Rush PCB Inc. is always on the lookout for advanced technologies that cut down on the time for assembly, while improving the quality of the PCBs. One of the bottlenecks in the assembly line was the Infrared or IR reflow, which involved profiling each PCB type and consumed a huge amount of time in the process.
In our search for improving the efficacy of the SMT assembly process, we came across the Vapor Phase Reflow or VPR oven, and an investment in these systems has proven to be successful in eliminating the hold-ups. Here, we compare the two methods of soldering.
Soldering a PCB assembly using the IR reflow process requires generating its unique reflow profile for uniform soldering across the board. This is because the Hot Air Convection Reflow or HACR process mainly uses hot air, and hot air convection is not always a suitable method for the soldering process.
The thermal mass of the PCB and components on it strongly influence the hot air convection reflow profile. An additional factor influencing the profile is the color of the components, as dark colored components absorb heat more than those with reflective or lighter coloring. These factors change the rate of transfer of heat from the hot air as it moves through the oven.
Profiling is the measurement of temperature change in the oven using thermocouples on a test PCB assembly as it travels through the oven. There are two drawbacks to this technique. The first drawback is, the activity takes time, as it is necessary to achieve the unique temperature profile for proper soldering across the board. The second drawback is, one requires several test PCB assemblies, and the customer may not be willing to provide them, especially for a small run.
Contrary to the HACR process that uses hot air to transfer heat to the PCB and its components as it travels through the system on a conveyor belt, vapor phase ovens use a vapor layer, which transfers its latent heat as it condenses. Inside the system, boiling a liquid at 235 °C creates a layer of vapor, and the transfer process of latent heat from the vapor is highly efficient—components of different thermal mass acquire the same temperature at the same time.
Early models of the VPR oven were not popular as much of the vapor escaped, making them unfriendly to the environment and to the user. Moreover, the reflow profile depended on the amount of fluid in the tank, and the temperature range rather limited. Passing the PCB assembly through the oven required manual handling, so it was possible to solder only small batches at a time.
The design of modern VPR ovens is entirely different, with the enclosure recovering almost the entire vapor medium. Manufacturers have added automated handling for the PCB assemblies, and it is possible to monitor and control the temperature profile with great precision. This has prompted many electronic manufacturers to set up VPR ovens and take advantage of their simplistic design and effective soldering method.
VPR Process of Soldering
Primarily soldering looks to be a simple process, as all it takes is application of adequate heat to allow the solder to melt. However, there is a lot more to the process when the system is using solder paste to solder SMT boards.
A good solder joint not only requires the entire solder paste to melt completely, it also requires the copper pad on the PCB and the component leads to reach a temperature above the melting point of the solder paste. Failure to achieve this results in a cold and failed joint. Moreover, the application of heat must not overheat the components, as this can damage them. The VPR soldering process transfers heat from a condensing vapor rather than by forced convection, radiation, or conduction from a soldering gun, as in other soldering processes.
The working process of a VP reflow machine is very simple. Before switch on, the vapor phase fluid rests as a liquid at the bottom of the fluid tank at room temperature. After switch on, a heater warms the fluid to boiling at 235 °C, the typical melting point of lead-free solder paste.
Manufacturers of VPR systems specifically engineer the boiling point of the vapor phase liquid to 235 °C, as this is the optimum temperature for melting solder paste. The boiling fluid creates a thick blanket of vapor with enough latent heat for transfer to the PCB assembly.
As the hot vapor is denser than air, it displaces all the air in the tank, creating an inert layer, just like a Nitrogen layer does. As a cold PCB assembly enters the tank, vapor condenses on it, transferring its latent heat to the components and the PCB pads, until they are all at the same temperature as the vapor is. The system cannot cause the PCB assembly to overheat.
Profiling with VPR process
VPR ovens have a smaller footprint compared to convection machines. Rather than pass the PCB assembly horizontally through it, VPR ovens require lowering the assembly vertically into the vapor layer and holding it in position until soldering is complete. The operator controls the rate of transfer of heat by adjusting the height of the PCB assembly within the vapor.
As this is a batch process, profiling several circuits simultaneously at different heights, while logging their profiles individually, saves a lot of time. All the circuits heat up at the same time, and cool down simultaneously at controlled rates.
Advantages of the VPR Process
The vapor phase reflow process offers several advantages over other conventional methods of soldering:
- Wide process window allowing batch profiling to cut down profiling time
- No risk of overheating components
- Reliable method even for small batches
- High repeatability for both low and high mass boards
Rush PCB Inc. recommends vapor phase reflow soldering as it is a simple and very reliable method for PCB assemblies. The method is suitable for and reliable for soldering not only regular SMT boards, but even those with complex components such as ceramic assemblies, flip chips, BGAs, and QFTs.
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