Soldering and Brazing

Soldering and brazing are similar in that a third material is added to form the joint.  Soldering and brazing are perhaps the oldest known welding processes used by humans, possibly because of their simplicity.  All you need is a heat source and a filler metal; you do not need focused laser light, flowing electrons, or ultrasonic vibrations - all recent inventions.

Quite often, soldering and brazing are distinguished by the melting point of the filler metal.  Typically, filler melting below 400C are considered solders and those above are considered brazes.  There are additional metallurgical differences as well that can play an important role in evaluating the joints.

Of all the joining processes, soldering/brazing offer the widest variety of heat sources that can be used.  Most common for manual soldering is the soldering iron; hydrogen torch is an option.  Others soldering processes, typically used for automated soldering include reflow ovens, wave soldering, and induction soldering.  Newer technologies that are now vying for attention include white-light (or laser light) soldering and ultrasonic soldering.  White-light soldering is similar to laser welding in that an intense beam of light is used to melt the solder.  Ultrasonic soldering uses ultrasonic energy (delivered through the soldering iron or through a solder pot) to remove the oxides on the surface and allow the laser to form a bond.  Ultrasonic soldering has been shown to work even with aluminum without using a flux.  Soldering is commonly used to form bonds where one of the main functions of the bond is to conduct electricity.

Brazing is typically used to form a structural bond.  Common heat sources are brazing torches and furnaces including hydrogen, induction, or radiation furnaces.

Ceramic Brazing

Contrary to popular belief, ceramics can be brazed.  Given that it is very difficult to machine ceramics, brazing offers a unique opportunity to produce a composite where the ceramic component is exposed to the harsh environment whereas on the other side a metallic component is able to facilitate easy connectivity.  Ceramics can also be brazed to glass and to other ceramics.  Of course, the ideal solution will depend on the geometry, materials, and application requirements.  Alloys used to braze metals are unable to react and bond to ceramics and hence the need for active brazing alloys.  As the name suggests, active metal brazes have an active component which is able to react with the ceramic.  The most common active metal is Ti used in the Ag-Cu-Ti ternary composition.  Other active metals such as Zr have been experimented as well.  The active element is usually limited to less than 5% and is sufficient to uniformly attack the ceramic surface, reduce the ceramic and form a strong bond.  Because the active element is very reactive, the brazing process has to be carried out in either vacuum or an inert environment such as under Argon.

Examples include brazing of ceramic turbines to steel shafts in turbochargers.  Some race-cars also have a engine valves made of ceramic on the hot end and a metal shaft on other; ceramic valves allow high temperature operation whereas the metal end allows impact loads against the rocker arms.  Ceramic-to-ceramic brazing has been successfully used in producing high voltage vacuum interrupters for power stations.

Lead-Free Soldering

Lead has been identified as a harmful element and has been eliminated from many applications where it was previously used such as lead compounds in gasoline (or petrol) and paint.  However, new regulations have forced manufacturers to remove lead from solders as well.  Regulations such as RoHS (Restriction on use of Hazardous Substances) initiative in Europe and similar such regulations all over the world are establishing new restrictions to use of lead.  Research into alternatives have narrowed down to a family of alloys called SAC for Sn (Tin) - Ag (Silver) - Cu (Copper).  Different continents seems to have zeroed in on slightly different compositions but the overall composition is 95-96.5% Sn, 3-4%Ag, and 0.5-0.7% Cu.  For wave soldering, the 0.5% Cu - SAC alloys are competing with 99.3%Sn-0.7%Cu alloys with small amount of Ni for stabilization.

With lead-free, one has to worry about not only the solder alloy but the plating as well.  For ease of solderability, most electrical components are plated; plating helps prevent oxidation of base copper alloys and provide good compatibility with the solder alloy.  Gold flash over Ni is a choice but can cause embrittlement of the joint by gold migration;  Silver platings have their own problems including tarnishing, migration, and formation of silver-sulfide dendrites.  Organic inhibitor coatings can mitigate problems with limited shelf life of silver and tin.  Surface finish of choice seems to be Sn since it is compatible with SAC alloys but poses challenges of tin whisker formation.  Tin finishes can be applied by electrolytic (1 to 13 microns), electroless ( less than 5 microns), and immersion (0.3 to 1.5 microns) techniques.

Lead-free solders have higher melting temperatures (217C or 423F) than traditional tin-lead eutectics alloys, collateral damage to components becomes an issue.  Higher melting point reduces the operating process window for the soldering operation since temperatures have to be high enough to melt the solder but low enough to avoid component damage.  Wave soldering operations have adapted well to a lead-free environment since the operating temperatures (250-270C) have remained the same as lead-based alloys. 

Reliability of solder joints with lead-free compositions is also of concern.  For consumer and telecommunications applications (0-100C hardware), SAC solders reportedly perform better than tin-lead compositions.  However, for military and underhood applications, (-55 to 125C) SAC solders do not perform as well.  Cross-compatibility of mixing leaded and lead-free solders and surface platings is also a concern.  Growth of tin-whiskers form 100% tin platings also remains a concern.  Issues with using lead-free solders in high-temperature (die-attach) and low temperature (optoelectronic and MEMS) applications are still being worked out.