Ultrasonic Soldering for Fluxless PCB Joints: 7 Process Rules

Ultrasonic soldering uses a vibrating tip to break up the oxide film on a metal surface while the molten alloy wets it. The vibration sets up cavitation in the liquid solder, and the collapsing bubbles scrub the surface clean at a scale no brush can reach. The result is a joint made without flux, which is why the process appears wherever flux residue cannot be tolerated or cannot be cleaned out afterwards.

It is a specialised method rather than a general one. It suits aluminium, stainless steel, ceramics and glass that ordinary flux cannot wet, and it is widely used for tinning and for attaching terminals where a flux free joint is required. It is not a replacement for reflow or wave soldering, and treating it as one is the quickest way to produce joints that look right and fail in the field.

How the Vibration Cleans the Surface

Molten solder does not wet a metal that is covered by a tenacious oxide, and aluminium and stainless steel form exactly that kind of oxide within seconds of being cleaned. Ultrasonic energy at the tip is transmitted into the liquid alloy, where the alternating pressure produces cavitation: tiny bubbles form and collapse, and the collapse generates local shock that lifts the oxide from the surface and disperses it into the solder.

Because the cleaning happens under the alloy rather than in air, the freshly exposed metal never sees oxygen before it is wetted. That is the whole advantage of the process, and it explains why the vibration has to continue until wetting has actually started. A tool that vibrates for a fixed time regardless of what the joint is doing is leaving the result to chance.

Where a Flux Free Joint Is Needed

The obvious application is a material that flux cannot wet, but there are others. Medical and analytical instruments cannot carry residue that might leach into a sample. Vacuum and high voltage assemblies cannot tolerate ionic material that will track or outgas. Optical packages need a joint that can be cleaned without attacking the optics.

In all of those cases the cost of the process is justified by the removal of the cleaning step and the residue risk that comes with it. Where a conventional iron and a no-clean flux will do the job, they remain the cheaper and more repeatable choice, and the decision should be made on the requirement rather than on the novelty of the tool.

Alloy Selection for Ultrasonic Tinning

The alloy has to melt at a temperature the substrate can survive and wet the surface once the oxide is removed. Tin based alloys are the usual choice, and the working temperature is kept as low as the joint allows, because the ultrasonic action is more effective when the alloy is not overheated and oxidising on its own surface.

The alloy also determines what happens at the interface afterwards. A tin coating on aluminium forms an intermetallic layer that grows with temperature and time, and a joint that is made too hot will have a thick, brittle boundary from the beginning. Selecting the alloy and the temperature together, rather than picking an alloy and adjusting the temperature until it flows, gives a joint with a controlled interface.

Tip Material, Power and Dwell

The tip is a consumable. It erodes as it works, and the erosion changes both its shape and the way it couples energy into the joint. A tip that has lost its flat face will still vibrate, but it will transfer less energy and the operator will compensate by holding it longer, which heats the substrate and the surrounding components.

Power and dwell should be set from a trial on the actual materials, and both should be written into the work instruction. The correct dwell is the time it takes for the alloy to wet, plus a short margin, and it can be confirmed by watching the meniscus form at the edge of the tip rather than by watching a clock. Where several joints are made on one assembly, the tip temperature should be checked between them, because the tool heats up as it works.

Joint Design and Heat Input

An ultrasonic joint is usually small, and the heat has to stay in the joint rather than spread into the assembly. A pad or terminal with a large mass, or one connected to a plane, will pull heat away from the working area and stop the wetting before it has started. Where that is unavoidable, local preheating from below reduces the energy the tip has to deliver.

Components close to the joint have to be considered as well. The vibration is transmitted through the workpiece, and a brittle part or a wire bond next to the joint can be damaged by energy that was intended for the solder. Spacing and clamping are part of the process design, not details left to the operator.

Oxide Regrowth and Timing

The oxide that was removed starts to form again as soon as the surface cools, and on aluminium that happens within minutes. A surface that is tinned with ultrasonic energy and then left on a bench for a shift has lost most of the benefit, and the soldering operation that follows behaves as if the tinning had never been done.

The tinning and the final joint should therefore be part of the same operation, or the interval should be short and controlled. Where an assembly has to wait, it should be stored in a way that limits exposure, and the interval should be stated in the process instruction with the same authority as the soldering temperature.

Inspection and Acceptance

Visual inspection confirms that the alloy has wetted the surface and formed a fillet, and that no oxide inclusions have been left at the interface. A joint that has been made with too little vibration looks like a blob sitting on the surface, with the metal still visible underneath. A joint made with too much energy has a rough surface and often a discoloured, overheated appearance.

Where the application is critical, a peel or shear test on a sample establishes that the interface is sound, because visual inspection cannot see the boundary. A section through the joint is the most informative check, and it should be performed when the process is first qualified and whenever the alloy, the tip or the equipment changes.

Tool Care and Records

The transducer and the tip should be treated as a calibrated system. Tip wear should be checked on a schedule, the power setting should be recorded with the tip serial number, and the tool should be re-qualified after any repair. Where several operators use the same tool, the setting should be locked so that it cannot be changed to suit an individual technique.

The records should link the joint to the parameters. That is the same discipline that supports conventional copper surface solderability work and the judgement made from a contact angle check, and it is what allows a change in supplier or alloy to be evaluated instead of guessed. Where a conventional flux process is used instead, the alloy and the chemistry still have to be paired deliberately, as described in the alloy and flux pairing notes, and the alternative heating methods covered in the laser soldering guide are worth comparing before a process is chosen.

FAQ

Does ultrasonic soldering really need no flux? On the materials it is intended for, the mechanical action of the cavitation replaces the chemical action of the flux, so no flux is used. On a surface that has heavy contamination rather than a thin oxide, the process will not clean it and the part must be prepared first.

Can ultrasonic soldering be automated? It can, but the tool has to approach the joint with controlled force and the vibration has to be started at the right moment, so the fixturing is more demanding than for a conventional iron. It is normally reserved for a small number of joints per assembly.

Why does the joint fail after tinning and storage? Because the oxide regrows on the tinned surface, and on metals such as aluminium it regrows quickly. The tinning and the final soldering step should be kept close together in time, and the interval should be controlled rather than assumed.