Choosing the right wave soldering flux is crucial for achieving strong, reliable solder joints with optimal wetting and minimal residue. Whether you're working on high-volume PCB assembly or specialized electronics, the type of flux you use can make or break your soldering process. In this comprehensive guide, we’ll explore wave soldering flux types, the benefits of wave soldering flux no-clean and low residue options, and effective wave soldering flux application methods. Our goal is to help you make informed decisions for cleaner, more efficient soldering results in your projects.
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Let’s dive into the world of wave soldering flux and uncover the best practices to enhance your manufacturing process with precision and reliability.
Wave soldering is a widely used process in electronics manufacturing to solder through-hole components to printed circuit boards (PCBs). During this process, a wave of molten solder flows over the bottom of the PCB, creating strong connections between components and the board. However, for this to work effectively, the surfaces must be clean and free of oxides or contaminants. This is where wave soldering flux comes in.
Flux is a chemical agent applied before soldering to remove oxides from metal surfaces, improve solder wetting (the ability of solder to spread and adhere to surfaces), and protect the metal from re-oxidation during the high temperatures of soldering. Without the right flux, you risk poor solder joints, defects like bridging or incomplete wetting, and long-term reliability issues in your electronic assemblies.
Selecting the appropriate flux type is critical for achieving clean, reliable joints while minimizing residue that could affect performance or require extensive cleaning. Let’s break down the key considerations and types of flux available for wave soldering.
Not all fluxes are created equal. The type of flux you choose depends on your specific application, the materials being soldered, and the level of cleanliness required after soldering. Below, we’ll explore the main wave soldering flux types commonly used in electronics manufacturing.
Rosin-based flux, derived from pine tree resin, is one of the oldest and most traditional types of flux used in soldering. It comes in three activity levels: R (Rosin, mildly active), RMA (Rosin Mildly Activated), and RA (Rosin Activated). Rosin flux is known for providing good wetting properties and is often used in through-hole wave soldering for general-purpose electronics.
Wave soldering flux no-clean options have gained popularity in modern electronics manufacturing due to their convenience. As the name suggests, no-clean flux is designed to leave minimal, non-corrosive residue after soldering, eliminating the need for a cleaning step in most cases.
Water-soluble flux, also known as organic acid flux, is highly active and provides excellent wetting for challenging surfaces or lead-free solders. The residues left behind are conductive and corrosive, but they can be easily removed with deionized (DI) water, making this flux ideal for applications requiring a spotless finish.
Wave soldering flux low residue types are formulated to leave very little residue after soldering, often overlapping with no-clean fluxes. These are designed for processes where even small amounts of residue could affect performance or aesthetics.
Achieving optimal wetting and minimal residue in wave soldering requires more than just picking the right flux type. Several factors influence the performance of flux in your soldering process. Let’s explore these key considerations to help you fine-tune your approach.
The cleanliness of the PCB and components before soldering directly impacts wetting. Contaminants like oils, dust, or oxidation can prevent solder from adhering properly, even with a high-activity flux. Ensure that surfaces are clean and free of oxides before applying flux. Additionally, check that the flux is compatible with the materials being soldered, especially when using lead-free solders, which often require more active fluxes due to higher melting points (around 217-227°C compared to 183°C for traditional lead-based solders).
Flux activity refers to its ability to remove oxides and promote wetting. Highly active fluxes (like RA rosin or water-soluble fluxes) are great for difficult surfaces but may leave more residue or require cleaning. Low-activity fluxes (like no-clean or low-solids) are cleaner but may not provide sufficient wetting for heavily oxidized surfaces. Balance activity with residue concerns based on your specific needs.
Wave soldering typically operates at temperatures between 250-260°C for lead-free solders. Flux must withstand these temperatures without breaking down or leaving excessive residue. Ensure your flux is rated for your process conditions, as overheating can degrade its performance and increase residue. Also, minimize the time the PCB is exposed to high heat to reduce the risk of flux burnout or charring.
