S-Bond® products work with the addition of titanium and/or rare earth elements to conventional solder alloy bases. These active elements migrate to any interface and react with the opposing material surface to remove oxides and nitrides and transport them into the bulk of the solder as an inert material. This process occurs while the material is molten and once the thin “skin” that forms on the surface of the molten solder is broken to allow contact between the bulk solder and the substrate surface. The breaking of this skin is referred to as “activation” and is done by application of a low level of mechanical shearing action at the interface between the S-Bond material and the substrate. The level of shear required is small, and can be delivered by brushing or scraping the surface, sliding the joining surfaces relative to one another, or application of high frequency vibration to the parts to be joined.

Once the skin layer has been disrupted, the bulk solder reacts practically instantaneously and, in the case of a molecular bond, irreversibly with the substrate surfaces, creating a tightly held layer of solder on the substrate. This means that the resulting joint may be disassembled and reassembled by simply re-heating above the melting temperature of the SBT product and then re-joining the parts with some additional activation to insure reaction with the new solder. The bonded layer at the substrate surface will not be affected, so good interfacial bond strength is maintained and re-activation is not required.

The activation process for S-Bond products is not ultrasonic soldering, which is where a large amount of ultrasonic energy is directed against the surface of the substrate material to break up the oxide surface layer and allow the molten solder to circumvent and interpenetrate the layer to reach the base material. The oxide layer being broken in S-Bond® joining is very thin, is only on the solder material, requires very little energy to be disrupted, and does not remain as part of the bond.

A feature of S-Bond products that is they do not flow or wick into openings like conventional solders. Unless pushed, our materials stay where they are placed. This can be useful in situations where precision joining is required.

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The specific process to use SBT solder materials depends on the substrate materials, the joint geometry, and the production volume. But, in general, the process flow is to:

Bring the component parts to the proper joining temperature;

Using mechanical shear (usually brushing, scraping, or ultrasound) uniformly coat the joining surfac es of the parts;

Using additional solder material to insure a completely filled joint, bring the surfaces together with additional shear (usually sliding the parts relative to each other or ultrasound);

Clamp or hold the parts during cooling to maintain position and prevent the joint from spreading due to the surface tension of the solder material.

This general process applies to any metal or ceramic, with the resulting joint strength varying from 20-55 MPa (3-8,000 psi). Higher joint strengths in carbon based substrates (e.g. diamond, thermal pyrolytic graphite, foamed graphite); stainless steel, Kovar, and other high performance materials are achieved by replacing the initial coating step with a proprietary heat treatment process that insures a permanent chemical bond to the material surface. It may also be possible to skip the surface precoating steps, depending on the geometry of the bond area and the substrate materials.

Variations of this generic process are practiced today in the production of thermal management devices, sensor housings, and sputter targets. A process for semi-automated joining using a multi-station carousel has also been demonstrated. Please Contact Us if you need further information or help in defining the best process for your application, or see our Products and Services page for more information about specific products.

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The following section illustrates typical joint structures found when using SBT solder products with a variety of materials.
If you do not see your materials combination, please Contact Us

Metals

S-Bond alloys can wet and join all metals and all combinations of metals. There are; however, differences in the level of metallurgical interactions and in the strengths of the resultant joints. These figures illustrate typical joint structures. The first micrograph is a Cu-SB220 bond, done in air and without flux or surface treatment.

Below, stainless steel has been bonded to itself using SB220. A dense adherent joint has been produced with good wetting at the interfaces. Due to the low relative joining temperatures there is no discernable interaction, yet good joint strengths have been measured. With stainless steel, higher interface strength can be developed through use of a proprietary surface bonding treatment that creates a permanently solderable layer on the stainless steel prior to joining.

A dissimilar metallic joint is demonstrated in the joint shown in the micrograph below. Aluminum and stainless steel would normally be very difficult to bond to, however S-Bond products enable such joint to be made without flux, preplating, and in air.

The micrographs above show that SB220 has wetted and joined Ti to itself, using no flux and no pre-coatings. The detail, high magnification photograph shows that a continuous interface has been formed, but as in the case of stainless steel, the low joining temperature (250°C) has limited metallurgical interactions. The bonding strength to both titanium and stainless steel can be improved by use of our proprietary process to covalently bond our materials to the metal surface.

The figures below demonstrate the joining of aluminum in many combinations. On the aluminum side of these joints, a metallurgical interaction forming Al-Ag intermetallics has occurred; demonstrating that good interpenetration between the solder and the base metal has been achieved.

