PCB Surface Finishes Introduction and Comparison

While ordering PCBs, one needs to decide upon the substrate material of the PCB, solder mask, silkscreen, surface finish, size and thickness of the board, the thickness of copper, blind and buried vias, through-hole plating, SMT, panels, tolerances, and any other specifications before your circuit boards are fabricated. Of these factors, the choice of surface finish is most important due to its large effect on the reliability of an electronic product. Since the copper layers on the PCBs are easily oxidized, this oxidation can seriously degrade soldering quality and hence the reliability or functionality of final products. The surface finish prevents the pads from oxidation, ensuring good solderability for electrical performance.

Surface finish, known as surface coating, is an important process between PCB manufacturing and circuit board assembly, offering dual functions in its processes: first the protection of exposed copper circuitry, and second solderability for component attachment. As seen in Figure 1, the surface finish is applied onto the outermost layer of the PCB on top of the copper, which functions like a protective "coat" for the copper.

Types of surface finish

There are mainly two big categories for surface finishes of a PCB: metallic and organic. Metallic finishes include HASL, ENIG/ENEPIG, Immersion Gold, and Immersion Tin, while OSP and Carbon Ink are under organic finishes.

HASL Hot Air Solder Leveling

HASL is the conventional surface finish, which is always applied to PCBs. In this process, the board is submerged in a bath of molten solder; this flow completely covers all the exposed copper surfaces with solder. This excess solder is removed by passing the board through hot air knives as shown in Figure 2 below：

On account of concern for the protection of the environment, HASL itself has evolved into two variants: lead-based HASL and lead-free HASL. The lead-free version is compliant with RoHS regulations, first put in place by the EU.

ENIG and ENEPIG

ENIG is an abbreviation for Electroless Nickel Immersion Gold. A coating of electroless nickel is put on the PCB, followed by a small layer of immersion gold to keep the nickel from oxidizing. ENEPIG stands for Electroless Nickel Electroless Palladium Immersion Gold, which means that in addition to what ENIG does, a palladium layer will be added to act as a barrier, preventing nickel from oxidizing and diffusing into the copper layer. While ENIG and ENEPIG provide far improved solderability to the other surface treatments, their prices are significantly higher. The varied processes for creating ENIG versus ENEPIG are diagrammed below in Figure 3:

The electroless nickel step is autocatalytic, a deposition of nickel onto the palladium-catalyzed copper surface. The reducing solution contains nickel ions, which must be maintained in specified concentrations, temperatures, and acid levels. During the Immersion Gold step, the gold adheres to the nickel-plated areas through molecular exchange and covers the nickel, protecting it until soldering, and control of the gold thickness within specified tolerances ensures the continued solderability of the nickel.

Both ENIG and ENEPIG have their pros and cons. On its part, ENIG has potential advantages from the fact that it has a flat surface to its fast application and high-temperature resistance. On the other side, ENEPIG brags of strength in withstanding reflow cycles and truly reliable wire bonding. Depending on the application imperatives, each finish has its chastened uses. ENIG is mainly geared toward lead-free soldering, SMT, and BGA packages. Oppositely, ENEPIG is applicable for rigorous processes like THT, SMT, BGA, wire bonding, and press-fit technologies.

ImAg (Immersion Silver)

ImAg consists of a thin layer of immersion silver plating over copper traces. The standard procedure for making ImAg is as follows:

ImSn—Immersion Tin

ImSn uses tin instead of silver, and one of the main advantages of this process is the very planar finish that it provides to the copper pads, which in turn makes it highly suitable for SMT applications. Another fast became the fact that ImSn offers a surface easily detectable by common AOI techniques used.

OSP: Organic Solderability Preservatives

OSP belongs to the surface finishes using transparent organic material. The water-based organic compound can attach itself selectively onto copper, thus protecting it until soldering. The process of a typical OSP is as follows:

Summary

Each type of surface finish has different advantages and disadvantages. Selection should consider end-use application, performance requirements, cost, corrosion resistance, ICT capabilities, hole fill, and other applicable parameters. The more variables used to compare, the more accurate your selection will be.

Comparing Surface Finishes

• Cost: ImAg and OSP are the least expensive, with ENIG as the highest cost option.
• Corrosion Resistance: HASL and ImSn are at the top; ImAg is last on the list.
• ICT Capability: Essentially, all finishes are good, except OSP.
• Hole Fill: HASL and ENIG are excellent compared to other finishes.

Surface Finish Selection

Choosing an appropriate surface finish in the fabrication of printed circuit boards is very important for high process yields, low numbers of rework, low field failure rates, effective test capabilities, low scrap rates, and a lower cost. All significant factors related to assembly should be taken into consideration to ensure the final products are of a quality standard and performed accordingly.

Each occupant has different selection criteria. More often than not, the chosen surface finish meets particular requirements and convenience at the cost of quality, performance, and reliability of the PCB.

In light of the above descriptions of each type of surface finish, some attributes become very critical selection criteria. As one can tell from the form shown below, each of the types of surface finishes either has it or otherwise, giving a guideline for selecting the best option for the surface finish, considering the requirements and characteristics of your PCB products.

The ultimate in surface-finish selection should be to provide the optimal balance of the many functions. Each surface finish has its own list of pros and cons. Fortunately, there are techniques in engineering to help offset the negatives. Some examples are, to overcome the reduced wetting force of OSP, consider one or more of the following: improve the board solderability plating, alter wave solder alloys, or increase top-side preheat. It is all about taking every possible consideration to reach an optimum performance.

The growing importance of environmental issues in the electronics industry has prompted different regulations recently, including RoHS, or the Restriction of Hazardous Substances and Lead-Free, gaining its first appearance within the European Union through Directive 2002/95/EC. This directive bans the use of six hazardous chemicals in electrical and electronic equipment, and beginning July 1, 2006, all eligible products supplied in the EU market must be RoHS-compliant. RoHS compliance has a broad impact on the electronics industry, driving demand for lead-free surface treatments.

Check Prices with Different Surface FInish

PCBX provides a convenient and free online price calculator to help you estimate the cost of PCBs with any of the following surface finishes. Click the button below to access the PCB quote page and see how prices vary with different surface finish options.