Robert Voigt, DDM Novastar
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There are several different reflow methods to liquefy solder in a controlled way and not all of them can literally be called “ovens”.
For instance, the simplest form of heating is performed by conduction, that is, by contact with a heated surface such as a hot plate. The other heating methods are typically performed within a chamber (oven) but rely on different heating methods:
Q. What size do I need? A. How many zones can you afford?
Seems like an odd way to start a discussion about Reflow Ovens, but it’s not entirely unreasonable. As with any complex process, there will be tradeoffs between cost and capabilities, and more zones will always give you better flexibility and more control over your profile, but at a cost. The decision has to be qualified primarily on your anticipated throughput, that is, how many boards you process in a day or a week.
There are other considerations too, such as board size, component density, and appropriate thermal technology, but we’ll talk about those after identifying the work flow volume. The following is a guideline for zones relating to typical volume needs:
**In a typical low-to mid-volume production world, a small conveyorized horizontal convection oven meets most basic requirements.
Now for the explanation. A typical soldering operation in today’s world requires three main stages for temperature profiling: Preheat, Soak, and Reflow to perform these functions: 1) The preheat stage for a certain period of time to activation temperature, 2) the soak stage for a different period of time to activate the solder, and 3) the reflow stage where temperatures peak for yet a different time frame. After this, the board is typically cooled and removed. Depending on the material, e.g., leaded, lead-free or specialty materials such as epoxies, the heating profile for each stage will vary according to the manufacturer’s specifications to achieve optimal bonding.
Fig 1: Dual stage hot plate
CONDUCTION
With a hot plate (a direct contact device), it is nearly impossible to achieve three discrete stages with any degree of accuracy, so this technique is recommended only for the hobbyist who makes a couple boards a day and isn’t particular about the thermal profile. A dual-stage hot plate can do a little better, because with two digital temperature controls, the user can pre-set temperatures and manually move the board from one plate to another for specific times to accomplish each stage. However, no hot plate is really a technically professional method.
Pros: Price
Cons: Low volume; no profiling ability, thus no reasonable quality control
CONVECTION
Single zone batch ovens can improve quality and speed by virtue of a built-in multi-stage temperature controller. This system is appropriate for the serious hobbyist, budget-conscious customer, or test lab. It works by pre-setting the temperatures for each stage, placing the board in the oven, closing the lid, and removing it when complete. Adding a single or multi-device shuttle helps to stage one board for loading as soon as the prior board is done and eliminating the need to open the lid. Obviously, this technique is limited by speed but also by size, since batch ovens typically handle boards only up to 8” x 8”.
Pros: Inexpensive; fair controllability for lead and lead-free profiles; small footprint (tabletop); low power consumption
Cons: Max board size 8”x8”; limited profiling capability due to single heat zone construction; low volume
Fig 2: Multi zone oven with convection heating
Conveyorized multi-zone ovens,
starting with three zones to achieve each stage of preheat, soak and reflow, are the best solution for any type of production volume. They provide complete control of each stage for temperature and timing. Moving via a constant speed conveyor, each board spends just the right amount of time in each stage based on the zone length (or alternately, the number of zones), to achieve the perfect ramp temperature rate for the material profile, board density and geometry.
Pros: Economical production system for >100 boards/day; will typically accommodate board sizes up to 12”x12”; offers precise control of ramp temperatures to handle most common leaded and lead-free soldering profiles; can be reconfigured easily to run different board profiles, an important feature for contract manufacturers.
Cons: More expensive than other methods
Now back to the original question: Why is it always a good idea to buy as many zones as you can afford? Or rather, when might you need more than three zones in an oven?
The answer is a function of how much flexibility you need in your assembly operation, and that’s driven by controllability, volume efficiency, and cost. More zones, along with more length will provide greater flexibility to handle more complex profiles, greater board component densities, and higher speeds (number of boards processed). As a general rule of thumb, ramp temperatures must not exceed 2 to 4 C per second for proper profiling, so more zone flexibility will provide better control.
Larger ovens can also be built with other features such as the ability to handle larger board geometries and special capabilities such as double-sided soldering. For the contract assembly operator who handles a wide variety of products, more zones provides the ability to capture more business that can’t be done by competitors with smaller ovens.
Fig 3: Discrete zone schematic in conveyorized oven
Heating Technologies
Regardless of the technology used for heating boards, the primary objective is to control the thermal profile to prevent undesirable conditions – such as: 1) Tombstoning (a condition that causes the component to stand up due to micro-explosions in the solder); 2) De-lamination of the substrate (a partial separation of the layers of circuit board material, similar to blisters); and 3) Poor adhesion/contact caused by over-heating and under-heating.
