Soldering of heated contacts in automotive rear windows
- Soldering of superconductors, components, ceramic parts
- Vacuum sealing of glass tubes, bonding of hardware
- Sealing of photoconductive glass fibres (metal hoop bonding)
- Electrode bonding to solar cells (crystalline, thin-film) front/back contact
- Bonding in metallic glass, liquid crystal glass, crystal oscillators, hybrid integrated circuit leads (dots)
Ultrasonic Solder Basic Soldering Method
When coating glass, ceramics or metal oxides with Cerasolzer solder, full contact must be maintained between the solder and the substrate, eliminating by friction the small air bubbles present in the boundary zone substrate surface and the soft solder. Ultrasonic vibration stops the air layer quickly, resulting in a coherent solder joint without air bubbles in the boundary area. If the substrate is highly heat-absorbing, an auxiliary heat source, such as a hot plate, should be used to maintain proper soldering conditions.
The most appropriate practice is to apply Cerasolzer on a transparent slide, the reverse side of which is the adequately established boundary zone and looks like the reflective surface of a mirror.
Two-step method (pre-soldering + bonding)
In the first step, Cerasolzer is fed to the tip of the ultrasonic soldering iron using high temperature and ultrasonic vibration and coated on the substrate (glass, etc.). Then, metal fittings, wires, strips, etc., are soldered in the pre-soldering area with ordinary solder or the preferred Cerasolzer.
One-step method (direct welding)
When Cerasolzer is sandwiched in the gap between metal and glass, ceramic or metal oxide, ultrasonic vibrations act directly on the metal. The bonding is done in a short time. This method provides a slightly lower performance than the previous method.
Impregnation is the ideal method for coating large surfaces. cerasolzer solder is melted in a specific soldering pot with an ultrasonic vibrator. When the soft solder surface is rippled by ultrasonic vibrations, the coated part is immersed in water, and ultrasonic dip soldering equipment is recommended.