Brazed Fin Heat Sinks: Baked to Perfection

Thermal Design

What is Brazing?

Brazing is the process of melting a metal between two other pieces of metal in order to join them. Common metals for this process in the thermal management world include copper and copper based alloys, aluminum-silicon based alloys, and nickel alloys. This process allows us to create brazed fin heat sinks and expands our thermal management technology toolbox.

Brazing is not to be confused with braising, which is a cooking process where your first sear the meat at a high temperature then cook it low and slow with some amount of liquid in a closed container like a pot. Brazing metals is more like high temperature baking anyways.

Not Brazed- Braised Short Ribs Courtesy of BonAppetit
Yum. Courtesy of Bonappetit.com

 
Brazing, if applied well, can generate tight joints since the process relies on capillary action wicking the braze material between the parts. Braze material is an alloy that melts at a temperature lower than the parts you’re joining together. Stronger and more consistent joints are made with materials that closely match the mating parts. In all brazing processes, mating parts are fixtured together to ensure they retain their geometric relationship to each other during the brazing process. Once parts cool, they’re either ready to go or need some finishing like residual flux removal or final straightening or machining.
 

What’s the difference between brazing and soldering?

For those of you wondering, brazing sounds awfully like soldering. Brazing essentially is soldering, just at a much higher temperature (above 450°C, or 840°F). The higher temperature opens the door to a bigger material selection, which includes one of our favorites, aluminum. Both processes use a filler material, like a solder, to join the metal pieces together. If the metals and atmosphere require it, flux is also added to the mix to prevent oxidation of the joint.

Alternative to "Flux" Used in Brazed Heat Sinks - Back to the Future Flux Capacitor
Flux Capacitor Image Courtesy of Back to the Future (Universal Pictures and Ambin Entertainment)

 
In Aavid Genie, Brazed Fin Heat Sinks refer both to brazed and soldered assemblies. Sometimes we only solder copper heat sinks, but we wanted to include those heat sinks as part of the options available to Aavid Genie users.

Brazed Fin Heat Sinks: Bonded Fin’s Older, Stronger Sibling

Brazed Fin Heat Sinks geometrically are nearly the same as bonded fin heat sinks. Like bonded fins, brazed fin heat sinks consist of a base, fins, and braze material to join the parts together. Bases typically have tight grooves for the braze material to wick into. Brazed fin heat sinks have the added bonus of a stronger joint with less interface resistance between the base and fins.

Brazed Fin Heat Sinks - Double Sided

 

Pick your Process: The Different Types of Brazing

While they’re many types of brazing, we’ll go over the common ones used in thermal management.

Controlled Atmosphere Brazing

Controlled Atmosphere Brazing (CAB) is conducted in an oven with a specific mixture of air surrounding the parts. This mixture is primarily inert gasses and not oxygen that can oxidize the joints. Since the parts are placed in an oven, the braze material flows horizontally outwards where it’s not pulled by capillary action. This process is great for making a brazed fin heat sink since the base grooves are ideal in wicking the braze material.

Controlled Atmosphere Brazing - CAB Oven for Heat Sinks and Liquid Cold Plates

 

Vacuum Brazing

As the name suggests, vacuum brazing is conducted in a vacuum chamber. This is to remove any sort of oxygen that can oxidize the braze joints or other gasses that could get caught in tiny pockets in the joints. In practice, vacuum brazing has an advantage over other types of brazing since it doesn’t require flux to prevent oxidation. That makes vacuum brazing popular for complicated assemblies like heat exchangers and liquid cold plates, since they’re tough to clean any residual flux out.

Vacuum Brazing Ovens for Heat Sinks, Heat Exchangers, and Cold Plates

 

Dip Brazing with a Salt Bath Flux

Now, dip brazing is awesome to watch. You put braze material between your joints and fixture your parts together like you would for a vacuum brazed or CAB assembly. Then you put that assembly on a stainless steel fixture. With lift that looks like a giant fork, you lower the whole thing into molten salt.  The heat of the molten salt conducts through the aluminum assembly and reflows all the braze material. Since it’s submerged, there’s no need for flux and reflow can spread in multiple directions, not just horizontally.

Dip Brazing for Thermal Management Chassis and Enclosures - Molten Salt Bath

 

Torch Brazing

In a few occasions, we’ll use torch brazing to make tricky prototypes and proofs of concept. Torch brazing utilizes a gas flame to melt the filler material between the two metal parts being joined. Since this typically is a manual process, it’s reserved for small quantities. Since we don’t have a tightly controlled atmosphere that aluminum brazing requires, this process is normally reserved for copper and nickel alloy brazing.

All in All

Brazing gives us a whole bunch of options when it comes to constructing high quality or high complexity thermal management products. Aavid Genie offers both copper and aluminum brazed fin heat sinks in the technology selection page.