Typically, there are three main methods used for reflow oven heating. They are:
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Vapor Phase
Vapor phase is a simple and reliable method of soldering. Complex components and assemblies can be done in a small process window, but this comes at a cost, so understand the process and what is involved before making a purchase decision. Vapor phase heating is performed in a batch process using a special liquid with a very particular vaporization temperature of 240F, ensuring that materials are never overheated and damaged. Parts are placed in a single chamber above a liquid bath, which is then heated to create a vapor at the perfect melting point of solder. The board is enveloped in the vapor to accomplish the soak and reflow over time, and the chamber temperature is reduced to condense the vapor back into the liquid bath. The parts are then removed and replaced with a new board (or set of boards).
Galden®, a proprietary heat transfer liquid from Solvay, is the most popular material typically used in these processes, and is quite expensive, although most of the material condenses back into the bath at the end of a heating cycle, so it can last quite a while. Vapor phase is a very predictable method of heating, but because it is a very expensive system, it is more appropriate for extremely high-end precision applications such as MIL-spec electronics.
Fig 4: Vapor Phase reflow oven showing board
suspended above heat transfer fluid bed
Pros: Very precise temperature control; eliminates the need for a profile based on product mass or size; less overall programming required since a single recipe fits all.
Cons: Very high cost, up to $100,000 versus a comparable convection oven at $15,000; material very expensive; typically batch process only with slow cycle times (10-20 min/cycle versus 4-5 minutes for convection).
Infrared (IR)
IR systems were widely used throughout the s because they were relatively inexpensive to buy and operate. IR heating can be used in either batch or continuous conveyorized units. While still available for specialized applications, IR heating is affected by the color of the target components, so different colored components absorb heat at different rates and can produce different heating profiles, not entirely desirable.
Today, IR heating is often used for special applications where heat ramp rates need to go quickly with very little soak time. The direct heating technique of IR makes modulation and control very difficult. Factors that can be used to vary the heating profile include distance, speed and heat intensity. For these reasons, IR is not often used in low- to medium-volume production environments which need to accommodate a wide range of board configurations and heat profiles.
Pros: Appropriate for specific applications requiring direct heating, but no longer widely used in circuit board assembly.
Cons: Very unforgiving heat; boards may be easily damaged by overheating, delaminating, tombstoning, etc.; heat profiles difficult to set based on material type and color; trials are usually necessary, making it impractical for short runs.
Convection
Convection heating is the most widely used method for reflowing circuit boards today. In this system, air is circulated throughout one or more chambers (or zones), either vertically or horizontally, to surround the entire board assembly with even, uniform heat. Each zone in a multi-zone oven retains its own heat profile very reliably. Both the vertical and horizontal methods of heat distribution share the same end result – even heat around the boards.
A plenum heat distribution system typically directs air vertically from above and below toward the board surfaces. They often require preheating plenums to condition the air temperature before heating the board, but directing airflow from two different directions (top & bottom) allows the user to adjust heat settings independently and with good control to create a very precise profile. This type of system is usually more complicated to build and thus more expensive than a horizontal convection type.
Pros: Even heat distribution; individual zone temperatures are extremely close to product board temperatures, making it easier to profile than IR; less trial and error testing needed
Cons: Certain methods of air circulation may not be as effective or easy to manage as horizontal convection
In a horizontal convection chamber, heat is uniformly distributed throughout the entire chamber, resulting in even heat distribution around the board itself. The length of each zone and the conveyor speed determine the amount of time a board resides in a particular heating environment, ensuring very reliable and repeatable processing quality. As opposed to air circulation moving vertically above and below the board, in horizontal reflows the air is circulated in one direction across the top of the board, and in the opposite direction beneath the board. This is a key benefit that prevents hot spots, and allows the parallel airstream ‘angle of attack’ to infiltrate the spaces beneath the components.
Pros: Even heat distribution, as the top & bottom of the board receive the air from the outside to the center – which counters the common ‘center hot spotting’ sometimes present in traditional reflow ovens; individual zone temperatures are extremely close to product board temperatures, making it easier to profile. Lower overall equipment cost, since a plenum is not required.
Methods of board transportation
There are two common techniques used for conveying boards through a multi-zone oven:
A conveyorized mesh belt, typically stainless steel or other non-corrosive alloy, is the most common method. Air is able to flow through the belt for even heating on the top and bottom of the board.
The pin or edge-type conveyor carries the board by the edges in a continuous line, allowing unobstructed airflow around the board. This method can be integrated within a modular in-line system with other assembly machines.
Either method works equally well for single- or double-sided boards. Edge conveyors can add some cost that may not be necessary. For integration with other in-line systems, consult SMEMA (Surface Mount Equipment Manufacturers Association), which writes connection standards for modular circuit board assembly integration.
