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Myths Regarding Lead-Free Solder Products and Joining Techniques

Soldering Techniques - Joing Methods

Brush Application

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

Read more about Myths Regarding Lead-Free Solder Products and Joining Techniques

Accounting for Material Thermal Expansion and Torsional, Tensile Strength

S-Bond material joining applications enable engineers to use multiple materials, such as materials and ceramics, in a variety of applications. However, just because aluminum and steel can be joined, as one example, does not mean that the joining process cannot introduce deformations or other issues.

Thermal Expansion Concerns in Bonding

When soldering two different types of metal together, both surfaces have to be heated in S-Bond can effectively assemble a wide variety of components with its unique bonding attributes - See more at: http://www.s-bond.com/solutions-and-service/s-bond-joined-components/#sthash.6FQ20NgZ.dpuforder for the solder to bond to both components. Materials expand as they heat, and different metals do so at a different rate. This can create problems as the joining site cools back down.

Two examples of materials that react easily to heat are aluminum and magnesium, which can expand at twice the rate of carbon steel and iron. If an aluminum sheet is soldered to a sheet of carbon steel, during the cool-down period the combined piece will warp with a slight curve. With more brittle components such as those made with ceramics, the combined part can shatter based on expansion during bonding and later cooling.

Solutions for Dissimilar Coefficients of Thermal Expansion

Read more about Accounting for Material Thermal Expansion and Torsional, Tensile Strength

S-Bond Joining of SiC Tiles in Microwave Beam Dampers (Absorbers)

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

Contact us to see how S-Bond joining can solve your ceramic to metal bonding challenges.

S-Bond Repair of Brazed Aluminum Heat Exchangers

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.


Please Contact Us to see how S-Bond Solder repair of aluminum can be used to salvage leaking aluminum heat exchangers and save on rework costs.

S-Bond Joined Phase Change Materials (PCM’s) Heat Sinks

S-Bond® active solders are being used extensively as a high conductivity bonding solution for foam cored phase change materials foam cored heat sinks. Increasingly, thermal management in electronics is the limiting factor in performance and/or life of electronics as higher power and higher speed in electronic devices generate more intense heat. High brightness LED’s, high speed/high bandwidth telecommunications, avionics, satellites and solid state conversion devices all have transient and steady power states where intense heat is generated and needs to be channeled away from the electronic device to prevent performance loss or permanent damage. The electronic industry is relying on a host of devices from conventional heat sinks with fins and fans to heat pipes and vapor chambers to more exotic materials and composites that include pyrolytic graphite or diamond. S-Bond materials and processes have been proven to be a good solution when bonding these various components and materials with a metallic “thermal interface material (TIM)” rather than filled polymeric bonding agents.

When electronics have a high transient heat output thermal engineers are using “phase change” heat sinks. Such heat sinks utilize “phase change materials (PCM’s)” that when exposed to heat absorb it very quickly and effectively as the material “changes phase”… either going from solid to liquid or liquid to vapor. Materials with high latent heats of fusion or latent heat of vaporization at or near the maximum temperatures that electronics are being used in the core of such heat sinks. In PCM heat sinks during the phase change there is the potential to rapidly absorb a high heat load… that can later be more slowly released to the atmosphere with cooling fins as the phase change is reversed and the heat is released away from the electronic device.

Two of the most used PCM’s are paraffin and water… each has a high latent heat of fusion or heat of vaporization, respectively. The challenge in the use of PCM’s is to overcome their relatively low thermal conductivities. For example, in heat sinks with paraffin as the PCM, when the heat transfers from the electronic device into the heat sink package, the outer layer of paraffin melts and then slows the transfer of heat into the solid paraffin core. To offset this heat flow limitation, designers are incorporating metallic or graphitic foams into the core of the heat sinks. The foams’ cells separate the PCM’s into small reservoirs that are surrounded by high thermal conductivity cell walls that then transfer the heat to a small PCM filled pore in the foam and therefore quickly melts the paraffin or vaporizes the water. Later in the “reverse” cycle, the conductive foam “cell walls” transfer the heat out of the PCM filled pores to solidify or condense the full volume PCM in the heat sink.

