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Desoldering: Thin layer of solder left on pads

Hi,

I'm not sure if this is a problem or not. When I desolder resistors
from a PCB, the desoldering wick sucks up enough solder to remove the
resistors, but leaves a thin layer of solder on the copper pads that is
_extremely_ difficult to remove. Is this a normal desoldering
experience? Do I have to remove all of the solder before inserting a
new component, and if the answer is 'yes' do you have any
suggestions on how to do this?

The materials I'm using are:

Desoldering braid dipped in liquid rosin flux
25-watt Weller soldering iron
97/03 tin/silver solder

Thank you very much.


P.S. Also, I would like to thank the people that responded to my
voltage polarity question a couple of weeks ago. You guys really
helped a lot.
 
J

John Popelish

Jan 1, 1970
0
...When I desolder resistors
from a PCB, the desoldering wick sucks up enough solder to remove the
resistors, but leaves a thin layer of solder on the copper pads that is
_extremely_ difficult to remove. Is this a normal desoldering
experience?

Yes. What is left is called a tinned copper surface and is perfectly
fine. It will help the new solder to coat the copper easily.
Do I have to remove all of the solder before inserting a
new component, and if the answer is 'yes' do you have any
suggestions on how to do this?

No. Once enough solder has been removed to easily remove the original
component and insert the new one, you have removed enough solder.
The materials I'm using are:

Desoldering braid dipped in liquid rosin flux
25-watt Weller soldering iron
97/03 tin/silver solder
(snip)

That solder has a high melting point that will easily damage many
circuit boards, separating the copper from the substrate. Must you
use lead free solder? 63% tin, 37% lead solder melts at a much lower
temperature. 62% tin, 36% lead, 2% silver melts almost as easily, and
gives a fine looking joint.
 
John said:
That solder has a high melting point that will easily damage many
circuit boards, separating the copper from the substrate. Must you
use lead free solder? 63% tin, 37% lead solder melts at a much lower
temperature. 62% tin, 36% lead, 2% silver melts almost as easily, and
gives a fine looking joint.

I've tried 60/40 tin/lead solder and noticed a huge difference between
the solder with lead and the stuff without, especially when I was using
a 15-watt iron from Radio Shack. That iron could barely melt the
lead-free stuff. Most of my solder joints looked like crumpled up tin
foil.

The main reason I was using lead-free solder was to make it safer for
people/pets, because I'm working at home. I'm not sure if that's a
good reason or not. I just wasn't crazy about having lots of little
pieces of lead laying around for unsuspecting critters to walk in and
track around the house, or the best way to clean the mess up, so I
avoided the problem entirely. :)

With that said, I may switch back now, as long as I can keep my work
area squeaky clean. What sort of steps to you take to keep everything
tidy? Just a decent broom? Or do you even worry about it?

Thanks again for the help.
 
J

John Popelish

Jan 1, 1970
0
I've tried 60/40 tin/lead solder and noticed a huge difference between
the solder with lead and the stuff without, especially when I was using
a 15-watt iron from Radio Shack. That iron could barely melt the
lead-free stuff. Most of my solder joints looked like crumpled up tin
foil.

The main reason I was using lead-free solder was to make it safer for
people/pets, because I'm working at home. I'm not sure if that's a
good reason or not. I just wasn't crazy about having lots of little
pieces of lead laying around for unsuspecting critters to walk in and
track around the house, or the best way to clean the mess up, so I
avoided the problem entirely. :)

With that said, I may switch back now, as long as I can keep my work
area squeaky clean. What sort of steps to you take to keep everything
tidy? Just a decent broom? Or do you even worry about it?

I do a lot of soldering on the kitchen table and I try to remember to
wipe it clean before making a sandwich there, but I don't worry too
much about it. The lead in 63 37 solder is pretty well locked up in
the alloy as far as absorbing it in the body, compared to lead oxides
used in old paint, etc. I think there is very little risk from
ingesting an occasional tiny solder ball.

Make sure you have a good quality, activated rosin core solder
designed for electrical work. For hand soldering, I have less success
with the no clean solder. The flux just doesn't do as good a job at
helping the solder wet the board and component leads. Never use
plumbing solder paste flux on a circuit board. It leaves a residue
that conducts electricity and corrodes metals it contacts.

If the flux and temperature are right, the joints should have a bright
shine, especially when eutectic alloys are used. These do not go
through a pasty stage (crystals of the excess metal growing in the
eutectic liquid before the freezing temperature is reached) but
solidify very suddenly.

Here is an interesting document on some flux choices:
http://rocky.digikey.com/WebLib/Kester/Web Data/285 RMA Solder.pdf
 
J

John Popelish

Jan 1, 1970
0
[email protected] wrote
I've tried 60/40 tin/lead solder and noticed a huge differenc betwee
the solder with lead and the stuff without, especially when I wa usin
a 15-watt iron from Radio Shack. That iron could barely melt th
lead-free stuff. Most of my solder joints looked like crumpled u ti
foil

The main reason I was using lead-free solder was to make it safe fo
people/pets, because I'm working at home. I'm not sure if that's
good reason or not. I just wasn't crazy about having lots o littl
pieces of lead laying around for unsuspecting critters to walk i an
track around the house, or the best way to clean the mess up, so
avoided the problem entirely. :

With that said, I may switch back now, as long as I can keep my wor
area squeaky clean. What sort of steps to you take to kee everythin
tidy? Just a decent broom? Or do you even worry about it
I do a lot of soldering on the kitchen table and I try to remember to
wipe it clean before making a sandwich there, but I don't worry too
much about it. The lead in 63 37 solder is pretty well locked up in
the alloy as far as absorbing it in the body, compared to lead oxides
used in old paint, etc. I think there is very little risk from
ingesting an occasional tiny solder ball

