Vapor Phase (Re)Visited
Out of favor for decades, the reflow process is undergoing a renaissance.
Vapor phase was the original and principal mass reflow method for a few years in the early 1980s. Somewhat displaced by IR and, eventually, convection dominant reflow, it has remained in limited use, primarily for prototypes, lower-volume production and thermally complex assemblies. In the past few years, vapor phase has experienced renewed interest primarily because of its advantages in achieving the small process window for peak temperatures with Pb-free assembly.
The first vapor phase reflow systems used the same configuration as a traditional vapor degreaser: a vertical, open-topped tank with a heater at the bottom and a cooling coil around the top opening. In the original single-batch versions of vapor phase, a large vat, so to speak, contained a fluid that had a specific boiling point – usually around 210°C, not coincidentally the full liquidus temperature of SnPb37. A heater boiled the fluid, generating vapor. Because the vapor was significantly heavier than air, it remained at the bottom of the tank. As more vapor was generated, a region of saturated vapor called a “blanket” expanded upward, pushing air out the top opening. The cooling coils defined the top of the blanket, preventing (or at least minimizing) vapor losses from the top opening. Assemblies were lowered into this vapor blanket and kept there until reflow was complete. Heat from the transition of vapor to liquid was transferred to all surfaces of PCB and components continuously until they reached saturation (i.e., boiling) temperature. These systems were expensive to operate because of continuous loss of vapor through the open top, and they offered no provision for recovery of fluid condensed on the products.
The first economical systems for commercial production incorporated a “secondary” vapor blanket of a different fluid, which floated on top of the primary vapor, minimizing vapor losses and providing a zone for vapor recovery. Originally Freon R-113 was used because it was low cost, has a low boiling point (48°C), has a vapor density between those of air and the primary vapor, and is inert. Later, more environmentally friendly secondary fluids were developed.
The single-vapor inline machine was developed for high-volume production. It used a single (primary) vapor in a closed top chamber with a continuous inclined conveyor (down on inlet – up on exit) entering and exiting through long chilled ducts. These ducts minimized vapor loss, and the exit duct provided vapor recovery.
‘A complete soldering process.’ The dual vapor and inline single vapor systems made the vapor phase reflow economical, but did not provide a complete soldering process. In dual vapor systems, the Freon secondary blanket was a solvent to the solder paste chemistry. Therefore, the paste on the assemblies had to be “dried” prior to vapor phase, usually in a batch oven. This provided an acceptable process, and also was used with single vapor inline systems. However, in both machines, high defect levels of tombstones, solder ball, and solder wicking indicated that preheating immediately before entering the vapor was required. This was part of the general increase in understanding of the total process of reflowing solder paste: evaporating solvents, activating flux chemistries and permitting time for the reduction of oxides on soldering surfaces, controlling heating rates for critical materials within the assembly, etc.
Inline IR preheat was incorporated into single and dual-vapor systems along with active cooling/vapor recovery, providing for the first time the complete reflow process in a single machine; this configuration remains available. However, at the same time, convection IR ovens demonstrated the capability for complete reflow with adequate uniformity at a much lower cost. Reflow capability was significantly advanced with the development of reliable convection dominant ovens, which are the principal technology in use today.
Vapor recovery started simply as dwell time within a chamber with chilled surfaces. Latent heat within the assembly evaporated (hopefully all) condensate from external, principally horizontal, surfaces. The vapors produced were recondensed on chilled surfaces and captured. More advanced systems added active air circulation (usually also providing cooling to the assembly) coupled with filters, condensers, stacked columns, and so on, to capture fluid more effectively.
As mentioned, vapor phase is experiencing something of a renaissance because of its advantages in achieving the small process window for peak temperatures with Pb-free assembly. The vapor phase process offers some distinct advantages over the convection dominant methodology that may make its use in Pb-free reflow more feasible.
Vapor phase offers absolute maximum control of temperature. As noted, the entire assembly contacts only vapor, which cannot be hotter than the boiling point of the fluid (e.g., 215°C for SnPb or 235°C for SAC). There is no possibility of overheating small components or critical materials. Heating is fast and relatively uniform. As an assembly enters the vapor zone, fluid condenses on all external PCB surfaces, and components, independent of size, shape, location IR or convection ovens, must use higher temperatures to achieve rapid heat transfer, which opens the door to potential overheating of small components.
While IR and convection dominant (forced convection) ovens aggressively have been attempting to reach lower levels of oxygen, vapor phase is inherently inert. (Indeed, another moniker attached to the process a few years ago was condensation inert soldering.) Fluids used in vapor phase reflow are completely chemically inert both in the liquid and vapor states. The vapors are significantly heavier than air and, therefore, exclude air/oxygen from the heating zone. In some instances, an inert reflow atmosphere may provide the advantage of increased wetting for Pb-free solders.
So with all these advantages, how did vapor phase succumb to IR convection and ultimately convection dominant reflow ovens? Why doesn’t everyone use it? In short, there are some severe limitations to the vapor phase process.
The greatest limitation of the vapor phase reflow process is the difficulty in controlling the heating rate (slope) throughout the process. If an unsoldered assembly at ambient temperature is immersed directly into saturated vapor, extremely fast heating occurs, which is uncontrollable, as long as there is adequate vapor present. Condensation proceeds, transferring heat to all surfaces until saturation temperature is reached. Condensation heat transfer has been measured to be as high as 10x that of convection, resulting in board/component heating rates as high as 50˚C/sec. Heating this quickly may not permit adequate time for fluxing actions, and can result in material damage and significantly increase defects such as tombstones, solder balls and solder wicking.
