Vapor phase reflow

The soldering process has always presented challenges in large scale PCB applications. Perhaps the largest challenge is the risk that the high temperatures of tooling and soldering could cause damage to the structure of different components being mounted to the board or even the board itself. Many companies are moving to a leadless solder base, which means that even higher temperatures will now be required to melt the solder. This only increases the risk that during assembly there will be higher occurrences of heat-based damage and failure on the electronic units. 
Currently, one of the best methods of mitigating the risk of heat damage is to use a system called vapor phase soldering. This provides distinct advantages over traditional IR reflow or laser methodologies since issues with board layout, topography and temperature fluctuations are virtually eliminated. 

Vapor phase soldering is quite simple in concept. In most cases it relies on a three-layer chamber to create all the components and conditions conducive to a successful solder: 
1.  Base liquid layer The base liquid layer is the chemical chosen to provide the vapor necessary for the soldering to take place. The liquids are chosen based on many factors, most importantly their boiling temperatures, environmental impact and corrosiveness of the produced vapor. The temperature at which the liquid boils is the temperature at which the solder on the PCB will be immersed, so there is an exact and predefined value. Pressure can be manipulated within the vapor phase chamber which can change the boiling temperature, but it can also be operated at ambient air pressure. Therefore, it is absolutely essential the correct chemical composition is used so the components are not damaged by high temperatures or the solder is not melted enough at the low end. Another important feature of the liquid base is the non-reactive aspect of the vapor produced. It is important that the vapor be inert, so there is not a negative reaction with the boards being soldered. This helps to prevent oxidation and mitigates failed or defective joints.

1.  Vapor layer Whereas the liquid layer solely exists to provide an appropriate vapor, the vapor layer is where the action happens in vapor phase soldering units. There are distinct advantages of the vapor layer which is why this technique is becoming more popular in the industry today. Because the vapor is the result of a boiling liquid, it is always at a constant temperature, the exact temperature of the liquid. This is true even if the heating element has fluctuations in temperature, which would only result in more vapor being produced while avoiding an increase in temperature. Boiling liquids are very effective thermostats that have little to no meaningful variation. Vapor produced is also designed to be far more dense than air. This means that when the liquid is boiling, the vapor will form right above it and will not escape or float away from the surface. Cooling coils are also added to the chamber to ensure the vapor does not escape. It also displaces any other gaseous materials in the way and forms a clean, solid vaporous layer which is ready for soldering. This means there is an inert gas at a constant, predefined temperature. There is no air to oxidize any board elements, and there is an encompassing vapor which can surround and uniformly heat a unit regardless of shape or design. 

2.  Preheat zone In order to prevent various soldering defects as well as to provide a more solid and conductive joint, PCBs are preheated in most soldering applications today. For vapor phase, this is usually done by hovering the boards directly above the vapor layer in the phase chamber for a predetermined amount of time in order to sufficiently heat the solder paste and the components. The methods for doing this can vary from machine to machine. In many machines, there is a secondary vapor phase which is produced at a cooler temperature than the main vapor layer. No soldering takes place here, only a temperature raise. Other machines rely on IR preheating or air temps above the vapor to heat the element and prepare for soldering. In any case, there must be some board temperature conditioning before entering a temperature at which the solder liquefies. 
The actual soldering process on the board as it enters the vapor level begins with a condensing of the vapor onto the board, which induces a heat transfer between the vapor and the entire board structure, including the solder. The heat transfer is very uniform and the risk of overheating is virtually nil. The heat transfer rate can be controlled by the heating elements under the liquid base, which can increase the vapor generation rate but will not increase the actually temperature. As the board is completely immersed in the vapor it continues to condense until the PCB is at the same temperature as the vapor. This happens much more quickly than IR or air heated boards due to the excellent heat transfer capabilities of the vapors in use today. After a time, the board is removed from the vapor level, and the condensed chemicals evaporate off the still-heated components, leaving a residue-free and fully soldered unit.

With improvements to the quality and process of vapor phase soldering, along with the continued push to partially or fully leadless components in SMT applications, vapor phase is becoming a more popular alternative. It boasts excellent specs in soldering time, energy consumption and the risk of overheating drastically, all of which are very appealing to industries all over the spectrum.