Inerting Systems
If an inerting system is advised for a specific product application, Nitrogen is the most common gas used for reducing or eliminating oxidation during reflow. However, in most low- to medium-volume assembly environments, inerting is typically not needed. When specified, an inerting system can add approximately 10% more to the cost of a basic reflow oven, and most manufacturers offer it.
Methods of control
Any reflow oven, regardless of the type and number of heating zones, needs a method to control the temperature and recipe profile of the product being processed. These can either be built-in to the oven (i.e., on-board) or PC-based and connected via a communications cable. As with everything else, there are advantages and disadvantages to each type.
On-board control
On-board control means that the oven has a stand-alone controller built in to the system. It requires no PC or external source of information to create recipes to run the process. It can be controlled via a simple user interface displaying input prompts and readouts. In most cases, the interface is fairly basic and generally rather small.
Pros: Very good functional performance; generally more affordable than external PC-based system.
Cons: Some restrictions in storage capacities for recipes; software updates can be more limiting due to the machine’s limited hardware capacity; user interface is typically minimal, displaying the bare essentials of inputs and outputs.
PC-based Control
In this scenario, the oven plugs into a PC or laptop which hosts the software via a communications cable. (It is not common to see wireless connections for this equipment. Hard-wired connections are considered more stable.)
Pros: Software updates are more frequent, allowing on-going usability enhancements offered by the manufacturer; A PC interface delivers a full screen color graphical user interface for feature-rich visual presentation of data; user can see profile graphs not available on most on-board systems. Users can purchase their own computers with all the capacity they may need now and in the future to accommodate a much greater number of recipes/profiles than would be available on an on-board system.
Cons: Can add $500 - $ in cost versus an on-board control system.
Product Profiling
The product profiling process determines and/or validates the actual oven settings in the controller, regardless of whether it’s on-board or PC-driven. Profiling kits are available from most manufacturers and include thermocouples, all accessories to attach the probes to the board for use during the process, and a how-to guide with instructions.
Profiling is done using one or more thermal probes (thermocouples) attached to the product and running it through the entire process to validate or modify a recipe based on the solder past or product requirements. Multiple probes are desirable for profiling to ensure that the board surface temperatures from one side to the other don’t vary more than 2-4 C.
An optimal reflow profile is one of the most critical factors in achieving quality solder joints on a printed circuit board (PCB) assembly with surface mount components. A profile is a function of temperatures applied to the assembly over time. When graphed, a curve is formed that represents the temperature at a specific point on the PCB, at any given time, throughout the reflow process.
Fig 8: Sample profiling kit including probes,
accessories and user guide
On-board profiling
Profiling can be done with some on-board control systems; however, they are limited in visual representation by size and configuration of the display, and they usually don’t have a utility to print or save and retrieve a file off-line, which is necessary for ISO and other quality assurance documentation. So, if you are considering an on-board control, make sure it has the option available to export/output data for QA documentation if you think you will need it in the future.
PC-drive profiling
A PC-driven profiler will provide a feature-rich user interface to enhance the operator’s use. The Windows™ operating system is familiar to everyone, and is very intuitive for graphical presentations, seamless printing and file saving functions to ensure proper record-keeping.
Third-party profiling
A profiler purchased from a third party provider typically combines more enhancements than either on-board or PC-driven systems have available. Profiler devices are offered by a number of manufacturers and can be found easily using a search for “reflow oven profile manufacturer.”
How to select a Reflow Oven 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.
Whether you're an electronics hobbyist repairing old circuit boards or an engineer working on next-generation prototypes, reflow soldering is an essential process in your toolkit. Using controlled heat to "reflow" molten solder allows for the precise simultaneous attachment of numerous tiny surface-mount components with unmatched efficiency.
Reflow soldering has revolutionized electronic manufacturing, enabling the miniaturization and complexity we now take for granted in our smartphones, laptops, and countless other devices. Gone are the days of painstaking manual soldering under a microscope. Today's state-of-the-art surface-mount technology relies on reflow's ability to seamlessly attach minuscule components mere millimeters in size.
Have you ever wondered how reflow soldering is done? What equipment is required? What processes ensure uniform heating and reliable connections?
In this guide, we'll explain everything you need to know to successfully use reflow soldering in your work.
Reflow soldering is one of the major manufacturing processes used in the surface mount technology (SMT) production of PCBs. It is a procedure for soldering the electronic components (such as ICs, resistors, and capacitors) to the conductive pads of a PCB.
In reflow soldering, a thin layer of solder paste is screen-printed to the PCB pads with a stencil. This solder paste contains a mixture of solder spheres and flux which hold the spheres together. The surface mount components are next aligned onto the pads by use of the pick and place machine. Components such as the board are then transported to the reflow oven where they are heated under well-defined thermal profiles.
