What is surface treatment on PCB

As you may already know, printed circuit board traces and components must withstand the effects of harsh environments and remain as elements that can be worked on. Since the environment might wear off some of the features and components of a PCB such as vias or pads, soldering and manufacturing could become problematic, and hence, the use of a PCB surface treatment becomes handy.

FPCmanufacturer learned the PCB surface treatment is a finishing layer that allows protection to the PCB for its late manufacturability (shelf-life), and they come in different options and applications depending on their type, as described below.

What is the purpose of surface treatment

The objective of providing a PCB surface treatment is that of protection and assurance of the board manufacturability in terms of soldering and guarantee the provisioned component electric characteristics. Say, to prevent corrosion and wear-off from PCB pads that might happen after some time, an extra finishing layer will give it protection for the environment it was designed to work on.

How many types of surface treatment are there

Hot air solder leveling

Hot Air Solder Leveling, also known as HASL, is the most common finishing due to its low cost of application.

This finishing consists of a coating with tin and lead through the PCB while hot air bursts (hot air knives) are utilized to remove excess material from the surface.

Although this finishing provides good life expectancy (having been a long-time industry standard, it is not good when it comes to environmental impact due to having lead present in the process. However, recent developments have led to lead-free HASL that utilizes materials such as nickel and germanium as lead replacement, but unfortunately this process requires higher temperatures to be effective.

Among other disadvantages presented by HASL is its lack of compatibility for very small pitch SMT and BGA components.

OSP

OSP is an organic based protective film, hence the terminology it stands for: Organic Solderability Preservatives (also known as preflux). This leads to having RoHS compliant PCB fabrication, for boards that need to get involved in standard moisture environments, for rust prevention.

However, the consistency of the preflux does not have long shelf-life applications, nor a good protection for plated holes, hence the low planarity conservation. Moreover, since the component is sprayed, there is no guarantee on a specific thickness measurement for the organic coating. However, compared to other cheap finishing such as HASL, it has wire bondable applicability for aluminum.

IMMERSION GOLD

Also known as ENIG (Electroless Nickel Immersion Gold), it is a PCB surface treatment consisting of a dual coating surface.

The description is simple. Nickel material is deposited on the copper from the PCB, having a thickness from 3 to 6 µm typically. Then, a thin layer of about 0.05 to 0.1 µm gold atoms is deposited over the nickel layer.

Gold is known for its great antioxidation capabilities, which in fact will provide protection for the previously deposited nickel on the PCB.However, the fact of using gold makes this PCB surface treatment very expensive, and it is not guaranteed that control on the phosphorous levels on the board can be controlled during the process, which might make the solder joints to be separated from the nickel underplate, which is known as black pad effect.

There is another variant called as Hard Gold, which deposits a higher amount of gold and nickel, which is widely used for connectors and special contacts. But again, this finishing is very expensive and must be used where it is a differentiator (quality signal transmission).types-of-surface-treatment

ENEPIG

ENEPIG stands for Electroless Nickel Electroless Palladium Immersion Gold, and comes in a similar finishing fashion as ENIG, with the difference of adding a middle layer of palladium, which can provide better wire bonding from ENIG and eliminating the black pad effect.However, this technique is fresh and complex due to its maturity, making it a high-cost PCB surface treatment yet.

IMMERSION SILVER

This PCB surface treatment provides an additional layer of silver of around 0.1 to 0.3 µm, which will diffuse into the copper, providing planarity and flatness to the PCB with RoHS compliance.It can be cheaper than gold, but it can get some rust or tarnish in the long end, which will require special storage with anti-tarnishing packaging.

This is a good option for BGA fine pitches and reworkable PCBs, but it is prone to a very short timeframe for assembling once the packaging has been opened.

IMMERSION TIN

It is similar on its application as immersion silver as the tin deposited will get diffused with the copper.Also, it needs special storage with anti-tarnishing packaging, making it to have low shelf-life, needing to get assembled during thirty or less days from its manufacturing.

Again, immersion PCB surface treatments provide good planarity and flatness for the board and allows small sized components such as BGA packages suitable to be worked with, however it has some drawbacks for the creation of special crystalline structures that can form on Zinc or tin structures known as whiskers (intermetallic diffusion problems) which can create transient short circuits on the places where they might become present.

PCBsupplier learned this article described about different types of finishing and discussed about the appropriate application for them.  Hopefully, this short guide can provide suitable reference for choosing a finishing option to a specific design.