[sdiy] Transition to Lead-Free Hand Soldering

Mike Beauchamp list at mikebeauchamp.com
Thu Aug 19 21:45:40 CEST 2021

I've recently started transitioning into Lead-Free solder for all of my 
projects. Now that surface-mount assembly is affordable in small 
quantities, it's very easy to get boards made using lead-free finishes 
and solders but the hand-soldering is a more difficult transition.

After a conversation with Kester on the telephone (it's amazing how 
helpful companies can be over the phone), they suggested a few products 
for me to try and I've now gone through a few rolls to give some opinions.

For Water-Soluble flux, I am using Kester Part # 24-9574-6403
which is a .031" K100LD solder with the Organic 331 Flux.

For No-Clean solder, I am using Kester Part # 24-9574-7618
which is a .031" K100LD solder with 3.3% Mildly Activated Rosin 275 Flux.

K100LD is described as "a eutectic Tin/Copper alloy with controlled 
metallic dopants  to control the grainstructure  within  the  solder 
joint,  and  to  minimize  the  dissolution  of  copper  into  the 
solder  pot.    K100LD virtually  eliminates  the  occurrence  of 
common  defects  such  as  icicling  and  bridging.    The  improved 
grainstructure also results in shinier solder joints than traditional 
lead-free alloy alternatives".

I'm not sure what all of that means, but it's a cheaper alternative to 
SAC305 at less than half the price for a roll and seems to be designed 
with hand-soldering thru-hole components in mind.

In use, both solders are "OK". I increased my tip temperature to those 
recommended by Kester right away, but was still noticing that the 
lead-free solder remains "goopier" and doesn't wet and flow into gaps 
nearly as fast, or sometimes at all. As I continued to increase the tip 
temperature, this did change and the solder started acting more like the 
Lead I've used for 20 years.

I now have my Hakko 936 set to 800 degrees and things are mostly 
working, but not getting great flow on TH components that are being 
attached to the ground plane (even with thermal reliefs on the board). 
Also at this temperature both solders, especially the 275 Flux is 
spattering pretty crazy and eye protection is probably recommended. The 
flux residue from this solder has also completely gummed up my fume 
extraction tube as well, so that is also something to look out for and 
to be changing pre-filters often. I'm also noticing that soldering pins 
of cheap plastic IDC headers, it's very easy to melt the plastic with 
the 800F heat, so I don't dwell on them for long at all. Maybe better 
parts have higher heat tolerance?

These solders work good enough to get the job done the first time, but 
if you try to rework something or correct a mistake, the solders will 
suddenly change to sticky goo so I usually just suck the solder off 
completely and start with new solder.

Kester has a good PDF to troubleshoot lead free hand soldering issues: 

So there's definitely a compromise, but those two Kester parts are a 
good substitute for the leaded versions I was previously using (as 
suggested by Paul S. on here years ago). Next time I redesign my PCBS, 
I'm going to use a more aggressive thermal relief on ground plane 
thru-holes and I think the tip temperature should be adjusted based on 
the physical dimensions of the parts being soldered.

I'm not sure about the long-term reliability of Lead-Free solder. In my 
repair shop, I get a lot of music gear coming in with broken solders on 
PCB-mounted jacks, pots, tube sockets, etc and I can't help thinking 
that the lead-free solder is more brittle and prone to failure in these 

Have you converted to lead-free in your hobby as well? I'm curious to 
hear others' suggestions and experiences.


More information about the Synth-diy mailing list