Published byPinsheng Electronics Co., LtdAugust 21,2020
What is PCB surface finish?
PCB surface finish is between the metal face of the components and the pads on PCB, for protecting the copper foil from oxidation and providing a solderable surface for PCB assembly. Every PCB surface finish has its pros and cons, could be customized as the requirements.
"The PCB is dipped into molten solder so that all exposed copper surfaces are covered, excess solder is removed by 'hot air knives'".
The composition of hasl treatment commonly is Tin (63% ) and Lead (37%), typical thicknesses range from 30-1500 μ'', of which the melts point is about 183 degrees C.
For hasl lead-free, the common alloys refer to Sn/Ag/Cu (SAC), Sn/Cu/Co, Sn/Cu/Ni/Ge, etc., of which the thicknesses range from 300-1000 μ''. The hasl PCBs (lead-free) even can bear a high temperature of 260 degrees C. Hasl lead-free is the predominant PCB surface finish for circuit boards for it is RoHS&WEEE compliant.
Two layer metallic coating - ENIG cover the copper using thick, well-electrically nickel-gold, commonly 2-8 μ'' Au over 120-240 μ'' Ni, in which nickel is deposited on palladium-catalyzed copper surfaces, gold adheres to the nickel-plated area by molecular exchange, which protects the nickel (actual solder surface) until the soldering process, also Au provides low contact resistance and good wetting.
ENIG technology answered the request for the flat surface used in fine pitch assembly and lead-free, even though it is a little bit expensive than others, but no doubt ENIG has already become themost commonly-used PCB surface finish for many categories, such as cellphone or power supply.
ENIG PCB finish
Flat surface excellent for fine-pitch (BGA, QFP...)
The same two-layer metal coating, what differs from ENIG is that this PCB surface finish is electroplated rather than chemical immersion, in which nickel layer gets plated before gold plating. Since the PCB must act as an electrode, the electroplating must be done before the solder mask process.
Because of unstable solderability, it is currently only used in IC carrier boards (PBGA/FCBGA/FCCSP...), or in the gold finger that is explicitly required.
Excellent shelf life (12 months+)
Especially suitable for contact switch or "gold finger"
A solution to "Black phosphorus pad" of ENIG. By introducing a layer of palladium (4 to 20 μ''), prevent Ni layer (120 – 240 μ'') from corroding overly by Au (2 to 8 μ''), ENEPIG got come up in the 1990s, also brought in a better option to wire bonding.
Although ENEPIG it is not a commonly-used PCB surface finish at present, the development grows rapidly.
Excellent shelf life (12 months+)
Good for IC carrier PCBs
Excellently suitable for gold/aluminum wire bonding
On the clean bare copper surface, a layer of organic film is chemically applied. This film is a water-based organic compound that selectively combines with copper and provides an organic metal layer to protect the copper surface from rust in the normal environment. In the subsequent soldering, it will be quickly removed.
OSP PCB finish
Flat surface, good for fine-pitch pads (BGA, QFP...)
Very thin coating
Can be applied together with other finishes (e.g. OSP+ENIG)
Not good for PTH
Short Shelf life (<6 months)
Not suitable for crimping technology
Not Good for multiple reflow
Copper will get exposed at assembly, requires relatively aggressive flux
Difficult to inspect, might cause issues in ICT testing
Tin will be applied directly on copper for protection. It does not bring in any new elements to the soldering surface, especially workable for communication backplanes. But copper and tin have a strong affinity with each other, the diffusion of one metal to another will inevitably occur, which will directly affect the shelf life. ISn requires good storage, while it is restricted due to the presence of carcinogens.
A good alternative to ENIG, with a moderate shelf life (about 12 months) and good electrical properties unmatched by other PCB surface finishes, but it is susceptible to sulfur dioxide, which tarnishes the surface and forms an AgS2 layer. This layer adversely affects solderability. To avoid this contamination, we package the PCB in silver-plated paper and sealed packaging.
Flat surface for fine pitch
Aluminum wire bondable
Cost effective (than ENIG)
Low high frequency signal loss (skin effect)
Great Shelf life (6-12 months)
Sensitive to handling, electrical test, packaging
Difficult to inspect
Not good for micro-vias of ratio > 1:1
Not suited for compliant-pin intertion (Ni-Au Pins)
Not sure which type of PCB surface finish is suitable for your PCB boards? Now is the time to contact us for a further manufacturability suggestion.