Some fluxes contain volatile organic compounds (VOCs) or hazardous materials that may be restricted in certain regions due to environmental or safety concerns. Opt for low-VOC or halogen-free fluxes if your production must comply with regulations like RoHS (Restriction of Hazardous Substances) or REACH (Registration, Evaluation, Authorization, and Restriction of Chemicals).
How you apply flux in the wave soldering process is just as important as the type of flux you choose. Proper application ensures even coverage, effective oxide removal, and minimal waste. Let’s look at common wave soldering flux application methods and best practices.
Spray fluxing is the most common method in modern wave soldering machines. A fine mist of flux is sprayed onto the bottom of the PCB as it passes over the fluxing unit. This method provides uniform coverage and allows precise control over the amount of flux applied.
In foam fluxing, air is bubbled through a reservoir of flux to create a foam head that contacts the PCB as it passes over. This method is older but still used in some setups due to its simplicity and low cost.
For small-scale or rework applications, flux may be applied manually using a brush or dropper. While not common in high-volume wave soldering, this method can be useful for prototypes or repairs.
To maximize the effectiveness of your chosen flux and achieve optimal wetting with minimal residue, follow these best practices during your wave soldering process.
Selecting the right wave soldering flux is a balancing act between achieving optimal wetting for strong, reliable joints and minimizing residue for a clean, efficient process. Whether you opt for a traditional rosin-based flux, a convenient wave soldering flux no-clean solution, or a high-performance wave soldering flux low residue option, understanding your application’s needs is key. Combine this with proper wave soldering flux application methods like spray or foam fluxing to ensure consistent results.
By considering factors like surface cleanliness, flux activity, process conditions, and regulatory requirements, you can streamline your wave soldering process for better productivity and quality. Use the insights and best practices shared in this guide to elevate your PCB assembly outcomes and meet the demands of modern electronics manufacturing.
At ALLPCB, we’re committed to supporting your success with resources and expertise for every step of your PCB production journey. Explore our services to see how we can help bring your designs to life with precision and efficiency.
Robert Voigt, DDM Novastar
The various types of through-hole (or thru-hole) soldering techniques are:
While Manual soldering is still done today, it is usually not found in a production environment because it involves highly skilled and labor intensive work producing very low numbers of boards. Dip and Drag were introduced many years ago as lower cost alternatives to Wave soldering, but have become outdated forms with the advent of more affordable and highly accurate Wave systems.
Today, Wave soldering is the most common and most efficient form of throughhole soldering available. It involves a solder pot large enough to handle the width of the largest boards you expect to process. By pumping hot solder through a nozzle in a way that the bottom of the board surface encounters the wave caused by the nozzle, the resultant hot solder waterfall creates a single point of contact across all the connection joints on the board, eliminating any potential bridging. The system usually integrates a fluxing station, a pre-heat station, and a wave station in a conveyorized system, using fingers or pallet-type board mounts.
Wave soldering is a time-tested technique, but there are many variations of systems to look for depending on your needs. In all of them, three operations are involved:
The first step in selecting a Wave system is to identify the largest board size (length x width) the machine will be expected to process. This will determine the machine size and, just as importantly, its cost, from a benchtop unit handling 8” boards (starting in the $10,000 range), up to free-standing machines with capacities up to 24” using a very large solder pot (costing as much as $80,000 or more). The larger sizes will also accommodate much more solder, which also contributes to the cost of ownership.
A larger system provides greater flexibility. It can not only handle larger boards, but it can also handle multiple smaller boards conveyed side-by-side to increase throughput. A prospective buyer may consider production throughput by calculating number of boards through a total cycle time from start to finish varying from 2.5 to 4 minutes from flux to solder.
Features of wave solder
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There are two types of wave:
Some machines are available with dual pots that can be used independently or together on the same board. SMD components are often processed in a turbulent wave machine.
How a wave solder system works
Based on the machine’s control system, a wave is created by a high pressure chamber pushing solder out through the solder nozzle. A chamber holds the nozzle, pump motor that drives an impeller inside the pressure chamber.
