Selective Soldering: A Partial and Precise Soldering Process

The miniaturization trends in circuit board fabrication have necessitated selective soldering to prevent component damage. 

Reflow and wave soldering might still work on boards with purely SMT and through-hole components but can damage mixed assemblies. 

So it is necessary to explore the selective soldering process, and here’s all you need to know about it.

What Is Selective Soldering?

As the name suggests, selective soldering is the partial welding or attaching select components to the PCB.

 Usually, reflow and wave soldering machines weld the entire board by heating the applied solder or wetting the board.

The other process that can achieve partial electronic component welding is manual soldering. 

But the method is slow, time-consuming, labor-intensive, costly, and not as accurate.

A technician manually soldering PCB components.

So selective soldering is the most viable option for high-volume mixed-technology circuit board assembly.

Why Use Selective Soldering?

The traditional way of assembling mixed technology circuit boards begins with soldering through-hole components, then surface-mount parts.

 So wave soldering comes before reflow soldering.

But with the growing popularity of SMT devices due to their compact size, THT components are reducing in modern PCBs.

 However, we cannot do away with through-hole parts entirely. 

Although SMT parts take up significantly less real estate on circuit boards, it is impossible to make all components SMT.

So modern compact boards usually feature multiple surface-mount devices and few THT parts.

 Subjecting these components to the thermal shock created by the molten wave of solder can damage them.

 And remember, SMT pieces are costlier than their THT counterparts. So damaging them will be expensive.

Therefore, from an economic point of view, it is less risky to assemble the SMT components using reflow soldering first.

 Afterward, the board can undergo selective soldering to attach the few through-hole pieces.

A reflow oven

So selective soldering machines must carefully work around the already soldered SMT components by targeting specific areas. 

And since boards have different designs, this machine must be programmable, precise, and quick (to reduce the soldering time).

Selective Soldering Assembly Processes

Assemblers can use either of the following selective soldering processes.

Selective Aperture Tooling Over Wave Solder

This selective aperture tool covers the previously soldered SMT parts, exposing the areas requiring soldering. 

Next, this tool and PCB go into a wave soldering machine to weld the THT components.

Mass Selective Dip Solder Fountain

This process is a variant of the tooling process above. It involves using a specialized tool with holes to allow solder through to the areas requiring soldering. 

So these holes must match the THT component location, meaning each aperture tool is unique to a specific PCB assembly.

After attaching the two, they go above a selective solder fountain where all the soldering spots get welded simultaneously. 

The soldering occurs as the board lowers into the fountain.

A selective soldering machine

Miniature Wave Selective Solder Fountain

As the name suggests, this process uses a small wave soldering machine to pump a round solder wave. 

This wave is the size of a crayon. The setup can have a fixed liquid metal bath and a moving PCB.

Alternatively, it can have a fixed PCB and an adjustable solder pot under the board.

But both solder the PCB sequentially, making the process slower than the two above. However, it is toolless and more precise.

Laser Selective Soldering

Selective laser soldering is relatively newer than the other three.

 It imports the PCB CAD designs to identify the areas that require soldering, then positions the laser to hit these spots.

Due to its non-contact soldering design, the process eliminates thermal stress and creates high-quality solder joints. 

Also, it is efficient because the soldering time per joint averages one second.

And you can eliminate solder masks and stencils to lower the manufacturing costs when using selective laser soldering.

Other less popular options include:

• Robotic soldering iron with a wire solder feed
• Induction soldering with hot gas and solder paste, solder preforms, or solder-laden pads

An automated selective soldering machine

Selective Soldering Challenges

All seems well to this point. But the process is not without its challenges. And the most prevalent one is ionic contamination.

Because the selective solder machine heats specific board areas, it might not activate the solder paste flux as required. 

So the flux residue can spread to other PCB areas.

Flux usually has ionic contaminants, and they are not easy to detect. 

If undetected, this contamination can cause dendritic growth over time.

Solder flux

But there are solutions to this issue. One is preventive (preheating), and the other is corrective (contamination tester updating).

Preheating the entire surface activates the flux better and prevents flux migration.

On the corrective side, detecting ionic contamination is easier if you update your contamination testers.

These devices usually compute ionic contaminants across the entire board surface to measure potential harmful/unwanted board conductivity. 

But you can calibrate them to measure only the flux path. This programming will give you more precise ionic contamination tests.

Factors To Consider When Choosing a Selective Soldering Process

Consider these factors for fast, efficient, and precise selective soldering.

PCB Size

The maximum board size you want to process will determine the machine you can use. 

If too broad, you’ll have to buy a big solder machine and process only one board at a time. 

Alternatively, you can introduce a second solder pot to hasten the process.

But if small, you can configure the machine to have two conveyor belts to process two PCBs concurrently. 

Machine Footprint

Selective soldering machines vary sizewise, so consider the available floor space in your work area. 

You can get compact machines that process a single board at a time. 

Or pick a sizable in-line unit for higher volume production.

Solder Type

Solder melts at different temperatures depending on its composition. 

Most assemblers use lead-free solder, which melts at around 270-300°C.

 But some newer variants liquify at lower temperatures of about 250°C. 

The machine’s pot temperature must reach the melting point of the solder you use.

Lead-free solder wire

Solder Pot

Some selective soldering systems can use more than one solder pot per module. This setup gives you more flexibility to do the following.

• Place different solder alloys in separate pots
• Have a variable Z-axis on both to fit different nozzle sizes
• Vary the Y-axis between solder pots to process two PCBs on one panel concurrently on a single conveyor system. Alternatively, you can solder two PCBs on a dual conveyor system.

Maintenance

Cheaper machines use impellers to drive the solder, which can generate dross, meaning more maintenance work. 

But the costlier versions use electromagnets to push the solder. 

These produce minimal dross impurities and generate a steady wave that enhances the soldering accuracy.

Solder dross recycling in a wave soldering machine

Nozzles

Nozzles vary sizewise, with the thin ones being more precise and ideal for reaching restricted areas.

 However, these narrow-diameter nozzles wear out quickly and lengthen the processing times.

Nitrogen Supply

Dross forming on the nozzles affects the solder quality and repeatability. 

So manufacturers use nitrogen to prevent the formation of these oxides in the liquid solder.

Solder dross

You can obtain nitrogen from pressurized bottles, liquid nitrogen tanks, or local generation.

 The last option is more expensive to set up but is cheaper in the long run.

Other factors to consider include:

• Flux
• Preheating
• PCB handling
• Machine options
• Machine configuration

Benefits of Selective Soldering

• No need to glue SMT devices
• Consumes less flux and solder
• Does not need to heat all board areas
• Customizable to accommodate different parameters, such as component pitch
• Repeatability

Drawbacks of Selective Soldering

• Slower than wave soldering
• Complex setup
• Not ideal for large-scale production

Wrap Up

As you can see, selective soldering is not the fastest soldering process. 

And it is considerably slow compared to wave soldering. 

But it is a safer process because it does not subject SMT components to thermal shock. 

And you have several selective soldering methods to consider. That’s it for this article. 

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