[Elecraft] reflow solder K-pod power resistor

[Ron D'Eau Claire]

Bob: Your technique works FB for me for reflowing when there already is some
solder on both ends of the connection. 

Flux is important to me only when soldering to a clean, un-tinned surface
such as a bare component lead or bare copper trace. Heating the bare metal
causes instant oxidation which the flux cuts nicely. But if I have two
surfaces tinned with solder adding a tiny bit more solder via the soldering
iron tip is quite sufficient.

Many SMD parts, especially low value resistors conduct heat so well that
heating one end to melt the solder usually melts the solder at the other end
too. I've learned to hold the part in place with a toothpick while reheating
the joint. Otherwise the part sometimes seems to "disappear". It's actually
stuck to the soldering iron tip, Hi! 

At the other extreme, when I'm adding a small SMD part to a bare un-tinned
trace, I usually start with a tiny drop of liquid rosin flux on the
aforementioned toothpick applied to the bare metal, then set the part in
place, hold it steady (again with the toothpick) while I pick up a tiny drop
of solder and touch the joint to flow it between the part and trace. 

A bottle of liquid flux is a great "tool" for any solder jockey's bench. I
have a little 2 fl oz (50 ml) bottle that I bought at least 15 years ago.
I've used about one tenth of it so far.

73, Ron AC7AC 


-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Bob
Novas
Sent: Monday, December 26, 2016 4:47 PM
To: donwilh at embarqmail.com; elecraft at mailman.qth.net
Subject: Re: [Elecraft] reflow solder K-pod power resistor

Don - I literally touch the iron to the solder, and then touch the part with
the iron conveying the solder and flux. You have to be quick.  Bob

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Don
Wilhelm
Sent: Monday, December 26, 2016 7:24 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] reflow solder K-pod power resistor

Bob,

It is good to hear that you were successful.
However, I have a reservations about your recommended technique.
Yes, a small bit of solder on the iron is required to conduct heat to the
already soldered junction, but you should add a wee bit of fresh solder to
the joint while reflowing.  The reason -- the heat of the soldering iron
will vaporize the flux quickly, probably before you can touch the iron to
the joint -- as a result, there is no flux left, and that is what the solder
connection likely needs most - a bit of added flux.

So, yes put a small bit of solder on the iron and then touch it to the
connection.  As soon as the solder begins to flow, touch the joint with your
flux core solder to complete the job.  The use of small diameter solder
prevents you from getting excessive solder on the joint/board/etc.

73,
Don W3FPR

On 12/26/2016 7:03 PM, Bob Novas wrote:
> I successfully reflow soldered the surface mount 6.8 ohm resistor 
> across R82 in my K3S today. It took me a while to get my courage up, 
> but it worked out well.
>
>
>
> In the kind of reflow soldering I used, you melt some solder and flux 
> on the tip of a temperature controlled iron, and then, while holding 
> the SMD part to be soldered to the board in its final location, touch 
> the iron tip to the surface mount part one one side and then the other 
> until it heats up and solder flows (the "reflow").  It's hard to hold 
> everything perfectly still, but you can take a couple cracks at it 
> until
you get it right.
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