Alloys for Gold Soldering
Indium-lead solder for thick gold metallization
Anyone who spends time perusing the various solder compositions will quickly realize that tin is one of the main constituents in most solders. However, tin has an affinity for alloying with precious metals such as gold. Studies indicate that 63Sn/37Pb at 200ºC (392°F) will dissolve one micron (~40 micro-inches) of gold/second/unit area. As tin reacts with gold, a brittle Au/Sn intermetallic forms. When the concentration is high enough, these intermetallics have a deleterious effect on the thermal fatigue characteristics of the joint, and make it susceptible to fracture during thermal cycling. For tin-bearing solders in applications with gold plated materials, it is advisable to keep the gold layer thin, < 0.38μ (15 micro-inches), thereby reducing the concentration of Au/ Sn intermetallic that can form. However, many applications such as optoelectronics packages and defense/space electronics call for thicker gold metallization. In such scenarios, in which the need for reliability is high, tin-bearing solders are not appropriate. Unlike tin, indium has a much lower affinity for precious metals and dissolves gold at a rate 13 to14 times slower than tin. Also, indevices with operational temperatures below 125ºC (257°F), the intermetallic that forms between indium and gold is of a much more compliant and ductile nature, and is not susceptible to embrittlement. Therefore, the family of In/Pb solders is beneficial when soldering against thick gold film metallizations.The In/Pb alloys are a solid solution system in which the liquidus and solidus temperatures are close for all compositions (near-eutectic at all compositions). The indium-lead system offers alloys of varying melting points, with indium-rich compositions having a lower melting range, and the lead-rich compositions having a higher melting range.
Solder is typically provided in these common forms:
- Bar/Ingot: Typically cast and used in solder pot or wave soldering applications.
- Shot: Small tear-drop shaped pieces of alloy. The relatively small size offers flexibility in applications in which the alloy has to be weighed to a particular amount, such as filling crucibles for vapor deposition.
- Spheres: Also called precision solder balls, spheres are supplied with diameters from 0.012 to 0.032 in. They are deposited as bumps on electronicpackages such as BGAs (ball grid arrays).
- Ribbon and foil: Typically thin (0.002 to 0.010 in.+ thick) pieces of solder, foil often has a square or rectangular geometry. Ribbon, on the other hand, is more of a long, narrow strip wrapped on a spool. Both can be hand cut to form simple preforms or to make shims and thermal interfaces.
- Wire: Often applied in rework or cut to lengths and formed into rings or other simple shapes, wire diameters typically range from 0.010 to 0.030 in. However, smaller and larger diameters are available, depending on the alloy. Solder wire can be produced with a flux core.
- Preforms: Typically punched, these thin pieces of solder are manufactured as squares, rectangles, frames, disks, washers, and custom geometries. Solder preforms can be applied in surface mount technology (SMT),which is common to the manufacture of most consumer electronics such ascellular phones and computers. Preforms separately attach a component to a pad, or they augment the solder volume of the solder paste. Washers serve as pin connectors or other through-hole components.
Generally RMA,RAand Liquid flux can be used but specially for gold soldering we advise to use halide free fluxes or such fluxes which can be forming gas on 320—350°C is effective and may be used instead of flux. An inert atmosphere will increase flux efficiency and simplify cleaning issues in high temperature applications where a flux is still required. For fluxless soldering with highindium alloys, an inert atmosphere is obligatory
The most suitable alloys are given in below:
|Alloy No||Composition||Melting temperature||Working temperature|