Extended time above liquidus (TAL) can be a drawback of vapor phase reflow. Because condensation heats so effectively, more heat may be stored in the assembly materials, especially thick PCBs and high-mass components. This can slow the cooling process, extending the total TAL. Some vapor phase machine cycles do not start active cooling until after all condensate is re-evaporated, which can further increase TAL.
Expensive fluids. Vapor phase is not inexpensive. Because of their special characteristics, the fluids are very expensive. Fluid for Pb-free reflow with a boiling temperature of 230˚C can cost well over $1,000 per gallon! It is vital to the economic viability of the process to avoid fluid losses, either as liquid or vapor. There is no inherent consumption of fluid during reflow, and the fluid does not degrade and require replacement if proper filtering is performed to remove contaminants, principally flux chemicals. Any vapor phase reflow machine should incorporate a fluid filtration system and be designed and fabricated to comprehensively prevent leaks.
In any vapor phase system, at the end of the reflow heating cycle, the entire product is completely covered with condensed liquid: all surfaces, under all components, inside all cavities! Drag-out of this condensed fluid on the completed assemblies exiting the system constitutes a continuous (potentially high) cost and must be prevented. Recovery of condensed fluid is accomplished by re-evaporation after reflow, in some systems as part of the cooling stage.
Previously, complete vapor phase reflow systems have been available that used IR preheat and convection-based vapor recovery/cooling. Newer systems feature vapor-based preheat, and advanced recovery sometimes uses vacuum systems.
Frankly, we find it surprising that most published information on currently available vapor phase reflow systems provides very little detail on the techniques and performance of the vapor recovery methods employed to prevent “drag-out” of condensed fluid on assemblies exiting the system after reflow. Given the fluid expense, this factor is vital to the economic operation of any system in actual production.
Today’s systems. Most systems available today offer some form of preheat before entering vapor phase reflow. As noted, both batch and inline systems are available with traditional IR preheaters. These configurations have been successful for decades, but IR heating’s limitations are well known, including a lack of speed and uniformity, especially on thermally complex assemblies.
Preheat using vapor is offered on several newer machines. The exact techniques appear to be unique to each manufacturer, some protected by patents. But they appear to fall into a few generic categories.
In dual vapor systems, assemblies dwell within the secondary vapor blanket for controllable intervals, providing some level of preheat before being lowered into the primary vapor zone for reflow. Some single vapor machines use the primary vapor to accomplish preheating through varying strategies of timed exposure to a zone of carefully controlled, non-saturated vapor. This provides a zone of lower temperature and energy density, permitting lower heating rates while maintaining uniformity across product surfaces. The result is the capacity to generate the complete heating profile shapes required for high-quality paste reflow, including low heating slopes, soak sections, etc., while maintaining good thermal uniformity, even across complex assemblies. Again, exact strategies differ among equipment manufacturers. Some use timed immersion into the non-saturated vapor zone, created directly above the saturated vapor zone (reflow section). Other systems enclose assemblies in a sealed chamber and inject measured amounts of primary vapor at programmable intervals to achieve the desired heating profile.
All these systems have produced test results demonstrating typical ramp-soak-spike profiles with low ΔTs. Satisfied users in actual production environments have validated this performance.1 However, long-term experience has shown that, in direct contrast to the saturated reflow zones, non-saturated vapor zones tend to be unstable and difficult to control. Therefore, as with any piece of production equipment, a potential user should validate the temperature profile on their own product, attaching their own thermocouples and confirming repeatability of the process.
Several vapor phase systems come with vacuum systems to enhance processing capabilities. The principal benefit claimed is the reduction or elimination of voids within the solder joint.2,3 By applying some level of vacuum to an assembly following reflow, while the solder joints are still molten, voids formed during the heating/melting stages can be drawn out and eliminated.3 Power electronics are said to benefit specifically because of the improved heat dissipation through solid joints with no voids.
The application of vacuum after reflow should also offer more capability and speed in recovering condensed primary fluid from product surfaces. Relying totally on the stored heat to completely evaporate all condensate can be problematic for component geometries that tend to trap volumes of liquid such as connectors, which may contain upward facing “cups” that trap deep pockets of liquid. Vacuum “drying” to remove trace amounts of liquid is a well-established technique used throughout process industries and should provide more reliable economies for complex assemblies.
Vapor phase will see some level of continuing growth in the next few years, based on its capacity to reflow the most complex Pb-free (or SnPb) assemblies with no chance of high temperature damage to components or materials. The extent of this growth will depend on the ability of the new breed of systems to compete on price-performance (cost of ownership-performance) with well-established families of convection dominant ovens, which already have been proven to provide acceptable reflow for a wide range of Pb-free products. Through it all, vapor phase will always provide the “ultimate reflow zone.”
References
1. Chris Munroe, “Beating the RoHS Heat,” Circuits Assembly, March 2008.
2. Fraunhofer IZM, izm.fraunhofer.de, January 2008.
3. John Bashe, A New Approach to Vapor Phase Reflow Soldering, SMT, November 2006.
Jim Hall is principal consultant and Six-Sigma Blackbelt with ITM Consulting, and a pioneer in vapor phase soldering. Phil Zarrow is president and a principal consultant at ITM Consulting (itmconsulting.org); phil_zarrow@itmconsulting.org.
