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PCB Surface Finish Options Or Types

PCB Surface Finish Options Or Types
Issue Time:2018-12-19

PCB Surface Finish Options Or Types

The basic purpose of PCB surface treatment is to ensure good weldability or electrical properties. Due to copper easily oxidized in the air, copper oxide layer on the welding has a great influence, it is easy to form false welding, welding, caused serious welding plate and components cannot be welded, as such, PCB in production, there will be a process, in the welding plate surface coating (plating) after a layer of material on, protect welding plate from oxidation.
Lead-free Hot Air Solder Leveling Due to environmental concerns, the need for lead-free solutions.  To create the HASL finish, the printed circuit board will be submersed in molten solder (tin / lead). This solder will cover all the exposed copper surfaces. Upon retraction from the solder, high pressure hot air is blown over the surface through air knives, this levels the solder deposit and removes the excess solder from the board surface.Hot air leveling bisector for vertical and horizontal two, generally think of horizontal better, mainly horizontal hot air leveling coating is more uniform, can achieve automatic production.
OSP 

Organic Solderability Preservative  

OSP is a water-based, organic surface finish that is typically used for copper pads. It selectively bonds to copper and protects the copper pad before soldering. 

Advantages: simple process, very smooth surface, suitable for lead-free welding and SMT. Easy rework, easy production operation, suitable for horizontal line operation. Boards are suitable for multiple processes (e.g. OSP+ENIG). Low cost, environmentally friendly.
Disadvantages: limited number of reflow soldering (multiple welding thickness, film will be damaged, basically no problem for 2 times). Not suitable for compression bonding technology, wire binding. Visual inspection and electrical inspection are inconvenient. N2 gas protection is required for SMT. SMT rework is not suitable. Storage requirements are high.

Immersion Tin (ISn) is a good surface finish for planar, fine pitch products and is popular with press fit and backplanes. 

Advantages: suitable for horizontal line production. Suitable for fine line processing, lead-free welding, especially for compression welding technology. Very good flatness, suitable for SMT.
Disadvantages: good storage conditions are required, preferably no more than 6 months to control tin whisker growth. Not suitable for contact switch design. In the production process, there is a high requirement for the soldering film process, otherwise the film will fall off. N2 gas protection is preferred for multiple welds. Electrical measurement is also a problem.

Immersion Silver
Silver precipitation process is between organic coating and electroless nickel plating/gold deposition. Silver retains good weldability even when exposed to heat, moisture and pollution, but it loses its luster. Silver deposit does not have the physical strength of electroless nickel plating/gold deposit because there is no nickel under the silver deposit.
Advantages: simple process, suitable for lead-free welding, SMT. Very smooth surface, low cost, suitable for very fine lines.
Disadvantages: high storage conditions, easy pollution. Welding strength is prone to problems (micro cavity problems). Electromigration and givani corrosion of copper under soldering film are easy to occur. Electrical measurement is also a problem
ENIG coating PCB surface conductors with nickel before gold plating, nickel plating is mainly to prevent the diffusion of gold and copper. There are currently two types of nickel gold plating: soft gold and hard gold. Soft gold is mainly used for chip packaging when gold wire; Hard gold is primarily used for electrical interconnections that are not soldered.
Pros: longer storage time >12 months. Suitable for contact switch design and gold wire binding.
Weakness: higher cost, thicker gold. Electroplated fingers require additional design lines to conduct electricity. Due to the inconsistent thickness of gold, when used in welding, the welding point may be brittle due to the thickness of gold, which affects the strength.
HASL
Silver precipitation process is between organic coating and electroless nickel plating/gold deposition. Silver retains good weldability even when exposed to heat, moisture and pollution, but it loses its luster. Silver deposit does not have the physical strength of electroless nickel plating/gold deposit because there is no nickel under the silver deposit.
Advantages: simple process, suitable for lead-free welding, SMT. Very smooth surface, low cost, suitable for very fine lines.
Disadvantages: high storage conditions, easy pollution. Welding strength is prone to problems (micro cavity problems). Electromigration and givani corrosion of copper under soldering film are easy to occur. Electrical measurement is also a problem
Gold Finger
 A row of square contacts neatly arranged on the edge of PCB are intended to plug into another matching socket of a main board to connect both PCBs and they are often dealed with gold-plating,this edge connector is called gold finger. 
Tin-Plating
Tin plating is an industrial process where metal objects are coated with a thin layer of tin. This process can be applied to many types of base metals, including steel, iron, and copper. Tin plating improves the appearance of these metals for certain applications, and also helps to increase their performance and durability.
Flash Gold
Flash gold plating is less than 0.25 Microns and usually not done on connectors. For this purpose, Cobalt or Nickel hardened additives are used in the Acid gold bath and usually a thickness of >1.0 Micron gold is deposited and is subjected to tests such as contact resistance, wear resistance, porosity etc.
Gold Plating
Gold Plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver (to make silver-gilt), by chemical or electrochemical plating.
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