Fig 2: Benchtop wave solder system
The conveyor that holds the board attacks the wave at a 7 angle, and makes contact on all components only for a very short time, then exits the wave. The solder pot size is determined by wave size, the smallest holding around 100lbs of solder for a 7”-8” wave width, up to lb of solder for a 24” wide wave. Since solder is costly, the size of the machine contributes to the cost of operation.
Also, solder running in a hot wave is susceptible to oxidation and depletion so all these factors add cost. Inerting the area with nitrogen at the point of contact will produce a better quality joint and eliminate the possibility of a cold solder joint or oxidation. Nitrogen also helps to cut down on dross, which is defined as solder that builds up enough oxidation or flux residue to render the solder useless, somewhat like slag.
Contact on the component by the solder wave only takes place in seconds and once passed, cools almost immediately.
Flux systems
There are two common types of fluxing systems available:
Fig 3: Foam fluxer
Fig 4: Spray fluxer
Foam fluxing tends to be less expensive and easier to maintain than spray. The downside is that this will cause evaporation if it’s not contained, and the flux will degrade more quickly. Foam systems are low cost to operate, and easy to use and maintain.
Spray flux is contained in a high-pressure chamber which eliminates evaporation or degradation. However, spray systems are much more expensive, they’re difficult to set up, and require daily maintenance. They also involve a higher learning curve because it needs to be customized for each different board configuration.
Check out DDM Novastar's wide range of Wave Soldering Machines.
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Preheat Systems
Flux requires activation by heating, so any wave system requires a preheat station enclosed in a conveyorized chamber, where preheat temperatures reach 140-175 C. Both the components and the board are pre-heated to eliminate thermal shock during the wave.
Typical preheat technologies are:
There are many variables in every system, so it’s very important to discuss your needs with a vendor partner to determine what’s best for your situation.
For Wave Soldering, there are three common methods of running boards:
Automated In-Line System
This arrangement is usually tied in to a total PC board assembly line, where the conveyor simply moves assembled boards from the assembly stage through the Wave Solder machine and on to cleaning, finishing and other secondary operations. There is no manual interference at the solder machine; it’s a totally hands-off operation from beginning to end.
Wave machines that run this way are usually very expensive and are used in high-volume repetitive operations. SMEMA (Surface Mount Equipment Manufacturers Association) defines uniform specifications for in-line systems to assure that all the operations in an assembly environment transfer boards seamlessly from one machine to another, regardless of the manufacturer, machine model, etc.
Pros: Very efficient; reduces or virtually eliminates handling and manual labor
Cons: Very expensive, usually out of reach of low to mid volume contract assembly shops
Typical cost range: Often in excess of $100,000
Conveyorized System
This system uses titanium fingers or clips to retain and position the boards as they move through the wave machine. Boards are loaded onto the conveyor by hand and removed by the operator on the other side.
Pros: Considerably more affordable than an full in-line system. Less labor intensive than a palletized system (below)
Cons: Odd-shaped or thin boards can be difficult to run if they’re only supported by edge clips or fingers; this is where the benefits of a Palletized system come in
Typical cost range: $30,000 - $40,000
Palletized System
A palletized carrier is used to hold the circuit boards (which may include multiple boards) and the entire pallet is loaded onto the Wave solder machine. This is a batch process rather than an in-line system. The pallets are usually a fixed size according to the wave capacity (width and length).
Pros: Good support for odd-shaped boards or thin boards that could warp otherwise; can be customized to hold unusually shaped boards, or multiple boards; very easy to maintain pallets and fingers because everything can be done outside the machine; pallets don’t rely on a set of fingers internal to the machine that would require shutting it down for maintenance.
Cons: Boards must be loaded manually into each pallet and then into the machine.
Typical cost range: $12,000 - $30,000, but most high-end machines can use a pallet system
Wave Solder Pallet Types
There are a number of companies that specialize in designing and manufacturing custom wave pallets. Following are some highlights on pallet features and design options. The Wave Solder machine vendor you choose should be able to guide you in the selection and use of a proper pallet device.