S-Bond joining has found excellent application in paraffin based PCM heat sinks in combination with graphitic foams (PocoFoam® or K-Foam®). S-Bond can effectively bond to graphite and graphite foams to heat sink package materials such as aluminum, copper and many heat sinks composites such Al-SiC, Al-Gr, Cu-W or Cu-Gr. In such graphite core/paraffin heat sinks. S-Bond Technologies has S-Bond metallized the Gr-Foam preparing for it to be soldered directly to the heat sink package. After S-Bond metallization, various S-Bond solders and processing can be used to bond the graphite foam to the components of the heat sink.

Heat Sink for Laser Diode Packages

Figure 1. Paraffin “PCM” / Graphite Foam cored heat sink for laser diode packages.

Figure 1 shows a PCM core finned Aluminum heat sink box used to cool high power laser diode packages mounted on the flat side opposite the side with the fins. The aluminum box contains a core of graphite foam bonded to the walls and base of the aluminum enclosure. The aluminum enclosure is then heated to 100˚C and filled and infiltrated with paraffin PCM’s. After filling the enclosure is sealed and the assembly is a PCM heat sink. When the laser diodes are on for intermittent periods of time the graphite foams thermally bonded to the wall of the enclosure heat the PCM… later when the heat load from the diodes are off, the bonded fins with air convection assist, cool and solidify the paraffin PCM to get the heat sink ready for the next thermal cycle. S-Bond active solder joining enabled the graphite foam to have an excellent thermal interface to the enclosure without filling the graphite foam and compromising the graphite foam’s ability to hold and transfer heat quickly to the paraffin PCM.

Heat Sink for Hot Fluid Channeled into Core

Figure 2. Graphite foam cored PCM heat sink for hot fluid channeled into the core.

Figure 2 shows another type of PCM cooling. The alternating bonded fluid circulating aluminum tubes bonded with S-Bond sandwiched between S-Bond metallized and bonded graphite foam plates. The stack is later encased in an enclosure and paraffin PCM is infiltrated into the foam to make a large ~ 24” x 24” x 12” PCM heat sink. When heated fluids circulate in the aluminum tubes the PCM filled graphite foam core rapidly absorbs the heat from the fluid.

Graphite foam wrapped heat pipe PCM heat sink

Figure 3. Graphite foam “wrapped” heat pipe PCM heat sink.

Figure 3 illustrates another style of PCM heat sinks that are mounted around a central heat pipe. In this design, S-Bond metallization of the faces and ID’s of the graphite annular rings permitted a graphite foam outer core to be become an effective PCM heat sink for a heat pipe cored thermal management device.

Contact us to evaluate how S-Bond can be used to enable your thermal management components to be made and how phase change material (PCM) heat sinks can be effectively incorporated into your designs.

S-Bond Technologies “Industry News” Webpage

S-Bond Technologies has added a News section to its website. This news section is focused on industry and technology developments that may be of general interest to you or may impact your business. Our Industry News feed will be updated during each month as targeted technology and industry news is identified.  Dr. Ronald Smith, President of S-Bond Technologies says “Our Company is materials technology focused with solutions in metals and ceramic bonding that are well suited for dissimilar materials joining, aluminum bonding, and ceramic to metal bonding. Applications for our technology include thermal management, alternative energy, sensors, electronics, aerospace and automotive. We are providing this Industry News section focused on applications that benefit from bonding technology as a feature for our website visitors and customers.”

This industry news provided at our website www.s-bond.com centers on materials technology in industry where bonding/soldering/brazing challenges are prevalent. The News section articles will be available in an S-Bond News Team periodic email that will feature the most recent news articles.

Please if you want to opt-in on our periodic S-Bond News Team communication… please click the link below. If you do not opt-in, you can still see the news by visiting our website, www.s-bond.com . If you do not opt-in, you will remain on our S-Bond e-Newsletter that features S-Bond related technical articles.

