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Tue, 10 Dec 2002 15:31:16 +0800
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Hi, Sam,

I and Steve Gregory share a common story in our quest for rework stations.
My Company has an old Metcal hot-air rework station that was obsolescent
when we bought it, and we would have to get any new nozzles we need  custom
made since those of original manufacture are no longer available. It also
fell into disuse for some reason - I suspect largely because of its
inconvenient location, but that's another story.

Suffice it to say that we, like Steve, needed a rework station that would
handle the rework of all today's components. Our volume is much lower than
Steve's, though, so we couldn't justify getting the full-blown Lightmaster
Pro that Steve has. Ours will be a basic PDR1500 system with no vision
placement/alignment system, PC or monitor. We found we didn't need any of
them, even for fine pitch devices and BGA's. We intend to use the machine
just for removing components, with the exception of BGA's LCC's and any RF
components that have an inaccessible ground plate underneath them. These we
will use the PDR to mount as well.

The reason for not choosing all the expensive alignment equipment is quite
simple. LCC's, BGA's and so on self align very well when the solder is
melted. We can align these well enough manually, using a dynascope for
especially small and/or fine-pitch devices if necessary. The "Smart"
Controller we're buying with our machine gives a very generous process
window for re-mounting components, and the temperatures involved are much
lower than for hot air. Good joints can be obtained with the temperature
only about 10 degrees above liquidous instead of the usual 20 degrees or
more, though good solderability is required to achieve this.

The only real complaint I've ever heard any Hot Air Rework Station salesman
come up with is the perception that IR is not controllable. No other
reasons were put forward, but it's been enough to explain why Hot Air has
been popular while IR has languished in misunderstanding. The absence of
further reasons why IR is bad suggested to me that IR is a perfectly valid
process if controllability could be proved. The "Smart" controller does
this very well once you understand how it works in relation to particular
component material types. It takes a bit more thinking about than the hot
air systems, which give more direct feedback of temperatures measured,
that's all.

Our rep for the PDR is not a salesman in the usual sense. His core
business, which is quite small, is repairing unserviceable circuit cards,
mostly from the telecoms sector. He chose to use a PDR to remove components
in one piece, but being a very innovative fellow, he realised the
weaknesses in the system he'd bought and set about re-designing the
controller himself (the "smart" controller he now offers optionally with
the machines). Further he has proved to himself and others that evenness of
heating is not really an issue. Any parts of a component not quickly
absorbing heat directly from the IR source, are heated by conduction from
other parts of the component that do heat up more quickly. By the time the
solder melts, there are no more temperature differences across a component
than you would expect from a hot air system. If you find that there are
temperature differences, they can be compensated for by adjusting dwell
times at the various heating stages.

There are few advantages of one type of system over the other in terms of
how well they do the job. The advantages of the IR system, as I see them,
over a hot air system are:

1. IR doesn't require one nozzle for every different package type that
you're dealing with, so you potentially save quite a bit on nozzle costs.
You may need a different focal length lens if you take off especially large
components, and the cost of that is equal to about 4 nozzles, so it's a
toss up what's more worthwhile to you. IR's independance from nozzles makes
it infinitely more flexible in its application, as far as I'm concerned.
2. It's not so easy to burn yourself on IR equipment as it is with hot air
systems.
3. IR removes PTH devices as easily as it removes SMT parts - something Hot
Air does not do at all, or is much more difficult to do.
4. IR can (with some persuasion, admittedly) remove plastic connectors in
one piece, which Hot Air definitely cannot without melting the connector
out of all recognition.

Disadvantages of IR over Hot Air are relatively few:

1. To get the best out of the IR system, a lot more understanding of the
mechanics of what's going on is required than is the case for Hot Air. IR
is not so "Plug and Play". The understanding required is of the interaction
amongst all the elements involved in getting solder to reflow properly -
the heat being applied, how the component temperature is measured and fed
back (component emissivity vs temperature equivalent), profiling
requirements, formation of solder joints, behaviour of liquid solder, and
so on.
2. With no shielding like you'ld get with a hot air nozzle, the energy
input to the board is a little less confined to just one component - others
immediately round about may be affected. If this is a concern, then
measures have to be taken to shield them - such as by covering them with
cooking foil, shiny side outwards.

Don't know what else I can tell you for now. I've only had assisted trials
with a demo unit so far - no hard experience with "real" boards. Steve may
be able to fill in any gaps in my narrative, or even counter some of my
brief experiences from the point of view of the hardened veteran. He still
sounds like a kid with a new toy, as the novelty doesn't seem to have worn
off yet, so I guess he'll also give you all positives and few negatives.

If you're still in doubt about which to choose, get a demo unit and find
out for yourself! I think you'll be as impressed as I am.

Good luck,

Peter




Sam Abernathy <[log in to unmask]>      10/12/2002 11:02 AM

              To:  [log in to unmask], DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group@ST
              Domain
              cc:
              Subject: Re: Table Top Static Reflow Ovens








Hello Peter,

We are actually looking/evaluating BGA rework equipment (this would be our
2nd BGA system) for our lab/proto work.  You have some good things to say
about the PDR lightmaster.  We have not evaluated this system (in part due
to the fact that it is an all IR system...temeperature uniformity might be
an issue).

