PCB Fabrication

Different PCB Surface Finishes and Choosing the Right One

    Printed Circuit Boards (PCBs) have Copper traces on their surfaces. As Copper oxidizes and deteriorates if left unprotected, it can make the board unusable. To prevent such deterioration, PCB manufacturers provide a surface finish on the exposed copper. Forming a critical interface between the component and the Copper trace on the PCB, the surface finish has two essential functions. The first is to protect the exposed Copper on the traces and the second to provide a solderable surface during assembly of components to the PCB.

    So far, one of the most tried and tested methods of providing a surface finish on the PCB was Hot Air Solder Leveling (HASL). As long as the component density on the PCB was low, HASL delivered consistent and reliable assembly results. With circuit complexity and component density consistently increasing, HASL has reached its limits.

    With component pitches becoming finer, there is a growing need for thinner coating on the exposed Copper traces on the PCB. As a result, PCB manufacturers have developed alternative coatings to HASL that offer a far superior performance. Including HASL, these are:

    • Hot Air Solder Leveling (HASL)
    • Lead -Free HASL
    • Immersion Tin (ISn)
    • Immersion Silver (IAg)
    • Electroless Nickel Immersion Gold (ENIG)
    • Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
    • Organic Solderability Preservative (OSP)

    Hot Air Solder Leveling (HASL)
    One of the most common surface finishes, HASL is also the most affordable and easy to apply. Manufacturers simply dip the PCB in molten solder and blow the residue off using a hot air knife. The molten solder is an alloy of Lead and Tin.

    Apart from being economical, HASL has the advantage of being applicable to a wide range of PCBs, as it allows for a large processing window, while the solderability it offers is excellent. However, as it involves the use of Lead, HASL is not suitable for boards that must comply with RoHS requirements.

    Other disadvantages include differences in thickness over small and large pads, making it unsuitable for High Density Interconnect (HDI) products that require uniform leveling. It is difficult for manufacturers using HASL to control solder bridging on fine pitch circuits, and this makes HASL unsuitable as a surface finish for PCBs with fine-pitch SMDs and BGAs.

    Lead-Free HASL
    By immersing the PCB in molten Lead-free solder and blowing off the residue with a hot air knife, manufacturers offer the economics of HASL to boards that need to comply with RoHS requirements. An added advantage of Lead-free HASL surface finish is it can expose potential delamination issues of the PCB, as it exposes the board to temperatures of about 260 °C (509 °F).Other advantages of Lead-free HASL surface finish are it is readily available, eco-friendly, durable, re-workable, affordable, and has excellent solderability.

    The disadvantages of Lead-free HASL are similar to those of HASL—uneven surfaces (but better than HASL), solder bridge formation, and not being ideal for fine pitch components. In addition, subjecting the PCB to high temperatures may expose it to a thermal shock.

    Immersion Tin (ISn)
    Manufacturers also refer to Immersion Tin as White Tin, and apply it as a very thin layer of Tin on the Copper surface using a chemical process. However, not all PCB manufacturers offer it, as they offer other alternatives. The use of Immersion Tin began primarily as an alternative to lead-based surface finish.

    Primary advantages of using Immersion Tin as a surface finish are the extremely flat and smooth finish it offers, and its sustainability. The flat surface it produces makes it ideal for HDI boards and fine-pitched surface mount components. Surface finishes with Immersion Tin are highly reliable and suitable for a wide range of PCB types. The application process uses lower amounts of water and chemicals than other similar processes do.

    As Tin and Copper have a strong affinity to one another, it produces a diffusion of one metal into the other. This results in a shorter shelf life compared to most other PCB surface finishes, and often limits the performance of the PCB. Other disadvantages of Immersion Tin are formation of whiskers, damage to solder mask, difficulty in handling, and not being ideal for Plated Through Holes (PTH). The biggest drawback to using Immersion Tin as a surface finish is the material is carcinogenic and hence unhealthy for handlers.

