Guidelines for selecting the best PCB surface finish

Surface Finish Considerations

The PCB Surface Finish you select may be the most important material decision you make for your electronic assembly. The Surface Finish you select will influence the cost, manufacturability, quality, and reliability of the final product.

Just a few years ago, nearly three fourths of all the electronics were produced with SnPb or Hot Air Solder Level (HASL) as the finish. Today, other surface finishes have emerged, including: ENIG, ENEPIG, Soft and Hard gold, Silver OSP, and White Tin, and, SnPb solder now makes up just over 10% of the finishes in use.

More complicated board specifications are becoming common printed circuit board manufacturing requirements in the continuous drive to advance technology, such as: etching buried components, blind vias, drilling ever smaller holes, laser drilled blind vias, soldermask dams as low as 1.0 mil, and thicker high count multi-layer boards. These changes pose considerable assembly and integration challenges. Not to mention the increased demands to meet the restrictions in place to safeguard against the overuse of lead products and materials. Therefore, a number of Surface Finishes have grown in popularity and currently share the marketplace for PCB Manufacturing, each with attributes that make it attractive for certain applications.

When considering which surface finish to use, the following questions must be considered (in addition to general cost considerations):

• Does your board need to be lead free?
• What is the desired volume or quantities?
• Where will the assembly be marketed and used?
• What type and pitch of components will be applied?
• Does your board need to be wire bonded or wave soldered?
• What environmental issues and handling will the final product be exposed to?

All of these will influence your selection of the optimal PCB Surface Finish for your project.

Surface Finish Options And Application

Let&#;s explore a few of the more common surface finishes and their general use applications:

Hot Air Solder Leveling (HASL) &#; Leaded and Lead-Free

HASL Finish is the predominant &#;leaded&#; surface finish used in the industry.

HASL stands for Hot Air Solder Leveling. The circuit boards must be immersed in a tin/lead alloy for this finish. &#;Air knives&#; then remove the excess solder by blowing hot air across the surface of the board.

HASL Finish has many advantages when using the printed circuit assembly (PCA) process. It is one of the least expensive surface finishes available and the surface finish remains solderable through multiple reflow, wash, and storage cycles. For an In-circuit test (ICT), a HASL Finish automatically covers the test pads and vias with solder. However, the flatness, or coplanarity, of the surface is poor when compared to available alternatives. This considerably bumpy finish is not only an aesthetic issue, but can also be problematic when sending your boards to assembly. However, it&#;s corrosion resistance and testability are excellent.

Lead-free HASL Finish is a great alternative to the leaded HASL Finish. Not only is the coating planarity of most lead-free HASL Finishes reportedly better than leaded HASL Finish, concerns with  copper dissolution and heat damage to the circuit boards have largely been overcome with different solder alloys such as SnCuNi, SnAgCuNi or SnCuCo While lead free HASL Finish may not be the best coating for projects with small spacing between components due to its tendency to bridge across the gap during heating, it is currently being used on products with component pitch as low as 0.5 mm.

The most significant advantage of using a HASL Surface Finish, whether leaded or lead free, is its excellent solderability.

Electroless Nickel Immersion Gold (ENIG/ENEPIG)

These coatings have been used with great success on many boards despite their higher per unit cost.

The best features of using an ENIG Surface Finish is its flat surface and excellent solderability. The Electroless Nickel is an auto-catalytic process that deposits Nickel on a Palladium catalyzed Copper surface.

Immersion Gold is a replacement chemistry. In other words, it attaches to the Nickel by replacing atoms of Nickel with atoms of Gold.  The recommended Gold thickness is 2-4 µin. The purpose of the immersion Gold layer is to protect the Nickel surface and maintain its solderability.

While the Nickel serves as a barrier layer to Copper. eventually, it too will diffuse to the surface of the Gold and cause the same solderability issue, it just happens at a slower rate than Copper).

Typical ENIG specifications are defined by IPC- Specification for Electroless Nickel/Immersion Gold. The Nickel thickness must be in the range of 3-6 µm, which is sufficient to prevent penetrability through to the base layer of Copper.

An ENIG Finish provides many advantages, including:

• Excellent flatness for fine pitch components.
• Endurable through multiple reflow cycles.
• Supports alternate connections like wire bond, edge card connection, and push buttons.
• Good for In-circuit testing and press fit connectors.

The Gold readily dissolves into solder and does not tarnish or oxidize.  While the Nickel strengthens the PTH, increases thermal cycle life, and acts as a barrier that prevents Copper dissolution during wave solder and rework.

