Active Solder Joining for Semiconductor Processing

S-Bond Technologies’ active solders are finding wide application in semiconductor processing equipment. Their capability to join a wide range of dissimilar materials and the fact that as a filler metal, their bonds are thermally conductive make S-Bond solders far superior to many other commercial bonding/thermal interface materials. Semiconductor processing applications include…

In sputter targets, the deposition/target materials is sputtered (high energy ion impacts displace a target’s atoms) onto an opposite surface to form a functional coating or other element layer in a computer chip, TV screen or other semiconductor device. The high energy impact of the deposited films require “sputter targets” to be cooled to keep the target material layer from melting. To cool the target, water is circulated in an aluminum or copper base plate.  S-Bond active solders can bond all metals and most ceramics that are commercially used to produce sputtered films. Active solders can bond these sputter targets to either Copper or Aluminum, bonding them at low temperatures (which minimizes thermal expansion mismatch stresses), without flux contamination/entrapment (which will contaminate the sputtering plasma), and with excellent thermal conductivity (a metallic bond with nearly no voids).

For probes and sensors, S-Bond active solders can bond silicon or carbide/carbon based devices (MEMS or other semiconductor based probes) directly to metal and metallic conductor leads. The low temperature active solders, when joining semi-conductor devices, impart low residual stresses and the joints are electrically and thermally conductive… good for signal transmittance and cooling.

Piezoceramics such as Pb-Zirconates (PZT) that impart force/small displacement or create ultrasonic pulses, can be bonded direct to metals with active solders. Bonding is accomplished bonded below their curie points with acoustically sound interfaces that can transmit sound effectively.  Such piezoceramic based sensors and actuators are used in accurately measuring gas flow and can be used on gas control in MOCVD processes used to deposit and etch computer chips. S-Bond active solders can wet and adhere to most piezoceramics all without pre-plating and chemical fluxes… as such they are finding excellent application in probes and sensors used on semiconductor processing.

Wafers (silicon or other) are placed into energetic plasmas and other beams to deposit then etch a complicated surface morphology in layers to create semiconductor based chips. The high heat energy into the wafer needs to be removed through water cooled wafer handling devices such as the pedestals the wafer sits on in their processing chambers. If not cooled, the interdiffusion of the fabricated on the wafer. With the latest 300mm diameter wafer technology over a $1M work of chips can be on a wafer.

 

 

The high energy levels used in semiconductor processing requires well cooled and reliable handling equipment. S-Bond active solders can intimately join copper and aluminum as well as other thermal management materials such as AlSiC and pyrolytic graphite

If you would like to see how S-Bond active solders can improve your semiconductor processes and handling and measuring equipment, please Contact US.

Joining Thermal Management Graphite Composites

S-Bond® active solders enable graphite bonding and the joining of other carbon based materials to each other and to most metals within the constraints of thermal expansion mismatch. S-Bond® alloys have active elements such as titanium and cerium added to Sn-Ag, Sn-In-Ag, and Sn-Bi alloys to create a solder that can be reacted directly with the carbon surfaces prior to bonding using specialized S-Bond® treatments for solder joining. Reliable joints have been made between graphite and carbon based materials with all metals including steel, stainless steels, titanium, nickel alloys, copper and aluminum alloys.

In high power density electronics, there is a need to rapidly spread and dissipate heat generated by the high frequency operations in the electronics.  In order to improve the heat dissipation capacity of graphite based materials, Applied Nanotech developed a new passive thermal management material, CarbAl™, which is a carbon-based material with a unique combination of low density, high thermal diffusivity, and low coefficient of thermal expansion based on Figure 1.

Figure 1. Picture of the CarbAl-G high thermal diffusivity graphite composite.

Applied Nanotech reports that CarbAl™ has a density of 1.75 g/cmcompared to 2.7 g/cmfor aluminum and 8.9 g/cmfor copper. While copper has a slightly higher thermal conductivity than CarbAl™, 390 W/mK compared to 350 W/mK, CarbAl’s thermal diffusivity is approximately 2.9 cm2/sec compared to 0.84 cm2/sec for aluminum and 1.12 cm2/sec for copper.

S-Bond Technologies and Applied Nanotech have collaborated to make heat spreaders with CarbAl-G cores combined with copper and aluminum claddings to make the heat spreaders that are more robust and able to be fabricated to support a high density of high power electronic devices, yet be mounted in standard “PC” card configurations as seen in Figure 2.

Figure 2. Copper and Aluminum clad CarbAl-G circuit boards.

These clad CarbAl-G cored boards have utilized active S-Bond® solder layers to intimately bond and thermally connect the thin copper or aluminum claddings to the lightweight, high thermal diffusivity CarbAl-G composite sheets.