Moving on, in the reflow soldering process, the temperature steadily increases within the reflow oven, passing through distinct temperature zones tailored for various component and soldering requirements. In the reflow stage, the solder paste melts and the flux helps with the process by removing oxidation from the pads and component leads. It gives the solder the necessary time to wet the molten solder without bridging between adjacent pads. When the board cools down, the solder becomes hard and it forms a permanent physical and electrical connection between components and the printed circuit board.
Fusion soldering is the most common method employed in mass production, thanks to its high productivity and the high quality of solder joints. This is the ideal design for SMT boards with dense packages and miniature components. The ovens with reflow technology ensure steady and constant heat distribution for the large boards, thus preventing defects caused by temperature variations.
Correct control of time and temperature profiles is a must for solder joints to be generated properly as well as for the unharmed temperature-sensitive components of the device. It is in this sense that reflow soldering is becoming a necessity for the current production of sophisticated electronics.
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Reflow soldering involves several critical preparation and assembly steps to ensure precise placement and bonding of components. Here is a detailed overview of the steps you need to follow in a reflow soldering station.
The first stage is preparing the board and components for soldering. This involves applying solder paste and positioning the electronic components.
Solder paste is a mix of fine solder powder suspended in a flux vehicle. It is applied to the circuit board pads and lands where solder joints need to form. A solder stencil with precise openings is used so the paste is deposited accurately in just the right amount and location. This helps ensure good wetting and bonding during reflow. Most assembly lines use an automated stencil printer to do this repeatably at high speeds.
The stencil must be customized for the specific PCB design with openings that directly align to the pad locations and sizes. It is fabricated from thin sheets of stainless steel, brass, or polymer materials using laser cutting or etching techniques to achieve the required resolution and printing fidelity.
Several factors influence the stencil printing process and the quality of solder paste deposition:
● Stencil Design: Stencil thickness, opening geometry, land widths, reductions, and bridging elements all impact the solder transfer efficiency and print quality achieved. Thinner stencils allow for smaller deposits but are less durable, while thicker stencils are more robust but limited resolution.
● Solder Paste: The rheology, metal content, and particle size distribution of the paste must be optimized for the intended process. Higher viscosity pastes print well but release poorly from the stencil, while low viscosity pastes tend to bridge between pads more easily. Choosing the right paste for the application and equipment is critical.
● Printing Speed: Moving the squeegee over the stencil too quickly can leave solder paste strands behind or create an uneven deposit. Conversely, printing too slowly wastes time without significant benefit. Equipment and processes are tuned for an optimum speed range.
● Squeegee Angle & Pressure: The squeegee must be set at just the right angle, typically between 15-30 degrees, and apply the proper downward force to shear the paste and clear the stencil openings without disturbing the pads. Too steep an angle or light pressure leads to incomplete prints.
Monitoring and controlling these factors results in consistent placement of precise volumes of solder paste in the targeted locations on the PCB. This provides an ideal foundation for subsequent solder joint formation through reflow.
Once the solder paste is applied, the electronic components like integrated circuits, resistors, and connectors are placed on the board. For small production runs, this is done manually with tweezers or vacuum pick-and-place tools. Larger volumes use high-speed surface mount placement machines that can pick and place thousands of components very quickly.
High-speed contact pickup heads rely on vacuum or capillary action to securely grip components without damage. Modern reflow soldering machines can place thousands of parts per hour with placement accuracy within +/-50 microns or better.
Component recognition and orientation are crucial initial steps. Most machines employ overhead vision systems and component data files to scan, identify, and correctly rotate parts for optimum placement onto the board. Any misplaced components could cause defects or reduced yields.
Key factors that contribute to the efficiency and accuracy of the pick and place process include:
● Placement Speed: Modern equipment is capable of speeds over 200,000 cph for small components, maximizing throughput. However, speed must be balanced against accuracy demands.
● Machine Accuracy and Repeatability: Placement variability (3sigma) of less than 50 microns laterally and vertically allows for tight manufacturing tolerances and minimal rework. Accuracy is maintained across the full component range and operating envelope.
● Feeder Capacity and Changeover Time: High-capacity tape-and-reel or bulk feeders optimize runtime before replenishment is required. Fast, simple feeder changes when needed minimize downtime spent loading new parts.
The components self-align on the wet solder paste as they are gently pressed into place. Finally, any through-hole components like connectors are manually inserted and their wires soldered on the opposite side.
Now it's time for the magic of reflow soldering to occur. The prepared board enters the reflow oven for precise heating. During this stage, two processes take place.
Circuit boards are conveyed into the reflow oven on a stainless steel belt or conveyor. Inside, they pass through multiple heated zones including top and bottom heat sources. Infrared lamps, hot air impingement jets, and heated surfaces work together to apply exactly the right amount and distribution of heat. Temperature is carefully monitored and controlled with thermocouples, ensuring each board experiences the same thermal profile.
