PCB plating takes place in tanks filled with dissolved metals.

For some processes, fabricators must connect the PCB to the cathode of a DC power supply; for other processes, the PCB simply needs to be exposed to the dissolved metals for the plating to occur.  There are two types of plating: additive plating, where the overall thickness of the base metal increases, and displacement plating, where the overall thickness of the base metal remains constant

Immersion

Immersion processes are replacement/displacement reactions. The plating atoms from the solution encounter and replace the surface atoms of the base material.  Immersion reactions are self-limiting – the reaction stops once the new atoms replace all of the surface substrate atoms. The plating thickness increases asymptotically and is mostly independent of surface geometry.

Autocatalytic/Electroless

Autocatalytic / electroless reactions are reduction reactions that add new metals atop the original base metals.  Unlike typical electroplating reactions, no external current is needed for autocatalytic/electroless plating operations. The plating thickness depends on the time the metal spends in the plating bath.

These reactions sometimes require the surface to be “activated” with a precious metal before placing the board in a plating bath full of the plating metal and some reducing agent.  Atoms of the plating metal will continuously accumulate on the surface of the base metal as long as the board stays in the bath.

Electroplating

Electroplating is an additive process where an electric current drives atoms from sacrificial ingots connected to the anode of a power supply to conductors connected to the cathode of a power supply.  This process can produce plating of any thickness.  The geometry of the target metal and the location of that metal in the plating tank will affect plating thickness.

HASL

Hot Air Surface Level is a popular option for designers because it is very inexpensive.  For this method, fabricators dip a PCB in a vat of molten solder metals. When the board comes out of the solder bath, hot air knives blow the molten solder from the board’s surface into the bath.  A thin solder layer is left behind, owing to the adhesion between the copper and the solder metals.

Unfortunately, the process does have drawbacks.

For example, the liquid temperature of molten solder is dozens of degrees higher than the glass-transition temperature of the PCB substrate, which might cause the board to warp or de-laminate when submerged into the vat of molten solder.

OSP

The image above is an artistic interpretation of an OSP (ethyl-benzimidazole) molecule that demonstrates the approximate location of the pi-orbitals.   Nitrogen atoms (shown here in Blue) in the OSP molecules bond weakly with the copper atoms on the surface of a PCB, forming a barrier against atmospheric oxygen.

Organic Solderability Preservative is an organic chemical that temporarily coats all exposed copper with a thin layer of chemicals that provides a barrier between copper and oxygen. Fabricators dip a PCB in the chemical bath, and the nitrogen atoms in the azole group form weak bonds with the surface copper atoms.  During assembly, flux removes the OSP from the land pads, and the solder metals form permanent bonds.

ENIG/ENAG

Electroless Nickel Immersion Gold/Electroless Nickel Autocatalytic Gold are very common surface finish that coat all exposed copper with a layer of nickel followed by a layer of gold.  The nickel builds linearly over time to the desired thickness.  Then either immersion gold will replace the outermost nickel atoms in a self-limiting, asymptotic-thickness coating, or autocatalytic gold will build linearly to a specified thickness over time.

Unfortunately, ENIG has a potential flaw called black pad, wherein the immersion gold process selectively corrodes the nickel layer around grain boundaries.  Visually, it appears as black spots on a land pad.  Physically, these corrosion spots are weak spots that will eventually cause solder-joint failure.

ENEPIG

With Electroless Nickel, Electroless Palladium, Immersion Gold, an additional metal layer provides a buffer between the nickel and the gold atoms.  The palladium prevents the gold from replacing the nickel atoms, and with no atom exchange, the black-pad defect cannot occur.  Unfortunately, the additional steps to create these pads and the addition of a precious metal substantially increase the coating cost.

Summary

Of the finishes listed above, HASL is the least expensive and least coplanar.  OSP is inexpensive but fragile and has a short shelf-life.  ENIG is more expensive than the previous two options and offers great performance against a medium-range price.  ENEPIG is the surface finish of choice for the highest reliability applications, but it comes with a premium price point.

To reduce the chance of failure of your SMT design during assembly, avoid HASL and use an assembly shop that uses jet-paste printers.