Guide

A Beginner’s Guide to PCB Surface Finish Types

PCB Surface Finish

Upon visiting the fabrication house website, PCB surface finish types are one of the options we are faced with when we decide to create a prototype for our design. The different PCB surface finish types make a huge difference in the storage lifetime and quality of the PCB,  protecting the conductivity of copper on the board.

Different PCB surface finish types have different properties. To reduce the exposure of copper traces to external air, we should learn about the pros and cons of each type.

In this guide, we will learn about the definition of PCB Surface Finish, and what PCB  Surface Finish Types are available that suit the best project specifications. Further, we will look into similarities among the options and points to keep in mind while doing PCB surface finish types selection activity.

Let us begin!

What is the PCB Surface Finish?

PCB Surface finish is a coating step in the PCB fabrication process that ensures that the PCB is guarded from external exposure to erosion and rusting. It is applied on the solder pads and open copper areas of the board. Open copper areas are left exposed to make the heat transfer from the board when the temperature of the PCB rises.

It provides aid during the PCB assembly process when electronic components are to be soldered.

With the correct selection of PCB surface finish types, we can ensure robust solder joints. The surface finish acts as a shield between copper on the board and the external environment. It supports the soldering process and extends the PCB lifetime.

There are different PCB surface finish types namely HASL, ENIG, OSP, Immersion Tin, SENIG

Different PCB Surface Finishes Types and Their Pros and Cons

PCB Surface Finish Types

HASL: Hot Air Solder Levelling

As one of the PCB surface finish types, this surface coating is widely used in most of the board fabrication. The process is to dip the PCB in the melting solder pool containing lead and either use Hot air knives or a hot air gun to smoothen out the extra solder deposited on the copper region. In these kinds of PCB finish types, it creates a smooth surface finish which helps component soldering placement and creates durable solder joints.

The solder material used in this one of the PCB surface finish types is a mixture of Tin and Lead. HASL can also be lead-free where the solder pool is a composition of 99% tin and 1% copper. Both the HASL and Lead-free HASL which is one of the PCB surface finish types, have slightly different production processes, which have to be adapted accordingly.

Advantages

  • Long-lasting shelf life
  • Cost-effective solution
  • Lead-free HASL is not harmful
  • Soldering assisted by easier metal-to-solder bonding
  • The copper coverage area is more.

Disadvantages

  • Not suitable for small packaging components
  • Due to high heat, chances of board damage
  • Lack of flatness due to the levelling process

 

OSP: Organic Solderability Preservative

It’s another one of the PCB surface finish types. In this surface finish process, a film of water-based organic layer is deposited on the bare copper of the board. The molecules used in the chemical process are benzotriazole and imidazole, both have anti-rust. The thickness of the layer is 0.2um to 0.45um. The process is very simple and budget-friendly. The steps of these kinds of PCB finish types followed are roughening the bare copper, performing etching, and then cleaning with pure water. The organic layer of film is coated which has to be repeated many times tens or hundreds as the layer keeps absorbing copper due to the very paper-thin layer.

In the final step of these kinds of PCB finish types, cleaning is performed again and treatment is completed. OSP layer is performed using the dip, spray, or brush method, depending on the thickness of deposition we require. In these kinds of PCB finish types, bare copper is accessible during reflow.

Advantages

  • Inexpensive Process
  • Simple procedure
  • Easy to repair
  • For better performance, a flat surface is achieved
  • Free from Lead
  • Nature friendly

Disadvantages

  • Bad storage life
  • Protection is low, due to the thin layer thickness
  • Not helpful in strong pad wetting bond during soldering
  • Film thickness is very sleek to manage

ENIG: Electroless Nickel and Immersion Gold

ENIG

In this one of the PCB surface finish types, chemical electroplating is done on the copper layer of the PCB.

A layer of Nickel is deposited initially followed by the deposition of gold layer. The thickness of nickel deposition is 3 to 6um which acts as a guarded shield against the corrosion and decay of the copper on the board. In this one of the PCB surface finish types, the thickness of the Gold layer deposited can be as thin as 0.05um and up to 0.120um. The gold layer used in these kinds of PCB finish types adds an extra layer of protection for the nickel surface. It adds an aesthetic look to the board which makes it look appealing to the eyes in this one of the PCB surface finish types.

