by Conrad Weiss - NN6CW
I received a number of technical questions re: the Weller WTCPT solder
station and solder tips. Since I'm *not* a real "Doctor of Solder," I decided
to call one on the phone. I had a nice chat with Mr. Larry Smith in Weller's
engineering support division, and a thirty-year veteran in the soldering
industry. Here we go with, "Ask Dr. Solder."
Q: Why is a solder station "better" than a non-temp controlled (unregulated)
soldering iron?
A: A non-temp controlled iron *continues* to increase in tip temperature
the longer it's plugged in -- up to 1100 degrees-F ! A solder station controls
and maintains an exact tip temperature. In the case of 700 degree-F tips,
you'll be soldering at a tip temperature of 400 degrees *less* than a non-temp
controlled iron. A lower tip temperature with good technique, translates
to better joints with less chance of damage to sensitive components --
IC's, etc. (The 700 degree-F Weller tips will *not* exceed 710 degrees-F.)
Q: Which tips do I use -- 700 or 800 degree-F ?
A: The 700 degree-F tips are typically recommended for single layer
and double-sided PC boards -- even four-layer PC boards. This includes
practically everything in the world of amateur radio kits & homebrew.
The most common tip for the WTCPT station is the Weller PTA-7 (1/16" screwdriver
tip). You might also want a PTD-7 for much bigger pads/lugs, and a PTJ-7
for longer-reach work -- or <gasp> rework ;) If you want to try surface
mount, pick up a PTS-7 for a start. The 800 degree-F tips are recommended
for serious multi-layer boards -- 4 to 8 layer PC boards -- *not* typically
used for ham radio kits ;)
(Note: Elecraft recommends the WTCPT station
with PTF-7 1/32" tip for K2 construction.)
Q: Is the tip of a WTCPT really grounded ?
A: YES !!! All Weller solder stations have grounded tips. You can check
the integrity of the tip ground as follows:
Turn the solder station OFF and let it cool. Pull the plug OUT of the
AC mains and using your DMM (ohm meter) check for continuity between the
ground plug on the Weller AC plug (you know the one) and the tip of the
soldering iron. You should be reading approximately one ohm. If your resistance
is considerably higher, you probably need to clean the tip (with 600 emery
cloth, etc.) and possibly remove the tip and the "barrel" (the hollow part
that screws the tip to the iron) and clean the threads with a brass/bronze
brush to remove any oxide build-up. Never use a 3-prong to 2-prong "ground
cheater plug" -- obviously, this defeats tip grounding and ESD-safety.
Q: What is the meaning of "ESD-safe" in soldering stations?
A: "ESD-safety" guarantees that static energy from your body (hands,
etc.) will be "dumped" to ground via the plastic soldering iron handle
or solder station plastic housing. Otherwise, the ESD would transfer to
the soldering tip, thus destroying the IC you're soldering. The solder
station essentially discharges your body's static charge to ground every
time you pick up the soldering iron. (Naturally, you should take additional
ESD precautions if you're handling *extremely* sensitive components --
and not rely on the solder station for all ESD-safety in the lab!)
Q: How long should it take to make a "good" solder joint on a common
solder pad?
A: Tricky question! The time spent on the pad is called "dwell time."
A dwell time of 4 seconds at 700 degrees-F might be considered destructive
for some components -- however, a dwell time of 2 seconds with an 1100
degree-F unregulated soldering iron can be much more destructive. A 700
degree-F dwell time of 3-4 seconds is not uncommon, to allow for "filling
the pocket" -- that's the volume of the thru-plated hole, flush to the
component side and properly "feathered" evenly to the circumference of
the solder pad -- on the solder side. The "pocket" should be filled --
but not over-filled -- and the solder in the center of the pad should rise
evenly to the component lead that will be trimmed. Never have a solder
pad with an under-filled "dimple" on the pad. The solder level at the component
lead should be higher than the edges of the pad. A soldered pad should
never look like a "round BLOB of solder *sitting* on the pad." Wire soldering
is more of an art than a science, and with practice -- 1 to 3 second dwell
times can be achieved, while safely meeting all of
the above criteria.
Q: What about residual flux that's left on the board?
A: Flux removal is an issue that's best answered by the solder and
board manufacturer. However, the *color* of the flux -- after soldering
a pad -- is a good indicator of whether your dwell time is too long. Your
residual flux should look "honey-colored" rather than "caramel colored."
If you're starting to burn the flux, your dwell time is too long, OR your
tip temp is too high, OR you need to consider a different tip shape, OR
applying heat to the pad/component lead from a different angle. Heat primarily
the pad, and catch the lead with the side of the tip -- then practice,
practice... practice. Have a friend time you and inspect your work if you're
serious.
Q: Any closing remarks?
A: Soldering stations allow for greater soldering consistency when
compared to non-temp controlled irons. Overall, this translates to fewer
cold joints, fewer heat-damaged components and fewer heat-damaged pads
and traces. The quality of your work will be superior, compared to soldering
with non-temp controlled irons. Remember to wash your hands after you're
finished handling solder. Never smoke cigarettes or eat while soldering.
Never allow kids to play with solder. And, practice soldering! It's still
an art -- not a science!
There you have it! A conversation with "Dr. Solder."
Happy soldering,
Conrad Weiss - NN6CW -- remember, I'm *not* a real doctor ;)
!
Elecraft Note: The Weller WTCPT station is available from WASSCO, http://www.wassco.com/was/wtconoutstat.html