Phil,
The problem is that the solder wick acts as a large non-uniform heat sink.
Even though parts of the wick appear to be molten, other parts, particularly
those in contact with some thermal mass or surface area may not be. So the
part touching the pad is below melting temperature, and the part where the
iron is actually touching is molten. And the wick is a most excellent heat
sink. Lots of surface area, and radiating out into the air. It sounds like
possibly your tip temperature is too low, or the iron is too low a wattage
to be effective with wick. I've been soldering/desoldering through-hole and
SMD for many years, both in a hobby and a production environment, so I've
been through everything you've had happen to you, at one time or another.
All copper material on the board is bound to the underlying material
(fiberglass, whatever) with an adhesive. These adhesives have a rated
delamination temperature, at which point they let go pretty quickly. I used
to know the temperatures of typical PC board material by heart, now I have
to look it up. Really good irons (like Metcals), offer 3 tip temperature
ranges. A temperature above typical 63/37 solder melting points but below
the delamination point, a temperature slightly above the delamination point,
and stupidly hot. Never really had an application for the last one.
Many times people get away with using too hot an iron for a couple reason:
They don't put any (or much) mechanical pressure on the pad, so even if the
adhesive goes above the delamination point, the pad won't shift. However,
you can often see signs of beginning liftage, usually expressed as ripples
in the pad surface as the adhesive starts going through phase change.
Another is that there is sufficient thermal mass in the joint to keep the
pad temperature below the delamination point. That more or less amounts to
luck, but if you keep the iron applied, the pad will eventually lift as
everything reaches a steady state that's above the delamination temperature.
Properly selected tips are above the solder melting temperature and below
the delamination temperature. In fact, with a good well regulated iron, you
can leave it applied to the joint all day and never damage the board (cook
the chip by exceeding junction temperature, sure).
Solder wick is the gift of the gods for SMD work, but usually not the right
thing for through-hole.
For desoldering, there are several basic technologies. Wick, solder bulb,
solder pump, and electrical pumps. Wick is pretty obvious. Wick is good at
removing gross amounts of solder, but without adequate temperature control
and application, you can lift pads. Solder bulbs are junk. In fact,
they're more useful for blowing air *though* a hole than sucking solder.
Solder pumps are spring actuated suckers that are triggered by pressing a
release button. Generally effective, assuming the tip is in decent shape,
and the plunger has dried out. These need to be cleaned and lubricated (use
vaseline) periodically, or they don't suck well. A good indication is if
you fire it against your moistened thumb, and you can still feel suction a
few sections later, it's a good one. I haven't seen any without ESD tips
lately, but you *really* want that. A plunger moving through a plastic body
can build up a helluva charge on a plastic tip. A good tip doesn't remove
that possibility, but instead keeps the pump the same potential as the
board. Electical pumps (for lack of a better name), are devices like Pace
machines that have a soldering iron with a hollow tip and a vaccuum pump and
a filter system. Removed solder is tpyically pulled in a glass tube, and
the tube periocally emptied. Pace units, in particular, are lousy at
temperature control. Also, using an improperly sized or conditioned tip can
result in rapid board destruction.
It is impossible to adequately preach on the subject of decent irons. Many
people get through life with a poor soldering iron, and just write the
occasionally lifted pad or damaged board to lack of experience. Many people
also think, for example, that a Weller WTC-201 iron is a decent iron. The
reality is it's shit for temperature control. That whole magnetic tip
temperature control thing under goes 100 degree temperature swings in either
direction. Put a pyrometer on one and start using it, you'll be appalled.
In my book, there is only one iron to own, and it's a Metcal MX-500 series.
I'm sure there are some others out there claim (and do) match the stability
and usability of a Metcal, but they're few and far between, and I don't have
any experience with them.
A Metcal is unlike anything you'll ever use. Tip temperature stability
under any load is less than 10 degrees. The tip heats up in about 10
seconds, cools down in about 20. There are dozens of tips, fine enough to
solder < 25 mil leads to 15 mil pads, and large enough to solder a stack of
5 pennies to a piece of 2oz copperclad board.
