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www.us-tech.com
February, 2013
Vapor Phase: Today’s Premium SMT Technology. T
By Patty Chonis, President, A-Tek Systems Group, Longmont, CO
oday’s reflow systems need another look — particularly when compared to the founding technology without which early SMT assem-
blies would not have been possible. That founding technology — vapor phase — is still with us, and for good reason. It can handle the tough jobs that the ever-popular convection systems really can’t handle effectively. Here’s
why: l
Superior joint integrity. l Oxygen-free environment.
l Most complex product capability. l Competitive cost of ownership.
better than convection. l
l
inates a source of process control error. The use of an inert fluid with a specific boiling point (240°C for example) ensures the circuit and all of the compo-
very energy-efficient. Typical power usage when evaluated against a comparable convection system is less than 50 percent and start times are just 20 min- utes from cold.
Oxygen-Free Environment Vapor phase operates in an oxygen-free
l Low-to-high-volume product run flexibility. l Environmental safety.
l Operational energy efficiency; 50 percent Reliability of the end product.
Fast start: 20 minutes from cold start, 1 minute from standby. First recall how vapor phase works. The process is brilliantly simple, and can be com- pared to a boiling pot of water on the stovetop, but rather than water, a Perfluorpolyether (PF- PE) fluid is used. PFPEs have high boiling points between 200 and 260°C that can be selected to meet the needs of the alloy being used to solder the board.
environment and by its very nature, the process places less stress on the flux chemistry than any other system. It gives the OEM or CEM an option that might allow the reaction of active agents in the flux. The role that fluxes are de- signed to play can be exploited to the full extent possible in a vapor phase machine because the maximum temperature limit is determined as the boiling point of the fluid which protects the fluxes from excessive thermal stress. It is not uncommon in a convection system
Asscon VP6000 vapor phase reflow with vacuum.
nents mounted on it cannot possibly reach a temper- ature that is higher than 240°C.
Profiling Reflow Time The second element of reflow profiles is time.
In convection oven, excessive source temperature can affect low-mass parts.
This, as with any boiling fluid, limits the maximum possible temperature any component can be ex- posed to in an absolute physical manner, rather than by a machine’s program parameter. This elim-
With the vapor phase system the process duration is a simple two step profile —the immersion period fol- lowed by the cooling period. The immersion period is determined by the size and mass of the circuit being reflowed. The larger the circuit, the greater the en- ergy needed to raise its temperature to the 240°C level. Heat transfer in the inert PFPE environment is significantly better and homogeneous than a gaseous environment. PFPEs have a higher specific heat capacity and a given unit volume of vapor car- ries a greater energy load. They also have a higher thermal conductivity so they are more efficient in the transfer of the energy from the vapor blanket to the circuit board and the attached components. This lev- el of thermal efficiency also means the process is
processing a high mass/high mix PCB for fluxes to become exhausted around smaller compo- nents as they reach elevated temperatures in the period of dwell while waiting for the high mass parts to reach an adequate peak temper-
ature. Good joints can quickly become bad joints when elevated temperatures increase the surface re-
Vapor phase boiling point determines peak temperatures for all components.
activity in the joint in an oxygen rich environment. Most engineers dismiss vapor phase reflow as being employed on the fringes of production opera-
Continued on next page
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