While it has been several years since manufacturers began moving to lead-free solder procedures, in part due to the European Union’s Restriction of Hazardous Substances Directive, some still believe myths that have long been inaccurate regarding the use of alloy joining materials that do not require flux and are based on lead and tin.
Temperatures Can Be Enough to Destroy Components
The first round of lead-free solder options to join metals and other materials were comprised of tin, silver and copper, which do have a slightly higher melting point of 217 degrees Celsius compared to existing solder’s 183 degrees Celsius. That disparity could cause problems regarding PC board damage.
However, newer products including several offered by S-Bond have significantly lower melting points that make it easier to join metals like aluminum. At the lowest temperatures, some materials can be joined at just 115 degrees Celsius.
Issues Regarding Silicon Will Require Other Materials
Argonne National Laboratories selected S-Bond active solder technology to make water cooled high power microwave beam dump in its Advanced Photon Source which is a user-facility to producing extremely brilliant x-ray photon beams. The Advanced Photon Source uses high energy microwave beams to steer and create such x-ray photon beams. These beams once started cannot be shut down or restarted easily, so to facilitate the use the various beam lines, the microwave beams are diverted to beam dumps. These beam dumps consist of microwave cavities that are lined with SiC tiles bonded to water cooled rectangular copper enclosures that are heavy water cooled. SiC is a well know high efficiency absorber of microwave energy and thus is used in dampers.
The challenge faced by the Argonne engineers and physicists was to find a stable process for bonding the SiC tiles to copper bases that would provide thermal and electrically conductive interface and be able to take the thermal expansion mismatch during the bonding processes and in service. Active brazing and active soldering were considered since active brazes and solders are able to form metallurgical bonds with the SiC tiles. Active brazing, using Cu-Ag-Ti was tested and it was found the residual stresses stemming from the coefficient of thermal expansion (CTE) mismatch of SiC and copper led to the fracture of the SiC tiles upon cooling from the 860˚C brazing temperature to room temperature. S-Bond active soldering was selected as good alternative to active brazing since the solder bonding temperature of 250˚C yielded much lower CTE derived stresses and created a more compliant bond line that would better accommodate the heating and cooling stresses in service.
Figure 1 below show the S-Bond joined SiC tiles being bonded into one half of the microwave beam damper cavity indicating how S-Bond successfully joins SiC to copper. Figure 2 is an ultrasonic C-Scan of the bonded interfaces under each tile in the damper half
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. (more…)
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. (more…)
The Issue of Coefficient of Thermal Expansion (CTE) Mismatch
Yes, S-Bond can join a wide variety of materials, including aluminum, copper, stainless steel, refractory metals and ceramic to metal brazing with aluminum oxide, aluminum nitride, silicon carbide and other oxide, nitrides and carbides… however, with this wide variety of materials joining capability, we have a lot of inquiries about aluminum soldering to stainless steel or aluminum oxide, graphite bonding to aluminum, titanium to silicon carbide, etc. (more…)