Over the years, large scale PCB applications have faced significant challenges due to the soldering process. One of the top difficulties PCB designers and PCB manufacturers encounter has to do with the high temperatures of PCB welding. Such temperatures might damage the board's structure or the various components.
In this comprehensive guide, we tell you all you need to know about the vapor phase reflow.
1. What is Vapor Phase Reflux?
Vapor phase reflow refers to an advanced PCB soldering technology that uses an inert liquid with a high boiling point. This technology has, in recent years, gained massive popularity among large scale PCB assemblers.
To solder electronic components to PCBs, manufacturers usually need high temperatures. It’s a complex process that has become even more challenging as electronic components become smaller.
Vapor phase reflow soldering ensures that the PCB board and its members don't face temperatures higher than necessary. For that reason, parts don't overheat. This soldering process provides the ideal wetting of the details. Moreover, the entire soldering process takes place within an inert atmosphere. In the end, PCB assemblers get the highest possible PCB soldering results.
Automated soldering machines are the norm in vapor phase reflow.
2. Why use Vapor Phase Reflux?
Vapor phase reflow if quickly replacing other soldering technologies in the market. There are many reasons for this development. We look at two of them in this section.
2.1 The Welding Problems Faced
The vast majority of manufacturers now opt for leadless solder bases. These bases need manufacturers to use even higher temperatures to melt the solder effectively. As a result, the risk of damage to the electronic units due to overheating is higher.
Vapor phase reflow technology helps counter this problem by virtually eliminating temperature fluctuations, topography, and board layout.
2.2 PCB Circuit Board Components
PCB circuits need require parts of different masses, which makes the soldering process a complicated one. That’s because the heat these components store varies significantly based depending on their groups. The result is that there’s uneven heat distribution, which could lead to warping of the PCBs. To reduce these issues, PCB assemblers and designers opt for vapor phase reflow.
Vapor phase reflow averts problems resulting from varying PCB components.
3. The Working Principle of Vapor Phase Reflux
To understand how vapor phase reflow works, we need to look at three different things:
- Heat adjustment
- Level adjustment
- Void formation
Heat adjustment involves PCB engineers adjusting the temperature gradients in vapor phase reflow equipment. They do so by regulating the power that goes to the equipment’s heating elements. As more power goes to the heaters, there’s more vapor production. It means more heat transfer to the PCB assembly, even though the liquid’s boiling point remains the same.
As we hinted earlier, the vapor phase reflow takes place in an atmosphere that's entire oxygen-free. This atmosphere reduces the likelihood of oxide formation. It also improves wetting.
Usually, there's a slight delay in the creation and the subsistence of vapors. Thus, there's no creation of sophisticated temperature profiles. To solve this challenge, manufacturers developed soft vapor phase (SVP) machines.
One of the top benefits of SVP machines is that they allow you to control immediate temperature gradients. During the SVP process, the PCB's temperature rises as it moves deeper into the vapor blanket.
To achieve the ideal preheating of the PCB board, hold it at a specific depth. Note that as you increase the center, the board will move towards liquidus temperature. Assemblers and designers can create just about any thermal profile they want. They do so by automatically preselecting and controlling the soldering time.
As soon as you're through soldering, the SVP machine will move the board up from the depths of the vapor blanket to the vapor boundary. Here, the board's temperature lowers. You can then lift the board out of the vapor to allow it to cool down.
Usually, the SVP process doesn't need any extra mechanisms to stop overheating. That's because the liquid's boiling point limits the PCB's peak temperature.
The void formation is inevitable with vapor phase reflow. Voids can compromise or diminish both the thermal and electrical conductivity of a solder joint.
So, how do engineers mitigate void formation? One effective way is through a vacuum controlled procedure that outgasses these voids. When you reduce the pressure over a liquid, its boiling point reduces, and vice versa. Using a vacuum can extend the time above liquidus by 30 seconds or so, reducing existing voids and stopping new ones from forming. Of course, you should conduct void reduction when solder joints are liquids.
Applying the paste to PCB boards before the vapor phase reflow process
4. Composition of Vapor Phase Reflux
The vapor reflow process depends on the three chambers. These chambers create the conditions required for successful welding assembly. We discuss these three layers in detail below.
4.1 Basic Fluid Layer
The base liquid layer refers to the chemical that provides the vapor you need for soldering. Without the smoke, the soldering can’t occur. When choosing the ideal base liquid, designers and assemblers use a variety of factors, including:
- The boiling temperature of the liquid
- The liquid’s environmental impact
- How corrosive the vapor is
Remember, you will immerse the PCB board at the temperature that the liquid boils. That’s why you need a predefined value for the boiling temperature of the base fluid. Of course, you can still manipulate the pressure within the phase chamber of the vapor or opt to operate at ambient air pressure.
