Ultrasonic soldering technology

Ultrasonic vibrations, cavitation, flux free soldering, how ultrasonic soldering works
Ultrasonic soldering technology

Ultrasonic soldering is a flux free method which is considered more eco-friendly comparing to conventional soldering methods. Instead of chemical agents, surface oxide layers are removed from the soldered surface by using vibrations and a cavitation phenomenon.

Ultrasonic soldering technology is different from ultrasonic plastic welding, in which the vibrations generate heat to melt the parts being joined. Ultrasonic soldering uses the same basics as the ultrasonic cleaning process where the vibrational energy induces cavitation in the water bath or the cleaning solvent. Immersed parts are scrubbed clean by a strong erosive effect of collapsing vapor bubbles in the liquid medium.

Ultrasonic soldering diagramDuring the ultrasonic soldering process, heat from a separate energy source melts the solder before vibrational energy is applied. Molten solder then acts as an acoustic transfer medium for the ultrasonic vibrations. When high-frequency vibrational energy is applied to the molten solder, controlled acoustic cavitation is induced at the tip of the soldering tool in order to break up and disperse surface oxides. The cavitating micro bubbles burst, cleaning all surfaces and allowing the liquid solder to wet and bond pure metal.

Vibrations also ensure that the solder joint is free of voids, vibrational energy forces the liquid solder into crevices and micropores in the substrate. It helps to seal the parts and increases the surface area to which the solder can bond. Ultrasonic vibrations also press gas bubbles out of the liquid solder therefore this method makes joints suitable for high vacuum applications where hermetic seals are required.

Ultrasonic soldering allows for joining dissimilar materials and can be used for materials that are difficult to solder with conventional methods. Because it requires no flux, users save time and costs of cleaning flux residues while reducing corrosion and increasing the durability of soldered joints.

Ultrasonic soldering method can be easily adopted to manual soldering with a handheld ultrasonic soldering iron or even mass production ultrasonic soldering machine.

Ultrasonic soldering on glass, ceramics, stainless steel, aluminium

During many years of extensive research of glass-metal bonding, Japanese engineers developed special solder alloy called CERASOLZER. This active solder alloy is specially formulated to work with ultrasonic soldering method and has very unique bonding abilities that can replace commonly used silver baking, indium soldering, molybdenum-manganese and resin bonding methods.

CERASOLZER chemical bond (glass substrate)Besides direct metal-to-metal bonding, Cerasolzer creates strong chemical bond with soldered substrate. Alloy consists of same main components as standard solder alloys (Pb/Sn), however in addition to that it includes small amount of elements such as Zn, Ti, Si, Al, Be and Rare Earth, which have a very strong chemical affinity with oxygen.

During the bonding process, these additional elements combine with ambient oxygen to form an oxide which is chemically bound to various materials including glass glass, ceramics, aluminium, stainless steel, conductive oxides and many other substrates previously considered as unsolderable. Resulting oxide unites with the soldered substrate to form an extremely strong chemical bond (RO) at the interface.

Accordingly, if oxygen is competely eliminated by substituting the air surrounding the bonding equipment with an inactive gas, for example nitrogen, the adhesion of Cerasolzer will be lost. It has been found that the critical oxygen concentration for proper adhesion is about 2%.

Melting temperature of Cerasolzer alloys ranges from 155 to 297 °C and because of ultrasonic vibrations, soldering method is flux-free. Actually, if flux is applied with our ultrasonic soldering method, it will destroy the oxygen bonds and corrupt the whole soldering process so it should not be used.

Basic bonding methods for ultrasonic soldering

When coating glass, ceramics or metal oxides with Cerasolzer solder, it is necessary to keep full contact between the solder and the substrate by employing friction to eliminate small bubbles which exist in the boundary zone between the surface of the substrate and the molten solder. Ultrasonic vibrations remove air layers in an extremely short time and as a result provide coherent joint with no bubbles in the boundary zone. In case that substrate has a high heat absorbtion, supplemental heat source such as a hot plate should be used to maintain proper soldering conditions.

Best example of correct conditions provides Cerasolzer coated on a transparent glass slide, resp. its reverse side where the properly established boundary zone looks like the reflecting side of the mirror.

Two-step method (pre-soldering + bonding)
In the first step, Cerasolzer is fed to the tip of ultrasonic soldering iron and coated on substrate (glass etc.) by using heat and ultrasonic vibrations. Then the metal fittings, wires, ribbon, etc. are soldered on the pre-soldered area using ordinary solder or preferably Cerasolzer.

One-step method (direct bonding)
While Cerasolzer is sandwiched in a slit between metal and glass or ceramics or metal oxide, ultrasonic vibrations are applied directly to the metal. Bonding is performed whithin a very short time. This method provides slightly lower performance than the previous one.

Dipping Method
Dipping method is ideal for large surface coating. Cerasolzer solder is melted in a specific soldering pot which is equipped with an ultrasonic vibrator. While the molten solder has a ripple on its surface caused by ultrasonic vibration, the part to be coated is dipped in.

Surface cleanliness of the soldered substrate does significantly affect the bonding performance. Staining with resin, finger-prints, flux for ordinary solder, acid, alkali, powder, carbon etc. must be avoided. Best cleaning results are achieved by using chemically clean acetone.

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