In all honesty, I am a bit confused about which is better forced convection
or IR.  According to previous evaluations, IR is a plus when we need a fast
ramp (up to 5 degrees per second, compared to up to 4 degrees that could be
achieved with forced air systems)however, 3 degrees per second is the max
that we have ever used.  Forced Convection is a plus in achieving a unifom
temperature across the component.  Most rework systems that I know of are
forced convection systems.  Every rep that demoed their equipment said that
forced convection is the prefered heating source thus most rework systems
on the market are forced air.  Not exactly a scientific answer, but is a
consitent one.  Can you provide some insight on the benefits/disagvantages
of IR/Forced Convection?

The price for the PDR system really makes it affordable.  All of the
systems that we have seen are in the $30-$40k range.  Is $14k for a new
unit with the vision/BGA alignment system, PC, monitor, etc.?  Sounds too
good to be true...lol

Thank You

Sam A.

On Tue, 10 Dec 2002 09:39:11 +0800, [log in to unmask] wrote:

>Hi, Steve,
>
>Yup! We've taken the plunge and bought one after getting the rep to
>demonstrate the removal and replacement of all kinds of things from 2 inch
>square PGA's to plastic and ceramic BGA's, plastic and ceramic LCC's -
even
>plastic connectors! We were hugely impressed with it. The guy even showed
>that you could leave the system cooking away while going for a cup of
>coffee and you wouldn't come back to find everything cooked to a crisp.
>Many folks avoid IR because of the misconception that it's not
controllable
>- but it is, isn't it? I'm looking forward to having some fun with it once
>it arrives.
>
>I'm interested on your take regarding the thermally consuctive tape. I was
>going to buy a couple of reels of the stuff along with the machine, but
was
>told that the tape's function is not actually to transmit heat. Rather,
its
>purpose is to improve the emissivity of things like metal-bodied
components
>for the feedback sensors that control the amount of heat being put into
the
>component. Our rep uses the machine himself for his core business of
>repairing elderly comms equipment cards, and is thus quite an expert in
>using it. It was he who also re-designed the original controller. His
>explanation is that it's component emissivity to the sensors that
indicates
>the component temperature and governs the amount of heating that a
>component receives. His experiments have shown that even using the tape,
>indicated temperatures can be out by 20 or more degrees compared with
>actual temperatures. His approach is not to use the tape at all, but to
use
>the natural emissivity of the component and derive a number of profiles
for
>different component body materials using thermocouples and noting the
>temperature reading on the controller. So if the thermocouple reading
shows
>the component is at 190 degrees, for example, but the controller only
shows
>150, that's OK. If you like, he's calibrating the controller readings to
>suit a range of component body material types. It makes sense in theory,
>but I don't have the benefit of practical experience yet to prove to
myself
>that what he says is valid. It means, though, that I'll have to sit down
>and tabulate what the controller readings should be for the components we
>rework where they're metal or ceramic - the systems "naturally" works best
>for plastic-bodied devices. WHAT D'YA THINK?
>
>I'm relieved to hear that you're still enthusiastic about your machine.
>Maybe we can compare notes from time to time on our findings and
>experiences.
>
>Peter
>
>
>
>Steve Gregory <[log in to unmask]>   10/12/2002 08:34 AM
>Sent by: TechNet <[log in to unmask]>
>
>Please respond to "TechNet E-Mail Forum."; Please respond to SteveZeva
>
>              To:  [log in to unmask]
>              cc:  (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
>              Subject: Re: [TN] Table Top Static Reflow Ovens
>
>
>
>
>
>
>
>
>Hi Peter!
>
>So, you guys are buying one? Pretty cool, ain't they? I've had mine for
>about 10-months (or there abouts) now, and love it!! One part that
>sometimes can be a problem I've found out, is PLCC sockets. Just gotta
make
>sure that the thermal conductive tape contacts, and is seated well against
>the leads at the top of the sockets so that you get good thermal transfer
>down to reflow the joints.
>
>But otherwise, I haven't run across a part I couldn't rework...even with
>boards being bonded to big, thick, heatsinks. Couldn't even get the solder
>plastic with hot air systems.
>
>-Steve Gregory-
>
>
> Hi, Bev,
>
> Not a reflow oven, but a rework station if you're interested in such a
> thing as an adjunct to your main oven. We have just ordered a PDR
> Lightmaster IR component rework station from a local company here in
> Singapore. They have redesigned the controller unit and the operating
> principle so that it now has a proper feedback loop that controls the
> power
> being put into the components very nicely, and doesn't involve all those
> nozzles that are required for a hot air system and which can cost quite a
> bit.
>
> The basic system costs about US$14K and seems to handle just about
> anything
> once you understand about the emissivity of the different component body
> materials. The company selling the re-modelled systems is Vanco Pte Ltd,
> but they don't have a website. Contact me off-line if you're interested
> and
> I'll give you more details.
>
> Peter
>
>
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