Bonding Graphite –Ceramic – Stainless Steel Composite Component For Los Alamos National Laboratories

Fabricating Parts for Proton Collimator With S-Bond® Active Solders®
The unique capability of S-Bond solders to join graphite and ceramic to metals was the solution for Los Alamos for fabricating core elements of their Proton Collimator used in its Proton Radiography facility. Conventional brazing was considered but their large differences in Coefficient of Thermal Expansions (CTE’s) limited brazing since on cooling from brazing temperatures (over 800°C), the resultant CTE derived residual stresses would have likely cracked the ceramic, graphite or torn the bond interface. Figure 1 illustrates the graphite – ceramic-stainless steel composite assembly that required stable, thermally and electrical conductive connection between the assembly’s elements.

Los Alamos researchers reached out to S-Bond Technologies to use its S-Bond solders to join these disparate materials. Normally plating would have to be used to make the ceramic and graphite materials solderable. In the case of S-Bond joining, the same solder and soldering process was used to make the joint between the graphite base, the insulating alumina sheet and the stainless steel plate, as depicted in Figure 1.

Figure1AProtonColimator

Figure 1. Illustration of the proton collimator elements joined with S-Bond solders.

The soldering of this composite started the S-Bond metallization of the bonding surfaces of the graphite base and the alumina insulator. In this process, S-Bond metallization paste was applied to the one surface of the graphite and the two opposite sides of the alumina paste. The graphite and the alumina sheet with pastes applied, were heated to 960C in a vacuum furnace in order to react the elements in the paste with the graphite and ceramic surfaces to create a chemical bond between the solder and the graphite and alumina.  After metallization these parts’ surfaces are solderable with a well bonded interface. The Graphite base, the alumina insulator plate and the stainless steel header were heated to 250C where S-Bond 220 solder filler metal was applied via melting on and mechanical activation (spreading by heated blade or bush) to pre-tin the faying surfaces of the assembly. Once the S-Bond solder filler metal was pre-placed (pre-tinned) the parts kept hot at 250C, were placed together in an alignment fixture to align the constituent parts accurately and then pressed / loaded with 50 lbs of deadweight as the bonded assembly was cooled.

Figures 2 illustrates the solder bonded composite proton beam collimator component. The pictures show the two S-Bond solder interfaces connecting the water cooled Stainless steel end plate, to the ceramic insulator plate, then connected to the graphite cathode.

Figure2aProtonColimatorFigure 2a. Back of S-Bond joined collimator part. Stainless Steel/ceramic insulator/graphite base (from Top to Bottom)

Figure2bProtonColimatorFigure 2b. Side view of S-Bond joined collimator part.

Figure 3 illustrates the Proton Collimator with the S-Bond joined parts being assembled at Los Alamos National Laboratory (LANL). There were two S-Bond joined parts per assembly. These bonded component assemblies worked very well and enables LANL engineers to successfully implement their design.

Figure3ProtonColimator

Figure 3. Proton Collimator with two S-Bond joined composite being mounted.

LANL engineers were able to utilize S-Bond’s unique capability to solder join stainless steel to ceramic to graphite. If you have such joining challenges, Contact Us for incorporating S-Bond joining in your assemblies.

 

S-Bond® Solders At the Interface of the NanoBond® Process

Figure 1. Illustration of the NanoBond® / NanoFoil® heating process® (from www.indiumcorp.com)

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

S-Bond Joining of High Brightness LEDs

S-Bond active solder joining is emerging as an effective method to bond heat sinks to the back of High Brightness Light Emitting Diodes (HBLEDs). Active solders can wet and adhere to many of the thermally conductive ceramics (AlN, BeO, etc.) that are being used in HBLED’s and enable effective and thermally stable and conductive joints. Read more about S-Bond Joining of High Brightness LEDs

Soldering Silicon Carbide (SiC) for Electronics and Optics

Figure 1. Steel fitting S-Bond joined to SIC

Figure 1. Steel fitting S-Bond joined to SIC

S-Bond active soldering of silicon carbide (SiC) has recently been demonstrated on a range of electronic and optical components, providing for metal to SiC joints in plug, mounting and/or water cooling fittings. Silicon carbide is ceramic semiconductor with good thermal conductivity (120 W/mK) and low thermal expansion ( 4 ppm / °C). Thermal conductivity is comparable to aluminum with 1/8 of aluminum’s thermal expansion coefficient (CTE), making it a very stable material. The manufacture techniques for SiC and Si:SiC have recently developed to permit more complex SiC based components. As a ceramic, SiC is very difficult to machine so normally powder sintering and infiltration and/or slip casting and sintering followed by infiltration is used making for making complex shapes. Because of its thermal, electrical and optical properties, SiC and SiC composites are seeing increased industrial application in electronics and optics thus driving an interest for robust SiC joining methods. For high temperature SiC applications vacuum active brazing has proven effective; however, for lower temperature electronic and optical applications, there has been interest in solder joining methods. Read more about Soldering Silicon Carbide (SiC) for Electronics and Optics

S-Bond 220M Developed for Silicon/Silicate Joining

The direct solder joining of silicon is difficult posing solder wetting and adherence challenges for many applications including electronic “die” packages, sensor chips and solar panels. The direct solder bonding to silicon (Si) has been limited by the wetting resistance of angstrom thick nascent silicon dioxide (SiO2) layers that naturally forms on silicon. To combat these solder bonding challenges, metal plating (vapor deposition of Ti and Ni) has been used. To address this challenge, S-Bond Technologies has developed and has recently been awarded a patent for its S-Bond 220M alloy which is a Sn-Ag-Ti-Ce-Ga + Mg alloy that has been optimized for direct Si solder bonding without flux nor plating. The new alloy bonds well to silicon, silica, and glass silicates based on a solder formulation that adds magnesium (Mg) in low enough levels that does not change the solder melt behavior but enhances the “active” nature of S-Bond alloys to interact with oxides of silicon and many other metals even more effectively than other active solders. These Mg modified active solders wet and adhere very well to silicon based on mechanical activation used in other active solders. Read more about S-Bond 220M Developed for Silicon/Silicate Joining