ENIG is an electroless nickel layer capped with a thin layer of immersion gold. It is a multifunctional surface finish, applicable to soldering, aluminum and copper wedge wire bonding, press fit connections, and as a contact surface. The immersion gold protects the underlying nickel from oxidation/passivation over its intended life. The nickel layer forms a diffusion barrier preventing the diffusion of the underlying copper into the gold overcoat. The nickel also strengthens the thru hole by acting as a rivet. In addition the nickel protects the copper from dissolution into molten solder at assembly. Chemically, gold is the ideal element for the external coating of PCBs. Gold does not form an oxide, so it is virtually unaffected by temperature and storage conditions that might reduce the shelf-life of other finishes. In addition, gold dissolves readily into solder, promoting superior wettability to the nickel substrate.

IPC-4552 ENIG Specification issued in 2021 has stated the electroless nickel thickness shall be 3 to 6 μm [118.1 to 236.2 μin], and the minimum immersion gold thickness shall be 0.05 [1.97 μin] at four sigma (standard deviation) below the mean; the typical range is 0.075 to 0.125 μm [2.955 to 4.925 μin]. There was no upper limit set.

OSP is very thin organic coatings used to preserve the solderability of PCB surface copper. OSP forms a complex organometallic compound with the copper surface of the PCB. The primary function of OSP is to provide a solderable surface finish, suitable for all surface mount and through-hole assembly applications and with an appropriate shelf life. The coating has demonstrated the ability to meet category 3 coating durability (shelf life of 12 months per J-STD-003 and industry data), when produced per the required vendor specifications. Some OSP coatings, may not necessarily meet this requirement, but may meet category 2 or 1 coating durability. In spite of OSP being limited to solderability preservation, OSPs are widely used worldwide on products that are not fine pitched or with high aspect ratio holes.

Immersion tin is a thin coating of pure tin, typically 0.6 to 1.2 μm thick, which protects the underlying copper from oxidation and provides a highly solderable surface. Tin is deposited using a galvanic displacement process and can be applied in vertical or conveyorized equipment. Tin boards are used primarily for solderability and have very good compliant pin connector functionality. IPC-4554 Immersion Tin Specification has specified a lower limit for thickness. The relatively thick value of 1 μm was chosen to ensure that enough virgin tin would be available at the surface for soldering after storage. It is well understood that tin forms an intermetallic (IMC) layer with the underlying copper, and that this layer continues to grow in thickness over time.

Immersion silver forms a thin immersion deposit over copper. It is a multifunctional surface finish, applicable to soldering, press fit connections and as a contact surface. It has the potential to be suitable for aluminum wire bonding. The immersion silver protects the underlying copper from oxidation over its intended shelf life. Immersion silver is a galvanic displacement process with a typical thickness range of 0.1 to 0.4 μm. The fabrication process is easy, but the coating can suffer from tarnish when left exposed after assembly in field use. Heavy tarnish can be a predictor for corrosion and functional loss.

IPC-4553 Immersion Silver Specification states: The thickness of the immersion silver deposit should be 0.12 μm [5 μin] minimum to 0.4 μm [16 μin] maximum at ± 4σ from process mean as measured on a pad of area 2.25 mm² or 1.5 mm × 1.5 mm [approximately 0.0036 in² or 0.060 in × 0.060 in]; typical value between 0.2 μm [8 μin] to 0.3 μm [12 μin].

The electrolytic nickel-gold process employs galvanic electroplating. The circuit board is placed on a rack and electrically connected to a power supply (rectifier). When immersed in a solution of metal ions, the metals are reduced as a metal coating on the PCB by the supply of electrons. The metal thickness grows in a very predictable way depending on the time, current, area plated, and efficiency of the reaction. More frequently, the coating is applied selectively in combination with another finish such as OSP. Electrolytic nickel gold is commonly used on features requiring high-force physical connections or gold wire-bonding. Several micrometers of nickel are deposited, and 0.50 to 1.5 μm of gold as the final mating surface. It is not a recommended soldering surface as the thicker gold will create embrittlement within the intermetallic phase of the solder joint. If the surface is designed as a contacting surface with multiple insertions hard gold must be used. Gold is hardened by alloying the deposit with ~3 percent of nickel, chromium, or iron. For a large number of PCBs that employ edge tab connection, the PCB is electroplated in a special plating line where only the exposed edge features are immersed into the plating cell. If the application is gold wire bonding, only soft gold (99.99) must be used. For gold wire bonding a thick gold layer ensures that the underlying nickel will not defuse all the way to the surface where it would have an adverse effect on the bonding process.

(ENEPIG) is sometimes referred to as the “universal” finish, because of the versatility of its applications. ENEPIG is suitable for soldering, gold wire bonding, aluminum wire bonding, and contact resistance. ENEPIG is formed by the sequential deposition of electroless Ni (120 to 240 μin) followed by 2 to 12 μin of electroless Pd with an immersion gold flash (1 to 2 μin) on top. It is a multifunctional surface finish, applicable to soldering and to gold, aluminum and copper wire bonding. In addition, it is also suitable as the mating surface for soft membrane and steel dome contacts, “low insertion force” (LIF) and “zero insertion force” (ZIF) edge connectors and for press-fit applications.

ENEPIG has the advantage over ENIG of being a gold wire bondable surface. ENIG is not an ideal gold wire bonding surface because over time the electroless nickel will diffuse thru to the surface along the crystal boundaries of the immersion gold and the bonding would fail. ENEPIG’s palladium layer is a diffusion barrier to the nickel, the gold remains unadulterated and is bondable with gold thickness as low as 1.2 μin or 0.03 μm. A thicker gold coating as thick as 0.1 to 0.2 μm or 4 to 8 μin would open the wire bonding operating window. Thicker gold is beyond the capability of standard immersion gold. If a thicker gold is specified then a modified immersion gold like a “reduction assisted immersion gold” would be the preferred choice.

The electroless palladium requires a reducing agent. The most commonly used reducing agent is sodium hypophosphite, this produces a phosphorus palladium deposit with 4 to 5 percent phosphorous in the deposit. The presence of phosphorous renders the palladium deposit amorphous (noncrystalline), making it the ideal diffusion barrier. Other reducing agents may produce a nonphosphorus palladium which tends to be crystalline in structure.