Opt-In for the S-Bond News Team communication.

New Lower Temperature Active Solders Developed

S-Bond Technologies has developed and proven a new, lower temperature active solder that melts from 135 – 140°C. The solder, S-Bond® 140 is based around the Bismuth-Tin (Bi-Sn) eutectic composition. This new solder is a lower temperature active solder that enables multi-step soldering where previously soldered connections/seals are not remelted. Active solders that melt below 150C are also finding use in thermally sensitive applications where Sn-Ag based solders that melt over 215°C can thermally degrade the component parts being assembled. Lower temperature soldering also can more effectively bond dissimilar materials where thermal expansion mismatch many times fractures or distorts an assembly’s component parts.

S-Bond 140 is already finding application in glass-metal seals in electronic packages where higher temperature soldering alloys would have damaged the packages’ components. S-Bond 140 is also being used to bond heat pipes and vapor chamber thermal management devices to protect the thermally sensitive phase change fluids from damaging the devices when solder bonding to electronic and LED devices.

Electro-optical package to be bonded to heat sink with S-Bond® 140

Electro-optical package to be bonded to heat sink with S-Bond® 140

Joining of Heat Pipes and Vapor Chambers

Figure 1. Illustration of vapor chamber heat spreader with CPU heat source

Figure 1. Illustration of vapor chamber heat spreader with CPU heat source

Heat pipes and vapor chambers are used to transfer and/or spread heat from concentrated heat sources such as high brightness light emitting diodes (LEDs) and high computing speed CPUs. These active thermal management devices are enclosures/tubes that have porous wick materials lining the walls that provide condensation surfaces and small connected pores that via capillary force, transfer condensed fluids that were originally vaporized at heat source surfaces. When the vapor is transported via convection to the cooler surface to condense, the fluid is then channeled back to the heat source surfaces in a continuous cycle, in effect pumping the heat out of the package without using external power surfaces. Figure 1 illustrates a vapor chamber used to cool a mounted CPU.

Figure 2. Light emitting diode package bonded to vapor chamber

Figure 2. Light emitting diode package bonded to vapor chamber

Thermal management is critical in the life and performance of such electronic components that all employ a variety of thermal interface materials (TIMs). With increased power and speed, the polymer-based TIMs being used today are limiting and metal bonding with solders is growing in application. Conventional Sn-Ag soldering temperatures can overheat the thermal fluids in heat pipes and chambers while Indium (In) solders are expensive and do not bond as well as active solders. Responding to this need, engineers at S-Bond® Technologies have announced its latest alloy, S-Bond® 140 as an effective TIM for bonding CPUs or LEDS to heat pipes and vapor chambers.  The Bi-Sn-Ag-Ti alloy can wet and join to all metals including aluminum and to most ceramics and glasses.  S-Bond® 140 is lead free, does not require plating and flux thus keeping electronic and LED packages clean.

Figure 3. S-Bond 140 bonded heat pipe assembly

Figure 3. S-Bond 140 bonded heat pipe assembly

Figure 2 illustrates a high brightness LED array that has been bonded to a Ni-plated copper vapor chamber with S-Bond 140 solder. This technique provides a high strength and high thermal conductivity metallic solder bond. Figure 3 is another example showing S-Bond 140 solder bonding copper heat pipe tubes (water as the phase change fluid) into aluminum slots to enhance the cooling from the heat pipe to the aluminum package base without plating and flux.. Normally when soldering heat pipes over 200°C, the water in the heat pipe goes to vapor and the resultant pressure distends/distorts the thin copper tube walls. Lower temperature metallic solders, such as S-Bond 140.

Contact us for more information and to order our S-Bond products.

Design Considerations for Solder Bonding

Solder bonding is a versatile lower temperature bonding process that is used in joining a range of metals, ceramics, glass and metal: ceramic composites. By definition, solders are joining filler metals that melt below 450°C. Solder bonding is typically used in the assembly of structures for its good thermal and/or electrical contact or for creating seals. The advantage of solder bonding stems from lower temperature exposure (less that 400°C), compared to brazing when joining thermally sensitive materials.  Alternatively, compared to bonding with epoxy adhesives, solder bonding is a more conductive bond, but does require higher temperature exposure and the wetting of the molten metal to the bonding surfaces.

