When crafting jewelry, there are several options for joining pieces of metal together. These include: soldering, laser beam welding, fusion welding, diffusion soldering, and adhesive joining. Today, we’re going to focus on soldering: It’s definition, principles, and application.
As A Joining Option
Simply put: Filler metal (or solder) is a metal alloy with a melting temperature lower than those of the metals being joined. When heated, this alloy “wets” and flows across joint surfaces to create a metallurgical bond between the solder and joined pieces. Flux (a chemical agent used to displace oxides from surfaces being soldered) promotes the wetting and flow of solder over the surfaces being joined.
Working Properties At-a-Glance
- Solidus: The temperature at which a material (in this case solder) begins to melt.
- Liquidus: The temperature at which a material (in this case solder) becomes liquid, yet still contains crystals to form a metallurgical bond.
- Flow Point: Or working temperature of the solder is the temperature at which the solder becomes liquid and begins to flow. This is typically 25 – 75°F above the liquidus temperature of the solder.
- Melting Range: Knowing the solidus and liquidus temperature of different types of solder will help you in making the right choice for your project. For close-clearance joints (e.g. less than 0.005”) a solder formulated with a narrow temperature range flow easily into the joint. Solders with a more “sluggish” flow are designed to fill wide gaps. NOTE: It’s important to remember that solder, first and foremost, is best used for joining precious metals in jewelry and not as filler.
As mentioned before, this chemical compound is used to prepare joints for soldering. It does this by removing oxides from joint areas, promoting solder wetting, and preventing further oxidation during soldering.
Fluxes come in two types: Chemically active and chemically passive. Dandix flux (which contains halide) is an example of a chemically s active flux. Borax or boric acid fluxes are considered chemically passive. It’s good to note, as the karatage of work decreases, more active fluxes are needed to keep your soldering project clean and free from oxidation.
Lastly, the best flux for a project is one that becomes completely fluid and chemically active before the solder melts and after it solidifies. It also:
- Dissolves oxide scales
- Provides a surface cover to inhibit the formation of oxides during heating
- Promotes the flow of molten solder and its wetting of surfaces
- Is easily removed when the soldering operation is complete
Success with Soldering
Though application techniques are unique to each jeweler, the criteria for ensuring successful soldering operations are standard. Here’s a quick review.
- To avoid localized melting of the parent metal being joined, the working temperature of the solder must be lower than the parts being joined. The ideal temperature difference between the solder’s working temperature and the parent metal’s solidus temperature should be at least 100°F.
- Applying flux ensures joint surfaces are clean and prepared to allow proper wetting conditions for the solder. This also prevents oxidation.
- Capillary force draws molten solder into joint gaps. Therefore, the smaller the joint gap, the farther the liquid solder travels into the joint clearance. To achieve adequate filling and a strong bond, a joint gap in the 0.0004-0.004” range is recommended.
- Assisting the spread and flow of molten solder gives you a better-filled joint. Abrading joint surfaces with 400-grit paper and applying radius to the edges of the gap entrance where possible are both good practices.
We hope these basics on soldering gave you a solid understanding of what is needed in a successful soldering operation. For projects and use scenarios, check out our library for articles on soldering specific metals.
Also, if you have comments or a soldering story to contribute record it here, or contact us to be a guest contributor.
[Adapted from: The Stuller Metals Catalog, volume 76]