It is easier to get a flat and smooth surface with an ENIG surface finish, which maximises the storage life of the board compared to other PCB surface finish types.

Advantages

  • Easier smooth finish for soldering
  • No harmful lead composition
  • Excellent storage life for long-time
  • Good conductor for current flow
  • Suitable for Plated through hole vias

Disadvantages

  • Not budget-friendly
  • Chances of signal loss
  • The process is complicated and it demands strict attention to each step
  • Likely to get Black Pad syndrome, a barrier between nickel and gold

ENEPIG: Electroless Nickel Electroless Palladium Immersion Gold

ENEPIG

ENEPIG which is another one of the PCB surface finish types is a blend of three metals namely electroless nickel, palladium, and immersion gold(Au). It is a better version of ENIG we learned before. Similar to other surface finish layer processes we deposit nickel palladium and then gold on the copper areas of the board. ENEPIG as one of the PCB finish types is commonly used in most PCB applications, it prevents rusting from oxidation, considered a finish type that is acceptable to all PCBs. These PCB surface finish types give good strength to the solder joints.

The nickel layer imparts a shield against copper oxidation, whereas palladium takes care of wire bonding and the immersion gold sheet is for solderability

All three layers have separate thicknesses respectively 3um to 5um, palladium is 0.05um to 0.16um, and the gold(Au)film is lesser than ENIG but it is still costlier in ENEPIG of thickness 0.03um to 0.049um. Compared to other PCB surface finish types, ENEPIG finish caters to the very good performance of mechanical and heat management.

Advantages

  • Excellent stronger solder joints, and wire bonding
  • Compatible with all assembly types
  • Better resistance to oxidation and corrosion
  • Long term benefits
  • Lead-free
  • Black pad problem-free

Disadvantages

  • Expensive due to many layer deposition process
  • Wear and tear easy on a gold film
  • Trained professionals required
  • Process limitations

Immersion Tin

In these kinds of PCB surface finish types, copper exposed on the PCB is dipped in the pool of chemical compound consisting of Tin(Sn) metal. Another name for these PCB  surface finish types is white tin(Sn) plating. It has a benefit to RoHS compliance. The thickness of a tin layer deposited is below 1 micron. After melted Tin gets deposited it combines with copper and diffuses together.

Solder is also made of tin. To avoid the issue of tin whisker, during the tin deposition process, we add organic particles to the solution and the tin becomes granular, thus supporting soldering and heat management.

The applications that use Immersion Tin are in consumer electronics, automobile electronics, and communication devices such as Mobile phones, which have high-density components of small pitch, routers, and switches. compared to other PCB surface finish types, it is used in medical-grade types of equipment where we require excellent soldering capacity.

Advantages

  • The surface finish is flat and smooth, like that of HASL
  • Extended shelf life
  • Good for SMT parts, with small form factor
  • Utilized for multiple reflows
  • Good for press-fit type connectors and parts

Disadvantages

  • Exposed tin(Sn) is susceptible to corrosion and decay over time
  • Limited storage life

Gold Plating

Gold Plating

As another one of the PCB surface finish types, Hard Gold is mixed with other metals like iron, cobalt and gold. It forms a fine thin layer that is sturdy and durable. For these kinds of PCB surface finish types, the application areas are in edge connectors on the board, touchpads, and sliders, due to the better conduction property. Pure gold is expensive 24 karat is used for the surface finish. It is also called Electrolytic wire bondable gold. There are some similar gold plating PCB surface finish types electrolytic hard gold, Immersion nickel gold,

Advantages

  • Long storage life
  • Best ruggedness, no wear and tear for many years
  • No friction in the surface film
  • Visually attractive from the outside
  • Possess the best conductivity quality

Disadvantages

  • Does not provide coverage to all sections
  • Chances of copper oxidation, as a gold sheet, is the thin deposition
  • Requires skilled craftsmanship to handle chemicals

 

From the various PCB surface finish types, it is necessary to remember the factors to check during the selection of PCB surface finish types for the project work.

Designers should look into the project cost assigned, the application of design is high-speed signals, the type of signals in the circuit, what type of storage life is expected from the product, and also consider the field of operation for the design.

 

Keeping all the information on PCB finish types aligned with the PCB manufacturer, engineers can accomplish a flat, smooth, reliable finish for the boards that match the optimised board performance.

Additional Resources:

    Request for Quote

    Related Posts