Introduction

Wave soldering, also known as flow soldering, is a bulk soldering process used in the assembly of printed circuit boards (PCBs). It involves passing the underside of an assembled PCB over a wave of molten solder to simultaneously solder all solder pads and component leads on the board.

Wave soldering is an efficient and cost-effective method for soldering high volumes of PCB assemblies. It provides a means of soldering large numbers of joints reliably and quickly compared to selective soldering or manual soldering.

How Wave Soldering Works

The wave soldering process consists of the following main steps:

• PCB Preparation – The assembled PCBs are prepared by applying solder paste or flux to the areas to be soldered. This removes any oxides and contaminants to allow the molten solder to properly wet and adhere to the metal surfaces.
• Preheating – The PCBs are preheated on a conveyorized oven to a temperature of 100-150°C. This helps evaporate volatile flux constituents, activates the flux, and prevents thermal shock to the components and PCBs.
• Flux Application – Additional flux is sprayed on the bottom side of the heated PCBs as they exit the preheater. This further removes oxides immediately before soldering.
• Soldering – The PCBs are conveyed over a turbulent wave of molten solder (around 245-260°C). The underside makes contact with the rising/falling wave, allowing the solder to wet and adhere to the exposed solder pads and component leads.
• Cleaning – The soldered assemblies are conveyed through a cleaning station to remove flux residues from the soldering process. This typically involves spraying solvent or aqueous cleaners.
• Inspection – The completed boards are visually inspected and often pass through an automated optical inspection (AOI) system to verify all solder joints meet quality standards.

Main Components of a Wave Soldering Machine

Wave soldering systems are complex pieces of machinery designed to deliver a controlled soldering process. The main components include:

• Solder Pot – Holds the molten solder alloy. Usually heated by electrical elements and pumped to create a smooth laminar wave. Constructed of steel, titanium, or ceramic-coated materials.
• Nitrogen Blanket – An inert nitrogen atmosphere is maintained over the solder pot to prevent solder oxidation. This reduces dross formation.
• Fluxer – Applies flux to the bottom side of the heated PCBs before soldering. This is done by spray fluxers or foam fluxers. Ensures proper solderability and wetting.
• Preheater – Heats up the PCBs to the necessary preheat temperature to activate the flux and prevent thermal shock. Consists of IR heaters, convection heaters, or a combination of both.
• Solder Wave – The pumped, turbulent wave of molten solder that contacts the underside of the PCB to form the solder joints. Carefully designed to ensure an even solder coat.
• Conveyors – Transports the PCBs through the soldering machine over the solder wave at a controlled speed. Made of stainless steel or heat-resistant polymers.
• Cleaning System – Removes flux residues from the soldered PCBs. Typically spray washers that use solvent or water-based cleaners.
• Exhaust System – Safely collects and filters out fumes or vapors from the soldering process. This includes flux vapors and cleaning agents.

Advantages of Wave Soldering

Wave soldering provides the following benefits over other soldering methods for PCB production:

• High Throughput – Capable of very high throughput up to thousands of boards per hour. Much faster than manual soldering.
• Low Labor – Does not require a skilled operator at each joint. Only one or two operators needed to oversee the automated machine.
• Consistency – Provides a consistent soldering process across all joints. Joint quality is not operator dependent.
• Capacity – Can handle very high lead counts with hundreds or thousands of joints per board. Not practical with selective soldering.
• Cost-Effective – Lower capital cost than equivalent selective soldering capacity. Also has a lower cost per board at high volumes.
• Reliability – The controlled process produces highly reliable solder joints with consistent quality. Detects unsoldered joints.
• Simple Operation – Training requirements are minimal compared to manual soldering. Easier to find operators.
• Flexible – Machines can handle a wide variety of board designs, components, and solder alloys. Quick changeovers.

Limitations of Wave Soldering

However, wave soldering does come with the following drawbacks and limitations:

• Lead-Free Incompatibility – Lead-free solders require higher soldering temps incompatible with many component temperature limits.
• Limited Component Types – Cannot solder large plastic components, tall components, or connectors/sockets since they interfere with the solder wave.
• Shadowed Areas – Shaded areas underneath large components may receive insufficient heat transfer and solder coverage.
• Solder Balls – Excess turbulence can generate solder splatter which leads to solder balls that must be removed.
• Voids – Air bubbles can get trapped in solder joints, creating voids that may impact reliability.
• Dross Formation – Oxidized solder contaminants build up on the solder pot walls and must be periodically removed.
• Flux Residues – Flux residues left after soldering can lead to corrosion or electrical leakage if not properly cleaned.
• Single Sided – Only allows soldering on one side of the PCB at a time. Two passes are needed for double-sided boards.

