How To Solder Aluminum – A Complete Guide

Aluminum needs heat to accept solder, usually to the tune of 300°C or more. Aluminum is a great heat sink, so the heat source will need to be even higher to get the aluminum up to the right temperature. Soldering aluminum is not like soldering copper; it’s not enough to just heat up and melt the solder.

Aluminum and aluminum base alloys can be soldered by techniques that are similar to those used for other metals.

Abrasion and reaction soldering is more commonly used with aluminum than with other metals. However, aluminum requires special fluxes. Rosin fluxes are not satisfactory.

Do not use solder if anything soldered comes in contact with heat levels that are higher than the melting point of the solder.

Soldering Of Aluminum Alloys

The most readily soldered aluminum alloys contain no more than 1 percent magnesium or 5 percent silicon.

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Alloys containing greater amounts of these constituents have poor flux wetting characteristics. High copper and zinc-containing alloys have poor soldering characteristics because of rapid solder penetration and loss of base metal properties.

Aluminum needs heat to accept solder, usually to the tune of 300°C or more. Aluminum is a great heat sink, so the heat source will need to be even higher to get the aluminum up to the right temperature. Soldering aluminum is not like soldering copper; it's not enough to just heat up and melt the solder.

Joint Design

The joint designs used for soldering aluminum assemblies are similar to those used with other metals. The most commonly used designs are forms of simple lap and T-type joints.

Joint clearance varies with the specific soldering method, base alloy composition, solder composition, joint design, and flux composition employed. However, as a guide, joint clearance ranging from 0.005 to 0.020 in. (0.13 to 0.51 mm) is required when chemical fluxes are used. A 0.002 to 0.010 in. (0.05 to 0.25 mm) spacing is used when a reaction-type flux is used.

Joints must fit snugly, but not so snug that the solder cannot get into the gap.

Preparation

Grease, dirt, and other foreign material must be removed from the surface of aluminum before soldering.

The surface must be clean. A stainless-steel brush or steel wool works well. In most cases, only solvent degreasing is required. However, if the surface is heavily oxidized, wire-brushing or chemical cleaning may be required.

Area prepared with a stainless-steel wire brush to remove any grease or oil. Blowtorch used to heat base metal first followed by melting Harbor Freight Alum Weld Aluminum Welding Rods.

Soldering Techniques

The higher melting point solders normally used to join aluminum assemblies plus the excellent thermal conductivity of aluminum dictate that a large capacity heat source must be used to bring the joint area to proper soldering temperature. Uniform, well-controlled heating should be provided.

Tinning of the aluminum surface can best be accomplished by covering the material with a molten puddle of solder and then scrubbing the surface with a non-heat absorbing item such as a glass fiber brush, serrated wooden stick, or fiber block. Wire brushes or other metallic substances are not recommended. They tend to leave metallic deposits, absorb heat, and quickly freeze the solder.

How To Solder Aluminum

Soldering aluminum is notoriously difficult when compared to soldering other metals. This is particularly true when it comes to aluminum alloys. This is because all aluminum is coated in a layer of aluminum oxide, where the metal comes into contact with the atmosphere.

Aluminum oxide cannot be soldered, so must be scraped off. Soldering must then be done very quickly, before more aluminum oxide forms. Aluminum also has a relatively low melting point of around 660 degrees, meaning you’ll likely need a special soldering iron with a lower temperature.

Heat up your soldering iron. It can take around 10 minutes for it to reach the ideal temperature. It’s a good idea to keep a damp sponge beside your iron in order to wipe off any excess solder and get the neatest finish. It’s also recommended that you wear a safety mask, goggles, and gloves whilst soldering.

You next need to remove the aluminum oxide from the aluminum. This can be done with a steel brush. Extremely oxidized aluminum may require more intensive sanding, or cleaning off with acetone. Apply the cleaning agent, called the flux, to prevent aluminum oxide from reforming as quickly.

You can then heat your solder until it becomes soft. The solder can then be applied to the aluminum. If it doesn’t bond, the likely issue is that aluminum oxide has reformed and the piece needs to be brushed and cleaned again. The other issue may be that your aluminum is actually an aluminum alloy that cannot be soldered. If this is the case, you’ll need to bond your metals using a ready-bought aluminum adhesive.

Heat the areas you want to bond with your soldering iron. This prevents an easily cracked “cold join.” Heat your solder, and using both the iron and the solder apply your solder to the areas you want to bond.

Once your solder is dry, which should take only a few seconds, you’ll want to remove any remaining flux. If it’s water-based, it can be rinsed in water, but if it’s resin-based your piece will need to be cleaned in acetone.

Solders

The commercial solders for aluminum can be classified into three general groups according to their melting points:

  • Low temperature solders. The melting point of these solders is between 300 and 500ºF (149 and 260ºC). Solders in this group contain tin, lead, zinc, and/or cadmium and produce joints with the least corrosion resistance.
  • Intermediate temperature solders. These solders melt between 500 and 700 ºF (260 and 371ºC). Solders in this group contain tin or cadmium in various combinations with zinc, plus small amounts of aluminum, copper, nickel or silver, and lead.
  • High temperature solders. These solders melt between 700 and 800ºF (371 and 427ºC). These zinc base solders contain 3 to 10 percent aluminum and small amounts of other metals such as copper, silver, nickel; and iron to modify their melting and wetting characteristics. The high zinc solders have the highest strength of the aluminum solders, and form the most corrosion-resistant soldered assemblies.

Rules Of Aluminum Soldering

  • Before starting any aluminum soldering project, clean the metal to remove grease and oils
  • Joint fit must be snug but with a gap for the solder
  • Don’t let the parts move while soldering, this will result in a poor result
  • Review the manufacturers instructions for the right amount of heat
  • Use the correct flux.