The board labeling certainly helps with the lead vs. lead free
problem, but the link you posted also states that the different lead
free alloys should not be mixed. The do seem to imply that mixing lead
in with the lead free is worse, but I can't tell whether the problem
there is an engineering problem or a legal one.
We should all remember that the big boys also had soldering problems
before the lead free mandate. AFAIK, those problems were mainly due to
trying to run the boards over the wave soldering machines too fast. I
doubt that present day production quotas are any less compelling.
I'm glad to see that you do understand the difference between heat and
temperature, and the causes and effects of heat flow, but we won't
settle this argument until someone can post the actual melting point
of 63/37 and the solidus/liquidus of some of the lead free solders.
ISTR that the good old eutectic stuff melted at something like 370 F,
but I wouldn't bet on it. I'm pretty sure that it's somewhere below
400 F, so another 50 F is not such a big deal.
I don't find an actual tip temperature in the Weller link, but they do
say to use the lowest temperature possible. Since I find that I have
no trouble (really!) soldering with a 700 F tip, and everyone in our
lab uses the same irons with the same alloys, I see no reason why
anything hotter would be justified.
It is also quite true that I do not work under a quota requirement,
nor am I paid by the joint, so I don't mind if the joint takes 3
seconds instead of 2. OTOH, I'm not really much aware of any change in
my soldering habits or technique since I used my first WTCP iron in
about 1977.
I think they would do just as well if they ran more slowly at lower
temps, but they probably find that is even more expensive than the
higher temps.
We won't really know the answer until we know the actual liquidus
temps of the Rohs solders. A check here gives us some facts to
consider:
http://kester.com/en-US/technical/alloy.aspx
I note that most of these range from 420 to 450 F, with 2 eutectic
alloys that melt at 430 and 440. 700 F seems like plenty of overhead
for working at those ranges, at least to me.
-
All points noted, and for the most part, agreed with. I think what I am
saying, and probably Graham too, if I understand him correctly, is that for
most modern service work, a small tip is needed, which is less than ideal
for transferring heat from the iron's element to the actual joint, which
should be being made by the joint itself being heated, and the solder 'fed
in', as is the case with leaded solder. The result of that less than ideal
sized tip is that inevitably, as a large joint leaches heat from it, the
temperature drops, perhaps 50 or more degrees. With leaded solder, this is
of little consequence, as the drop in temperature is still well above the
point where the solder is able to be worked, to create a guaranteed good
joint. However, that said, try it on a big enough joint, and that won't be
the case. With lead-free, the drop in tip temperature is of greater
significance, as it readily causes poor workability of the solder, a 50 or
more degree drop taking you a lot nearer to the point where the solder works
'pasty' rather than fluidly.
So in this case, a tip that starts off at 800 deg and then drops towards 700
'under load' appears to represent a tool better suited to the job 'on
average', bearing in mind that as service engineers, we are seeing many
types of equipment that need, in theory at least, a similar variety of
different soldering equipment. So what we are using is a working compromise,
that has to be able to cope with leaded as well as lead-free solder, and
everything from IC pins to BNC connectors or worse.
To some extent, the points raised are moot in that there are now much better
tools on the market for coping with the modern situation, without having to
compromise. Someone mentioned Metcal stations for instance. Any station with
a tight control loop, is much better than a TCP for general service work
these days. I run an Antex temperature adjustable station, which I keep
idling at about 680 degrees. I turn it up to 750 when using lead-free, as I
find from a purely personal point of view, that this temperature suits both
me, and the Ersin 306 alloy that I use. I still keep my TCP running, with a
700 deg tip mostly, more out of comfort as it has been with me many years,
but I do find that I use it less now.
As far as the manufacturers having trouble with the stuff goes, I don't
think that it is to do with deadlines per se. I can remember when PCBs were
first around, and the technology advanced very quickly, driven mainly by the
big Jap consumer market manufacturers, to the point where bad joints on
their equipment were virtually unheard of, and that has remained the
situation for many years now. Given that wave and reflow soldering were
fully mature and largely trouble free and reliable technologies, one would
have expected that the degree of understanding that they must have of the
processes involved, would have allowed them to slip seamlessly into
similarly reliable manufacturing with lead-free. This doesn't appear to have
been the case, and equipment is still coming out of factories with less than
satisfactory joints, which tells me that the problem is with the technology
itself, not how the manufacturers are incorporating it into their overall
manufacturing time budget. When push comes to shove, it is a replacement
technology that arguably wasn't required in the first place. The original
technology involved in soldering was the right one, as has been shown over
many years, and the replacement uses materials deemed to be ecologically
better, but which don't actually readily do the job that they are needed to.
The lead-free solder is being 'made to fit' on the back of the "green"
ticket if you will, and sod the consequences ...
Arfa
