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Proper Exhaust Pressure for Reflow Ovens?



Proper Exhaust Pressure for Reflow Ovens?
What is the proper exhaust pressure for a reflow over? Where should we measure the pressure? How should we measure pressure? Phil Zarrow and Jim Hall, The Assembly Brothers, address these questions and share their own experiences.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript


Phil
Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, pick and place or place and pick. We're here to answer your process questions regarding SMT and the associated realm. What's today's question.

Jim
This question comes from E.R. You had mentioned about proper exhausting of the reflow oven. At what exhaust pressure normally should the oven be measured at?

Well, certainly measuring pressure in an exhaust system is one way to quantify it. You've got to have the right exhaust and it's important. The real critical parameter that most ovens will specify is a flow rate in cubic feet per minute or cubic meters per hour or some other volumetric flow rate.

Then the static pressure, if it's outside the oven, to create that flow rate is going to depend upon the geometry and dynamics of your exhaust piping. Some ovens may give you a pressure tap inside the oven within their known piping. But the bottom line is you go to the manufacturer.

If they give you a flow rate, then you go to your piping and make the conversion using standard duct flow procedures.

Phil
What instrument would E.R. be using to performs these tests?

Jim
For measuring static pressures, it's just any kind of a pressure gauge or manometer. Often you see these little red mercury or alcohol manometers that they use in duct work. The key is pressure is only an indication of flow rate.  And what you're really concerned about is getting the proper volumetric flow rate and that it's stable. That it doesn't vary over time such as when the people down at the end of the exhaust duct turn on the spray booth or something and the static pressure, and therefore the flow rate, changes and your oven thermal process is upset.

Phil
The only thing I want to add is when you do your ducting, make sure that the ducting you're using to connect from the oven to the rest of your exhausting system is rated for the proper temperature for the reflow oven.

Jim
Depends upon the way the oven is ducted internally and so forth. But yes, high tempered and cleanable, you know, all the manufactures do their best to put filters for flux-capture inside, but always anticipate the possibility of some flux getting into the duct and condensing somewhere. Make sure that you put in provisions to take the duct apart and clean it.

Phil
I hope we answered E.R.'s question. The only thing I could add to that is no matter how you're exhausting your reflow oven ...

Jim
Don't solder like my brother.

Phil
Don't solder like my brother.

Comments

Very good point. Talking about reflow ovens using nitrogen. If the reflow oven exhaust flow is too high, the oven may be "sucking" air into its doors and them contaminating your reflow zone with unwanted oxygen from air.
Luiz Felipe Rodrigues, Air Liquide
A single-stack exhaust for each oven certainly does not work very well. This will NOT provide a stable air flow or stable pressure, indeed you will see just the opposite. The reason is the same as the example of multiple vents connected to a main exhaust plenum, variation in building pressure. If you have multiple stacks in a building and each has its own blower, the building pressure will change depending on how many individual stacks are being used. This would include heating and AC systems as well as reflow ovens, convection ovens, test chambers, number of doors used, number of dock doors open/closed, and the list goes on and on.

For something like this, you need a real Viking to solve the problem. https://www.tjernlund.com/specified_systems_content.htm. This is just one example, there are many regulated-pressure automatic exhaust damper systems that are really not that expensive. In fact they pay for themselves many times over in energy costs if properly designed into the building ventilation system when you considered the heat/AC losses that several uncontrolled exhausts present. The cost of each exhaust port going through the roof can be a huge expense. With an automatic damper on each stack, they can all be connected into a single main exhaust, yet they all are being held to a constant air flow, each set for the desired air flow of the oven, chamber, or room exhaust they serve. Remember, Vikings do it better. Always have, always will!
Odin Stadheim, Aker Solutions
The simplest install is 1 exhaust fan/1 oven. Costs more, more roof penetrations (leak opportunities) but is stable.
Do not recommend big exhaust requirements to mix into the system. As noted turning on a the paint booth will change the airflow at the oven.If a multiple oven set up is used, monitor the static pressure and check the air flow on install and periodically. Changes creep into these multiple systems (some one moved a damper or it slipped, oven added or deleted, the fan motor changed but not quite the same, the work space went from a positive to negative pressure because they put the conformal coat line or wave solder in at the other end of the room). Moving from sea level to 6k feet will cause all the setting on the oven to change.
Larry Dzaugis
Yes, settings are very machine specific as the internal design of the exhaust will dictate settings.

General rule of thumb however is evacuation & containment. IE keep fumes from escaping into workspace but not so much as to be sucking heat or inert atmosphere out of the machine. I have often seen relocated ovens never perform the same as in their original location as the exhaust characterization was never duplicated
Ray Chartrand, Chartrain Consulting

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