1. Apply flux precisely

Flux helps remove oxides and wet the surfaces to be soldered. In selective soldering, the application must reach the intended connections without unnecessarily affecting adjacent areas.

SASinno Ultra systems use drop-jet fluxers for localised application. Alongside position, the dispensed quantity matters: a partially clogged nozzle can continue to deliver material while the application has already changed.

What matters in practice

  • Match application position and quantity to pad, lead and plated-through hole.
  • Follow the processing instructions for the flux used.
  • Check nozzle condition, cleaning and uniform application.

Available monitoring functions must be checked against the specific machine and software configuration.

2. Prepare the assembly thermally

Preheating supports flux preparation and brings the assembly to a suitable temperature for subsequent soldering. Both the PCB and populated components play a role.

Thick multilayer PCBs, large copper areas and thermally demanding connections can absorb and dissipate more heat. Suitable top and bottom heating helps match heat distribution to the assembly.

Measure instead of setting generically

A heater setpoint does not necessarily represent the temperature at the solder joint. Temperature measurements at suitable points on the actual assembly are therefore useful during setup. Temperature rise and dwell time must be compatible with the flux, PCB material and component limits.

In inline production, the available preheating time must also be considered. Systems such as the Ultra-i2/52 offer a longer preheating section. Its benefit should be evaluated together with the required cycle time.

3. Control the solder wave and heat input

During soldering, the local solder wave contacts the intended joints. Solder temperature, contact time, travel path, nozzle shape and wave height together determine the process conditions.

Stable wave height

A change in wave height changes contact with the assembly. Automatic wave-height calibration on the featured Ultra systems helps control this parameter. It supplements regular process monitoring.

Suitable nozzle and protective atmosphere

The nozzle must suit the joint’s accessibility and heat demand. Large connectors and leads tightly spaced among SMD components have different requirements. Nozzle cleanliness and wetting condition also affect the solder wave.

Nitrogen can limit oxidation around the molten solder. Specific settings must be matched to the qualified process; general temperature or consumption figures cannot replace a trial on the assembly.

What does this mean for system selection?

Base the choice on your assemblies: which joints must be reached, how much heat is needed, and how should the machine fit into production?

The Ultra-i2 offers two solder pots in an offline system. For inline operation, options include the Ultra-i2/41 with two and the Ultra-i4/41 with four electromagnetic pumps. Actual suitability depends on the process trial and configuration.

Evaluate quality against agreed criteria

Soldering results must be checked against the acceptance criteria defined for the project. A machine feature alone proves neither a standards-compliant joint nor a particular throughput.

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Frequently asked questions about selective soldering

What three steps does selective soldering include?

The sequence comprises targeted flux application, assembly preheating and soldering with a local solder wave. Settings must work together for the assembly, solder alloy and flux.

What is the right preheating temperature?

There is no universal setpoint. The flux processing instructions, the PCB’s thermal mass and the components’ permissible exposure are decisive. A measured temperature profile on the actual assembly tells more than the heater setpoint alone.

Why is nitrogen used?

Nitrogen reduces oxygen contact around the solder wave and can thus limit oxidation. Purity, flow rate and any heating must be matched to the system and process. Nitrogen does not replace suitable preheating.

Are two solder pots inherently better than one?

Two solder pots can, for example, use different nozzles for different joints. Whether that offers an advantage depends on accessibility, heat demand, program sequence and production requirements. More solder pots do not automatically mean a particular throughput.

Is 75 percent hole fill always sufficient?

A blanket percentage is not enough to assess quality. The applicable standard edition, product class, joint type and contractual requirements must be established. The figure in the original article is therefore not presented here as a universal acceptance threshold.

What information helps with a soldering trial?

Useful inputs include sample assemblies, PCB dimensions and construction, information on THT components and spacing, and the solder and flux used. Quality criteria, desired cycle time and planned line integration are also needed.