Advantages and Disadvantages of PCB Surface Finishes
That Printed Circuit Boards (PCBs) have copper tracks and planes on them is a fact that all users of printed circuits and those within the PCB industry know. If left unprotected, the copper surface easily oxidizes, tarnishes, and deteriorates, making the circuit board unusable. Therefore, eminent PCB manufacturers such as Rush PCB provide a surface finish on their PCBs, forming a critical interface between the SMD component and the PCB.
A surface finish on the copper of the PCB serves two essential functions. The first is to protect the exposed copper circuitry from the vagaries of the environment. The second is to offer a solderable surface, especially during assembly of components on the PCB.
Earlier, the most tried and true method of providing a surface finish that most PCB manufacturers offered was the HASL or Hot Air Solder Leveling. With the steady increase of circuit complexity and component density, the HASL method lost its importance, as it failed in its capabilities of providing an even layer of horizontal solder layer that modern PCBs demand.
With component pitches becoming even finer, there was an increasing need for an even thinner surface coating. With HASL, PCB manufacturers hit a process limitation, and had to develop alternative coatings using both the immersion and electrolytic processes. We discuss below some popular surface finishes that PCB manufacturers offer.
HASL — Leaded and Lead-Free
In the electronic industry, HASL happens to be the predominant surface finish, so far as the complexity and component density of the board allows. The process is simple, consisting of immersing the circuit boards in a molten alloy of Tin and Lead (solder). Air knives then remove the excess solder from the surface, by blowing thin hot air across the board. For Lead-free boards, manufacturers replace the molten alloy with one that does not contain Lead.
As the HASL process exposes the board to temperatures near to 265 °C, it inadvertently exposes any potential delamination issues well before the assembly of any expensive component on the board.
Having excellent shelf-life, the HASL finish offers a significant advantage such as making the board re-workable. The process is simple and low-cost, and the ingredients are widely available. The typical thicknesses involved in HASL are between 70 micro-inches (0.07 mils) to 200 micro-inches (0.2 mils).
However, there are several disadvantages of the HASL process as well. Chief among them is the final uneven surface, which is detrimental to fine pitch components, often causing solder bridging and plugging or reducing the diameter of Plated Through Holes (PTH). Presence of Lead is another disadvantage, and so is the thermal shock the HASL process administers to the PCB.
The Lead-free version of HASL, or LFH, is not popular due to the complicated process steps it involves. The LFH process requires two passes, as the first pass leaves the surface grainy and dull. Although the second pass improves the surface finish to a coating that is shiny, flat, even, and smooth, the PCB must suffer exposure to two heat cycles. Moreover, two dips in the molten solution leaves excess copper in PTH walls, which may make the PCB unacceptable as per the IPC standards.
Immersion Tin — ISn
The trade Association Connecting Electronic Industries, IPC, defines Immersion Tin (ISn) as a metallic finish that manufacturers apply directly over the basic metal of the circuit board using a chemical displacement reaction. The main functionality of ISn is to protect the underlying copper surface from oxidation.
With Copper and Tin having a strong affinity for one another, the tendency of one metal is to diffuse into the other. The diffusion ultimately impacts the shelf life of the deposit and the performance of the finish suffers. Tin also suffers from whisker growth, leading to shorts between adjacent circuits.
However, during its intended shelf life, ISn offers several advantages such as a flat surface on the PCB with no presence of Lead. The surface is eminently re-workable, and for manufacturers who will be press fitting pin inserts into PCBs, ISn is a very desirable choice. The typical thicknesses involved in ISn are between 20 micro-inches (0.02 mils) to 50 micro-inches (0.05 mils).
One of the major disadvantages of ISn is that the surface is not suitable for multiple reflow or assembly processes. It is also difficult to measure the thickness of the deposit for ensuring a uniform level. While the Tin surface is prone to handling damage, the exposed Tin on the final assembly has a tendency to corrode. Growth of Tin whiskers is another potential problem, and that the process uses a Carcinogen is a potential health hazard.
Immersion Silver — IAg
Unlike ISn finishes, immersion Silver does not react with copper. However, Silver tarnishes when exposed to air. Therefore, PCBs with IAg surface finish need storing in anti-tarnishing packaging.
Usually, storing in proper packaging enables PCBs to remain solderable for about a year. However, removal from the package will require the PCB to undergo the reflow assembly process within a day. An additional layer of Gold plating over the Silver usually increases the shelf life.
Although IAg has low shelf life, it offers excellent flatness, is good for fine pitch components such as BGA, and is re-workable.
However, IAg is very sensitive to handling and tarnishes easily, leading to cosmetic concerns. It requires special packaging for storage, and offers only a short operating window between assembly stages. IAg surface finishes are not compatible with masks that require peeling off.
Electroless Nickel Immersion Gold — ENIG
Electroless Nickel Immersion Gold is a two-layer metallic coating on the Copper surface of the PCB. The first layer is Nickel, which forms a barrier over the Copper. The Gold forms a very thin layer to protect the Nickel during storage. The importance and growth of the RoHS regulation has popularized ENIG as the most used finish in the PCB industry.
