In article 39,
bz wrote:
Smitty Two wrote in
news
In article 39,
bz 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 Henlein, paper doesn't
burn till you get to 451.
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.
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.
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. It is
virtually impossible to solder with a 500 F iron, yet many components
supposedly can't stand even that for more than 5 seconds.
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, 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.
Now, a solder bath, having an immense thermal mass, as well as providing
virtually total joint immersion, can solder well at 500. But hand
soldering with any measure of expediency requires *at least* 700, and in
my experience, 800 is far better.
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. Why
would that be, if the very low temps you advocate are actually
sufficient?
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.
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.
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.