[log in to unmask] said:
>It isn't that our oven has ever exhibited heat loading
>problems. In fact, I plan on some heat loading experiments
>to prove this. I just have an external customer who likes
>to help. The problem is they are 100% convinced of the
>following:
>
>1) We absolutely have to have nitrogen on our reflow oven.
This is "absolutely" wrong, unless you have some unusual requirements.
>2) We have to place buffer boards before and after each
>production assembly or else our heaters won't handle the
>load properly.
And this is extremely dubious.
>I do appreciate all the responses so far, although more
>opinions are welcomed and encouraged.
If you look at the instructions of 10 year old lamp or panel emitter heated
furnaces, you would see suggestions regarding the use of dummy boards. This
is to break laminar air layers in the older style of furnace which did not
fan force the air. Especially the first few boards going into an idling
machine. Even in such older furnaces, most of the heat transfer to the work
is due to air (or N2), so when the air mixing is not good, neither is the
thermal gradient on the work.
Regarding N2 use, my experience is that it is of little benefit. In order
to gain the theoretical advantages, you must exclude oxygen to the ppm
level, and this becomes difficult, especially in the cooling. And the work
is unfortunately "contaminated" by oxides and gaseous O2 permeating the
materials before it reaches the furnace. Real life results I have seen were
not nearly so impressive as the lab photos.
And there are many more than two rules of thumb in my experiences regarding
SMT reflow, I'd suggest these in no particular order:
Buy a furnace with sufficient heated length to run at the conveyor rate
necessary to keep up with your placement line. Any longer is just that many
more steps to take a few thousand times in your life, and that many more
motors and bearings to break.
Make sure you have satisfactory exit transfer or built-in cooling to assure
safe handling of boards leaving the heated zone.
Don't pay for thermal transfer, zone isolation, and cooling schemes related
to N2 if you don't intend to use it.
Make sure you have the power distribution in your building to meet the
needs of phase and voltage selection. If possible, run multiphase high
voltage (480 or more) to improve effiency.
Find a furnace which is not too noisy. Stand by one running for an hour or
so to see how noisy it might be.
Compressed air is not cheap and easy to provide on a continuous basis, and
it tends to make some ugly noises.
Look hard at the mechanics of the conveyor drive, this is what will likely
break first.
The difference between car salespersons and SMT equipment salespersons is
that the car people realize when they are lying. ;-)
Your mileage may vary, void in New Jersey, use at your own risk, etc.
Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com
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