Re: Hot air smd rework station. SMD removal??? Defective Machines???

In article <Xns9A6A48F49B718WQAHBGMXSZHVspammote@xxxxxxxxxxxxxx>,
bz <bz+ser@xxxxxxxxxxxxxxxxxxxx> wrote:

Smitty Two <prestwhich@xxxxxxxxxxxxx> wrote in
news:prestwhich-12B49F.22303422032008@xxxxxxxxxxxxxxxxxxxxxxxxxxxx:

In article <Xns9A699E492EBACWQAHBGMXSZHVspammote@xxxxxxxxxxxxxx>,
bz <bz+ser@xxxxxxxxxxxxxxxxxxxx> wrote:

Smitty Two <prestwhich@xxxxxxxxxxxxx> wrote in
news:prestwhich-2AF65F.11475722032008@xxxxxxxxxxxxxxxxxxxxxxxxxxxx:

In article <Xns9A692879FD52DWQAHBGMXSZHVspammote@xxxxxxxxxxxxxx>,
bz <bz+ser@xxxxxxxxxxxxxxxxxxxx> wrote:

There is no reason to go about 225 C (437 F), for any normal solder,
and, if you remember the book title by Robert Heinlein, paper
doesn't burn till you get to 451.
[correction, Ray Bradbury, not RH. Sorry!]

I disagree. In order to get the solder to acquire melting temperature
in any reasonable length of time, the heat source has to be far above
the temps you mention. It is tedious to hand solder with a tip that
is less than 800 F, and desoldering requires similar temps. The
problem lies in heat transfer and loss.

You mistake the transfer of heat (calories) for temperature.
This is a common mistake.

I don't believe I'm mistaking the two, but I'm willing to be corrected
if you'll point out my error more clearly.

If the explanation that I gave was not clear, enough, I suggest the study
of thermodynamics and heat transfer. http://en.wikipedia.org/wiki/Heat is
a good place to start.

It isn't your explanation that lacks, only your suggestion that I'm
confused on the topic.


The raising of the temperature to the melting point of the solder requires
the transfer of a sufficient number of calories to the component. This does
NOT require a high temperature, it just requires efficient heat transfer.

I agree. Unfortunately, efficient heat transfer is difficult to
impossible in many hand soldering applications. The excess temperature
to a small contact area makes up for that.



The rate of heat transfer depends on TWO things: the difference in
temperature and the heat conductivity. In soldering there are other
important factors that often comes into play, the heat capacity of the
soldering iron and the power of the heating element.

Contact area is the holy grail of efficient heat transfer.


For soldering irons, there are two philosophies.
1) use a hot enough iron to rapidly transfer heat to the target and
remove the iron before the temperature gets high enough to damage the
parts.
Often the iron does NOT have enough power to raise a large object to
the iron temperature. This can make for both over heated components
and cold soldered joints.

2) use a temperature controlled iron that has enough heat capacity and
conductivity to rapidly heat the target to the soldering temperature.
In the second case, it is NOT important to remove the iron quickly
because it will NOT overheat the components.

I keep thinking that you speak from theory, not practice.

Incorrect. I soldered my first connections in the late 50's. When I was 8
years old.

First connections were made with 50/50 solder. Well I remember trying to
hold the lead perfectly stationary while my fingers burned.

I was first licensed as a ham in 1961 as WN5DQP at age 16.
I built Heathkit TVs and Scopes in the early 60's. Using 60/40 solder.

I worked on a resistor/capacitor production line in the late 60's, early
70's.

First as a Process Technician then as a Process Engineer for Sprague
Electric Co. I WROTE process specifications for Sprague's production line
for soldering capacitors to their leads in the early 70's.

After the 'mini recession' in the early 70's I went into consumer
electronic service. I owned a consumer electronics repair shop in the
70's. Got a bit of practical experience there.

In 74-76, I fixed radars and electronics on ships on the Mississippi. Got
a bit of practical experience there also.

I did board repair on DEC and DG computers in the late 70's. Component
level repairs on PCBs.

A couple of years ago, I built an Elecraft K2/100 ham transceiver.

Recently, I built several SoftRock RXTX SDR radios, using SMT components.

I think that qualifies as a bit of practice to go with a bit of theoretical
knowledge, a BS in Chemistry, 1970.

Credentials accepted.


It is
virtually impossible to solder with a 500 F iron,

I don't find that to be true. All one needs is a good clean iron properly
tinned, good 63/37 solder, and a good flux pen, clean, pretinned leads on
the components, a clean, pretinned PCB and proper technique.

