PCB FINISHES, the main options !

When it comes to finishes, the customer have a wide range of different types according to the needs, budget and applications. We will review the mains types of finishes available on the market from the cheapest to the most evolved.

What is a PCB finishing ?

It’s an important part of the manufacturing PCB process, which comes after applying the solder mask on the panel. The goal of the finishing is to protect the copper from oxidation, which can make the PCB unusable. The finishing protects the pattern from oxidation, but also from deterioration but it must provide a perfect solderability when the component of the assembly will be placed and welded on the board.

The different options available at ICAPE Group

With more than 20 years in the PCB industry, ICAPE Group has developped a strong knowledge in the different types of finishings. With its 25 factory-partners based in Asia, ICAPE Group can manage and control all the steps of the processes, which ensure a full satisfaction to its 1700 customers all around the world.

Lead Free Hot Air Solder Leveling - LF HASL

The panels are immersed in a bath of molten tin, during few seconds at 265°C and the excess is removed with high pressurized hot air (air knives) across the board. To dive the board in such a hot bath will help to identify defects or damages before assembling the components. This process was one of the most use surface finishes but reach its limitations. Due to high density boards, this process leaves uneven surfaces, can create electrical shorts and is not suitable for fine pitch component.

- Excellent solderability
- Cost efficient
- Widely available technology

- Possible difference in thickness on the surface
- Non suitable for fine pitch component
- Aggressive with the copper, depending of the allow
- Thermal stress to the board

Immersion Tin

Immersion Tin or ISn, is a metallic finish chemically deposited, applied over the basis metal of the PCB. The typical thickness of less than 1µm gives to this finishing a perfect flatness but it also make it very sensitive and the PCB must be handled carefully with gloves. Copper and Tin can create an intermetallic layer which can grow by Tin and XXXXXXXXXXXXXXXXX which reduce the shelf life to 6 months. Tin whiskers can also be a problem but usually, this finish ensure a great reliability and is good for fine pitch and small components.

- Perfect flatness
- No Lead used
- Possible rework
- Top choice for pressfit

- Sensitive, possible damage when handled
- Environmental issues with Thiourea used in processingt

Immersion Silver Ag

Immersion Silver is a Chemical process which provide an excellent flatness of the surfaces. The thickness of the finishing vary from 0.12 to 0.40 µm and the shelf life is 6 months. Mostly used in USA, it can be reworked and it’s a competitive solution for a lead free finish. Plus, this technology is aluminium wire bondable but it also brings some constrains like it’s sensitiveness to handling, the requirement of a special packaging, and the reduced supply chain options to support this finish.

- Excellent Flatness
- Perfect for fine pitch and small components
- Competitive price
- No lead
- Rework possible
- Aluminium wire bondable

- Sensitive to handling
- Reduced supply chain options
- Short window between operations

ENIG – Electroless Nickel Immersion Gold

This process quicky become the most use surface finishes. It’s a double layer metallic coating with Nickel and Gold. A layer of 3-5 microns nickel is chemically deposited in a bath. It’s a barrier to the copper and a surface where components are soldered. Afterwards, 0.05 micron of pure Gold is applied on the boards to protect the Nikel during the storage time. It’s a lead free process which provides a very flat surface and an excellent solderability and reliability. But all top technology have a cost and the cost of the ENIG finishing is one the highest on the market. and still have limits.

- Flat surfaces
- Strong solderability
- Bonding friendly

- Not cost effective
- Electroless nickel/phosphorous can have undesirable magnetic properties
- Aluminum wire bondable, but not gold wire
- Oxidation of Nickel can cause a solder failure

ENEPIG – Electroless Nickel Electroless Palladium Immersion Gold

The process is the same than ENIG, but even if ENIG is a top quality finishing, an oxidation of the Nikel can cause failure when it comes to solder the components on the board. To take that risk away, it is possible to apply a palladium layer between the Nickel and the Gold, which is resilient against corrosion. But, the solderability could be affected if the palladium layer is too thick.

- Flat surfaces
- Strong solderability
- Bonding friendly
- Palladium layer eliminates potential corrosion from immersion reaction

- Expensive technology
- Electroless nickel/phosphorous can have undesirable magnetic properties
- Solderability performance is reduced due to too thick palladium layer
- Slower to be wet

OSP - Organic Solderability Preservative

OSP is a water-based finish, applied in a bath and generally used for copper pads. This organic soldermask links to the copper and protect it from corrosion. If the mask is very flat due to its small thickness, but the shelf life is short. This process is environmentally friendly.

- Flatness
- Rework possible
- Clean and environmentally friendly
- Low cost

- Limited Shelf life
- Limited thermal cycles
- Difficult to inspect
- Request relatively aggressive flux at assembly