Surface finish, or surface coating, is the most important step in the process between PCB manufacturing and PCB assembly with two main functions, one of which is to preserve the exposed copper circuitry and the other of which is to provide solderable surface when soldering components to the PCB.
Listed below are some more common surface finishes used in PCB manufacturing.
HASL / Lead Free HASL
HASL is the predominant surface finish used in the industry. The process consists of immersing circuit boards in a molten pot of a tin/lead alloy and then removing the excess solder by using ‘air knives’, which blow hot air across the surface of the board.
Advantages: low cost, long shelf life, and HASL is reworkable.
Disadvantages: uneven surface for soldering, Contains Lead (Not RoHS Compliant), cannot hold tight tolerances on plated holes.
Immersion Gold (ENIG)
Electroless nickel immersion gold (ENIG) is a type of surface plating used for printed circuit boards. It consists of an electroless nickel plating covered with a thin layer of immersion gold, which protects the nickel from oxidation.
Advantages: excellent surface planarity (particularly helpful for PCBs with large BGA packages), good oxidation resistance, and usability for untreated contact surfaces such as membrane switches and contact points.
Disadvantages: expensive, signal loss for signal integrity applications, black pad.
Immersion Silver is the standard finish on PCB Train and is RoHS compliant. It is an improvement on HASL being exceptionally flat, and can replace Immersion Gold over Electroless Nickel (Au/Ni) for most applications. The use of Immersion Silver has no effect on customer’s assembly processes. Immersion Silver Metallic Solderability Preservative is a 0.1 to 0.2 micron layer of metallic silver coated with an organic layer of solderability preservative.
Advantages: flat surface to solder to, Lead-Free and RoHS compliant, tighter tolerances can be held for plated holes, low loss for Signal Integrity Applications.
Disadvantages: handling the PCB can cause soldering issues, it is more cost effective than ENIG but less cost effective than ISn, and the finish can tarnish and oxidize.
ImSn is mostly the same as ImAg except that tin is used in ImSn while silver is used in ImAg. In terms of the advantages of ImSn, it provides an extremely planar finish on the copper pads, making it very suitable for SMT applications.
OSP (Organic Solderability Preservative) or anti-tarnish preserves the copper surface from oxidation by applying a very thin protective layer of material over the exposed copper usually using a conveyorized process.
Advantages:Flat/planar; Short, easy process cycle ; Inexpensive;Reworkable ;Not affect finished hole size; Copper/Tin solder joint;
Disadvantages:Multiple reflows ; Limited shelf life ; Not conductive ; Difficult to inspect; Limited thermal cycles ;
Gold – Hard Gold
Hard Electrolytic Gold consists of a layer of gold plated over a barrier coat of nickel. Hard gold is extremely durable, and is most commonly applied to high-wear areas such as edge connector fingers and keypads.
Unlike ENIG, its thickness can vary by controlling the duration of the plating cycle, although the typical minimum values for fingers are 30 μin gold over 100 μin nickel for Class 1 and Class 2, 50 μin gold over 100 μin nickel for Class 3.
Hard gold is not generally applied to solderable areas, because of its high cost and its relatively poor solderability. The maximum thickness that IPC considers to be solderable is 17.8 μin, so if this type of gold must be used on surfaces to be soldered, the recommended nominal thickness should be about 5-10 μin.
Advantages to Hard Gold: durable surface, Lead-Free and RoHS Complaint, and long shelf life.
Disadvantages: extremely expensive compared to other finishes, bus plating can be required, and additional labor required.
All in all, it is important to select the appropriate surface finish for your project by considering the various options while factoring in performance requirements and material costs.
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