The process of brazing aluminum to aluminum or to other materials accurately must be in a vacuum because aluminum is so dirty and quick to oxidize. Vacuum brazing does a great job of de-oxidizing metals as well as vaporizing undesirable high vapor pressure elements that might be seen as contaminants to ensure a strong braze joint.
We vacuum braze aluminum assemblies in high-vacuum furnaces that are specifically designed with the multi-zone controls that are required to accurately braze alumimum. At Altair Technologies, a core competency and a core portion of our brazing services dedicated to vacuum brazing aluminum for the semiconductor, aerospace, medical and defense industries.
Today Altair has innovated ways to braze Copper, Stainless Steel, and other refractory metals to Aluminum while avoiding brittle intermetallic phases, leaving a strong and hermetic bond. We have also developed a braze technique for achieving a 360-degree thermal bond on a heating element surrounded by Aluminum. The semiconductor community has been using heating elements for decades, but the full circumferential bond allows for higher power densities and also provided better temperature uniformity.
Unlike dip-brazing, vacuum brazing is not plagued with dip-brazing salts that invariably leach out over time and contaminate the vacuum systems in which those assemblies are used. Also, if welding versus brazing is a concern, please take a look at the photo below showing the difference of a homogenous joint versus a welded joint.
For the best possible appearance, vacuum brazed assemblies are typically final machined after vacuum brazing, offering a nice, aesthetically pleasing appearance where the braze joint is barely noticeable. The vacuum-tight assembly shown in the photo (above right) was made from four separate Al 6061-T6 components and has two cooling channels hidden within the walls of the assembly. Vacuum aluminum brazing typically uses a Silicon-rich aluminum alloy such as 4047 (brazing filler), which has a slightly lower melting temperature (~600°C) than the base metals to be brazed.
The Challenges of Oxidation
Aluminum can be a difficult metal to braze because it forms an oxide layer at room temperature; therefore components are chemically etched/cleaned and sealed before use. The process of assembly is quick and once the thermal cycle begins, it is not uncommon to introduce a partial pressure of Mg gas to help strip residual oxides allowing for good wetting and bonding of the base material. As the thermal process takes place, the Silicon is both vaporized into the furnace’s vacuum and diffused into the surrounding components. The resulting braze joint is very close in composition to the base materials and is both strong and hermetic.
At Altair Technologies, the process of vacuum brazing is highly controlled and boasts a nearly perfect first-pass yield. We are able to help customers save costs and time with our batch processing capabilities. For one of our semiconductor ESC Cooling Assemblies, we brazed over 2,700 consecutive assemblies with a perfect 100% yield.