As the board on the reflow soldering conveyor warms up according to the thermal profile, the solder paste gradually reaches its melting point. Flux activators release gases that help clear away oxides on the component leads and board surfaces.
Reflow soldering allows molten solder to wet these clean metal surfaces, forming a metallurgical bond when cooled. All of this occurs seamlessly within a few minutes in the reflow oven, permanently joining the components in their places. The oven exhaust filters capture any smoke or fumes generated during the process.
The result is durable, high-quality solder joints that provide mechanical mounting and electrical connections. Surface mount components with their tiny termination widths are reliably assembled this way.
A key part of reflow soldering is managing the temperature rise accurately. Component damage must be prevented while ensuring optimal soldering conditions.
Boards enter the reflow soldering toaster oven at room temperature and are slowly warmed through multiple thermal zones. Infrared heaters and air impingement jets heat the board and components gradually from all sides. This avoids any thermally-induced mechanical stresses. A gentle ramp rate of around 1-3°C/second is typical.
Each component has a maximum temperature rating that must not be exceeded. More heat-sensitive parts like oscillators, crystal filters, and sensors require even lower temperatures. The reflow profile closely follows the thermal requirements of the most temperature-sensitive component used. Multiple thermocouples accurately measure and control the temperature at different points, ensuring uniform and safe heating.
Most reflow profiles involve four distinct stages of heating to prepare, activate, and finally melt the solder. These are the different stages that it involves.
The ramp to soak zone is the initial heating stage in reflow soldering. In this stage, the temperature of the PCB assembly is gradually increased in a controlled manner. The ramp rate, which is the rate of temperature increase, is typically between 1-5°C/second. A slower ramp rate helps ensure even and consistent warming of the entire board and its components to prevent issues like thermal stresses.
As the temperature starts to rise during ramping, the volatile organic compounds (VOCs) in the solder paste begin to evaporate out. Solder pastes contain solvents that keep the solder powder in a viscous, paste-like form suitable for printing or dispensing. These solvents need to fully evaporate before reflow to form a quality joint. If any solvent is left behind in the joint, it can cause defects like solder balling or joint voids.
The goal of the soak zone is to bring the entire assembly up to a consistent pre-heat temperature before continuing to the next stage. The typical pre-heat temperature range is 150-160°C for most alloys. Holding at this temperature for 1-3 minutes allows residual solvent evaporation to complete and prevents defects caused by uneven heating of components. It also pre-heats the assembly to facilitate rapid, even heating in subsequent stages.
Controlling the temperature and duration precisely is critical. Too high a temperature or long soak can cause defects like joint embrittlement or damage components that can only withstand lower temperatures. Too low/short and solvents may remain trapped. Proper profiling is determined based on the specific solder paste and assembly.
The reflow zone is the primary phase where the solder is melted. In this zone, the temperature is ramped higher than in previous stages to surpass the liquidus temperature of the solder.
The liquidus temperature is the point at which the solder first starts to melt, and is typically 30-50°C below the melting point. Most Sn-Pb and Sn-Ag-Cu alloy solders have liquidus points between 180-200°C.
The peak temperature is the maximum endured during reflow. For leaded solders, this is typically 20-40°C higher than the liquidus. Lead-free solders require higher peaks of 5-10°C above their significantly higher melting points.
Holding briefly at the peak ensures full wetting and flow of molten solder before cooling. The ideal peak time is typically 15-60 seconds depending on assembly size, density, and alloy used. Too short and solder may not fully melt and flow, while too long risks component damage from overheating.
During reflow, the molten solder wets and flows around the component terminals, joining them firmly to the PCB pads below. Simultaneously, flux activation helps remove any oxidation to ensure clean, void-free joints. Precise temperature control and profiles are vital for optimal solder wetting and flow without damage.
After the peak temperature holds, the assembly enters the cooling zone. In this final stage, controlled cooling brings the temperature back down in a controlled manner. The cooling rate is just as important as the heating rate to affect joint quality.
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Gradual cooling prevents defects caused by rapid thermal shock like internal joint cracks or component cracks. The ideal cooling rate is typically 1.5-6°C/second, depending on assembly size and alloy characteristics. Slower rates permit a finer-grained joint microstructure for better mechanical integrity.
The cooling zone temperature ramp continues until ambient assembly temperature is reached, usually below 100°C. At this point, the reflow cycle is complete and heat treatment of the solder joints is finished. They should be able to withstand normal operational stresses and thermal cycling loads.
Reflow soldering offers many benefits over other soldering methods. The key advantages of reflow soldering include:
Automation and Consistency:Reflow soldering is a fully automated process that can consistently place and solder surface mount components on a printed circuit board. This high level of automation and consistency reduces defects and improves yields. Having a repeatable process ensures the quality and reliability of the solder joints.