How long does it take you to make one solder connection on a 1/4 watt
through-hole resistor on a typical board two-sided board with plated
through holes, at 500F? I ask because in production, 3 seconds is far
beyond unacceptable.


Of course, ANY oxide (and solder does oxidize rapidly) on the tip of the iron

and you have just added thermal resistance. The iron tip MUST be freshly
cleaned and tinned.

yet many components
supposedly can't stand even that for more than 5 seconds.

Many components CAN'T.

Then why do you assert that with low temps, component damage is
impossible?



Given good heat conductivity (clean and tight joints in the iron) and
sufficient heat capacity (plenty of watts), a temperature controlled
iron is much better. The iron temperature should be set slightly higher
than the melting temperature of the solder. There is no need for dozens
or hundreds of degrees in excess of the melting temperature.

We disagree quite strongly on this point

As I said originally, there are two philosophies.

And one of them works in practice!


, and I wonder on what you base
your perspective? Many, many years ago, the military presumed to insist
on 600F, and that proved to be woefully inadequate for hand soldering.

This is true, IF the iron is not clean or is underpowered. The point at
which a temperature controlled iron measures the temperature is also
important. The nearer the tip, the better.

Now, a solder bath, having an immense thermal mass, as well as providing
virtually total joint immersion, can solder well at 500.

Correct.

If I recall correctly, our 95/5 (tin/silver) solder pots ran at
495F(257C). The 62/36/2 pots ran considerably cooler but I don't remember
the numbers. There was a layer of hot wax on top of the solder to 'preheat'
the parts and protect the solder from oxidizing.

But hand
soldering with any measure of expediency requires *at least* 700, and in
my experience, 800 is far better.

Once you get the joint above the melting point of the solder, there is no
need for higher temperature. Proper heat transfer is the key.

See above.



In the case of a hot air gun, when there is sufficient heating capacity
and air flow, everything within the area of the air flow will be
quickly brought to the set temperature.

Hot air guns are much, much hotter than the melting point of solder.

Depends on the settings of the gun. My 'CSI Hot air gun 2'
<http://www.web-tronics.com/hotairgunwdi.html>
solders and desolders quite well at temperatures not much greater than the
melting point of the solder.

I have soldered and removed 48 lead SMT ICs without damaging the
surrounding SMT components. I have removed and later reused 8 lead SMT
ICs. This was done without badly charing the paper dams around the parts.
The paper WAS heated to a light brown so I believe the digital read out on
the hot-air gun.

Why
would that be, if the very low temps you advocate are actually
sufficient?

My hot air gun WILL go hotter, but after playing around with it for some
time, salvaging parts from old PCBs, I found that I could work quite well
with much lower temperatures than I had first tried. Nozzel size and air flow
rate are very important. The nozzel must be large enough to heat the area and
the flow rate must be sufficient to do so quickly.


caveat: Many components are rated for limited exposure to higher
temperatures. I don't know of ANY transistors or ICs that are rated to
withstand 800 F degrees for any length of time.

True. And they needn't be exposed very long. Through-hole ICs and
transistors may need approx. 1 second per lead. With a typical 16 pin
surface mount IC, three seconds is plenty to skate down one side and
solder all 8 pins, at 800F.

Or even at 440 F, with 63/37 solder, and then to 'mop up' with clean, well
fluxed solder braid.

One 48 lead component I worked with recently has an absolute max of
300C(572F) for 10 seconds.

Your 800F is 427C. There is a very good chance of damaging such an IC at
that temperature.

We do it all day long every day. Less than 1/2 second per lead. No
damage.



Final point: using the correct solder is important also. True eutectic
solders are best because they melt and 'freeze' at a single
temperature.

Non eutectic alloys, like 50/50 or 60/40, pass through a 'plastic
stage' where crystals of one of the components start to form.

And we had a discussion here not long ago, begun with the question of
why anyone would use anything other than 63/37, assuming a leaded
formulation. As I recall, no one offered any very plausible reason to
use anything else.

A small percentage of silver helps prevent leaching of silver from some
components and is vital for the mounting strips in some Tektronix scopes.
A small percentage of copper helps prevent leaching of copper from PCB
traces.

I'll accept your assertions. The original dialogue to which I refer
concerned only tin/lead mixtures.


Any movement while the joint cools through the 'plastic stage'
temperature range results in a 'cold solder joint' where there are
actually separate crystals of the component metals and conductivity is
unreliable.
.

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