Introduction
Selective soldering and wave soldering are two techniques used for soldering electronic printed circuit board assemblies. While both methods attach components by flowing molten solder onto joints, there are important distinctions between these widespread soldering processes.
This article provides an overview of how selective and wave soldering work, key differences, applications where each excels, equipment considerations, and factors guiding process selection for an electronics assembly line.
Printed Circuit Board Soldering
Attaching components to PCBs requires depositing melted solder alloys onto metallic joint surfaces to form both mechanical and electrical connections. This soldering process happens in assembly operations like:
Through-hole Parts – Components with wire leads inserted through holes in the board must be soldered on the opposite side to form a joint.
Surface Mount Devices – Parts placed onto surface PCB pads sit with terminals contacting pads ready for soldering.
Connectors and Pins – Mated connectors aligned to boards as well as pins inserted into plated through-holes rely on soldering for permanent attachment.
Reflows – Secondary rework requires re-melting existing solder joints to install new components or replace damaged parts.
Various soldering methods exist to handle this broad range of printed circuit board assembly soldering requirements.
Wave Soldering Overview
Wave soldering is a bulk process suitable for soldering most through-hole PCB components. Key characteristics include:
- Conveyor Transport – Boards progress on a conveyor through cleaning, fluxing, preheating, soldering, and cooling zones in an in-line assembly sequence.
- Solder Wave – Molten solder is pumped from a reservoir through a nozzle forming a standing wave. Bottom board surface passes over this wave.
- Non-Selective – All exposed metallic surfaces on the underside contact the wave depositing solder globally across the board footprint.
- High Throughput – Continuous in-line processing supports high volume production. Capable of over 15,000 components per hour.
- Mature Technology – Wave soldering is a proven process used in electronics for over 50 years. High reliability achieved through process control.
Wave solder systems efficiently solder both plated through-hole and surface mount bottom-side terminations in a single pass.
Selective Soldering Overview
Selective soldering is a targeted soldering approach applied only where needed on a PCB:
- Solder Jet Nozzle – A precision pump and nozzle dispenses a small volume of solder only onto specified joints. Nozzles come in various sizes and shapes.
- Localized Heating – Targeted infrared lamps or lasers preheat specific component areas requiring soldering minimizing thermal exposure for sensitive parts.
- Selective Control – Vision systems identify parts and joints. Robotic mechanisms position and align the board under the solder jet to selectively deposit onto intended wettable surfaces.
- High Mix – Selective system flexibility supports soldering a broad mix of through-hole and surface mount parts on the same board without setup changeover.
- Reduced Thermal Exposure – Localized preheat and soldering minimizes effects of process heat on temperature sensitive components.
Selective soldering provides a programmable, flexible alternative to wave solder with less process exposure for fragile boards.
Key Differences Between Wave and Selective Soldering
| Wave Soldering | Selective Soldering |
|---|---|
| Bulk process solders all exposed joints | Programmable selective deposition only where needed |
| Constant wave of molten solder | Metered solder jet pulses |
| Non-discriminating solder coverage | Computer vision targeting of joints |
| Entire board thermal exposure | Localized preheating at components |
| Limited to PTH parts on bottom side | Solders PTH, pads, connectors on both sides |
| High throughput for volume boards | Flexible changeovers between small batches |
| Mature process with lower equipment cost | Advanced automation requires higher capex |
Wave Soldering Applications
Some typical application examples where wave soldering excels:
- High Volume PTH Boards – Efficiently solders bottom-side plated through-holes on mass produced assemblies.
- Two Sided TH Boards – Flips board after first pass then run again to solder both sides.
- Multi-Wave Systems – Specialized nozzles allow soldering of boards over 12 inches long.
- Rework – Damaged boards can sometimes be salvaged by re-exposing to solder wave to reflow joints.
- Selective Coating – Mask areas enables soldering only portions of board.
- SMD and PTH Combinations – Wave deposits solder on bottom SMD parts during PTH soldering.
For broad soldering of high volume plated through-hole assemblies, wave solder provides a proven cost-effective process.
Selective Soldering Applications
Selective soldering is well suited for:
- Prototyping – Allows flexibility in soldering varied boards in low volume without extensive setup.
- High Mix Production – Easily switches between different assemblies unlike fixed in-line wave.
- Reflows – Capable of re-soldering joints, replacements, and repairs by targeted jet.
- Pin-in-Paste –Solders bottom leaded components inserted into paste deposits through stencil printer.
- Tall Components – Able to solder tall parts that would bump or contaminate a wave system.
- Heat Sensitive Parts – Localized heating reduces thermal stress versus whole-board exposure.
