R & D VaporTech
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Vapor Phase Reflow, or Condensation Heating, has progressed through many variations since its invention in 1973. It uses an inert liquid chosen for its boiling point, that when heated creates a very stable, uniform, and safe heat transfer medium in the form of vapor. Traditional reflow methods using infrared and convection heat may suffer from various problems that are solved by using a condensation form of heat transfer such as vapor phase.
Vapor Phase utilizes heaters immersed in the vapor phase fluid to boil this fluid and generate vapor. This vapor is seventeen times heavier than air so it remains in the bottom of the vapor tank. Cooling coils then condense any excess vapor returning it to the boiling liquid to maintain a set height of the vapor blanket. The earliest vapor phase systems used only vapor to heat the product from start to finish. Later, preheat was added to better control the heating thermal curve of the product being reflowed. The product was lowered on an elevator system into the preheat or "secondary" vapor which hovered above the primary or "reflow" vapor.
Current designs of most modern vapor phase systems have removed this "secondary" vapor in favor of an IR or Convected Zone type of preheat. This design change took place to provide better control of the preheat thermal curve and allow better solder paste conditioning prior to the reflow stage.
Most system utilize some sort of conveyor system to transport the product though the critical stages of reflow: preheat, reflow, flash off and cooldown.
With lead free initiatives taking place in the semiconductor industry, attention must be paid to the reflow process. Lead based solders such as Sn63/Pb37 melt at 183 C* with a typical reflow temperature of 200 C* - 205 C*. Industry practice has established a safety margin of 30 C* to reduce potential for defects caused by excessive heat. Lead free solderswith a eutectic temperature of 217 C* - 221 C* require a minimum peak reflow temperature of 230 C*. The same 30 C* safety margin brings the maximum reflow temperature to 260 C*. These higher temperature requirements only amplify the challenges faced by traditional reflow methods, since to ensure reflow across the whole assembly their temperature must be increased much more that the increases in the solder's reflow temperature but vapor phase has none of these problems. Simply choosing a fluid with a higher boiling point will continue to ensure complete reflow while making it impossible to exceed the temperature of its boiling point. Quite simply, lead free solder reflow can be done perfectly by never exceeding 230 C*.
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