    Immersion Silver (IAg)
    The passing of directives such as the WEEE and RoHS has made Immersion Silver a widely popular surface finish ideal for fine pitch components. Manufacturers use Immersion Silver mostly for wire bonding, EMI shielding, and membrane switches.

    Apart from being ideal for fine pitch components and suitable for boards compliant with RoHS directives, Immersion Silver is highly stable compared to the stability of other surface finishes. It is also cost-effective and affordable.

    Immersion Silver has shortcomings including a high fraction co-efficient that makes it unsuitable for compliant-pin interaction. Earlier, Immersion Silver was unable to cover micro-vias with aspect ratios of 1:1, and was also susceptible to tarnishing. However latest developments have been able to overcome these shortcomings, and customer have realized that Immersion Silver is a good surface finish unless the board will be exposed to Sulfur.

    Electroless Nickel Immersion Gold (ENIG)
    One of the more popular surface finishes in the PCB industry, ENIG overcomes several shortcomings of other surface finishes. The application process for ENIG consists of two parts—the first part requires the application of a layer of Nickel to serve as both a barrier on the copper surface and act as a surface excellent for soldering. The second part involves applying a layer of Gold to act as a protection for the layer of Nickel, so long as the board is in storage before assembly.

    ENIG is an ideal surface finish for the latest generation of complex surface mount components such as fine-pitch BGAs and flip chips. As both layers of Nickel as well as Gold are very thin and even, the surface finish presents and ideal flat surface. Absence of any hazardous substances makes ENIG compatible to RoHS directives. ENIG also has a long shelf life and is ideal for PTHs. One disadvantage with ENIG is the migration of Nickel into the Gold layer. This shows up as the black pad syndrome, and results in fractures and faulty board connections. ENIG is also not ideal for reworking.

    Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
    Manufacturers have developed ENEPIG as an upgrade to ENIG, where Nickel migrates into the Immersion Gold layer and turns it black. Placing a thin layer of electroless Palladium in between helps to avoid the migration, thereby preventing blackened pads.

    In the PCB industry ENEPIG is also known as the universal finish as it is applicable to a wide range of boards, especially the modern, highly advanced ones with multiple types of surface packages. Similar to the ENIG surface finish, ENEPIG also offers an excellent flat surface, is easy to process, and has no toxic effects on the skin. It is also ideal for multiple reflow cycles, and as there is no lead, ENEPIG is compatible to RoHS directives. This surface finish has a long life, and is compatible with various Lead-free solders.

    The main disadvantage of ENEPIG surface finish is it is more expensive compared to most other surface finishes, including ENIG.

    Organic Solderability Preservative (OSP)
    The search for a low-cost, Lead-free, and fine-pitch suitable surface finish has led to the development of Organic Solderability Preservative, commonly known as OSP. Producing a thin and uniform protective layer, OSP is a superior surface finish over the more traditional ones, especially in regards to its co-planarity and solderability.

    Although ideal for fine-pitch component assembly, OSP is an organic coating that is susceptible to degrading effects of acid from fingerprints, and therefore requires very careful handling. Thermal exposure can break down the OSP film, thereby reducing solderability of through hole vias. Moreover, as OSP is a non-conductive coating, it presents difficulties with circuit testing.

    Choosing the Right Surface Finish
    Differences in PCB surface finishes make them suitable for certain uses and unsuitable for others. Some factors that users should consider when looking for the ideal surface finish are:

    Surface Flatness Poor Fair Excellent Excellent Good Excellent Excellent
    No No Good Good Excellent Excellent Excellent
    Solderability and Wettability Excellent Good Good Excellent Good Good Good
    Aluminum or Gold Wire Bonding No No No No Yes Yes Yes
    Shelf Life 1-Year 1-Year 9-Months 9-Months 9-Months 9-Months 9-Months
    Solder Cycles Multiple Multiple Multiple Multiple Multiple Multiple Single
    No Yes Yes Yes Yes Yes Yes
    Cost Low Low Medium Medium High High Low
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