Fig 7: Adjustable pallet holds multiple
board sizes for fast changeover
Through hole technology remains the typical method for soldering connectors and many high power components such as transformers and regulators. This, in turn, has led to a style of mixed technology circuit boards with some or all of the following characteristics:
To wave solder these connections, a Conformal Selective Wave Solder Carrier (CSWSC) is called for. This is a tooling pallet which has been designed to:
Key Elements of a Wave Solder Pallet:
Some common options for Wave Soldering include:
Preface to cleaning options
Process requirements, based on IEEE, IPC or other standard, will usually dictate the type of flux to be used in a particular process. IPC standards, such as J-STD, are used in the electronic assembly/manufacturing industry. Among other things, this standard dictates the type of solder, materials and processes to conform to specific specifications. If a highly active acidic flux is used, it will often leave a residue if not cleaned in process, whereas a ‘no-clean’ flux is burned off during pre-heat and doesn’t usually require cleaning after wave soldering.
For no-clean fluxing systems, flux should be completely dry prior to its entering the solder wave. Many wave solder systems today using no-clean flux will include an extended preheat time (that is, a longer preheat travel distance) for activation, to ensure completely dry boards and provide an effectively clean board. The solder flux manufacturer will provide these recommended specifications.
Air Knife Cleaning
An air knife is used for removing excess flux by blowing it off the bottom of the board before entering the wave. It is usually used with foam fluxing methods but can also be used with spray fluxing.
Pros: Eliminates most post-wave cleaning; returns excess back to the flux reservoir to conserve material.
Cons: Adds a little cost but saves on post-wave manual cleaning.
Typical cost range: Usually runs less than $1,000 as an add-on to a Wave solder system.
Fig 9: Flux air knife
Fig 10: Diagram: Flux air knife
Recirculating Cleaning System
This is a cleaning system for finger-type or board edge conveyors. It sprays a solution onto the conveyor just before new boards are loaded. The system includes an alcohol detergent solution along with brushes.
Pros: Reduces maintenance labor and downtime caused by flux residue buildup.
Cons: Flux is a fairly sticky substance and can cause production problems if not dealt with regularly.
Typical cost range: Usually adds between $1,000 – $2,000 to a Wave machine purchase.
Inerting Systems
Fig 11: Finger cleaner
Inerting systems are used to introduce nitrogen during the soldering process. Nitrogen is contained within the solder pot by a hood to reduce oxidation in the pot and in the critical moments while soldering the boards. Nitrogen inerting is recommended for lead-free solder, which will oxidize rather quickly otherwise and can adversely affect joint quality. Nitrogen is by far the most commonly used gas in an SMT application.
Pros: Ensures high quality joints to meet IPC standards when required.
Cons: Adds a significant cost.
Typical cost range: Can add up to $4,000 to a Wave solder system.
Rollout Solder Pot
Fig 12: Rollout solder pot
On larger wave solder systems, the solder pot itself, when full, can weigh up to several hundred pounds. A rollout option simplifies maintenance by facilitating the removal and cleaning of the pot, which should be done weekly to remove solder dross in a typical production environment.
Pros: Simplifies cleaning and maintenance, reduces opportunity for personnel injury
Cons: May add $2,500 to a large wave solder machine with a heavy solder pot
PC Interface
This feature connects your wave solder system to a PC or networked computer to enable fault monitoring or data logging for QC purposes, or to allow the user to connect to the machine for remote control.
Pros: A data logging interface is important for ISO certification and record keeping.
Cost: Will range from $1,500 - $3,500
How to select a Wave Solder Machine Provider
When evaluating proposals from reflow oven vendors, try to find references or reviews before you buy. You can often find them on Internet search, and while not altogether scientific, you may get some clues regarding construction quality or temperature stability that could influence your decision. Some offshore products may look attractive from a pricing perspective, but may not perform to your standards; plus, aftermarket support can be problematic if you need help.
Contact us to discuss your requirements of Cast Iron Stick Electrodes. Our experienced sales team can help you identify the options that best suit your needs.