With a long shelf life, ENIG offers several distinct advantages. Most prominent among them being the absence of Lead. ENIG also offers a flat even surface, and is good for PTH. The Nickel layer is typically 100 micro-inches (0.1 mil) to 200 micro-inches (0.2 mil) thick, while the thickness of the Gold layer is only 2 – 4 micro-inches (0.002 – 0.004 mil).
However, ENIG is expensive in comparison to other surface finishes as it follows a complicated process, and is not re-workable. At times, ENIG can lead to a buildup of phosphorous between the Gold and Nickel layers, leading to black pads, fractured surfaces, and faulty connections.
Organic Solderability Preservative — OSP
Organic Solderability Preservative, as the name suggests, is a thin anti-tarnish protective layer of organic material that manufacturers apply over the exposed Copper. OSP helps preserve the surface from oxidation.
Using a water-based organic compound, OSP bonds to Copper providing an organometallic layer, as it protects the Copper prior to soldering. In comparison to other common Lead-free surface finishes, OSP is extremely environmentally friendly.
As the OSP process is simple and cost-effective, it is popular in the electronic industry. Additionally, other advantages OSP offers are a flat, re-workable surface, free from Lead. Typical thicknesses of OSP surface finish ranges from 4 micro-inches (0.004 mils) to 24 micro-inches (0.024 mils).
However, OSP is not PTH friendly, and the surface thickness is not uniform, with no way to measure its thickness. The shelf life for OSP is short, and it often causes connection issues with In-Circuit Testing machines. OSP is also prone to damage during handling, and can expose Copper during final assembly.
Hard Electrolytic Gold
Manufacturers first prepare a barrier coat of Nickel over the exposed Copper on the PCB. Over this, they electroplate a layer of Hard Gold, forming an extremely durable layer. Manufacturers apply this Hard Gold coating on fingers of edge connectors and keypads, as these are high-wear areas.
Hard Gold plating has the advantage that the manufacturer can control the plating thickness by controlling the duration of the plating cycle, unlike in the ENIG process. Typical plating thickness is:
Class 1 PCB — 30 micro-inch (0.03 mils) Gold over 100 micro-inch (0.1 mil) Nickel.
Class 2 PCB — 30 micro-inch (0.03 mils) Gold over 100 micro-inch (0.1 mil) Nickel.
Class 3 PCB — 50 micro-inch (0.05 mils) Gold over 100 micro-inch (0.1 mil) Nickel.
However, poor solderability and high cost of Gold does not allow its applications on the solderable areas of the PCB. According to IPC, the maximum thickness of Gold for proper solderability is only 17.8 micro-inches (0.0178 mils). Therefore, if the manufacturer must use this type of Gold finish on solderable surfaces, the nominal thickness they should use would be about 5-10 micro-inches (0.005-0.010 mils).
Although Hard Gold finish has a long shelf life, no presence of Lead, and offers a hard, durable surface, the process is very expensive and labor intensive. For instance, the manufacturer must electrically connect all surfaces that they intend to plate, while covering with resist or mask all surfaces that will not require plating. Except in finger areas, Hard Gold finish may not fully encapsulate sidewalls of traces, leaving them exposed to oxidation.
Summary of PCB Surface Finishes
|HASL – Pb||Low Cost Excellent Solderability Allows Large Processing Window Re-workable Excellent Shelf Life||Difference in thickness/topography between large and small pads Not Suitable for Fine Pitch < 20 mil Pitch Contains Lead Thermal Shock Plugging or Reducing PTH Solder Bridging Not ideal for HDI products|
HASL ||Excellent solderability Relatively inexpensive Allows large processing window Multiple thermal excursions||Difference in thickness/topography between large and small pads High processing temperature – 260-270 degrees C Not suited for < 20mil pitch SMD & BGA Bridging on fine pitch Thermal Shock|
|ISn||Good Flatness Lead Free Re-workable Top Choice for Press Fit Pin Insertion||Easy to Cause Handling Damage Carcinogenic Process Tin Whiskers Tin Exposed on Final Assembly can Corrode Not Suitable for Multiple Assembly/ Reflow Processes Difficulty in Measuring Thickness|
|IAg||Excellent Flatness Lead Free Good for Fine Pitch / BGA components Re-working Possible||Very Sensitive to Handling Easily Tarnishes – Cosmetic Concerns Requires Special Packaging for Storage Short Operating Window Not Recommended with Peelable Masks|
|ENIG||Good Flatness Lead Free Long Shelf Life Good for PTH||Expensive Not Re-workable Complicated Process Black Nickel / Black Pad Damage from ET Signal Loss|
|OSP||Good Flatness Lead Free Simple Process Re-workable Cost Effective||Difficulty in Measuring Thickness Not Suitable for PTH Short Shelf Life Can Cause ICT Issues Exposed Cu on Final Assembly Handling sensitive|
|HG||Hard, Durable Surface Lead Free Long Shelf Life||Very Expensive Extra Processing / Labor Intensive Etching Undercut May Cause Flaking / Slivering Difficult to Solder Above 17 μin Finish May Not Fully Encapsulate Sidewalls of Traces, Excepting Finger Areas|
Selecting the Appropriate Surface Finish
For a designer, selecting the appropriate surface finish may need balancing the various options available while factoring in performance requirements and the cost of materials. Rush PCB also advises considering component types and production volumes. Other important factors are durability and environmental impact.