S-Bond® CarbAl-G bonding and joining was thermally activated using S-Bond Technologies proprietary process, which prepared the CarbAl-G surfaces and developed a chemical bond to the surface, through reactions of the active elements in S-Bond® alloy to the graphite in CarbAl-G. These joints start with processing the graphite/carbon surfaces at elevated temperatures in a protective atmosphere furnace with S-Bond® alloy placed on the graphite-carbon surfaces to be joined. At these elevated temperatures, the active elements in S-Bond® (Ti, Ce, etc.) react with the ceramic to develop a chemical bond. After the CarbAl-G is prepared / S-Bond® metallized, the CarbAl-G is then S-Bond® soldered to the aluminum or copper sheets to for a metal clad CarbAl-G composite plate that can then be machined into a heat sink plate to which high power electronic devices be mounted.

The S-Bond® solder joints produced:

  • Are ductile, based on Sn-Ag or Sn-In alloys
  • Exceed the strength of the CarbAl-G
  • Are thermally conductive, with S-Bond® alloys having k = 50 W/(m-K)
  • Are metallic and this electrically conductive with a metallurgical bond

S-Bond Technologies has developed extensive experience in active, S-Bond® solder joining of graphite, carbon and carbide to metals. Contact Us to evaluate our joining solutions for your graphite joining applications. For more information on CarbAl-G, please contact Applied Nanotech. Inc

S-Bond® Active Soldering of High Purity Fused Silica for Optical Devices

S-Bond®  Ultrasonic Active Soldering of Silica

Ronald Smith    S-Bond Technologies Inc., Hatfield, PA,

Lawrence W. Shacklette, Michael R. Lange, James C. Beachboard,
Harris Corp., Melbourne, FL

and Donna L. Gerrity     E&S Consulting Inc., St. Augustine, FL

Packaging of optical devices often requires the need for creating strong bonds between metal and silica. The most convenient and cost effective approach would be to directly solder to both silica and metal without requiring pre-metallization of the silica. Soldering to oxides and oxidized surfaces has been accomplished with various solders containing metals with strong affinity for oxygen, known as “active solders”.

S-Bond Technologies worked with Harris Corporation to understand S-Bond® 140 active solders, based upon a tin-bismuth eutectic with activating additives of cerium, gallium, and titanium, to produce seals between metals and silica. Titanium and cerium are energetically capable of competing for the oxygen in silica, and are therefore capable of reducing or forming mixed oxides with silica under appropriate conditions. The bond between such an “activated” solder and high purity fused silica (HPFS) has been characterized by Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). Two variations of solder produced by S-Bond Technologies, S-Bond®140 and S-Bond®140 M1 were bonded to silica using a fluxless ultrasonic technique.

Figure 1 illustrates the ultrasonic soldering process where the resultant cavitation of the molten solder layer continually disrupts the oxides forming on the molten solder surface, enable the active elements to be in direct contact with the base materials own oxide surface, in this case silica, SiO2.

Figure 1. Illustration of ultrasonic soldering process with active solders.

To compare the influence of the active elements on the strength of the soldered bond silica to metal interfaces when using non-activated solders and active solders, a simple overlap soldered coupon ( ~ 1” x 1” ) was used on a compression lap shear test. Figure 2 illustrates the soldered specimen shear test configuration.

Figure 2. Illustration of compression lap shear test configuration.

Table 1 summarizes the lap shear strengths of the active S-Bond® 140 M1 and compares its shear strength to hose of typical non-active element solders.

Table 1. Compression Lap Shear Test Results

The results in Table 1 show that the active S-Bond® 140M1 solders far exceed the shear strengths of non-active elements solders.

To characterize how the active elements are increasing the bond silica-metal joint strengths, Time of Flight – Secondary-ion mass spectrometry [TOF-SIMS] was used to characterize the bond interface. TOF-SIMS measured the distribution of the various S-Bond® elements as a function of depth through the interface. The results show that the activating elements (Ti, Ce, Ga) concentrate at the interface and that their oxides form the interfacial layer between the high purity silica (HPS) and the bulk solder.

The efficacy of these additives was established by demonstrating that the block shear strength of the bond to HFPS was increased by 7 times through the addition of the Ti, Ce and Ga reactive metals to the base Sn-Bi solder.

The resultant data from the investigation showed a significant increase in the concentration of all of the “active” elements present in S-Bond® 140 M1 within a 220 nm interfacial zone between the solder and HPFS.

Figure 3. Charts of Element concentrations made from S-Bond® 140M1 joints between Silica and metal using Time of Flight – Secondary-ion mass spectrometry [TOF-SIMS].

