The surface finishing of PCB forms a interface between the component and the printed circuit board. The finish is done to provide two main features viz. to protect the exposed the copper circuitry and the other is to provide the proper solderable surface when assembling the electronic component to the PCB. There are various techniques which have been emerged in the past in order to provide surface finish to the PCB. The primary function to give surface finish is to protect the copper area from oxidation prior to assembly and to form the foundation of the connection to the device. By these surface finishing processes the process of pcb assembly enhances, and it gives a reliable joint and improves the life. The available types of finishes are as follows.
- Sn/Pb Hot Air Solder level
- Lead-Free HASL
- Organic Solderability Preservative
- Immersion Silver
- Immersion (White) Tin
- Electrolytic Nickel/Gold(Hard and soft Gold wire bondable)
- Electroless Nickel immersion Gold (ENIG)
- Electroless Nickel Palladium Immersion Gold (ENPIG)
Few of them are discussed below in larger context.
- Soft Gold Surface Finish:
- There are two varieties as far as gold finish is concern one is of hard gold and one is of soft gold. The former consist of a layer of gold plated over a barrier coating of nickel. It has good durability compare to the soft gold. The soft gold surface finish is well suited for wire bounding operations. The soft gold easily forms strong metallic bond with copper circuitry and the wires. The gold is provided to protect the nickel layer from oxidation through immersion. The strong bond of gold and copper enhance the conduction characteristics. The process of applying soft gold is akin to the hard gold, both processed involved electrolytic process to apply the finish. During the process of finishing when soldering is done, the soft gold remains the alloy of gold and copper, and produces a strong weld joint. It provides high quality finishing.
The knoop hardness of order 130 to 200 can be achieved. It provides excellent wear resistance for surface. In terms of shortcomings it causes porosity within the layer. The copper exposed on the sides of the traces. The mask usually adheres on the gold.
- Electroless Nickel with immersion Gold (ENIG):
- Electro Nickel with immersion gold is a lamellar metallic coating of gold and nickel in proportional thickness. The gold severe two purposes here one is to protect the nickel coating from the external atmosphere and also provides low contact resistance.
It provides good surface coating for plated through holes. It does not have involvement of lead. On the other hand it is very expensive, not re-workable. It also shows the problem of signal loss in some situations. Because of its lamellar structure it has a complicated process initially. It has excellent corrosion resistance. Excellent flatness, good solderabilty, good surface contrast, good contact resistance, high Aspect ratio capability are few advantages which offered by ENIG.
- Hot Air Solder Leveling (HASL):
- HASL is the most extensively used process for finishing of PCB’s. The process starts with immersing the circuit board in a molten pool of Sn/Pb alloy. The excess solder is removed by using ‘air knives’. In this process hot air blows across the board to remove the excess material. The remaining layer of solder protects from the corrosion, while easing the task of soldering components to the board. One major advantage of this process is that it will expose the PCB to temperature in range of 265o It is a low cost process, and finishing has excellent shelf life. And its availability is very wide. This process is very cost effective amongst all the surface finish processes available.
Another advancement that took place overtime that Lead free HASL which is similar to the previous one. Only tin and copper are part of this soldering alloy, and it ultimately results a higher melting point compare to the lead solder.
On the other hand uneven surfaces are part of this surface finishing. Even the ‘air knives’ doesn’t give the required flatness. It has poor performance to thermal shock. It has amount of lead as well.
- Organic Solderability Preservative (OSP):
- This method uses water-based compound which essentially bound with copper (Selectively) and protects the copper until soldering. OSP is a chemical process to provide a organic layer coating over the copper surface, which has anti-oxidation, moisture resistant and heat resistance to protect copper from environmental effects. In this method azole family compound are used viz. benzotriazeoles, imidazoles, etc. They adsorb on the surface of copper by coordination bond with copper atom and form a thick film. A flat surface can be achieved; no lead is involved in this process. On the other hand OSP has its own shortcomings in terms of performance. It has short shelf life. Organic layer largely expose to copper.
Advantage and Disadvantage of surface finishing Processes:
If the copper surface over a printed circuit board is left unprotected the copper will oxidize and deteriorate, and the circuit board becomes unusable. The surface finish forms a critical interface between component and circuit board. Some of the process in surface finishing has involvement of lead, which causes health problems.
HASL is the most commonly used in printed circuit board processing. It has longer storage time and also suitable for lead free soldering. Its low cost makes it more popular for this task.
Similarly the OSP process provides a very smooth process after finishing and suitable to operate. This is also low cost and environment friendly. On the other hand it is not suitable for visual inspection and electrical measurement. It also has high requirement for storage.
All processes of surface finishing have their advantages and disadvantages. It depends upon the requirement that which process is going to be most suitable. The decision is based on cost, quality and the task for which the printed circuit board is going to be used.