Copper has superior cooling capacity than aluminum and is the preferred heat sink material for telecommunications and high power electronics. However, the weight and cost or copper limits the size of the heat sink packages. Therefore for larger electronic enclosures a hybrid design, using copper for a localized heat sink joined to an aluminum frame with good thermal contact can significantly improve the cooling performance of a heat sink package.
Joining copper to aluminum poses it challenges. Cu and Al cannot be welded easily due to the intermetallics that form when Cu alloys with Al in the weld pool. Alternatively, brazing cannot be done since melt point of aluminum is below the typical Cu-Ag braze filler metals (silver solders) used to braze copper. These issues leave “soldering” as the metal filler joining process of choice. But alone, soldering of Cu to Al has challenges. Solders, typically Sn-Ag based, cannot easily wet and adhere to aluminum without first plating the aluminum with nickel or using very aggressive chemical fluxes which themselves are incompatible with soldering to copper.
S-Bond Technologies, working with its customers has demonstrated its active solder, S-Bond 220-50, join Cu to Al in all configurations. The figures below show an example of where a copper- finned heat sink assembly was S-Bond joined into a finned aluminum package. In this assembly, the Cu-fins were individually S-Bond soldered into copper heat sink base, after which the Cu fin-base assembly was then S-Bond joined into the aluminum base at 250C. This soldering temperature well below the softening temperatures for the aluminum frame and low enough that the thermal expansion mismatch between Cu and Al did not distort the bonded assembly when cooling.
Hybrid heat sinks, combing the thermal benefits of copper with lightweight aluminum are taking advantage of the capabilities of active solder joining. For tough dissimilar materials and copper and aluminum bonding challenges, Contact us.
Aluminum soldering is used in making small area electrical and/or thermal connections or seals to other metal or ceramics, while aluminum bonding is used to join large areas either for thermal and/or structural purposes. Aluminum soldering finds applications in sensors, electronics, and electrical power where aluminum contact and/or wire leads are being utilized. Aluminum soldering has also been used as a means to seal and/or repair aluminum heat exchangers.
We have been contacted many times for assistance in solving the problem of small contact to aluminum without the use of aggressive chemical flux or cases where the chemical flux for aluminum was not compatible with the metals of the opposing side of the joint. Additionally, in many electronic packages the use of corrosive aluminum soldering fluxed are limiting When faced with these choices, active fluxless solders such as S-Bond become a good solution. Read more about Applications For Aluminum Soldering →
Bond assembly can be done via 1) mechanical attachment, 2) adhesive bonding of which epoxy bonding is one form of adhesive, 3) soldering bonding using lower melting filler metals (< 450˚C), 4) brazing using filler metals melting above 450˚C, and 5) welding such as resistance welding bonding, ultrasonic welding and friction weld bonding that uses locally melted parent metal.
Bonding is done for a variety of technical reasons a) mechanical attachment, b) thermal contact, c) electrical contact d) gas or liquid seal, or e) any or all combinations thereof. The choice of bonding method will then depend on the intrinsic properties of the bonding filler materials ( i.e. hermetic, electrical conductance, thermal conductance, thermal coefficient of expansion, adhesive bond strength related to the intrinsic fillers’ mechanical properties, and their adhesive and cohesive strengths). Read more about Epoxy Bond vs. Solder Bond Applications →
Metal to metal bonding is used in many applications for fabricating components where the metallic parts are too large or too complex to make from one piece of metal or the assembly contains dissimilar metals for various functions, such as: 1) physical properties such as electrical or thermal conductivity, 2) differences in thermal coefficient of expansion, 3) differences in corrosion, 4) differences in strength and/or modulus. For designers to utilize the optimum combination of metal properties, it is useful to have metal to metal bonding properties that optimally combine metals in an assembly.
Bonding technologies include: 1)Mechanical fastening, 2) Epoxy bond, 3) other Metal Adhesives, 4) Diffusion Bonding, 5) Explosive Bonding, 6) Weld & Weld Cladding, 7) Ultrasonic Welding, 8) Brazing and Soldering and 9) specialized active solder bonding. For strength mechanical fastening and welding are favored… for low cost, epoxy and other adhesive metal bonding are best but have limitations with regard to sealing, thermal conductivity, and stability over time. Diffusion and explosive bonding perhaps provides the best strength and interfaces between metals. However; for the best combination of bond properties and the least effect on base metal properties; ultrasonic welding, brazing, or soldering are the processes of choice. The choice of bonding process also entails the area of bond required, the joints’ physical properties and the effect the bonding process has on the base metal properties… all these are considerations when selecting bonding process. Read more about Metal to Metal Bonding →
Thermal interface materials are materials used in creating heat conductive paths at interfaces between components and thus reduce thermal interface resistance. These materials permit more effective heat flow between separate components where heat is being generated to a heat dissipation components such as solid state transistors to heat sink or a high frequency device connected to a heat spreader. Thermal interface materials’ purpose is to fill the air gap that occurs at contact interfaces with more thermally conductive compounds to permit more effective heat flow than poorly conductive air.
