The atomic energy of surface finishes: taking a smaller perspective with surface finishes helps to understand the benefits of each type of metal coating.
The simplest metal deposition mechanism is electroplating. When a PCB is placed into a bath of nickel sulfate, nothing happens, but if that PCB is connected to the negative end of an electrical power supply and the positive end to the plating bath, voila, a nickel plated PCB. The amount of voltage needed has to exceed the reduction potential of the nickel; we need to squeeze an electron into the shell of electrons orbiting the positively charge nickel cation. The amount of current applied can be directly correlated to the nickel thickness. At two electrons per nickel atom, it's a simple calculation.
When electroplating, we end up with several microns of neatly stacked nickel, arranged rather similarly to oranges packed in a crate. Considering the fact that the distance from one nickel atom to another is about 1 angstrom to 2 angstroms (a tenth of a billionth of a meter), there are 10 layers to 20,000 layers of nickel atoms stacked up on the copper. The thinness, stacking order, purity and type of metal at the atomic level give each surface finish its properties. The result of 20,000 neatly stacked nickel atoms is a somewhat brittle, yet strong, grayish film of moderate electrical conductivity and enough durability to resist mild handling stresses, like those of one PCB sliding across the surface of another. But what if we had the desire to get the metal on the copper without all that rigging and rectifiers? Enter immersion metal plating, more technically known as galvanic displacement.
Galvanic displacement represents one step toward increased complexity in metal deposition. If we dump a handful of, say, silver nitrate into a tank of water, the solid silver nitrate salt dissolves. The N[O.sub.3] detaches from the silver atom, taking with it one electron, and that silver ion stays soluble in water in happy equilibrium with nitrate anion. If a surface of exposed copper is placed into the silver nitrate solution, the silver ion steals an electron from the copper on the PCB. The electron-satisfied silver turns solid, and the victim copper, deprived of its electron, becomes a cation, drifting around in the water. (It actually takes two silver atoms to steal enough electrons from copper to make it soluble.) This happens because silver has a higher affinity for electrons. Silver's electron affinity of 126 kJ/mol is higher than copper's at 119 kJ/mol, so silver does the stealing and copper does the giving.
Immersion plating relies on the exchange of metals and the exposure of underlying copper, therefore, immersion deposits are quite thin, usually less than one micron thick--just a scant thousand atoms or so. Thinness contributes to the more sensitive handling required of immersion metal deposits. Only metals with strong electron affinity will deposit. The electron affinity correlates with more tightly held electrons, which leads to atomic stability and manifests as relatively corrosion resistant metals, which do not easily oxidize (gold, silver and tin.)
The selection of immersion metal plating is usually driven by the desire to deposit metal without the need for electrical contact and bussing. Sometimes, we want to deposit a metal that cannot be immersion deposited, and we don't wish to return to bussed circuitry designs. Here, we're stuck with the most complicated surface finish, electroless metal deposition. Nickel, with an electron affinity of 112 kJ/mol, will not galvanically immerse on copper, so electrons need to get to nickel some other way--electroless deposition. If we take an unstable chemical, like sodium hypophosphite, and place it in a solution with nickel cations, the hypophosphite will give up electrons and supply them to the nickel; the nickel achieves a stable full shell of electrons and deposits on the copper as a solid metal. In the process, the hypophosphite molecule goes through some tricky disproportionation maneuvers, and some of the phosphorous gets trapped in the metal deposit, too. This makes the nickel in ENIG physically stronger than electroplated nickel, increases its electrical resistivity and makes it more resistant to corrosion. We know that slight increases in phosphorous concentration reduce the corrosion of black-pad nickel.
Understanding these mechanisms helps us determine the best type of surface finish for any specific application. Remember, the next time you need to sort out some big problems, make sure you start by thinking small, all the way down to the atomic level.
DON CULLEN is managing director, photovoltaics for MacDermid, Inc. He can be reached at firstname.lastname@example.org.
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|Title Annotation:||FINAL FINISH FORUM|
|Publication:||Printed Circuit Design & Fab|
|Date:||Jan 1, 2009|
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