Ceramics

S-Bond alloys can join a variety of ceramics and has the capability to join ceramics to metals and other ceramics and glasses. The joint strengths are generally lower than metal-to-metal bonds, and the structures vary with the substrate materials. However, significant bonding strengths are achieved both and without use of our surface precoating process. As in any dissimilar material joint, the joint performance will be influenced by thermal expansion mismatch derived residual stresses related to the joint size and geometry. The figures below show how SB220 has joined 99.6% alumina (Al2O3) to copper. The highly magnified view shows an Al2O3 interface with S- Bond 220 that has been thermally exposed at 205°C (sub-solidus) for 10 days.

Aluminum nitride (AlN) has been joined to itself with SB220, as shown in the photomicrograph to the left. Note the ceramic is wetted with indication of interaction at the S-Bond 220/AlN ceramic interface.
An example of zirconia (ZrO2) bonded to stainless steel is show to the right. The zirconia ceramic is well wetted and good adherence was achieved.
S-Bond products can also be used to bond carbide materials including graphite, diamond, and cemented carbides as illustrated in the two figures below.

Composites

S-Bond® materials will bond to metal, ceramics and carbide composites individually or if they are mixed as part of a metal matrix composite (MMC's). S-Bond® products have a unique ability to bond to the metal matrix and wet and adhere to the ceramic particles and/or fibers. Alternative processes involve precoating or treating the MMC surface followed by a flux based joining process. The first figure shows how C:C has been bonded with SB220 to aluminum. The others figures below indicate joints in Al-MMC structures using SB220.

S-Bond products are used to join Al:SiC composites, as seen above. Such Al:SiC composites are leading candidates for low expansion bases for electronic substrate heat spreaders, replacing copper and aluminum. Al:SiC composites are also finding use in precision machine tools where low CTE and high stiffness are essential for positioning accuracy. S-Bond 220 joints between Al:SiC and Kovar® (a low CTE Ni-Fe-Co alloy).

To the left one can see that another type of Al-MMC that has been joined. It is an Al:Gr-fiber, metal matrix composite joined with SB220 resulting in a dense structure with good adherence.

Electronic Materials

Electronics use a range of metals, ceramics and an increasing proportion of composite materials, for function and to thermally manage electronic and opto-electronic packages. The joint structures depicted in this section show some joints with typical materials that are used in electronic packages and in electronics, including alumina (Al2O3), aluminum nitride (AlN), silicon, silicon carbide, aluminum, copper and many other metals.

The picture above shows how silicon (Si) has been wetted and bonded with SB220, which is potentially useful in direct solder die attach, enabling excellent potential for silicon device manufacture.

The joint to the left illustrates the structure of the S-Bond Alloy 220 joined to alumina, used as a substrate material in chips and other devices.

Aluminum nitride (AlN) is an emerging electronic substrate material and is emerging as the substrate of choice in power electronic devices that carry and switch more current and thus generate more heat. AlN is more conductive than alumina and thus functions better to spread and conduct the heat away from the Si devices. S-Bond has been found to join AlN to itself and to many metals and composites. The joint structure shown to the right shows the good interface interaction a condition for good bonding.

Electronic packages many times involve the joining of copper to aluminum oxide. The joint structure depicted to the right demonstrates the S-Bond Alloy 220 can produce such joints.

Foams

S-Bond alloys are finding increasing application for joining graphitic and metallic foams. The limited capillarity and the active nature of S-Bond alloys enable them to join against porous surfaces. One emerging use of graphite and metal foams is in thermal management for electronics due to their high specific surface area and low weight. Another application for these foams includes lightweight structures. The figure to the right and below show examples where graphite foam has been bonded to Al:SiC composites and graphite bonded to copper, respectively.area and low weight. Another application includes lightweight structures. The figure to the right and below show examples where graphite foam has been bonded to Al:SiC composites and graphite bonded to copper, respectively.

Such joints microstructure can be seen in the pictures below. Note that SB220 has wetted both Al and Graphite in these two examples and that the alloy is located at the interface and has encapsulate the foam without excessive penetration.

The major functional advantage of S-Bond alloys a re that their metallic joints permit excellent heat transfer as can be seen in the figure below where graphite foams and aluminum foams were S-Bond joined to plate as in the figures above. The test consisted of heating the plates with a specified power density then measuring temperatures in the water and the plates. Note the superior heat transfer capacity of the Gr-Foams plates

The figure above illustrates another type of foam joint. The photomicrograph illustrates how stainless steel foams are bonded to copper. Such joining may be useful for making fluid or air filters.

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