Think of the smartphone you hold in your hand, or of your tablet or laptop. The amount of processing power they contain dwarfs offerings from just a few years ago. Theoretically, the parts should be much hotter from doing so many more operations. They are not and one of the biggest reasons why is aluminum soldering and aluminum bonding applications.
Basics of Thermal Management in Manufacturing
Many of the advancements for electronics manufacturing, and other industries that benefit from aluminum soldering, revolve around miniaturization. Parts are smaller, which in some cases mean they are more fragile. In the case of solid-state lighting and LEDs, the advancement in efficiency comes at a price as well: the area of the product where the light is created can hit high temperatures of up to 150 degrees Celsius at the absolute maximum, according to Cree LED data sheets. Read more about Aluminum Bonding and Heat Management in Manufacturing →
S-Bond Technologies has developed and demonstrated a “solder welding” process that is finding application in the repair of brazed aluminum heat exchangers.
Aluminum heat exchangers and cold plates many times are brazed either by dip brazing or vacuum brazing. In these operations, aluminum braze filler metals are added to joint areas as pastes, brazing foils or braze alloy clad aluminum sheets. Depending on the complexity of the braze joint and the assembly, braze joints can on various occasions after the brazing cycle, be found to leak. Leaks at this point cannot be re-brazed since the aluminum braze filler metals cannot be melted without melting the entire component due to the interdiffusion of the silicon from the filler metal into the base metals. Thus brazed aluminum heat exchangers are normally weld repaired… but with limited success. Many times the weld will “chase” the crack and not seal it and the locally high silicon in the braze joint can also create inconsistent welds and if thin walls are part of the aluminum heat exchanger the high local temperatures from the welding process can “blow” holes in the thing gage. With these limitations on weld repair, solder repair is more viable.
Solder repair is viable since it has low heat input and is conducted below 250˚C, provided the solder filler metal can wet and adhere to the rework areas. Conventional soldering aluminum normally requires Ni-plating and or aggressive fluxes which complicate the rework procedures. S-Bond active solders bond to directly to aluminum and fill in crevices in aluminum surfaces without the need for flux and or preplating, thus it can be used to directly fill machined out leaks on braze joint in aluminum. The process consists of 1) locating leak areas (bubble testing is typically used) 2) grinding out the areas through and adjacent to the leaks, 3) deburring and degreasing the machined areas, 4) heating the plate locally or in its entirety to hold the repair area at 250˚C 5) melt the S-Bond filler active solder with the heat in the heat exchanger 6) spread the S-Bond solder into the joint to mechanically activate the solder to enable it to wet and adhere to the aluminum repair areas. NOTE: is has been found that ultrasonic activation using an ultrasonic solder tip can improve the reliability of the repair soldering process. The figures below illustrate the S-Bond solder aluminum repair on aluminum surfaces.
Figure 1a. Multiple plate and fin brazed aluminum heat exchanger
with indicated leak that has been machine out for repair.
Figure 1. Illustration of the NanoBond® / NanoFoil® heating process® (from www.indiumcorp.com)
S-Bond active solder layers have been shown in many applications to be the key ingredient that permits many ceramics and refractory metals to be bonded to largely coefficient of thermal expansion (CTE) mismatched metals such as aluminum and copper. Indium Corporation offers a NanoBond® process that uses NanoFoil ® as local heat source to remelt preplaced solder layers without the need for the bulk heating of assembled components that have large CTE mismatch. Active S-Bond solders are applied as prelayers and have Ti, Ce, Ga and Mg additions that permit them to wet any ceramic or metal surface. Once the S-Bond pre-layers are applied to ceramic and/or metallic surfaces, conventional solders can be reflowed onto the S-Bond layer to create the preplaced solder layers that are remelted and bonded via the heat emitted from an ignited NanoFoil®. Figure 1 illustrates how temperatures of over 1,400 K are generated by an ignited nano-engineered foil. Read more about S-Bond® Solders At the Interface of the NanoBond® Process →