Nicholas,
I have not been around long enough in the industry (only 8 years) to
have any "old" thinking on the topic, so let's look at the whole picture.
Generally speaking for most chemical reactions around room temperature (and
all the temperature ranges we are throwing around here are) reaction rates
double with every 10C rise in temperature. This of course assumes that
enough heat/alternate energy source is available to provide the activation
energy for reactions to even happen. We will also assume that the reactions
we are interested in are not diffusion limited. A little more on that
later.
With that being said, any chemist would tell you that the cooler
room idea is a good one. But now let's look at it in not such a theoretical
sense, but in terms of printed circuit boards. In the past, say the 1980's
and into 1990-91 one could still receive boards from a supplier, especially
from the humid Far East, and have problems with them in manufacturing
because of outgassing during manufacturing. This was usually due to
moisture explosively vaporizing inside the board and then coming out through
blow holes in the plated through holes, forming holes in the joints or even
blowing the majority of the solder right out of the joint. We are talking
about more than a thousand fold increase in volume here as the water turns
to steam. To counteract this people would place their new incoming boards
in an oven for x temperature for y time (can't remember the conditions!).
This solved the symptom, but not really the issues - poor bare board
manufacturing environment and poor hole quality! Today if you are buying
from a quality board vendor you should not be seeing this problem, even with
Thermount (Aramid) boards. At least we haven't and you can bet we've
looked. So we just store our boards on shelves in the air-conditioned plant
proper. Of course we are not a military provider that has signed a contract
to provide 10 radios today and another 10 just like it in 10 years either.
We go through our boards pretty fast. If you are in the same manufacturing
stream as us and have good vendors, why do anything?
There is another reaction I have not mentioned yet and that is the
growth of intermetalics between the HASL layer and the copper. With enough
heat and time and aided by a thin HASL layer, a warm oven can consume all
your free solder on the board. This one may be diffusion controlled, I
don't remember. The fellows at the British National Physics Lab could tell
you more about this possibility.
And getting REALLY esoteric, (hope you can tell I'm really enjoying
doing this :) ) if you are dealing with boards with a 100% tin finish
there is the possibility that your tin could go from metallic tin to gray
tin at 13.2C. Hey metalurgists, can this still happen in the presence of
lead and antimony?
If we look at even the bigger picture there are costs involved with
both heating and cooling, which will vary whether you are near Hobart on
Tasmania (where snow is possible) or near one of the great Australian
deserts (no map handy). Next, if you chose to keep your boards in a cool
room there is the possibility that your system could lose coolant. If you
have a very moist climate, and if the dew point is reached in the chamber
and this happens often you will be condensing water onto your boards, which
will enhance surface oxide formation to some small extent. Also coolant
loss would put even more CFC's, HCFC's or HFC's into the atmosphere and
contribute to what may or may not be happening - global warming.
regards,
Bev Christian
Nortel
> ----------
> From: Nicholas Kane[SMTP:[log in to unmask]]
> Sent: Thursday, May 14, 1998 9:53 AM
> To: [log in to unmask]
> Subject: [TN] Storing PCB's
>
> I am confused.
>
> For years, I have understood that the best way to store pcb's prior to
> assembly is in a heated, relatively dry environment. I am sure that the
> general consensus amongst most manufactureres is that this works best.
>
> Well, just recently I was told by a company setting up a new facility that
> they had been advised by a well known consultant to use a cold dry
> environment. They are planning a 12-14 degree Celsius storeroom, once
> again relatively dry.
>
> Is there new thinking on this, or have they been badly advised?
>
>
>
>
> [Nicholas Kane]
> [Axion Australasia]
> [Suite 3, 651 Canterbury Road]
> [Surrey Hills]
> [Victoria 3127 Australia]
> [tel: 613 9899 3511 fax: 613 9899 3811]
>
> ################################################################
> TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
> ################################################################
> To subscribe/unsubscribe, send a message to [log in to unmask] with
> following text in the body:
> To subscribe: SUBSCRIBE TechNet <your full name>
> To unsubscribe: SIGNOFF TechNet
> ################################################################
> Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional
> information.
> For the technical support contact Dmitriy Sklyar at [log in to unmask] or
> 847-509-9700 ext.311
> ################################################################
>
################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe: SUBSCRIBE TechNet <your full name>
To unsubscribe: SIGNOFF TechNet
################################################################
Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information.
For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311
################################################################
|