- Connector Soldering – Precision jet solders sides, pins, and mid-board connectors.
- Limited Runs – Economic low volume production avoids high wave line setup charges.
For flexibility, reduced thermal effects, and specialized soldering requirements, selective systems excel.
Wave Solder Equipment Considerations
Key aspects in configuring a wave solder system:
- Conveyor Width – Accommodates range of board sizes by extensions or multiple lanes.
- Process Length – 5-15 foot total tunnel matching heating zone lengths, conveyor speed and volume needs.
- Solder Pots – Small pots for low volume or multiple large pots for high capacity and redundancy.
- Solder Alloy – Common tin-lead and lead-free alloys compatible with processes and safety. May require multiple pots.
- Nozzle Design – Shape and opening size affect fluid dynamics including turbulence and oxides.
- Fluxer – Foam or spray fluxing matches solderability needs and cleanliness requirements.
- Preheat – IR modules or convection heaters ramp board temperature suitable for parts prior to wave.
- Nitrogen Blanket – Covering tunnel in nitrogen atmosphere prevents oxidation.
Selective Solder Equipment Considerations
Key aspects in implementing a selective solder system:
- Motion Mechanism – Robotic arm, linear XYZ stage, or movable bond head for board positioning.
- Board Holder – Fixtures securely hold boards in accurate alignment under the solder jet.
- Solder Pump – Volumetric accuracy and response speed affects solder deposition precision.
- Process Cameras – High quality machine vision for imaging boards, locating parts, aligning joints.
- Software – Creates program sequences for part recognition, motion planning, jet activation based on CAD data.
- Nozzles – Various sizes and shapes optimized for joints being soldered. Quick changeout for flexibility.
- ** Preheat Method** – Infrared lamps, lasers, or heated nitrogen provide localized preheating.
- Board Handling – Mechanisms to quickly load/unload boards for changeover between low volume batches.
Comparing Wave and Selective Solder Costs
Wave soldering has lower capital equipment costs but selective provides flexibility:
| Cost Factor | Wave Solder | Selective Solder |
|---|---|---|
| Equipment Purchase | $$ | $$$$ |
| Setup/Changeover | $ | $$ |
| Maintenance | $ | $$ |
| Process Engineer | $ | $$ |
| Floor Space | $$ | $$$ |
| Cycle Time | $ | $$ |
| High Mix | $$$$ | $ |
| Low Volume Runs | $$$ | $ |
Process Selection Guidelines
In general, wave soldering suits:
- High volume production
- Longer boards up to 24 inches
- Lower part densities
- Higher mix of through-hole parts
- Lower complexity assemblies
- Mature, stable products
While selective soldering suits:
- Medium to low production volumes
- Shorter boards less than 6 inches
- Higher surface mount part densities
- Greater mix of SMD and connector content
- Complex or high value boards
- Frequent product changeovers
- Rework intensive assemblies
- Prototyping and ramp-to-volume
However, many factors should be weighed including product roadmaps, changeover needs, heat tolerance, and quality goals. Utilizing both technologies as applicable provides maximum manufacturing flexibility.
Wave and Selective Soldering – Frequently Asked Questions
Can surface mount parts be wave soldered?
Yes, surface mount pads and termination on the bottom side of boards can be wave soldered. This is known as mixed technology assembly. Topside SMD parts require gluing or stenciled solder paste.
How are connectors selectively soldered?
Connectors require fixtures to present joints squarely against precision solder jet nozzles. Robotic mechanisms position boards under stationary nozzles. Certain tall connectors may be unsuitable.
What are common solder alloys used?
63/37 tin/lead or 60/40 tin/lead were once common but environmental restrictions limit their use. Lead-free alloys like SAC305 (96.5/3/0.5 Sn/Ag/Cu) are now predominant.
What determines the conveyor speed in wave soldering?
Maximum speed balances allowing sufficient thermal soak time versus avoiding bridging risks. 4-6 feet per minute is typical although high speed systems operate over 7 ft/min by utilizing preheaters.
How are selective and wave soldering programmed?
Vision systems image boards to map programs of recognized parts. CAD data imports orthogonally locate components. Software interfaces allow touchscreen teaching of locations.
Conclusion
Both wave solder and selective soldering provide viable manufacturing processes applicable across a range of through-hole and SMD assembly volumes and configurations. Wave solder excels at rapid in-line soldering of high density PTH boards while selective deposit provides flexible automated freedom to target specific component joints. Weighing factors like changeover needs, heat tolerance, emerging connector trends, and production economics determines optimal utilization of these complementary technologies. Blending the strengths of wave and selective solder equipment maximizes assembly line agility, quality, and efficiency.