Density and Miniaturization:Surface mount technology enables higher-density circuit boards by allowing for smaller components and tighter spacing between them. Reflow soldering is necessary to place and solder these ultra-fine pitch components. This density and miniaturization have allowed for tremendous reductions in the size of electronics over the past few decades.
Mass Production: The automated nature of reflow soldering makes it well-suited for high-volume, mass production applications. A single reflow oven can process dozens or even hundreds of circuit boards per hour. This high throughput enables the economical production of electronics in large quantities. The automated process is also less labor-intensive than hand soldering.
Low Thermal Stress: During reflow soldering, components are secured to the board before heating. This allows all parts to be heated up and cooled down gradually and uniformly. In contrast, hand soldering risks applying localized heat repeatedly to components, which can cause thermal fatigue and potential failure over time. Reflow soldering induces less thermal stress on components and connectors.
Process Control and Optimization: Modern reflow ovens provide precise control over the temperature profile components are subjected to. The ability to carefully control conveyor speed, heating zone temperatures, cooling rates, and more allows the process to be optimized for different board designs and component mixes. Process tuning can ensure defects are minimized cost-effectively.
When assembling electronic circuit boards, the two main processes used historically have been reflow soldering and wave soldering. Here is a comparison of these techniques on several key aspects to break down their differences and suitability for various applications.
Reflow soldering has become the dominant process for electronics assembly today due to its flexibility in handling higher density and miniaturized surface mount components. The process is very clean with controlled heating and cooling profiles ensuring high-quality, consistent solder joints even on complex multi-layer boards.
However, the upfront costs of reflow ovens and inspection equipment are higher. Wave soldering remains suitable only for lower-density through-hole designs and offers a cheaper capital outlay though with less control and more potential defects in the soldering compared to reflow.
While reflow soldering offers great advantages in electronics manufacturing, it also presents some challenges that can compromise quality if not properly addressed.
Here are several common issues encountered during reflow and suitable countermeasures to ensure a robust process.
Solder bead formation, also called solder spattering or splattering, refers to the undesirable scattering of small solder balls and droplets across the PCB during reflow. There are a few key causes for this defect.
First, excessive solder paste volumes can lead to overflow during melting, causing the solder to fling off components and form beads. Improper stencil printing with inconsistent or oversized apertures can also deposit too much paste. Additionally, an overly aggressive reflow profile with too steep of a rise to peak temperature can cause rapid solder paste explosion.
To combat solder bead formation, firstly optimizing the stencil design and solder paste printing is critical. Ensuring uniform, controlled deposition minimizes excess paste. The reflow profile should then provide a gentle rate of rise to avoid shocking the paste. A longer soak time just below the melting point allows any outgassing to occur gradually.
Some solder pastes also contain additives that reduce spattering through controlled off-gassing. Regular stencil cleaning prevents buildup which could disturb paste release. With proper process tuning, solder bead formation during reflow can be minimized.
Tombstoning occurs when a surface mount component lifts from the PCB during reflow due to uneven wetting forces. Reasons for uneven wetting include misaligned or tilted components, non-uniform pad metallization, and asymmetric part/board geometries which provide more solderable areas on one side. The paste underneath is then drawn up by surface tension.
To combat tombstoning, first ensure accurate, repeatable placement of symmetrical components onto well-designed mating pads. Adjusting pad dimensions or adding corner pads can promote balanced soldering where needed. Finer pitch pastes provide more controlled flows compared to chunkier variants. Carefully formulated no-clean and low-residue fluxes maximize wettability on challenging surfaces too.
A precise reflow profile with a long thermal dwell near the solder melting point also helps; this allows any partial tilting to self-correct before solidification. Post-reflow inspection catches residual tombstoning for rework. With these combined measures, component lifting defects can be effectively mitigated.
A solder joint that appears partially or completely absent after reflow signifies a missed connection. Common contributing factors are insufficient solder paste deposition or solderability issues. In terms of the former, reasons involve an underfilled or misaligned stencil aperture, depleted solder paste reservoirs during long print runs, or worn/damaged printing rubber squeegees.
Solutions here entail careful stencil and solder paste management practices. Regular printer/stencil maintenance and meticulous print parameter controls ensure consistent solder volumes transfer onto pads acceptably. It also helps to select pastes suited for longer printing intervals between refills/cleanings.
As for solderability issues, common remedies involve cleaning flux residuals or contaminants from the board, improving pad plating quality/coverage, and applying optimized profile dwell times over critical melting ranges. Often the root cause stems from multiple minor variables combining; their optimization creates a robust "failure-proofed" soldering process.