One risk to be aware of when using an ENIG Finish is its tendency to create &#;black pads.&#; While the actual cause of this phenomenon is still open to debate, the leading hypothesis is that it is mostly likely a contamination of the Nickel that then migrates into the Gold, turning it black. This tends to be particular problem when the gold plating process is not well controlled.

One simple solution to this potential issue, is using a similar alternative surface finish: ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold). ENEPIG Finish solves this issue by depositing electroless palladium over the nickel layer, which prevents any contamination from migrating to the Gold. Of course, for those on a tight budget it is important to note that ENEPIG is more expensive than the already costly ENIG Finish.

Immersion Silver

Immersion Silver is one of the more recent additions to the list of surface finish options. It has been used mainly in Asia and is continuing to grow in popularity in both North America and Europe.

Immersion Silver is a preferred surface finish for those concerned with solderability and being able to easily probe directly to the finish during ICT. During the soldering process, the silver layer dissolves into the solder joint, leaving a (6-12 µin) Tin/Lead/Silver alloy on the Copper, making very reliable solder joints for BGA packages. Another benefit to using Immersion Silver is the color contrast that enables easier inspection.

This surface finish received a boost in popularity after the Underwriters Laboratory performed temperature/humidity/bias testing with favorable results, in which no electromechanical migration took place.

However, when scaled up to higher volumes for commercial electronic production, the Immersion Silver showed a number of weaknesses. These includes: a tendency to cause micro-voids, tarnishing any exposed silver almost to the point of turning black, and &#;creep corrosion&#; when used in an environment high in air-born sulfur and humidity.

In recent years, however, the micro-voids issue has since been eliminated thanks to improved plating processes. Additionally, the tarnishing problem does not necessarily cause board failures. It is usually only an issue of perceived poor quality by customers based purely on aesthetics.

Immersion Silver is a good surface finish if one is confident that the product will not be exposed to sulfur during shipping or use of the product. It is a favorable surface finish for most other attributes.

Organic Solder Preservative (OSP)

Organic Solder Preservative, commonly referred to as OSP Finish, is the leader in low cost surface finishes. It is designed to produce a thin, uniform, protective layer on the copper surface of the PCB that shields the circuitry from oxidization during storage and assembly operations. While OSP Finish has been around for quite some time, it has only recently gained popularity as customers increase their search for Lead-free and fine pitch options.

OSP is a superior PCB Finsih over traditional HASL, particularly for PCB assembly, in regards to co-planarity and solderability. However, it does require a significant process change with the type of flux and number of heat cycles necessary. Also, careful handling is very important given the degrading affects the acid from fingerprints have on the OSP, thus potentially leaving the copper susceptible to oxidation.

OSP is an organic coating that is deposited with a wet in-line panel process. It is one of the most common finishes and is an excellent selection for less complex assembly projects. Unfortunately, this finish falls short when wave soldering is required for double-sided boards. This is because the surface mount thermal exposures can break down the film and allow oxidation of the Copper in the barrels, thus reducing the solderability of the through-hole vias.

This finish also encounters some difficulty during circuit testing. Since it is a non-conductive coating, probing through the coating is not recommended.

OSP Finish is ideal for fine pitch assembly since the smooth surface allows the stencil to press firmly against the surface of the copper pads. It is a great choice for high volume orders at a low price.

Immersion Tin

Another finish that is ideal for those looking for a flat surface finish is Immersion Tin. However, one significant problem with Immersion Tin is the fact that it is made up of the carcinogenic ingredient Thiourea.

Immersion Tin also has a tendency to cause whiskers and intermetallic formations. Whiskers are particularly problematic when working with fine line/spaces and part insertion, increasing the possibility of electrical shorts. Copper and Tin intermetallic formations often occur during deposition and continue to grow. This significantly shortens the shelf life of the stored parts.

This finish has also been listed as being particularly difficult for wave soldering after assembly. w exposed to elevated temperature, the thin tin layer can often almost completely be converted to SnCu intermetallic, leaving very little tin for soldering. This issue with solderability increases after the first reflow cycle or long term storage of the PCBs.

Surface Finish for PCB Assembly

Choosing the right PCB Surface Finish is essential for predicting the cost, manufacturing process, quality, and reliability of any printed circuit board. Each surface finish has important strengths and weaknesses to consider while looking forward to PCB assembly. One way to ensure that you select the optimum finish for your boards can be to determine what problems are most important to solve and making sure that they are satisfied. For example, soldering circuit boards will require the right surface finish opposed to other PCB assembly methods.

For more information, download our Surface Finishes Chart, to help you pick the right surface finish for your PCB needs.

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