Posts Tagged ‘copper brazing’

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

Friday, January 27th, 2012
NonoBond heating process1 300x149 S Bond® Solders At the Interface of the NanoBond® Process

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.

Figure 2 illustrates the use of S-Bond in the NanoBond® process in bonding sputter targets.

s bond applied nanobond process S Bond® Solders At the Interface of the NanoBond® Process

Figure 2. An illustration of S-Bond being applied in the NanoBond® process

NanoFoil® , sold by Indium Corporation, is used on the Nanobond® process as a heat source to only locally reheat a pre-soldered interface with an instantaneous release of heat energy for joining applications. NanoFoil® is a nano-engineered material fabricated by vapor-depositing thousands of alternating nanoscale layers of Aluminum (Al) and Nickel (Ni), as shown in Figure 1. When activated by a small pulse of local energy from electrical, optical or thermal sources, the foil reacts to precisely deliver localized heat up to temperatures of 1500°C in fractions (thousandths) of a second. As a sacrificial heat source in soldering and brazing applications, NanoFoil® is ideal for high-temperature applications. NanoFoil® becomes a non-functional part of the solder or braze joint, eliminating the need for an oven or furnace and allowing for the use of higher temperature solders.

NanoFoil® works by acting as a local heat source to melt adjacent solder layers without heating the target or backing plate materials. This allows the bonding of nearly any combination of sputter target material and backing plate material, including ceramics to metals, irrespective of the difference in coefficient of thermal expansion (CTE). S-Bond solders enable the NanoBond® process in many applications by providing an “activated / bonded layer” on the ceramic or metal interface to which conventional solders can wet and adhere.

Contact us for more information on how S-Bond can assist your NanoBond® applications.

NanoFoil® and NanoBond® are registered trademarks of Indium Corporation.

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Posted in Ceramic-Metal Bonding, Process Articles | No Comments »

Soldering vs. Brazing

Friday, July 8th, 2011

We receive many inquiries to silver solder, solder or braze components and many times there is confusion over this terminology and the various materials and processes used to bond metals, ceramic and/or glasses. This short article offers some clarification to the distinctions between soldering and brazing such that you can make informed decisions about your needs.

Brazing is a process where a molten metal is the joining agent (filler) between to materials where during the bonding process only the filler metal is melted. These molten fillers react with and adhere to the adjoining surfaces. AWS (American Welding Society) defines a material to be a braze filler when it melts over 450C°C (842°F). Example braze fillers include but are not exclusive to copper, copper-silver, Cu-P, and all copper alloys, Al-Si, NiCrBSi, Ni-P, FeCrBSi, gold, silver, palladium, etc.

Soldering is a related process to brazing and also employs molten metal fillers, with the exception (according to AWS definitions) that solder fillers melt below 450°C. Such fillers include lead-tin (Pb-Sn), Sn-Ag, Sn-Bi, Sn-Sb, Zinc, Zinc-Al, etc… these solders are used in electronics, plumbing, structural low temperature components, heat sinks and cold plates, sputter targets, etc.

Note there is some confusion over the term “silver solder”… in reality silver solder is a braze but this term has been adopted commercially since the fillers use copper-silver (Cu-Ag) alloys. These silver-solders are also associated with the term “hard solder” vs. soft solder… All “silver solders” are technically brazes since their melting temperatures are over 750°C (1382°F) and employ braze processes to make bonded components.

Is soldering or brazing more suitable? …. The answer is “it depends” on…

  • Strength requirements… brazed joints can be 3 – 10x the strength of soldered joints
  • Corrosion resistance… solders are generally more susceptible to oxidation and degradation from chemicals and salt since the fillers are Sn, Zn or Pb based.
  • Temperature assemblies can be exposed to… solders melt from 100 – 250°C and are generally used in electronics and other temperature sensitive parts.
  • Thermal expansion… differing CTE assembly materials benefit from soldering since lower joining temperature lowers distortion upon cooling and “softer” filler metals permit CTE mismatch to be accommodated.
  • Cost… soldering is generally a lower cost process with the filler metals being less expensive and the lower temperatures processing reduced post joining clean-up lowers overall joining costs.

So, chose the processes and filler metals most suited to your assemblies and their expected service temperatures, remembering soldering is generally less expensive and less sensitive to thermal expansion mismatch.

When comparing soldering to brazing and their related filler alloys, one begins to see the processing changes made necessary by the significantly different processing temperatures (100 – 450°C) for solders and 450-1,600°C) for brazing. The higher temperatures needed to melt brazing fillers makes oxidation of the filler metals and the base materials (being joined) much more of a concern and problem. Since oxidation and the subsequent oxides formed interfere with wetting and adherence, oxidation must be minimized and a means to remove any formed oxides must be used. Chemical fluxes are commonly used and the most effective fluxes melt and flow just before the melting temperatures of the filler metal and are not thermally decomposed in the range of temperatures where the filler metals melt. For solder filler metals, fluxes are normally rosin based or low temperature acidic compounds that when melted can react with tin, lead, silver, copper or nickel oxides.  For brazes, fluxes are normally organometallic salts, or higher temperature salts that when melted are acid and reduce the oxides forming on materials such as brass, steel, stainless steel and even aluminum. Normally to flux the more oxidation resistant materials (stainless steel and/or aluminum) very acidic and corrosive fluoride based acid are required.  Soldering fluxes, as a result of their composition, are much less aggressive and generally less corrosive than the fluxes used in brazing.

