Getting to Know Vapor Phase Reflow: The Safety of PFPEs
(A preview of our new eBook on “The Chemical and Physical Fundamentals of Vapor Phase Reflow Soldering”)
ChloroFlouroCarbons (CFC) were the initial chemical of choice for vapor-phase soldering. They were first used in the process in 1974, gained immense popularity in the early 1980s, and then were phased out in 1989 when the Montreal Protocol came into effect. These molecules were relatively light and difficult to contain, so they would easily escape manufacturing processes. What’s worse is that they were immensely damaging to the upper layer atmosphere. Chlorine atoms will break off from CFC molecules when hit with high-energy photons. The free chlorine atoms will readily bond with oxygen atoms and either break apart O3 and O2 molecules, or latch onto atomic Oxygen to form Chlorine Oxide (CO). Without O3 in the atmosphere to absorb high-energy UV particles, the particles can reach the surface of the earth, strike plants and animals, damage crops and cause skin cancer.
The CFC215 molecule is made of Carbon, Chlorine, and Fluorine. Each CFC215 molecule has enough Cl atoms to completely destroy 1 O3 molecule to form 3 ClO molecules.
So for years, vapor-phase soldering went by the wayside until scientists discovered a much safer chemical to use — PFPE.
Perfluoropolyethers (PFPE) are a class of chemicals that are commonly used as lubricants in aerospace and electronic applications as well as high-vacuum environments. PFPEs are inert, much denser than air, non-explosive, chemically stable over a high temperature range and resistant to outgassing. And importantly, the liquid-vapor transition temperature is related to the molecular weight — which is selected by choosing molecules with different numbers of fluorinated ether groups. PFPEs come in two general types — linear and pendant. Pendant varieties have a CF3 molecule that replaces one of the ethers’ Fluorine atoms.
Two linear PFPE molecules are shown above. On the left the atomic weight is 360 amu and the one on the right is 674 amus. They have the same basic structure — but the one on the right has additional fluorinated ether groups.
Two pendant PFPE molecules are shown above. On the left the atomic weight is 386 amu and the one on the right is 750 amus. They have the same basic structure — but the one on the right has additional fluorinated ether groups.
Linear PFPE molecules are more often seen in lubricant oils and greases. Pendant PFPE molecules are more often seen in vapor-phase heat exchange operations.
This pendant PFPE molecule has a molecular mass of approximately 882 amu and a predicted boiling point of approximately 200°C.
This pendant PFPE molecule has a molecular mass of approximately 1020 amu and might have a boiling point around 230°C.
When a group of liquid PFPE molecules that are near their boiling point are heated, the heat energy added to the system goes into breaking intermolecular bonds. Once the intermolecular bonds are broken, the molecules enter a gaseous phase. But the molecules are still much denser than air. So as long as they are kept in a walled container, both the liquid and gaseous molecules are very easy to contain and recover.
Since there are no Chlorine atoms in the entire molecule, there’s no danger to Earth’s upper atmosphere!
This selection is taken from our upcoming eBook “The Chemical and Physical Fundamentals of Vapor Phase Reflow Soldering.”