Like solder beads, solder balls are undesirable clumps that form during reflow rather than wetting properly. Flux chemistry imbalance primarily causes this when overly active types release excessive gasses upon heating. Other contributing factors are contaminated/oxidized solder paste or component/board surfaces lacking proper wettability treatment.
Good flux management plays a key role in solutions. Carefully selecting a type with controlled activity and optimum tackiness for the solder alloy minimizes off-gassing issues. Thorough cleaning removes residues that could foul wetting reactions. Ensuring solder paste freshness through controlled storage and use also prevents oxidization buildup. Gentle heating profiles provide a gradual gas escape to inhibit spitting/splatter.
Finally, confirming ideal surface finishes on board landings and component leads fosters reliable solder wetting every time. With minor adjustments to consumables and process tweaks, solder ball defects can largely be overcome.
Localized component overheating during reflow's peak temperature dwell is a potential cause for melting/scorching of sensitive plastic packages or printed markings. Typical reasons are uneven board heating, insufficient air circulation/convection, and inaccurate reflow oven zone calibration. Failure to adequately preheat before the spike can also cause shocking thermal stresses.
A well-designed reflow profile and high-quality oven are key to prevention. Sufficient preheating brings all assembly materials up to target temp in a controlled manner. Gentle temperature ramps and a profiling system ensure ideal thermal uniformity across zones delivering distributed, consistent heat treatment.
Where possible components most vulnerable to distortion or discoloring can have their orientation optimized to heat more gradually. Attention to reflow soldering toaster oven maintenance and periodic profiling validates zone performance over time too. These measures help eliminate hot/cold spots protecting components from reflow hazards.
Where molten solder fails to properly flow and wet the pads/terminations of a joint during reflow, incomplete soldering arises. Common root causes are inadequate solder paste deposition, flux activity issues, component/pad geometries inhibiting flow, and non-ideal temperature profiles.
Best practices here involve using well-designed, precision-tuned stencil printing optimized for each application and paste type. Flux formulations with suitable surface-cleaning properties promote proper metallurgical bonding. Component self-alignment features assist self-centering on pads for uniform solder-wetting angles.
Reflow profiles deliver appropriate heat soaking above solder melting temperatures, with suitably long heat/mass transfer times for through-curing. In some cases, paste additions like tackifiers can improve spreading and anchoring to avoid shorts. Overall, attention across all reflow-related variables helps ensure issue-free, robust solder joints form every time.
Quality inspection is a crucial step in any reflow soldering process to ensure solder joints meet specifications and electronic assemblies are free of defects. By implementing thorough inspection and quality assurance protocols, manufacturers can identify potential issues early, drive process improvements, and help reduce costs associated with rework and component failures.
Here are various inspection techniques used in reflow soldering and strategies for establishing an effective quality assurance program.
Visual inspection is typically the first stage of quality control in any reflow soldering process. Operators carefully examine solder joints and surrounding areas under magnification to identify common defects like solder bridges, insufficient solder, misaligned components, and more. Manual inspection allows for human judgment but can be time-consuming and subjective.
Many companies supplement manual inspection with automated optical inspection (AOI) systems. AOI uses high-resolution cameras and software to capture and analyze images of solder joints. The software compares joints to design criteria to find anomalies.
The AOI systems typically consist of several main parts including the following;
High-resolution cameras: Optical inspection systems utilize one or several cameras to take close-up pictures of the printed circuit board (PCB). Different angles can be chosen for the camera placement depending on the system configuration. The board can be viewed from several perspectives, thus increasing the chances of finding defects.
Lighting: Consistent and stable lighting is one of the major conditions for the correct image capturing. For example, the AOI systems might be based on several sources of light with various wavelengths and angles, which create the needed contrast and shadow minimization.
Image processing software: The software works with the images that have been captured by comparing them to the reference images or design data to verify defects. The latest AOI systems are based on the algorithms of machine learning that help to make the inspection process more precise and allow to adapt to the component appearance variations and the quality of solder joints.
AOI improves speed, accuracy, and repeatability over manual inspection. However, like the human eye, AOI cannot see inside components or inspect hidden joints.
Defects identified during visual inspection may include:
● Solder bridges: Unintentional connections between joints
● Insufficient/excessive solder: Weak or short-circuited joints
● Component misalignment: Poor electrical connections
● Missing/incorrect components: Potential functionality issues
X-ray inspection complements visual techniques by allowing inspection of hidden solder joints underneath BGA and QFP packages. An X-ray system passes beams through components to generate radiographic images of internal solder joints. Inspectors or software then analyze images for voids, cracks, bridging, and other defects not visible externally.
While powerful, X-ray also has limitations. False positives can occur, and differentiating similar-density materials like solder and flux can be challenging. Correlating X-ray results with other techniques helps minimize interpretation errors. X-ray also exposes operators to radiation, requiring appropriate safety protocols.