Alternatively, brazing is also commercial done in furnaces… and those furnaces can either produce a “fluxing atmosphere” such as in cracked ammonia (reduces oxygen activity while reducing oxide scales formed in steel and copper based materials).  Other furnaces exclude oxygen altogether by pumping the atmosphere out with vacuum pumps, then backfilling with inert or reducing gases (N2, Ar, or H2) pumping to high vacuum where the high vacuum really excludes oxygen and can in many metals reduce and/or evaporate the surface oxides on part. Thus “furnace brazing” many times is a preferred method over torch brazing due to temperature uniformity and part cleanliness after brazing.

Structural soldering (non-electronic) is generally not practiced in ovens since soldering temperatures are low.  Soldering irons (hot metal tips), propane torches (in brazing MAP and even acetylene torches are used for their higher heating capacity), hot air guns and hot surface plates are used. In high volume soldering for electronics, batch or continuous flow (belt) furnaces are employed to re-flow solder pastes that consist of a decomposable organic carrier, a flux and the solder filler metal powder. When heated in a “reflow” oven, reducing gases or more inert gases such as N2 can also be used, however, if mildly acidic or no clean fluxes are uses, air solder reflow ovens can be used.

The bottom line, once you have the terminology down, is to choose the most compatible (technically and economically) filler metal (solder vs. braze) then select the most suitable process compatible with that filler metal in combination with the assemblies’ components.

Feel free to Contact Us to assist in your selection process. We can evaluate the most suitable filler metals followed by the most appropriate process.

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Posted in Aluminum Soldering, Ceramic to Metal Brazing, Graphite Bonding, Soldering & Brazing | No Comments »

Joining Dissimilar Materials

Tuesday, April 5th, 2011

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.

Our answer is “Yes we can join them, BUT….”   The big BUT… it depends on the materials and the joined assembly size and geometry. Our response is based solely in the CTE mismatch of the materials being joined. Materials expand at different rates depending on the composition (atomic elements), structure (atomic arrangement) and thermal properties.  A material’s volume will change based on the relationship and when derived to any linear dimension, the relationship of the increase of length per unit length per °C (or °F) is established that leads to the linear expansion relation.

equation1 Joining Dissimilar Materialsequation2 Joining Dissimilar Materials

A table of common metals, ceramics and glass is seen below showing that materials vary widely. Many errors or “miscalculations” occur from aluminum soldering to any other metal or ceramics. With a linear CTE of   23 x 10-6 / °C, aluminum is one of the most expanding metals when heated. Alternatively, SiC, quartz and tungsten have almost zero or not much expansion at all when heated.

image3 Joining Dissimilar Materials

The most common design error made in aluminum soldering is to solder or braze bond large aluminum components to any other metal or ceramic. Many times aluminum soldering uses low temperature curing adhesives since soldering or brazing aluminum must be heated from 200 – 550°C. When the solder or braze is bonded, then the aluminum will contract to its room temperature dimension. An example, when S-Bond 220 joining aluminum to steel, if the component parts require heating to 250 °C using the CTE values in the table below, a 12” plate of Al will grow by almost 0.060” while the 12” plate of steel will only grow by about ½ that amount with a CTE of 10.4 ppm for steel vs. the 23 ppm. Thus the steel plate only grows at 250°C by about 0.030”… so upon cooling, the aluminum will try to return to length, by 0.060” where the steel will only return, upon being bonded at 250°C by 0.030” setting up a strain difference and leading to either bending of the plates, as seen in the figures below… or by the accumulation of stress from the strain mismatch, the stresses may be sufficient to begin the fracture of the joint at the edges of the S-Bond solder joint where the stresses were higher than the tensile strength of the S-Bond.

Note that if the steel plate were replaced by a ceramic plate, that the strain difference upon cooling, if the design permits fracture, will deflect the ceramic plate enough to fracture the plate.

image41 Joining Dissimilar Materials

Note that S-Bond bonding is a soldering process and compared to other brazing processes that have to heat assemblies to over 700°C, S-Bond solder assembly is a lower temperature, hence lower thermal expansion sensitive process than brazing.

So, S-Bond can bond most materials…BUT, when one is solder bonding dissimilar materials even when bonding at 250°C (480°F) one must still properly accommodate CTE mismatch into their assembly designs by some of the following techniques.

1)     Using better matched CTE materials (e.g. ceramic to Kovar®).

2)     Using multi-layers to over a distance accommodate CTE.

3)     Bond smaller areas/components or make a mosaic breaking the larger CTE materials into smaller pieces.

4)     Stiffening a design to resist bowing (may still fracture joint).

5)     Use lower temperature joining processes, such as exothermic materials that only heat the joint areas, a recent commercially developed nanofoil has been developed and can reheat and solder joints via a patented NanoBond® process.

Feel free to consult with me about your particular application, we are prepared to discuss bonding options using our S-Bond dissimilar materials bonding techniques.

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Posted in Aluminum Soldering, Ceramic to Metal Brazing, Ceramic-Metal Bonding, Dissimilar Materials Bonding, Graphite Bonding, Technology | 1 Comment »