Selective Soldering: Key Checks Before Release

Selective soldering is a process that solders individual through hole joints, or small groups of them, with a small nozzle rather than a full wave. It exists because a mixed assembly has a few through hole parts that cannot survive a wave, and because the wave cannot be used once surface mount parts are on both sides.

The process gives a much gentler thermal load than a wave, since only the area being soldered is heated. That control is bought with throughput, and the trade is the reason selective soldering is used for smaller volumes and for high value assemblies.

What Selective Soldering Is

A nozzle delivers a small, stable fountain of molten solder to the joint. The board is moved over the nozzle, or the nozzle is moved under the board, and the joint is filled in one or two seconds. Flux is applied beforehand, and the board is preheated from below before soldering.

Because the nozzle is small, the solder contacts only the intended pin and its pad. Neighbouring surface mount parts are not exposed to the wave, and there is no need for the masking that a wave process demands.

The Main Machine Types

A Dip machine has a single stationary nozzle that the board is lowered onto. A drag machine has a nozzle that moves along a path under the board, soldering a row of pins as it travels. A multiple nozzle machine has several nozzles working in parallel to raise throughput.

The choice depends on the number of joints and on the layout. A few isolated pins suit a dip machine, while a connector with a long row of pins is much faster on a drag machine, because the nozzle can solder the whole row in one pass.

Nozzle Geometry And Sizing

The nozzle diameter should match the joint rather than the pin. Too small, and the solder fountain does not reach around the pad; too large, and the solder contacts adjacent pins or lifts a nearby component. Interchangeable nozzles are used to cover a range of pin sizes.

Nozzle condition matters as much as nozzle size. Dross builds up on the rim, the fountain becomes uneven and the joint fill falls. The nozzle should be checked and cleaned at the start of every shift, and replaced when the rim is worn.

Flux Application And Its Control

Flux is applied by spray, by a micro dot dispenser or by a pin transfer tool, and the quantity has to be enough to activate the joint without flooding the board. An excess of flux leaves a residue that is difficult to clean and can insulate the joint.

The application is often localised to the area that will be soldered, which keeps the rest of the assembly dry. That is an advantage over the wave process, where the whole board passes through the fluxer.

Thermal Profile And Preheat

Preheat comes from an infrared panel, a forced convection heater or a combination of both, applied from below so that the surface mount parts on top are not directly heated. The profile must bring the joint close to soldering temperature before the nozzle arrives.

A nozzle dwell time that is too short leaves the barrel unfilled, while one that is too long overheats the laminate and the pad. The window is narrow on a thick board with heavy copper planes, and it should be established on a profiled sample.

Single Pin Versus Group Soldering

Soldering one pin at a time gives the finest control and the longest cycle. Group soldering with a wide nozzle or a mini wave covers several pins at once and raises throughput, at the cost of a larger thermal load and a greater risk of bridging.

The decision follows the pitch and the thermal mass. A 2.54 mm pitch connector can usually be soldered as a group, while a 1.27 mm pitch connector with heavy ground pins may need a pin by pin approach.

Common Defects And Causes

Insufficient fill is the most common defect, and it usually traces to nozzle blockage, a dwell time that is too short or a preheat that is too low. Bridging between adjacent pins is the second, and it comes from too much solder or a nozzle that is too large.

Icicles and solder balls form when the nozzle is withdrawn while the solder is still flowing, and lifted components occur when the thermal load reaches a nearby surface mount part. Each defect has a specific cause, which makes the process relatively easy to diagnose.

Comparing With Wave Soldering

A wave is much faster, with a cycle measured in seconds for a whole board, while selective soldering takes seconds per joint. For a high volume board with many through hole parts, the wave remains the right choice.

Selective soldering wins on flexibility and on thermal gentleness. It removes the pallet cost, it allows the same line to build many different products without tooling, and it does not subject the whole board to a second thermal excursion.

Process Control And Verification

Control rests on four parameters: flux quantity, preheat profile, nozzle dwell time and nozzle condition. Each is monitored and recorded, and the profile is verified with thermocouples on a sample at the start of a run.

Verification of the joint is visual and by cross section on a sample, with the fill measured against the acceptance criteria for the class. How that requirement is documented is described under PCBA development process.

Support And Handling

The board has to be supported while the nozzle works, because the solder fountain pushes on the joint as it flows. A thin board without support flexes, the pin moves in the solder, and the fill becomes inconsistent from board to board within the same batch.

Support fixtures are usually simple pins or a machined ledge under the board, and they have to avoid the surface mount parts on the underside. Where a lifted component is the concern, the nozzle dwell time and the preheat are the parameters to review, as described under SMT component shift causes.

Throughput And Cost Comparison

A selective soldering machine typically takes two to five seconds per joint including movement, so a connector with forty pins needs a minute or more. A drag nozzle covering a whole row reduces that to a single pass, which is why nozzle selection drives the economics.

For a few hundred boards a year the flexibility is worth the cycle time, and the absence of tooling makes changeover almost instantaneous. At high volume the wave remains cheaper per board, so the decision should be made on annual quantity rather than on batch size.

FAQ

Can selective soldering replace a wave entirely? For a product with a small number of through hole joints it can. For a board with hundreds of pins the cycle time makes it impractical, and a wave is still used.

Why is the nozzle cleaned so often? Dross accumulates on the rim and distorts the fountain, which changes the fill. Cleaning is quick and it prevents a drift in joint quality that is otherwise hard to see.

Does selective soldering need a pallet? Usually not, which is one of its advantages. A support fixture may be needed for a thin board, but masking is not required. The pin rules involved are described under PCB pad design standards.