[Elecraft] Use of flux.. and another rant on soldering

[Tom Hammond]

Good Morning Art:

Arthur Tan (AB3LR) wrote:
>I've read (and re-read) your soldering tutorial.  It's excellent.

Thank you, kind sir.

>I was wondering if you could comment on if and when to use solder flux 
>(either the paste of liquid variety) in terms of general soldering techniques.

Basically... never!

That having been said... IF, and ONLY IF, you find that you absolutely MUST 
use an axillary fluxing method for some really pesky soldering task, USE 
ROSIN FLUX ONLY, and in MINUTE quantities. Then, once applied and the joint 
has been soldered, CLEAN and then FLUSH AWAY ANY RESIDUE with alcohol 
(either medical-grade 99% isopropyl [available at your local pharmacy], 
wood alcohol, or drinking alcohol such as Everclear). Once you have cleaned 
(and flushed) away the dissolved rosin, use a hair drier to blow away 
(actually to evaporate) any residual moisture which WILL have accumulated 
as a result of the hygroscopic properties of the alcohol. If possible, use 
the flux on components which can be soldered-to or cleaned before they are 
mounted to the PC board, and if you must clean the PC board itself TAKE 
GREAT care to not allow the dissolved rosin (in the alcohol) flow into 
switches or controls. Once in there, the alcohol will evaporate, leaving a 
film of rosin to cover the contacts... bad things will then happen.

If you use a good grade of solder (Kester, MultiCore, ReliaCore, several 
others), containing either RA (Activated Rosin) or RMA (Mildly Activated 
Rosin) flux, there should be almost to need to ever use additional flux for 
electronic soldering. If there IS a need, then most times, It can be 
circumvented by first doing a really good job of cleaning the joint to 
remove any oxidation BEFORE you attempt to first make the joint. This is a 
MUCH better route to follow than to try to 'force-feed' solder using an 
axillary flux or any type.

Many times, builders feel they must use extra flux to complete a joint 
because they are getting poor quality joints to start with. Most of the 
time, this result is NOT a fluxing problem but one of not being able to 
either PROPERLY APPLY the heat, or not having enough heat TO apply. 
"Properly applying heat" has to do with the technique of positioning the 
tip of your soldering iron AGAINST both the PC board pad AND the lead to be 
soldered at the same time, and then applying to solder to that junction... 
THEN leaving your iron in place for another second or two, to ensure that 
the solder joint has been properly heated, allowing the solder to flow INTO 
the joint AND INTO the plated-thru hole (PTH), if one exists). I've found 
that, it generally takes about 3 seconds to complete soldered connection of 
an Elecraft PC board unless the pad being soldered is at a ground point, 
which takes just a bit longer since the heat is also being 'sinked' to 
surrounding copper as well.

Having ENOUGH HEAT TO APPLY has to do with using a soldering iron which 
either has a massive enough tip to continue to apply the required amount of 
hear needed to complete the joint even when the heat is being drawn into 
the joint, OR having a temperature-controlled soldering iron which is 
'smart' enough to know when additional heat is required to keep the joint 
hot enough to continue soldering. BOTH items address the same need... 
KEEPING THE JOINT HOT. As soon as the tip of your iron touches a joint to 
be soldered, it starts LOSING HEAT because its heat is being given to the 
joint itself. There must be a reserve of available heat in order for the 
joint to remain hot enough to adequately melt the solder AND to allow it to 
flow around the component lead AND to properly 'wet' the PC board pad and 
PTH. This is why I always use builders to 'dwell' at the joint just a bit 
longer, once the solder has begun to flow.

You should not have to dwell on the joint too long, but if you have enough 
heat, IF you haven't applied too much solder, and if the solder is flowing 
properly, you should see it 'spread' over the joint quickly and then, about 
1-2 seconds after it's begun to melt, you should see it be 'sucked' into 
the PTH. THIS is the indication that you've done your job properly....! If 
you have applied too much solder, you may never see the solder being sucked 
into the hole because there's just too much solder there. This is why I 
urge builders to use SMALL DIAMETER solder (0.020" to no more than 0.035"), 
with the smaller diameter solder being the best choice.

I must admit that, for the first FORTY years of my soldering experience, I 
used solder wire which was probably much larger than I should have used, 
especially when it came to assembling PC boards with plated-thru holes. At 
that time, I was using 0.040" diameter solder. It was a good quality 
solder, BUT what I didn't realize was that I was using WAY too much solder 
on virtually every joint.

It wasn't because I was applying too much solder... it was because I 
couldn't apply ANY LESS...! When I'd applied heat to the joint, and then 
just BARELY TOUCHED the tip of the solder to the heated component leas and 
PC board pad, the solder virtually LEAPED off and onto the joint. And it 
was already too much. It couldn't help it... the solder I was using was 
just too large in diameter and there was no way of metering out a smaller 
amount.

It wasn't until about 4 years ago that I realized the virtues of using 
0.020" diameter solder wire. While stumbling thru the flea market at the 
1999 Dayton HamVention, I found several rolls of Kester "285" RMA 63/37 
composition solder in 0.020" diameter for a price I just could not pass up 
($5/roll). I didn't buy the solder because I wanted the smaller diameter. I 
bought it because it was SO LOW COST I couldn't pass it up.

Once back home, I tried my newest purchase on a new Elecraft kit and found 
that, due to it's much smaller diameter, I was now able to mete out 
precisely the amount of solder required to complete each and every joint 
WITHOUT having excess solder 'jump' onto the joint. I found that I now 
could apply just a small amount of solder to the joint, have it 'flow' 
around and into the joint, and if more was required, I could still add a 
wee bit more to allow me to fill the plated-thru hole which not leaving an 
excessive amount of solder on either side of the board.  Remember, when you 
are soldering to boards with plated-thru holes, there's no reason to do 
more than fill the plated-thru hole with solder. Once this has been 
accomplished, any extra solder application is generally NOT needed, nor 
desired.

Sorry if it appears as if I've 'unloaded' on you, Art. That was NOT the 
intent. But very recent discussions (with others) have served to 
reestablish the need to remind builders that there is no substitute for a 
good soldering technique which produces repeatably-excellent soldered 
connections. You just happened to ask the question which allowed me to rant 
and rave a bit.

I am NOT AN AUTHORITY on soldering by ANY stretch of the imagination!!! I 
do however have 45 years of experience, and I've made a LOT OF HORRIBLE 
SOLDER JOINTS over that time... fortunately, I've also been able to make a 
bunch of pretty good joints as well. And I've LEARNED A LOT, mostly in the 
past 4-5 years. I'm just trying to pass on what I've learned to others so 
they don't have to wait 45 years to learn it.

73,

Tom Hammond    N0SS