Aluminium Brazing Foil & Fin Stock for Heat Exchangers

What is aluminium brazing foil?

Brazing foil is the thin-gauge end of the brazing-sheet family. A core alloy with a high melting point is clad on one or both sides with a filler alloy containing 7 – 12 % silicon, and the composite is rolled down to foil. It is then slit to narrow coils for fin mills, or cut and stamped into plates and separators.

The point of cladding is that the filler is already where it is needed. When a stacked or assembled core goes through a brazing furnace, the cladding melts at 577 – 615 °C, flows by capillary action to every contact point between fin and tube or plate and plate, and solidifies into fillets. Thousands of joints are made in one pass, with no filler wire and no manual work.

Not all heat exchanger foil is clad. In many designs only one side of each joint needs filler: for example, clad tubes with bare fins in radiators, or bare extruded multi-port tubes with clad fins in condensers. Choosing which component carries the filler is one of the first design decisions, and it determines whether you need clad brazing foil or bare fin stock.

Common clad structures and where they are used

Structures are written filler / core / filler or liner, from one surface to the other. Clad ratio, the thickness of each clad layer as a percentage of the total, is specified per side.

Combinations shown are standard industry practice. Exact clad ratios, core modifications and liner chemistry are set by the heat exchanger designer and agreed on the order.

Filler and core alloys explained

Filler alloys are chosen for their melting range and flow. All melt above the aluminium-silicon eutectic at 577 °C; the higher the silicon, the narrower the melting range and the more fluid the filler.

Tempers, formability and sag resistance

Fin foil is corrugated or louvred at high speed and must then stay upright through the brazing furnace without collapsing. Temper controls both behaviours. Brazing foil is mostly supplied in strain-hardened tempers:

Sag is the drooping of thin fins at brazing temperature. Erosion or liquid-film migration is the attack of molten filler on the core, which can thin a fin or perforate a tube. Both are controlled through core chemistry, temper and grain structure, and a long time at peak temperature makes both worse. Specify sag and erosion tests if your design runs close to the limits.

Supply range and weight per square metre

Fin buyers often think in square metres per kilogram. The table uses a composite density of about 2.72 g/cm³, which is close for 3xxx cores with Al-Si cladding; actual figures vary slightly with alloy.

Tolerances, testing and documentation

Fin mills are sensitive to thickness variation and flatness, because they change fin height and pitch. Brazed cores are sensitive to clad ratio and cleanliness. A complete foil specification typically covers:

• Thickness tolerance: tight, often a few micrometres on the thinnest fin gauges; agree the value on the order
• Clad ratio: nominal percentage per side with a tolerance, measured on cross-sections
• Width and camber: slit width tolerance and edge straightness for narrow fin coils
• Mechanical properties: tensile strength, proof strength and elongation as delivered, and after a simulated braze cycle
• Brazeability: test brazing (for example angle-on-coupon tests) and sag tests on request
• Surface: residual oil level and cleanliness, important for flux adhesion
• Certification: mill test certificate to EN 10204 3.1 with chemistry of core and cladding

Relevant product standards include EN 683 (aluminium finstock), EN 573-3 (chemical composition), EN 485 and ASTM B209 for sheet properties, and AWS A5.8 / ISO 17672 for filler alloy classification.

Which brazing process suits which foil?

Flux choice is covered in our aluminium brazing flux guide.

How to choose a brazing foil, step by step

1. Decide where the filler goesLook at each joint in the core. If tubes or plates are already clad, the fin can be bare; if tubes are bare extrusions, the fin must be clad. Double-clad sheet is needed wherever a part brazes on both faces.
2. Choose the brazing processCAB with flux points to 4343 or 4045 cladding on a low-magnesium core. Vacuum brazing needs 4004 or 4104. Flux-free systems need their own matched cladding.
3. Set the corrosion strategyDecide which component should corrode first. Zinc in the fin core, or a 7072 liner on the coolant side, makes that part sacrificial and protects the tube wall from perforation.
4. Check post-braze strengthAfter brazing, a 3xxx core is essentially annealed. If fins or plates carry pressure or vibration, consider a long-life or heat-treatable core such as 6951.
5. Match forming and temperLouvred fins need a temper that forms cleanly at speed and resists sag; deep-drawn plates need O or H24 elongation.
6. Fix gauge, clad ratio and widthThinner fins save weight and cost but are more prone to erosion and sag. Confirm the clad ratio with test brazes on real parts.

Applications of brazing foil and fin stock

A typical passenger-car condenser, for example, combines bare extruded multi-port tubes, clad zinc-bearing fins about 0.07 – 0.08 mm thick, and clad header tubes. One furnace pass brazes every fin crest, tube end and header joint at once, which is why consistent foil gauge and clad ratio matter so much on high-volume lines.

Related brazing materials

Aluminium (aluminum) brazing foil: frequently asked questions