Be sure to use the correct chemical composition to don’t end up with too high temperatures to destroy components. You also want to make sure that the temperatures aren’t too low to melt the solder enough.
It is also crucial that the resulting vapor is non-reactive. This way, no oxidation can cause defective joints.
4.2 Vapor Layer
While the base liquid's sole purpose is to provide the necessary vapor, it is in the vapor layer where the real action occurs. Because of the need to generate steam in a boiling liquid, so always remain constant. It is the same as the principle of liquid vapor. It remains true even when the temperature of the heating element fluctuates. Sure, the vapor volume may increase due to fluctuation in heating elements' temperature. But the vapor temperature doesn't change.
Generally, boiling liquids have barely noticeable variations, which makes them good thermostats. The boiling liquid’s vapor is a lot denser than air, so it stays just above the liquid instead of escaping or floating away from the liquid's surface. The moisture displaces any gaseous materials to form a clean layer that's ideal for soldering.
4.3. Preheating Zone
Preheating occurs in the vast majority of soldering applications currently. The primary goal is to prevent a variety of soldering defects. Preheating also helps provide joints that are more conducive and solid.
In the vapor phase reflow process, preheating happens by hovering the board above the vapor layer for a certain amount of time. This way, you can heat the solder paste and the PCB components enough.
Preheating methods vary from one machine to another. Some devices have secondary vapor phases where vapors are lower than those in the leading layer. Other devices depend on IR preheating or air temperatures above the vapor layer to heat the element in preparation for soldering.
5. Convection Reflow oven and Vapor Phase Reflow
As technology continues to evolve, the reflow process continues to take a life of its own. Changes in the reflow process are the result of the increasing demand for more accuracy. Tolerances have also been getting tighter. It means that PCB assemblers and designers must do their best to avoid costly mistakes and damages.
In recent years, two leading soldering technologies have become increasingly popular among manufacturers. These are convection reflow and vapor reflow soldering technologies. This section compares these two technologies, so it's easier for you to decide which approach works best for you.
Convection Oven Method
The convection method generally has four main heat phases:
- The preheat phase
- The thermal soak phase
- The reflow phase
- The cooling phase
Each stage makes up an area inside the oven, and the PCB board passes through each location on the conveyor belt. The flow oven has different recirculation zones, allowing you to control the process better.
Because the convection oven process is a part of the entire assembly process, you can achieve greater throughput by integrating it with SMT systems and automating it. The conveyor belt you use is essential to driving PCB boards from one step to the next within the system. You can refine the entire reflow process by merely adjusting the speed of the conveyor belt. It means you don't need to manipulate the heat within different zones.
One of the challenges PCB assemblers may encounter in the convection oven reflow is insufficient wetting of the PCB boards. It is especially the case when there's a dense population of components on the board.
Air convection reflow oven
Vapor Phase Method
Regardless of the solder paste, you utilize, you'll ultimately need to configure both the reflow process and the solder. The aim is to reduce voiding and ensure there's enough wetting. That's why you need to assess the process as a whole instead of looking at each element separately.
In general, the vapor phase reflow process requires more investment than other solutions. These solutions include the convection oven reflow process. This investment includes what you spend on the system initially and the fluids you'll need to invest in during operations.
In vapor phase reflow, you use Perfluoropolyether Fluorinated Fluids or Golden. The liquids' thermal stability makes them suitable for PCB reflows. These liquids also have a wide operating temperature range, which is excellent for the vapor phase reflow process.
The vapor process's reflow process involves heating gold plating. The aim is to provide the necessary temperature to wet the circuit board. This process is especially perfect for PCBs with a wide range of chip sizes. That's because there are minimal temperature fluctuations between components with different thermal mass.
Besides convection oven reflow and vapor phase reflow methods, there's a third reflow technology: infra-red (IR) heating. Manufacturers often use IR soldering for substrates with components on the surface. The process involves the conveying of substrates through a machine that contains a series of heater elements. The elements are usually above and below the substrates moving through the heating compartments.
IR soldering is quite popular among PCB assemblers and designers. That's because the approach is immaculate and environmentally friendly. IR soldering is also contact-free, and it's easy to control the heating power.
Infrared soldering oven
6. Advantages and Disadvantages of Vapor Phase Reflux
Thanks to its many benefits, the vapor phase reflow technology has many proponents among PCB assemblers and designers. But this technology has its drawbacks as well. Let's look at some of the pros and cons of vapor phase reflow and how to know whether it's the ideal solution for your needs.