They're not cheap, but had can be had for good prices on eBay. However,
quality is rarely ever low cost, and with Weller, Hakko, and others, you get
what you pay for. A usable iron. You get a Metcal, you'll look for things
to solder just to use it.
Anyway, I digress. There are some basic core hints for soldering and
desoldering. Never use the tip as a pry bar. This should be obvious why,
but beyond simply distorting the tip from it's natural shape, it breaks the
cladding on the tip, causing improper wetting and non-uniform heat
distribution. The tip will develop cold and hot spots, worse than usual
(even the best tips on the best irons seem to have "sweet spots"). Don't
"scrub" the tip for the same reason. Most tip wear is a result of scraping
the pin against pins. Unavoidable now and then, but you should bring the
tip to the joint, let heat briefly (usually until you see the existing
solder go shiny), apply solder, pull the tip away.
Use a clean tip, sponge it frequently, keep it tinned. Cheap tips are
sometimes subject to thermal shock when you wipe them on the sponge. This
will result in premature failure of the tip, as the cladding weakens.
Usually when you find a tip where the cladding has started flaking, this is
the core reason. Tips from manufacturers like Weller, Hakko, Metcal, etc
won't do this. The ones at K-Mart are more likely to. However, tips are
replacable for a reason. A clean tip with a medium life is more important
than a crappy tip with a long life. And, incidently, I have some 4 and 5
year old tips that I use on a regular basis that are still in good shape. I
have a couple I keep reserved for abuse. When a tip starts to fail, REPLACE
IT. Life is too short to make lousy connections and get in a bad mood
because a number of joints are turning out poorly. Just toss the damn
thing, it's only $10 or so.
Flux is important. When building up a bare board, I usually polish it
first on a piece of 20lb paper, or a paper towel. This will knock the gross
oxides without tearing up the soldermasks (and cheap board houses often use
a pretty thin mask). There are dozens of choices in solder these days.
Rosin core, water soluble, no-kleen, organic fluxes, 63/37, 62/37/1 (1%
silver), wire or paste, etc. For most through-hole work, I like a rosin
core .035 diameter 63/37. It's cheap and reliable, but requires a defluxing
agent to clean. This stuff is getting expensive, and since CFCs where
outlawed, most of the new defluxers are crap compared to the good stuff.
However, rosin is a very good flux. Most of the newer water soluble fluxes
just aren't the same. They work, but their not as effective, particularly
if you're using components that haven't been environmentally sealed. A flux
pen helps immensely. For SMD (most of my stuff these days), I use a 62/37/1
water soluble flux, and clean the board with a toothbrush and hot water.
Using a liquid flux (from a flux pen or a jug) helps a lot with both
contaminants and to help prevent bridging on fine legged SMD components. If
you use a lot of flux, you have to let the joint boil clean before bringing
in the solder. Otherwise you get some funky looking questionable joints.
No-kleen solders are usually for production environments, and they suck.
They leave a sticky residue on the board that's non-corrosive. However, you
always wash your hands after handling such a board.
Probably nobody has really read this far, and I can't think of anything
else at the moment, and my fingers are tired.
--John
-----Original Message-----
From: cctalk-admin_at_classiccmp.org [mailto:cctalk-admin_at_classiccmp.org]On
Behalf Of Philip Pemberton
Sent: Tuesday, December 10, 2002 18:14
To: cctalk_at_classiccmp.org
Subject: Re: One more screwup with the Ace...
Dwight K. Elvey wrote:
> If you use a solder sucker, don't use a small one.
> Use one with as big a bore and stroke as you can find.
Hmm... I've got an Antex (www.antex.com) "PRO-DESOLD" black desolder pump in
my toolbox. I take it that would be better than a roll of Soder-Wick (sic)?
Thanks.
--
Phil.
philpem_at_dsl.pipex.com
http://www.philpem.dsl.pipex.com/
Received on Tue Dec 10 2002 - 17:59:01 GMT