John Guy said:
>Jerry ,
> I agree with everything you said here, but feel the need to add
>one major point.
By all means...
> When using a Vapor Phase system, the need to profile for a new
>assembly is almost nonexistent. As you said, the heating is very
>efficient and even across the assembly.
Whether you "need to profile" can't be established on the basis of which
process you choose. One must consider the properties and type of components
of the assembly. I suggest the issue is more "can you profile if you need
to?"
Although the heating of the work may be "efficient" and the delta T good
when using VP; what about rate of heating?
VP may give you a rate hazardous to certain components, and it is difficult
or impossible to modulate. Even with an in-line VP machine, when the work
enters the vapor zone, it will change temperature very, very rapidly - no
matter how quickly it entered the zone. It can't be removed from the zone
until it reaches equilibrium, or it will drag out fluid.
> When using a Convective system, you get the added control and
>ability to profile, but only at a price. The need to profile for
>different assemblies is required. The Convective systems are very robust,
>but I wouldn't say that one profile will serve all your needs.
Neither would I, but you can have more than one profile if you need it, and
you couldn't with the VP machines I used a few years back. Superior
equipment is probably out there today.
> This means that time is required setting the oven up for a new
>product, however, this shouldn't take any more time that to program your
>pick and place equipment.
John quoting me:
>----------
>From: Jerry Cupples[SMTP:[log in to unmask]]
>Sent: Wednesday, September 11, 1996 9:05 AM
>To: [log in to unmask]
>Cc: [log in to unmask]
>Subject: re: Vapor Phase Reflow
>
>[log in to unmask] asked:
>
>>I have attended a few training sessions and vapor phase reflow has been
>>discussed, but how much is this process used in production these days?
>>Isn't the convection reflow process better, more cost efficient, etc.?
>>Inquiring minds wish to know?
>
>Makes a good foil to throw in for a minute to dazzle the audience.
>
>It _is_ a "good process" in that it will provide consistent results and it
>can heat a lot of work fairly quickly. The work cannot heat past the fluid
>boiling point, so the max temp is restricted.
>
>There are unique advantages - some "tank" type VP systems are gigantic, and
>you simply lower the work (like an entire assembly the size of your desk)
>into the vapor zone, wait a few seconds, and pull it back out.
>
>Trouble is, the heating is very rapid. The fluid (typically a
>perfluorinated FC) is boiled to a vapor, then when the work enters the
>chamber, it condenses on the work, then drips back to the boiling liquid at
>the bottom of the chamber. The phase change transfers the heat pretty
>evenly to any surface contacting the vapor.
>
>Some VP furnaces have two vapor zones (one with a lower boiling fluid), or
>some other type of pre-heating to reduce the thermal shock to the work.
>
>It is difficult to get the type of "profile" with VP which IR or convection
>furnaces can provide. Typically, the conveyor rate is much slower, as the
>vapor zone is pretty short.
>
>The fluids are _very_ expensive, and there is some drag-out or unwanted
>evaporation in most systems.
>
>To answer your question, it seems to me that most of the industry has gone
>away from it in favor of the convection furnace. I'd say cost, some
>environmental concerns (greenhouse effect), ability to "profile", and
>throughput are the main reasons.
cheers,
Jerry
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