Make sure you have a good quality, activated rosin core solder
designed for electrical work. For hand soldering, I have less succes

with the no clean solder. The flux just doesn't do as good a job at
helping the solder wet the board and component leads. Never use
plumbing solder paste flux on a circuit board. It leaves a residue
that conducts electricity and corrodes metals it contacts

If the flux and temperature are right, the joints should have a brigh

shine, especially when eutectic alloys are used. These do not go
through a pasty stage (crystals of the excess metal growing in the
eutectic liquid before the freezing temperature is reached) but
solidify very suddenly

Here is an interesting document on some flux choices
http://rocky.digikey.com/WebLib/Kester/Web Data/285 RMA Solder.pd
 
J

Jasen Betts

Jan 1, 1970
0
Hi,

I'm not sure if this is a problem or not. When I desolder resistors
from a PCB, the desoldering wick sucks up enough solder to remove the
resistors, but leaves a thin layer of solder on the copper pads that is
_extremely_ difficult to remove. Is this a normal desoldering
experience? Do I have to remove all of the solder before inserting a
new component,

no, only enough to get the new component in, if you're experimenting with
different components you can often temporarily mount them on the underside,
which can be easier, but through-hole mounting is mechanically stronger
and therfore reccomended for permanent replacements.

Bye.
Jasen
 
R

Rich Grise

Jan 1, 1970
0
Hi,

I'm not sure if this is a problem or not. When I desolder resistors
from a PCB, the desoldering wick sucks up enough solder to remove the
resistors, but leaves a thin layer of solder on the copper pads that is
_extremely_ difficult to remove. Is this a normal desoldering
experience? Do I have to remove all of the solder before inserting a
new component, and if the answer is 'yes' do you have any
suggestions on how to do this?

I have a suggestion for a thought experiment: If you've removed enough
solder that the old component falls out of the hole, what will happen
if you try to insert a new component lead into the same hole that the
old one has just fallen out of? :)

Then, part 2: Once the new component is in place, what gets put back
into the hole? :)

Should I leave spoiler space? ;-)

That's as clean as it gets. To get any more solder off of it, you'd
have to use sandpaper. The copper is _tinned_, that means "coated with
solder". If, after wicking, there are no more big huge lumps, then
your wicking job is done.

I apologize if I sound condescending and patronizing here; after all,
this is the basics group, and any question is acceptable.

But, I _do_ wonder, being a tech myself, I have to ask, haven't you just
tried it and see?

If, in fact, the question was, "am I leaving too much solder?", then the
answer is, "If the lead fits, no." :)

On the other hand, if you're having trouble getting solder out from the
inside of the plated-through hole, then the answer could be a solder
sucker:
http://www.google.com/search?q=solder-sucker

Sorry it took this whole tirade to get to it! Shoulda signed it "Rich
the stoner." %-}

Good Luck!
Rich
 
R

Rich Grise

Jan 1, 1970
0
I've tried 60/40 tin/lead solder and noticed a huge difference between
the solder with lead and the stuff without, especially when I was using
a 15-watt iron from Radio Shack. That iron could barely melt the
lead-free stuff. Most of my solder joints looked like crumpled up tin
foil.

The main reason I was using lead-free solder was to make it safer for
people/pets, because I'm working at home. I'm not sure if that's a
good reason or not.

It's not. People have been using 63/37 eutectic solder in their home labs
for ever since electronics was discovered, and nobody's reported any ill
effects yet.
I just wasn't crazy about having lots of little
pieces of lead laying around for unsuspecting critters to walk in and
track around the house, or the best way to clean the mess up, so I
avoided the problem entirely. :)

Just don't leave little pieces of _solder_ (you're not melting down car
battery plates, are you?) lying around where pets and children can lick
them up off the floor. And even then, the dead epidermis cells and dust
mite feces are probably more hazardous. ;-p
With that said, I may switch back now, as long as I can keep my work
area squeaky clean. What sort of steps to you take to keep everything
tidy? Just a decent broom? Or do you even worry about it?

Just do your normal, usual, day-to-day, boring, mundane, uninteresting,
tedious, dull, dreary, mind-numbing, tiresome, lackluster, unexciting,
monotonous, repetitive, wearisome, humdrum, uninspiring housekeeping as
usual. Stuff that falls on the floor, even little droplets of solder, is
just dirt. Just treat it like ordinary dirt, and you'll be perfectly fine.
:)
Thanks again for the help.

You're more than welcome. :)

Have Fun!
Rich
 
Rich said:
But, I _do_ wonder, being a tech myself, I have to ask, haven't you just
tried it and see?

I'm sure I could get the component to stick if I put it in there, but I
was mostly wondering if doing so was "bad practice," or would cause
poor connections/dry joints or something. I guess the answer is "no"
on both counts. I should have been more straightforward.
 
P

Pooh Bear

Jan 1, 1970
0
The main reason I was using lead-free solder was to make it safer for
people/pets, because I'm working at home. I'm not sure if that's a
good reason or not.

It's not. The whole lead free thing is a hyped up mess.

You need specialist soldering gear to use lead free reliably too.
Specifically, maintaing the correct soldering temperature is far more
critical.

Graham
 
R

Rich Grise

Jan 1, 1970
0
I'm sure I could get the component to stick if I put it in there, but I
was mostly wondering if doing so was "bad practice," or would cause
poor connections/dry joints or something. I guess the answer is "no"
on both counts. I should have been more straightforward.

Not a problem. Actually, it was one of our better questions. ;-)

But yeah, it's not bad practice. There might be some oxide flakes left
over from the old solder, but the new solder and its flux will clean
that right up, and the solder will pull itself into the hold by
capillary action and any crud will float out and get fluxed away. ;-)

Cheers!
Rich
 
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