N Cook said:
Don't know about different types of lead-free solder.
But distinguishing lead free I've found is
1/ conical rather than domed solder joints
2/ putting a stainless steel sewing needle in some of the solder after
melting and extracting needle while still molten, it is much more
difficult
to remove the cold solder from the needle than doing the same with leaded
solder.
And many boards now actually state that they are lead free or "PbF" on the
silk screening. As far as I have been able to tell, it's not so much about
mixing different types of lead-free alloys, which may or may not contain
small traces of other metals such as silver, but more a case of not mixing
lead-free with leaded solder.
To Jim. All of my experience with this stuff is from a service rather than
production point of view. You are of course right that manufacturers use
wave or reflow soldering, and have done for many years. The point I was
making about lead-free joints and Weller TCPs at 700 deg, versus
manufacturers' joints, was perhaps not grammatically well-made. What I was
basically saying was that the manufacturers, with all of their expertise and
expensive production soldering equipment, still can't get to grips with the
stuff themselves, and are still producing equipment littered with bad joints
from day one. So, if you are making consistently good 'production' joints in
lead free, using 700 degree hand soldering equipment designed way way before
any eco-prat had ever come up with the concept of taking the lead out of
solder, then you (your company) are doing, on average, better than the big
boys.
As far as heat and temperature are concerned, I take your point that they
are not the same thing, and I don't think that I am confusing the two. They
are however, inextricably linked to one another by external influences.
Energy, in the form of heat, is what has to be put into a body in order to
raise its temperature. All solder has to have its temperature raised to the
point where its liquid state becomes suitable for making a soldered joint,
and then maintained at that temperature until the joint is completed. The
temperature at which this condition occurs for lead-free solder, is higher
than that of leaded solder. If you are just making small joints, then this
is of no consequence, and a 700 degree tip is fine for the job. With a
leaded joint - even a large one that causes the tip temperature to drop by a
few degrees - that drop is again of little consequence, as there is plenty
of temperature 'overhead' available from a 700 deg tip. However, with
lead-free, 50 degrees of that overhead, have already gone, so if a joint is
any bigger than 'small', the additional temperature drop at the tip, caused
by the joint leaching heat from it, results in a less than adequate tip
temperature being maintained, to correctly complete the joint. The result is
a bad or 'cold' joint. A 700 degree tip simply cannot maintain enough
heatflow into the solder, to keep it at a sufficient temperature to do a
'good job' on anything other than a small joint, and this is particularly
the case where a 'typical' repair workshop tip of small dimensions is used.
I have a repair service for a particular board which uses lead-free, and I
see many of them where the shop that's sending it back to me, have attempted
some rework or component replacement, and it's quite obvious that they have
been trying to use their normal leaded soldering equipment to do the job,
with the inevitable consequences.
I can accept what Cooper say about not being tempted to increase the tip
temperature, but I think that they are probably talking more about not going
up far enough to get the same 'feel' with lead-free, as with leaded. Most
commentators on the subject, including soldering equipment manufacturers,
agree that a higher nominal tip temperature is required to work reliably
with lead-free.
One of the main reasons that manufacturers are having so much trouble with
the stuff, is that they have to run their soldering processes at a higher
temperature. This then brings them close to the maximums that some of the
components can tolerate for any length of time, so they have to compromise
and run the process at a slightly reduced temperature. That is fine until
you have to solder a connector or power semiconductor - particularly one
that is mounted on a heatsink, and that is where many lead-free bad joints
are occuring. Trust me, if manufacturers didn't *have* to run their
processes at a higher temperature, with all of the implications of that,
including a higher energy useage, to cope with lead-free, then they wouldn't
....
Arfa