Solder Pot Design

The solder pot is a critical component which holds the molten solder that forms the standing wave. The design of the solder pot impacts the consistency and quality of the soldering process. Key features include:

• Pump – Provides smooth laminar flow and the desired solder wave characteristics. Often an impeller, auger, or jet pump design.
• Heating System – Electrical heating elements maintain the temperature of the solder bath. May use cartridge heaters or ribbion heaters.
• Solder Nozzle – Shapes the pumped solder into a smooth standing wave as it exits the solder pot. Removes turbulence and oxidation.
• Dross Removal – System for periodically removing dross buildup from the solder pot walls. May be manual, automatic, or continuous removal.
• Solder Level Sensor – Maintains the optimum solder level in the pot. Prevents overflow or underflow of solder.
• Solder Pot Liner – Made of coated steel, titanium or ceramics to prevent solder erosion of the solder pot. Extends the pot lifetime.
• Nitrogen Blanket – Blankets the solder pot surface with an inert nitrogen atmosphere to prevent oxidation.
• Solder Filtration – Filters continuously recirculate solder to remove inclusions and impurities. Improves solder quality.

Solder Alloys for Wave Soldering

The solder alloy used in wave soldering must meet several critical performance criteria:

Wetting Ability – Allows the molten solder to readily wet and adhere to the PCB and component surfaces to be soldered. Affected by alloy composition and fluxing.

Fluidity – The liquidus temperature must be low enough to produce a smooth wave and fully wet all joints. High fluidity reduces dross formation.

Mechanical Properties – Must solidify into a solder joint with sufficient strength, ductility, and thermal fatigue resistance.

Electrical Conductivity – Must deliver adequate electrical connectivity across the solder joints. High tin content improves conductivity.

Availability – Popular alloys are widely available from solder suppliers at reasonable cost. Exotic alloys are expensive and hard to source.

Environmental – Regulatory compliance regarding lead content and other restricted hazardous substances must be considered.

The most common solder alloys used in wave soldering machines include:

• 63Sn/37Pb – A eutectic tin-lead alloy. Excellent solderability and low cost. Being phased out due to toxicity concerns over lead content.
• SAC305 – A lead-free alloy of 96.5Sn/3Ag/0.5Cu. Currently the most popular lead-free wave soldering alloy, but requires higher soldering temperatures.
• SAC105 – A lower temperature lead-free alloy of 98.5Sn/1Ag/0.5Cu. Lower cost than SAC305. Higher tin content improves conductivity and wetting.
• SN100C – A 99.3Sn/0.7Cu nearly-eutectic, lead-free alloy. Good fluidity, wetting, and joint strength. Higher cost than SAC alloys.

Thermal Profiles

Precisely controlling the thermal profile is critical for achieving high solder joint quality and reliability in wave soldering. The profile consists of:

Preheat Stage

• Heats PCBs and components to 100-150°C. Eliminates moisture and activates flux. Prevents thermal shock.
• Typically 60-120 seconds in length. Temperature uniformity is important.
• Achieved using IR heaters, convection ovens, or a combination of both.

Soldering Stage

• Board conveyed over solder wave around 245-260°C. Rapid heating to the soldering temperature.
• Short contact time of 2-6 seconds to form the solder joint without overheating.
• Depends on conveyor speed, contact length, and solder pot temperature.

Cool Down Stage

• PCB assembly cooled back down to handable temperatures under 100°C.
• Prevents components from being damaged by prolonged heating.
• Air cooling is typically used. May be assisted by fans/blowers.
• Takes 15-60 seconds depending on the PCB size and design.

Key Process Parameters

In order to achieve high quality soldering, the wave soldering process must be optimized by tuning the following parameters:

• Conveyor Speed – Primary factor determining the contact time of board over the solder wave. Higher speed gives less time for soldering. Typical range of 1.5 – 6 ft/min.
• Solder Bath Temperature – Higher temperatures improve solder fluidity and wetting but may exceed component limits. 245-260°C is typical.
• Preheat Temperature – Must be high enough to activate the flux but below the damage threshold for sensitive components. 100-150°C is standard.
• Solder Nozzle Depth – Depth the solder wave nozzle which determines the wave height. Affects solder coverage and turbulence.
• Solder Chemistry – Composition directly affects melting point, wetting, fluidity, and electrical/mechanical properties.
• Flux Type and Application – Proper fluxing prevents oxidation and ensures good wetting of metallic surfaces.
• PCB Design Parameters – Thermal relief pads, board thickness, solder masks, and finishes impact soldering results.

Careful tuning and control of these parameters helps ensure a repeatable process that maximizes soldering quality. Real-time monitoring is recommended to detect any variations.