Typical defects identified include:
● Voids - Air pockets weakening joint integrity
● Bridging beneath components
● Insufficient/excessive solder under devices
For exceptionally dense packages, manufacturers may perform a cut-away inspection. A sampling of components undergoes sectioning using techniques like focused ion beam milling. This exposes internal joints for high-magnification optical or scanning electron microscope examination. Cut-away is highly effective but destructive, so only a sample receives this treatment.
Beyond physical inspection, functional testing evaluates assemblies for electrical defects. Methods like in-circuit testing, flying probe testing, and functional testing help identify failures like intermittent joints or cold solder defects that show no visual anomalies. Manufacturers must balance physical and functional testing based on their unique product tolerances and test capabilities.
To maximize inspection effectiveness, companies integrate inspection activities into a comprehensive quality assurance program. Key elements of such a program include:
● Establishing acceptance criteria for visual, X-ray, and functional testing based on product requirements and industry standards.
● Developing sampling plans to statistically inspect products while minimizing costs. Destructive techniques only sample a percentage.
● Creating inspection documents and checklists to standardize inspections and enable pass/fail tracking.
● Training operators on inspection procedures, acceptance criteria, and defect-recognition. Formal certification ensures consistent accuracy.
● Calibrating inspection tools on a schedule and if system specifications change. Calibration maintains test reliability over time.
● Investigating failures from field returns to enhance inspection criteria. Feedback leads to continuous improvement.
● Implementing statistical process control to monitor reflow quality over time and catch shifts before extensive failures occur.
A well-designed quality assurance program coordinated across inspection, assembly, and testing establishes accountability while driving ongoing process enhancement. Reliable inspection techniques and clear pass/fail criteria give manufacturers ongoing confidence in their soldering quality and product reliability.
The reflow soldering process has dramatically changed the way of assembling circuit boards via its unique attributes of efficiency, accuracy, and reliability. Through the use of precise temperature control, dwell times, and conveyor speeds, manufacturers can reach high yields and densities on their SMT lines. The smaller the components are reduced and the newer the variants like BGAs are introduced, the reflow soldering will always be the mainstay in the electronics industry.
Even though the China reflow soldering may seem complicated, the engineers of PCBasic have fully mastered the process via thousands of production runs. Through the implementation of our MES system, we control every single variable that could be a stressor for the most complicated designs.
Let your next prototype or low-volume production project's reflow be taken care of by our experts at PCBasic. Contact us today for more information on how to start your next electronic product with our wholesale reflow oven soldering service.
Reflow soldering is a process where solder paste is applied to circuit boards using a stencil or screen printing. The boards are then heated up to melt the solder and form electrical connections between the components and the board. It's commonly used for surface mount technology (SMT) components which have leads that are placed directly onto the surface of printed circuit boards. Reflow soldering allows for much higher-density packaging compared to through-hole components.
In reflow soldering, solder paste is first applied to a printed circuit board using a screen or stencil. This ensures the paste is deposited in the right locations. Components are then placed onto the paste. The board then goes through an oven or chamber that exposes it to carefully controlled heat. As the board heats up, the solder paste first goes through a "reflow" stage where it melts and forms preliminary connections. Once cooled, strong solder joints are formed between the components and board. Proper cooling is important to avoid defects. The finished boards then go through quality checks.
There are a few main types of equipment used: reflow ovens, convection reflow ovens, and inline reflow soldering systems. Reflow ovens provide controlled heat exposure but require loading/unloading each board. Convection ovens offer a continuous conveyor belt for high-volume production. Inline systems integrate component placement, soldering, inspection, and more for fully automated assembly lines. Quartz/infrared heaters and hot air are common heating methods. Temperature profiling and monitoring ensure consistent results. Choosing the right equipment depends on your specific production needs and volume.
The most basic profile types are single-stage (simplified single peak), two-stage (lower preheat then higher reflow peak), and multi-stage (multiple preheat and reflow stages). The key stages are preheating, soaking, reflow, and cool down. Variables like peak temperature, time above liquidus, ramp rates, and cooling ramps differ. Nitrogen is often used for faster cooling. Choosing a profile depends on factors like component sizes/density, solder paste type, and board assembly. Standard profiles from solder manufacturers are a good starting point but optimization may be needed.
Here are a few tips to help ensure successful reflow soldering results: Use the correct solder paste for your process, thoroughly clean circuit boards before assembly, avoid leaving components partially soldered, allow proper preheat/soak times, control and monitor temperatures closely, minimize air exposure during reflow, allow full cooling cycles, perform solder joint and manufacturing inspections, and keep equipment well maintained. Proper technique, settings validation, and quality checks can maximize yields and help troubleshoot any issues that do come up.
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