In addition to this accumulation of “active” elements, the quantitative concentration of O was higher in the interfacial region than in areas away from the interface in the solder bulk.  These data support the formation of mixed oxides at the interface play a significant role in adhesion.  The data also support the notion that the interface comprises an oxide to oxide bond, that is, a silica to active metal oxide bond. All three active elements present in the solder seem to participate relatively equally in this bond formation.

Figure 4. Illustration of mixed oxide bond interface at S-Bond® 140M1 solder to silica surface.

It cannot be necessarily concluded that each active element (Ti, Ce or Ga) has the same contribution to bond strength, or whether having an intermetallic mixed oxide offers an advantage over a simple oxide of a rare earth or titanium.  Based on concentration alone, it appears that the role played by all three metals is essentially the same.  The thickness of the oxide layer (220 nm) and the observation of an interface layer with mixed oxides supports the model depicted in Figure 4.

The active elements accumulate at the interface because this is the available reaction site due to the presence of the substrate oxide (silica) and potential free oxygen. Once the oxidation reaction occurs, the active metal becomes bound at the interface, and thus accumulates there. The mechanism for movement of the “active” elements to the interfacial region against an apparent concentration gradient is presumably due to mechanical forces, but could also be aided by thermal convection.  The ultrasonic energy applied to the system is believed to play a key role in the observed movement of “active” elements to the interfacial region and possibly to an enhanced O level in this same region.  Ultrasonic or any other form of mechanical agitation can establish a mixing of the solder that would bring active metal to the interface.

REFERENCES

[1]          Nagono, K., Nomaki, K., and Saoyama, Y., US Pat. 3,949,118.

[2]          Ramirez, A.G., Mavoori, H. and Jin, S., “Bonding nature of rare-earth-containing lead-free solders”, Appl. Phys. Lett. 80, 3 21, 398-400: and US Pat. 6,306,516.

[3]          Tomáš Skála, Nataliya Tsud, Kevin C. Prince and Vladimír Matolín, “Bimetallic bonding and mixed oxide formation in the Ga–Pd–CeO2 system”, J. Appl. Phys. 110, 043726 (2011).

[4]          A.R. Lobato, S. Lanfredi, J.F. Carvalho, A.C. Hernandes, “Synthesis, Crystal Growth and Characterization of g-Phase Bismuth Titanium Oxide with Gallium”, Mat. Res. vol.3 n.3 São Carlos July 2000.

This investigation has shown how effective active element solders, such as S-Bond 140M1 are in bonding metals to silica (SiO2) surfaces. If you have applications requiring the bonding and sealing of fused silica or related glasses, please Contact Us… and we can assist in meeting your need.

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.

 

Rotating Graphite:Metal Seals

2014-06-20 036Carbon/graphite compressor seal rings are employed in many compressors and more and more metal backed graphite seals are being used in higher efficiency compressors. Frictional heating of seals can degrade metal backed graphite seals, therefore good thermal contact between the graphite seal ring and the metal backing is needed to improve cooling of the seal. S-Bond Technologies has developed active soldering methods for graphite bonding which is now being used to manufacture rotating metal backed graphite seals.

In the process, graphite rings are initially S-Bond metallized to create a chemically bonded solder to graphite interface. The sequence of bonding is shown in the figures below. After metallization the metal rings’ bonding surface are pre-tinned with S-Bond filler metals, via heating, melting of the solder surfaces on the metal followed by mechanical activation (ultrasonic solder tip agitation). The metallized graphite ring surface, at 250C is pressed against the pre-tinned metal backing ring surface and then the assembled ring is cooled to solidify the solder joint.

These S-Bond solder joined graphite to metal backing ring seals have endured 1,000’s of hours of running in natural gas compressors and are providing the customer with improved graphite – metal backed ring seal performance.

Please Contact Us for your graphite to metal bonding solutions.

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Fabrication of Hybrid Cu-Al Finned Heat Sinks

DSCN7334Copper 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.

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S-Bond Helping Recycle Water on International Space Station (ISS)

International Space StationCarrying water to the space station is a real challenge and cost, hence recycling water is critical. Waste water, sweat and other ISS water is constantly recycled in a complex system that evaporates and condenses clean water for reuse. For more information on the space station recycling system see the following link: Water Recovery System.

Read more about S-Bond Helping Recycle Water on International Space Station (ISS)

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

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.

Sapphire Window Sealing with S-Bond®

S-Bond® active solder enables the joining of sapphire to metals and provides an alternative to other sealing processes. S-Bond joining of sapphire/metal seals is proving to be a more robust and reworkable joining process while being simpler than many of the existing sapphire widow sealing processes, as this article presents. Read more about Sapphire Window Sealing with S-Bond®