Figure 1. S-Bond joined heat pipe assembly bonding copper pipes to aluminum base

Figure 1. S-Bond joined heat pipe assembly bonding copper pipes to aluminum base

Because of it excellent thermal and electrical conductivity, solder bonding finds application in the manufacture of sputter targets, heat spreaders and cold plates and other related thermal management components. Solder bonding is also used to seal ceramic:metal and glass windows used in optical based sensors and in other fluid cooled enclosures.  Figures 1 and 2 show several typical solder bonded parts.

Solder bonding (e.g. S-Bond®), despite being versatile and capable of joining most materials, one must consider several issues when active solder bonding…

  • Thermal expansion mismatch
  • Size and shape of bonded parts
  • Interaction with post solder bond processing
  • Galvanic corrosion coupling
Figure 2. Aluminum to copper cooling tubes and ceramic to plated copper sputter targets.

Figure 2. Aluminum to copper cooling tubes and ceramic to plated copper sputter targets.

In every application being evaluated for a solder bonding solution, the component and process design needs to consider the following issues.

  • Minimize CTE mismatch of bonded materials to prevent distortion or fracture.
  • Understand post bonding processes to prevent damage of bond interface.
  • Know Service Temperature and Thermal cycling effects on bond interface.
  • Understand effects of service environment on bond interface corrosion

Thermal expansion mismatch (CTE): solder bonding requires heating the component parts in an assembly to 120 – 400°C, depending on the solder filler metal being used.  When similar materials are being joined there is no CTE mismatch so it is not a concern. However; many times solder bonding is being used to lower the CTE mismatch… but despite the lower bonding temperature, it is not alone a “silver bullet” universal solution. Even when heating to 250°C, melting for Sn-Ag based solders, upon cooling once the solder solidifies it can transfer a strain. Then the CTE derived stresses can distort metal assemblies, fracture a glass or ceramic components or fracture the bond. Thus, one needs to minimize CTE mismatch stresses by selecting assemble component materials that are as close as possible in CTE.

When matching CTE is not practical, then one should design the component parts with size and thickness in mind… larger bond areas will “accumulate” more stress and lead to more distortion and/or fracture. A solution for larger parts is to “tile” the component parts; by tiling (mosaic) the strain mismatch accumulation is interrupted and lower the accumulation of stress in the assembly.

Post solder bond processes such as post solder bond heating either with another solder process, welding or bake outs to dry or cure components. Coatings may also be required on a bonded assembly where the heat and or chemical exposure of the coating process, as in electro-plating (see the coating blog article), interacts negatively with the solder bond.

When post processing a solder bonded part, temperature exposures typically should be below 90% of the solidus temperature (temperatures where solder alloys begin to melt) to maintain the bond. The thermal cycle itself can be damaging to the bond, even if the temperature is below this limit, especially with assemblies that have dissimilar materials. The processes that can degrade solder bonds include, other solder steps, welding, bake out or curing, and coating. Therefore; one needs to understand their impact on the solder filler metals and the solder bond interface.

Service conditions can also limit the performance and life of solder bonds. Temperature in service generally needs to be restricted to be below 80% of the solidus temperature of the solder filler metal (although active solders such as S-Bond® can be used up to 90%) to maintain sufficient bond strength. Thermal cycles are more damaging than constant temperature exposure and can be more damaging when CTE mismatched materials. Joint design can mitigate some of these effects by…

  • Selecting component materials to lower CTE mismatch
  • Minimizing area of solder bond and consider tiling, if practical
  • Using thicker cross sections, if possible,  to limit distortion
  • Orienting or mechanically supporting solder bonds/seals to lower bond stresses.

With proper design solder bonded assemblies can be superior to epoxy bonded joints and work very well and compete with brazed or welded joints

Contact us for more information and to order our S-Bond products and Services.