7. Electroless nickel/impregnated gold
Applications: Suitable for PCBAs with a large number of fine pitch devices (<0.63mm) and high coplanarity requirements. can also be used as a selective plating on OSP surfaces, as per keyboards.
Cost: High.
Compatibility with lead-free: compatible.
Shelf life: 12 months.
Solderability (wettability): high.
Disadvantages: Risk of embrittlement of solder joints/soldering seams.
Risk of "black disc" failure. Black disc is a defect with a very low probability of occurrence, which is difficult to detect with normal inspection methods, but the resulting failure is catastrophic and is therefore generally not recommended for fine pitch BGA pad surface treatment.
The immersion gold layer is very thin and cannot withstand more than 10 mechanical insertions and removals.
Supplier resources: more.
8. Sunken silver Im-ag
Applications: Suitable for PCBAs with a large number of fine-pitch devices (<0.63mm) and relatively high coplanarity requirements.
Cost: low.
Compatibility with lead-free: compatible.
Shelf life: 12 months.
Solderability (wettability): high.
Disadvantages: potential interfacial microvoids.
Incompatible with gold-plated crimp connectors because of the high friction between the two.
The silver-impregnated layer is very thin and cannot withstand more than 10 mechanical insertions and removals.
Non-soldered areas are prone to high temperature discolouration.
Easily vulcanised (sulphur sensitive)
There is a Giovanni effect and the depth of the grooves will generally be plated at around 10μm.
Prone to creeping corrosion in high sulphur environments due to the exposed copper of the Giovanni grooves.
Supplier resources: more.
9. Sink tin Im-Sn
Application: Recommended for backplanes (Back Plane). It can obtain satisfactory crimp aperture size, easily ±0.05mm (±0.002mil), in addition to a certain lubricating effect, especially suitable for PCBA mainly for crimp connectors
Cost: low (equivalent to ENIG)
Compatibility with lead-free: compatible.
Shelf life: 12 months.
Solderability (wettability): high.
Disadvantages: not recommended for single boards (Line Card) due to handprints and limited number of returns
The tinned layer near the plug holes tends to discolour after reflow soldering. This is due to the fact that the solder resist (commonly known as green oil) tends to hide the flux in the plug hole and reacts with the nearby tin layer when sprayed out during reflow soldering.
There is a risk of tin whiskering. The risk of tin whiskering depends on the flux used for solder immersion, with some fluxes producing layers of tin that are prone to whiskering and others less so.
Some sink tin formulation fluxes are incompatible with solder resists and are more severe in terms of solder resist erosion, making them unsuitable for fine solder resist bridge applications.
Supplier resources: more.
10. Hot melt tin lead
Applications: Generally used for backplanes.
Cost: Medium.
Compatibility with lead-free: incompatible.
Shelf life: 12 months.
Solderability (wettability): Medium. The biggest advantage of hot melt is that it is corrosion resistant, but the solderability is not very good.
Disadvantages: not suitable for single boards (Line Card)
Poor coplanarity between pad and solder pad, not suitable for PCBAs with high coplanarity requirements.
For plating with relatively large variations in relative thickness, to obtain a satisfactory finished metallised aperture size, compensation is required for the drilled aperture size.
Supplier resources: limited.
11. Chemical nickel-palladium/impregnated gold
Application: For very durable and stable inert surfaces without the risk of "black pan". Potential replacement for OSP or ENIG surface coatings for veneer applications.
Cost: Medium to high (lower than ENIG)
Compatibility with lead-free: compatible.
Shelf life: 12 months.
Solderability (wettability): high.
Disadvantages: Very few applications in the PCB industry and little experience.
Supplier resources: very few.
12. Selective chemical nickel/gold with OSP
Applications: Can be used for PCBAs requiring mechanical contact areas and fitted with fine pitch devices. in this application, OSP is used as a highly reliable solderable layer and ENIG is used as a mechanical contact area, like mobile phone keypad boards, where this surface treatment is mostly used.
Cost: medium to high.
Compatibility with lead-free: compatible.
Shelf life: 6 months.
Solderability (wettability): low.
Disadvantages: relatively high cost.
ENIG is not suitable for use as a ground edge plating for guide channel mounting as the ENIG layer is very thin and not resistant to rubbing.
Risk of Giovanni effect in OSP fluxes.
Supplier resources: more.
