soldering close-up

There's nothing more frustrating than aligning two pieces of aluminium, applying the perfect solder, and letting it cool... only to have the solder come unstuck and ruin all your hard work.

Solder not sticking is a common issue when working with aluminium due to the metal's natural properties. However, it's not impossible to overcome - you just need to take a few extra steps.

The Clickmetal team have decades of experience dealing with aluminium, so we've mastered the techniques for getting the most out of it. Below, we'll explore four common reasons why solder might not be sticking to your aluminium and how to fix them.

1. You Haven't Prepared the Aluminium Surface

Aluminium forms a thin layer of aluminium oxide almost instantaneously when exposed to air. This invisible barrier is what makes it resistant to rust and corrosion, but it's also what prevents the solder from making direct contact with the bare metal. This can hinder a good connection and cause the solder to become unstuck.

To overcome this, you need to break through the oxide layer and expose fresh aluminium. You can do this by using a stainless steel brush or sandpaper to vigorously remove the oxide layer just before soldering. For old aluminium with heavy oxidation, you may need to wipe it with isopropyl alcohol and acetone. Either way, work quickly - a new oxide layer can form within seconds.

2. You're Using a Standard Flux

The standard flux core found in most solder isn't powerful enough to prevent the aluminium oxide layer from returning. Instead, you should use a soldering flux specifically formulated to dissolve aluminium oxide. These fluxes often come in a paste form for easier application.

As soon as you've finished cleaning the metal, use a soldering rod or a small metal tool to spread a thin layer of flux where you'll be making the connection. This will stop new oxide from forming and help the solder flow smoothly along the joint.

Make sure the recommended temperature of your flux matches the melting point of your soldier. If your solder has a melting point above 450°C, you'll need a brazing flux instead of a regular one. Be careful, as some brazing fluxes aren't safe for thin aluminium sheets or wires. In those cases, look for a "dip brazing" flux specifically.

3. You're Using the Wrong Solder

Just as you need a special flux, you'll also need a special solder. If you use a standard solder, the heat required to melt it will also quickly flow away into the aluminium, making it difficult to get the solder hot enough to flow and form a good joint.

Instead, look for solders with a low melting point (typically made from a combination of aluminium, silicon, and/or zinc). These solders are often referred to as "aluminium solder" or "low-temperature solder".

4. You're Not Using Enough Heat

Soldering aluminium requires a higher temperature than soldering other common metals like copper. This higher temperature is necessary to melt the solder and to break through the aluminium oxide layer.

When you're melting the solder, make sure your soldering iron is set to a high temperature appropriate for the specific aluminium solder you're using. A higher wattage iron is helpful because it maintains the heat during soldering, preventing the aluminium from cooling too quickly and disrupting the process.

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