Silver Soldering Tips

The Principles of Silver Soldering - Capillary flow






The key to successful silver soldering is to adhere to the basic principles of the brazing process. You are brazing - not welding with an expensive filler rod. Basically a welded joint relies on the strength of the filler and parent metals. The strength of a brazed joint, if made correctly, will be governed solely by the strength of the parent materials be they copper, brass, bronze or steel. They will fail before the joint fails.

The basic principle of brazing is that the alloy is made to flow into a joint by capillary action. This is what you see when dipping a sugar lump into your tea! The capillary forces are incredibly strong and will defy gravity.

We achieve this by considering four factors.
1) Fluxing
2) Joint gap
3) Heating technique
4) Application of alloy

Fluxing
The joints must be free of oil and grease.
The flux removes the oxides present before and formed during the heating process.

The flux melts when heated, and flows by capillary attraction into the joint removing all the oxides present. The alloy melts and it then flows by the same capillary attraction into the joint over the clean surfaces.

Mix the flux into a paste with water to the consistency of yoghurt. Add the water sparingly as it is very easy to ''swamp'' the flux. Adding a couple of drops of detergent before the water offers some advantages.
1) it helps the mixing of the paste
2) it will help the flux to "wet" onto the components.
3) If it dries out a little (and it will!) it can be remixed instead of being consigned to the bin.

Ensure that you have a good covering of flux paste before heating.

The flux used will depend on the parent materials, melting range of the silver solder and heating time. The ideal flux will satisfy the demands of all three criteria. Regrettably there is not a universal flux.
 
In essence there are two types of flux. The vast majority of joints will be made by a fluoride based flux. We refer to it as EF flux. Use it with a low or medium melting point silver solder such as 440 and 455 on copper, brass or mild steel.

If using a higher temperature alloy such as 424 or you are brazing stainless steel or have a long heating time use a borate based flux. We call this HT5 - "high temperature flux." If there is a problem getting the alloy to flow - it's probably a flux problem. Contact us.

It is essential that you cover, and keep covered, the joint areas with flux during the heating process. Use plenty to avoid the flux becoming "spent" and allowing oxidation to re-occur - a common fault that leads to poor joints. This also applies if the need to reflow the solder or add more solder arises.

Avoid "hot rodding" ie warming the rod and immersing it in the flux powder. This is fine for adding additional flux but should not be used as the prime fluxing technique. It leads to excessive oxidation in the joint. It promotes the heating of the rod - (no - no - no!) and poor joints because the joint is not hot enough.

Full details of the alloys can be found on the ''Silver Solder'' page.

Joint Gap

No gap = no capillary flow = no alloy penetration = weak joint

For all alloys there is an optimum joint gap. This ranges from 0.01mm .25mm. Alloys with a narrow melting range operate at the lower end of this band. To bridge wider gaps use an alloy with a wider melting range.

Generally speaking, the ideal joint gap is 0.13mm. In tube/tube joints that gap is radial - a tight rattle!

Most silver solder joints are self-jigging. For those that are not and require clamping use a joint gap gauge to set and maintain the correct gap to promote capillary flow.

The strongest joints are achieved with narrow joint gaps. They are also the cheapest!

Heating Technique
Ensure that all the joint is at brazing temperature. Cold spots will freeze the alloy, preventing flow through the entire joint resulting in low strength and leaks. Heat the joint not the alloy. Where you heat the joint will affect where the alloy flows. Molten alloy will tend to flow towards the hottest point. Heat the joint in front of where you want to apply the alloy.
Avoid heating the rod directly. Let the alloy take its heat from the joint.
When brazing tube or pipe assemblies it is better to make a ring and put the alloy inside the tube and heat the joint from outside. The alloy will melt and flow to the hottest point ie flows though and fills the joint area producing a positive witness of alloy. This is very good confirmation of a sound joint.

Care should be taken when brazing copper of unknown quality. Use a more expensive ''oxygen free'' grade (C106) to avoid hydrogen embrittlement of the copper or ensure that you use an oxidizing flame. The flame can react with the oxygen to produce steam that causes embrittlement and internal porosity.

Alloy Application
There is little point in trying to apply alloy until the joint is hot enough. A good indication of joint temperature is the flux itself. During heating the flux bubbles and dries out. It becomes a clear colourless liquid at about 550 deg C. This is just below the melting point of your alloy.
Don't try to apply alloy until you see the flux melt. There can be a tendency to rush the process and apply the rod too early. You may be tempted to switch the heat to the rod. Don't!
Be a little patient. Watch the flux !
Apply your rod to the joint. The alloy will melt and flow into the joint. Once the joint is full remove the alloy. There is no advantage to be gained by building up a large fillet. It contributes nothing to the strength of a joint. Leave fillets to the welders!
Use the smallest diameter rod you can. It helps to keep the alloy usage and cost to a minimum.

SAFETY PRECAUTIONS

DO NOT OVERHEAT THE ALLOY It does not produce stronger joints. On the contrary, overheating is one of the main causes of poor joints.
Once the alloy has flowed into the joint remove the heat.

Never brazed before ?
Experiment with our starter pack of alloy, flux and a few written words.