Common Defects

Despite best efforts to optimize the process, defects can still occur in wave soldering. Typical defects include:

• Solder Balls – Spheres of solder that splatter onto the PCB surface. Caused by turbulence, board orientation, and excess flux.
• Solder Bridging – Unwanted solder that bridges between neighboring pads or component leads. Results from excess solder.
• Icicles/Solder Flags – Jagged spikes of solidified solder protruding from joints. Due to oxidized solder or poor wetting.
• Insufficient Solder – Incomplete joint with inadequate solder to fill the pad or cover the leads. Caused by low solder, low heat, or surface contamination.
• Excessive Solder – Too much solder accumulating on pads or leads. Leads to bridging and blobs.
• Voids/Porosity – Air bubbles trapped within the solder joint, reducing strength. From surface contamination or high temperatures.
• Solder Beading – Solder forms a thick bead along the board edge instead of a smooth fillet up the component lead. Poor wetting characteristic.
• Isolated/Cold Solder – Unsoldered areas separated from main solder fillet. Due to shadowing or localized cold spots.

Solderability Testing

Since achieving good wetting and solderability is imperative for wave soldering, solderability testing of PCBs, components, and solder alloys is commonly performed:

• Wetting Balance Test – Measures wetting force and contact angle. Indicates the relative solderability of different board finishes and solder alloy combinations.
• Meniscograph Test – Analyzes the wetting behavior and solder spreading on a copper plate. Provides solder alloy fluidity data.
• Surface Insulation Resistance (SIR) – Electrical test method to assess surface insulation properties and detect potential electrochemical migration risks from flux residues.
• Solder Ball Test – Solder is dripped onto a board while inverted to check for solder balling, beading, and other wetting defects.
• Surface Finish Analysis – Surface finishes on PCBs and component leads are tested for expected properties. Finishes must be solderable.
• Component Solderability Testing – Verifies production components meet solderability criteria through dipping in molten solder and examining wetting effects.

Maintenance

To sustain consistent soldering results over thousands of boards, wave soldering machines require thorough maintenance procedures:

Daily Maintenance

• Visual inspection of solder nozzle & pot condition
• Remove dross and inspect solder level
• Clean solder surface with paddle
• Check conveyor condition and track alignment
• Verify heating system operation
• Monitor wave characteristics and topside board heat
• Inspect soldered boards for defects

Weekly Maintenance

• Clean entire solder pot system
• Thoroughly clean & adjust conveyors
• Replace preheaters/heaters if cracked or degraded
• Remove flux buildup from foam fluxers
• Clean cooling fans and airflow pathways

Periodic Maintenance

• Send solder pot liner out for chrome coating replenishment
• Replace worn solder pot impeller/pump
• Overhaul conveyor drives, bearings, motors
• Replace degraded insulation and heating elements
• Major cleaning of solder filtration system
• Calibrate sensors and instrumentation

Proper maintenance extends the service lifetime of the wave soldering system and ensures soldering quality does not degrade over time.

Quality Assurance

Maintaining control and consistency of the wave soldering process requires several quality assurance measures:

• Statistical process control tracking of key parameters like conveyor speed, temperatures, and wave height. Monitoring for process drift.
• Regular verification of thermal profiles. Use thermocouples on boards to validate preheat and soldering temperatures.
• Visual inspection under microscopes of solder joints on coupons or prototypes to confirm acceptable quality.
• Automated optical inspection (AOI) to check all solder connections and identify any defects.
• Testing of solder pot samples to check alloy composition and impurity levels.
• Periodic audits of maintenance logs, operator procedures, training records, and machine settings.
• Analysis of defects to find root causes. Feedback into improving procedures.
• Correlating defect rates with field failure rates to prove soldering reliability.
• Continuous improvement programs and kaizen events to incrementally improve the process.

FQA

What are the main advantages of wave soldering?

Some of the main advantages of wave soldering are:

• High throughput capability, able to solder up to thousands of boards per hour
• Low labor requirements compared to manual soldering
• Consistent solder joint quality that is not operator dependent
• Can handle PCBs with high lead counts, unlike selective soldering
• Lower capital equipment costs compared to other equivalent soldering methods
• Produces reliable, repeatable solder joints due to the controlled process
• Minimal operator training requirements
• Flexible process that can handle a wide variety of PCBs and components

What types of defects can occur in wave soldering?

Common wave soldering defects include:

• Solder balls – splashes of solder across PCB surface
• Solder bridging between pads or leads
• Icicles or jagged solder spikes due to poor wetting
• Insufficient solder and incomplete joints
• Excessive solder accumulation leading to blobs or bridges
• Voids or porosity within solder joints
• Solder beading along board edge instead of a fillet
• Isolated/cold solder joints that lack adhesion to the main joint

These defects are generally caused by issues like turbulence, contamination, poor wetting, oxidized solder, shadowed areas, or non-optimized process parameters.

How are PCBs prepared prior to wave soldering?

PCBs require the following preparation steps before wave soldering:

• Solder paste or liquid flux is applied to any areas that will contact solder
• This removes surface oxides and contaminants that can interfere with wetting