PCB after the vapor phase reflow process
6.1 Vapor Phase Reflow Advantages
Excellent temperature control
One of the top benefits of using vapor phase reflow technology is that it affords excellent temperature control. At atmospheric pressure, the vapor's temperature is the same as the temperature of the boiling liquid. It means there are minimal temperature fluctuations. It is unlike what happens in other reflow processes such as convection oven and infrared heating. In case you increase the temperature, the rate of vapor production will also increase, but there'll be no change in the weather.
Perform uniform heating
Another considerable advantage of this technology is uniform heating. Factors like size, shape, color, or geometric of PCB components do not affect the vapor phase reflow process. VPR exposes each part of each element to precisely the same temperature. It makes the vapor phase reflow process perfect for reflowing multilayer boards and large boards. VPR is also ideal for panels with variously sized pieces and boards with a dense population.
Higher heat transfer rate
The vapor phase reflow process is that it requires a lower temperature to ensure an ideal solder. Most reflow methods need up to 35°C extra heat for perfect soldering. But VPR needs only 5 to 10°C over the solder paste's melting point to do the same task. That's because of the higher heat transfer rate of the VPR process.
The vapor phase reflow process also offers an atmosphere with no oxygen at all. For this reason, you get superior wetting at no additional cost. There's much lower energy consumption than in the convectional oven reflow process.
With the insulation in VPR, there is a minimal release of heat into the surrounding environment, which means you don't need additional air conditioning.
Vapor phase ovens also tend to be more compact than convection ovens. It is a considerable advantage if saving space is one of your concerns.
More environmentally friendly
The vapor phase reflow process is also one of the cleanest and safest. Initially, many older reflow machines used Freon, which can be volatile. Current VPR machines do not need volatile organic compounds to operate. Fluorinated fluids are inert, nontoxic, noncorrosive, nonflammable, and nonpolluting. These fluids usually evaporate entirely from the board and leave no residue behind. It makes VPR one of the most eco-friendly reflow processes around.
6.2 Disadvantages of Vapor Phase Reflow
The vapor phase reflow process’s main setback is that it is more manual than some other alternatives. Thus, the VPR process is more suitable for batch production, which can impact the potential throughput.
There, of course, vapor phase systems that cater to large-scale productions, but they tend to cost more than most convection ovens. Another drawback of the vapor phase reflow technology is that it can be more time-consuming than other solutions.
6.3 Choose the Right Technology According to Your Needs
So, which reflow method should you choose? Well, that depends on a wide range of factors. These include:
The PCB board's specific design and the components' content will determine which reflow method to use. For instance, if your panel has a dense population of features, then the reflow phase method is more suitable due to the inert atmosphere.
As we already hinted, some VPR solutions are only ideal for batch reflow processes. Convection ovens' design allows them to automate the SMT line, offering a higher throughput seamlessly.
It's important to note that some VPR systems feature designs that allow for more increased automation. This quality enables them to greater throughput, making them perfect for large scale applications. Note that this technology may need a significant amount of investment.
How vast is the range of PCB boards you manufacture? Sometimes, you intend to manufacture many different panels. If you use the convection oven process, know that you'll need to allow for the different zones to reset from time to time. In turn, this impacts turnaround times.
For what purpose do you intend to use the PCBs? If the PCBs are for highly sensitive applications, use VPR. In these applications, such as medical, aerospace, or medical equipment, you want to opt for a reflow process that guarantees the best possible results. In the vapor phase reflow method, there are more consistency and less likelihood of overheating the components.
Technological process of soldering and assembly chip components on the PCB board
7. Composition of Vapor Phase Reflux
You already know that the VPR process relies on three distinct layers that provide the ideal conditions for top-notch welding. These three layers are the basic fluid layer, the vapor layer, and the preheating zone.
In the basic fluid layer is the chemical that you heat to provide the necessary vapor you'll need for welding. For satisfactory results, it's best to choose a liquid that has the right qualities. We've already outlined some of these features.
The vapor layer handles the actual soldering process. The good thing with the vapor layer is that there's no temperature fluctuation. It means you don't need to worry about the overheating of components.
The preheating layer helps avert soldering defects. The actual preheating occurs when you hover the PCB directly above the vapor layer. Keep in mind that there's a variety of preheating techniques based on the machine you're using.
In a world where top-notch quality is a must, vapor phase reflow technology has proven revolutionary. This process is a viable alternative to other popular soldering technologies.
The introduction of perfluoropolyether fluid has, in particular, made the VPR process effective. PCB designers and PCB assemblers who opt for vapor phase reflow solutions can expect many benefits. These include better solder wetting, lower peak temperatures, and an oxygen-free atmosphere. In the end, VPR results in top-quality soldering results.
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