Flux coated silver solder

[kiwi2]

Hi All,
I had to buy some more silver solder the other day and all that BOC had was the flux coated material. I bought some and tried it for the first time today. The only good thing I can say about it is that it's relatively easy to chip the flux off.
I looked at the video on youtube posted by Johnson Matthey on how to use it before I started and tried to follow the procedure.
The problem is that there is a lot of flux on the rod and it goes all over the work instead of being localised in the areas you want it to be. I normally make up a thick paste of flux in water and apply it to the areas I want the solder to be with a toothpick.
What are the views of the rest of you on this?
Regards,
Alan C.

 

[Blogwitch]

Hi Alan,
Many years ago I had the same problem as yourself with this type of pre fluxed rod, it gave a good enough joint, but the flux spread everywhere.

I combatted that by using Tippex correction liquid (used by typists to cover up mistakes) around the joints I was doing. When the flux and solder hit it, it couldn't go any further, so effectively containing it around the joint.
I don't think that is readily available any more, but you could try the old trick of drawing around the joint with a pencil to prevent the spread of the flux and solder.
Like yourself, I always use a wetted flux (Tenacity #5) but rather than feeding the rod, I cut the rod, wire or sheet up into pallions or rings and place it onto the joint, then just heat up and the job is soon done without wasting any of your precious silver solder.
If I was you, just break off the outer flux coating and use you normal flux with the bare rod that is left.

Rings and flux pre applied to the job where it won't be seen afterwards.

Then by heating from underneath, the solder just flows perfectly into all the joints when the correct temperature is reached.

John

 

[fourstroke]

I agree with John
Any of the correction type fluids will work, paint it around the joint up to the limit of where you want the solder to flow, silver or soft, and it stops the flow in its tracks
Dougie

 

[10K Pete]

Soap stone works well, especially for lower temp solders. But I've
had success in silver brazing as well.

Pete

 

[kiwi2]

Hi All,
Thanks for all the comments. I'll certainly give the correcting fluid a go as a masking agent.
When I was buying the silver solder (45% Ag) I also bought some 34% Ag solder. It was a lot cheaper and I had a job where I wanted a big fillet on the outside of the brazing rather than having the solder run into a joint. I used an acetylene/air flame and it worked a treat. It was more like welding than brazing as the rod had to be fed into the job all along the joint.
Regards,
Alan C.