The increasing use of lead-free solder has renewed interest in the vapor phase reflow process.

To eliminate lead from their products, electronics assemblers will have to do more than just change solder alloys. They’ll have to change their reflow process, too. And while they’re at it, assemblers may want to take a new look at an old standby: vapor phase soldering.

Invented in 1973, vapor phase reflow is as old as surface mount assembly itself. Despite some distinct advantages over infrared and convection reflow, those processes gradually supplanted vapor phase as the soldering methods of choice, especially for high-volume applications. Vapor phase reflow was primarily relegated to prototyping jobs and to small runs of highly complex, highly critical boards.

"Vapor phase tends to be used more for low-volume, high-mix applications," says Dave Suihkonen, an engineer at R&D Technical Services (Burnsville, MN), one of the last remaining manufacturers of vapor phase reflow equipment. "As circuit boards get more advanced, we get more calls. Our customers have always been [manufacturers such as] Honeywell, ITT Aerospace, and [NASA’s] Jet Propulsion Lab—companies on the leading edge of technology. JPL might only build three boards of a certain design, but they all have to work and they all have to be reliable enough to go to Mars and back."

Now, with the increased interest in lead-free electronics assembly, vapor phase reflow is coming back in vogue. During the past few years, manufacturers of vapor phase reflow equipment and fluids have noted increased sales. "In 1998, we thought the market was slowly going away," recalls Doug Kelly, sales manager at Ausimont USA Inc. (Thorofare, NJ), which supplies the perfluorinated fluids used in vapor phase reflow. "Then last year, [sales] picked up nicely. We found that vapor phase has become more popular because the industry is moving to lead-free assembly."

The Process

In vapor phase reflow, the printed circuit board (PCB) is heated by vapor from a boiling perfluorinated fluid. The vapor surrounds the board and its components and condenses onto them. As the vapor condenses, it transfers heat to the leads, solder paste and pads equally and simultaneously.

Although vapor phase reflow is most often done in batch-style equipment, the process can be adapted for in-line operations. Condensing coils can be placed on either side of the reflow zone to keep the vapor, which is heavier than air, from escaping the machine.

Vapor phase reflow is compatible with all solder pastes and electronic components, including ball grid arrays, micro ball grid arrays and flip chips.

"If you can run it in a convection oven, you can run it with vapor phase equipment," Suihkonen says. "Solder pastes have become [thinner] over the years, to allow for longer reflow Arial in convection ovens. But, that does not adversely affect vapor phase reflow."

Vapor phase reflow is fast and efficient. Condensation transfers heat 10 Arial faster than hot air and eight Arial faster than infrared radiation.

The process is clean and safe. In the 1970s, some older vapor phase reflow machines used Freon, but today’s equipment does not require any volatile organic compounds. Perfluorinated fluids are inert, noncorrosive, nontoxic, nonpolluting and nonflammable. They evaporate completely from the board, without leaving any residue behind.

One of the main advantages of vapor phase reflow is temperature control. At atmospheric pressure, the temperature of the vapor will be the same as that of the boiling liquid. Temperature fluctuations are much less than with infrared heating. If power is increased, the rate of vapor production will increase, but the temperature will remain the same.

"The reflow temperature cannot increase above the boiling point of the fluid. If the fluid boils at 215 C, the reflow temperature will never get hotter than 215 C," says Suihkonen.

Vapor phase fluids are available with boiling points ranging from 155 to 260 C. The fluids can even be blended. For example, a fluid that boils at 165 C can be mixed with a fluid that boils at 200 C to get a fluid that boils at 180 C.

Another big advantage of vapor phase reflow is uniform heating. The process is unaffected by the size, shape, geometry or color of the components or the PCB. Every part of every component is exposed to precisely the same temperature. As a result, the process is ideal for reflowing large boards, multilayer boards, densely populated boards and boards with a wide range of component sizes.

"The nice thing about vapor phase soldering is that the vapor gets into every crevice. I don’t care how intricate the chip is," Kelly explains. "No part of the board is left untouched, and it’s all at the same temperature. With a convection oven, you may be limited as to what you can do with board design."

"Vapor phase reflow heats small components and big components equally, so you don’t have to overheat small components to get larger components to reflow," adds Suihkonen.

Another benefit of vapor phase reflow is that there’s no need to use inert gases, such as nitrogen. Because the vapor density of the soldering fluid is much higher than that of air, reflow takes place in an inert atmosphere.

The Lead-Free Problem

The biggest difference between tin-lead solder and lead-free solder is that lead-free alloys melt at much higher temperatures. The most common solder for electronics assembly is 63 percent tin and 37 percent lead. It melts at 183 C. In contrast, most lead-free alloys have melting points at least 30 degrees higher. Tin-silver alloys melt at 221 to 245 C, and an alloy of 95 percent tin and 5 percent antimony melts at 232 to 240 C.

Such high temperatures can damage components, and the problem is magnified if heating is uneven, Kelly says. With convection and infrared reflow, sheltered or shaded areas on the board are not directly exposed to the heat. As a result, the oven temperature must be increased, so that even the unexposed areas reach the desired temperature.

"Think of a convection oven as a hot wind blowing on a wall," Kelly explains. "The side of the wall directly exposed to the wind will be hot. The opposite side will be hot, too, but not as hot as the side facing the wind.

"You can think of an infrared oven the same way, only with light shining on the wall. The side of the wall directly exposed to the light will be hot, but the opposite side will be less hot.

"To heat an entire board to 240 C, you might have raise the oven temperature to 260 C."

By ensuring even heating and limiting maximum temperature, vapor phase reflow may help assemblers shave a few extra degrees off their lead-free reflow profiles.