PCB Design and Outer Layer Surface Finishes

Although it might seem to be very downstream in terms of overall product development considerations, determining which PCB outer layer surface finish is the correct one is something that needs to be addressed at the forefront of the PCB design process. There are a variety of outer layer surface finishes available with both advantages and disadvantages to nearly each one. Which one is best suited to your particular design is highly dependent on the final application of your product. This article will describe the various classes of surfaces available; their history and evolution; the merits and drawbacks of each; where in the PCB fabrication a particular surface is applied and the current state-of-the-state relative to where the industry is today.

The Role of Outer Layer Surface Finishes

It’s a fair question to ask, “What does the surface finish have to do with my high-speed design?” It may seem that the answer would be “nothing” but, through the years, we have seen dozens if not hundreds or more elegantly executed high-speed designs fail due to unreliable solder connections that could be traced to using the wrong surface finish. When this occurs, the only solution is to throw away the whole assembly and start over. This can result in huge costs and schedule hits and may delay a product from meeting its market window and failing to generate anticipated revenues.

The Evolution of Surface Finishes

The purpose of any outer layer surface finish—regardless of the type and where in the overall fabrication process it is applied—is to protect the copper surface areas from corrosion in those places where components will be soldered.

The types of surface finishes available include:

• Electroplated solder
• Electroplated tin
• HASL
• ENTEC 106^® or other organic protection
• ENIG
• ENPIG
• Electroplated gold over electroplated nickel
• Immersion Tin
• Immersion Silver

It’s important to note that the application of these outer layer surface finishes occurs in different phases of the PCB fabrication process. Additionally, where the application process happens has a direct impact on how well each finish works. These two different phases include: 

• Plating finishes on right after the copper plating step.

In this instance, the plating serves as an etch resist when it’s time to remove the unwanted copper from the outer layers. The finishes in this category include electroplated solder, electroplated tin and electroplated gold over nickel.

• Applying finishes after the PCB has been fully plated and the outer layers have been etched with their final pattern.

In this instance, the plated solder or plated tin is used as the etch resist when etching the outer layers. This plating is removed prior to applying soldermask, leaving exposed copper where solder connections are to be made. These outer layer surface finishes are comprised of anticorrosive materials to protect the component mounting pads until soldering is complete. The details regarding their use are noted below. The specific finishes in this category include HASL, organic coatings such as ENTEC 106, ENIG, ENEPIG, immersion tin and immersion silver.

The evolution of these finishes occurred as follows:

1. Electroplated solder is the original surface finish used by the PCB industry. It is the lowest cost finish as it does two tasks in one operation. First, it serves as an etch resist. It also serves as the protection for the copper surfaces onto which the components will be soldered. The problem is that the solder mask which is applied over this surface finish can fail when the solder under that mask on the traces melts away during wave soldering. This leads to shorts between the traces. Also, this finish has an undesirable amount of lead content. As a result, it has been replaced with the next finish.
2. Electroplated tin came on the scene as the RoHS replacement for electroplated solder. The attraction of this finish is that the solder mask could be applied directly over the tin and it could be used as the final surface finish. This was tried on a number of products, including backplanes, with mixed results. The problem is that pure tin as well as some tin alloys will develop very tiny tin whiskers between circuits of different voltages. As the whiskers grow, they lead to short circuits and leakage paths. These whiskers usually appear some time after the PCB is through assembly and in the hands of the user resulting in a field return. For this reason, any form of tin as a surface finish is not a reliable choice for PCB implementations.
3. SMBOC (solder mask over bare copper) as a process was developed to address the solder short problem. Here, solder or tin is stripped off the traces prior to the application of the solder mask. Since the solder mask adheres very well to bare copper it solves the solder short problem. However, it leaves the mounting pads exposed as bare copper that corrodes very quickly. This means some sort of anticorrosive material is required to protect the copper mounting pads. These anticorrosive materials are noted above and more information regarding them follows.
4. Electroplated gold over electroplated nickel as a surface finish is as old if not older than electroplated solder. The advantages of this finish are that it is easy to plate on, it’s great protection for copper surfaces and it looks great. However, it is the most expensive finish to use so we only use it on high-priced PCBs. However, it does have some small risks. When this finish is used on PCBs that have small, high aspect ratio holes there is a challenge with the etching process that is used to remove unwanted copper from the outer layers of the PCB. The nickel and the gold protect the copper in the plated through-holes from being etched away. If the plating process used to deposit the nickel is not very well controlled, the copper in the center of a plated through-hole will not be covered, and it is etched away as a result. This is not a problem if the copper is completely etched through because it shows as an open circuit and the board can be discarded before it goes through further processing. The challenge is that small amounts of copper remain in the hole and this is enough to pass bare board testing. However, after the soldering process the copper often fails and this results in a bad PCB. There are ways to address this type of copper failure.The first is to plate extra copper in the holes to allow for the etching process. The second is to plug the vias on the top and bottom of the PCB with a photo-imageable material prior to the outer layers being etched.

As noted above, the following finishes are applied after the PCB has been fully plated and the outer layers have been etched with their final pattern and soldermask applied. The finishes that fall into this category include: 

• HASL (hot air solder leveling) was the first finish developed for SMOBC PCBs and it remains the first finish of choice for them. It is the least expensive finish after plated solder. HASL is applied by immersing the finished SMOBC PCB in a vat of molten solder to coat all of the exposed copper. The PCB is drawn out of the solder bath through a pair of air knives that blow away the excess solder, leaving enough solder on each pad to provide corrosion protection. The disadvantage of this finish is that the height of the solder pads is not uniform so fine pitch QFPs and BGAs ended up with solder shorts or open circuits. In addition, with HASL, each PCB is subjected to a severe thermal shock when it is immersed in the molten solder. With thick PCBs containing large numbers of small plated through-holes this can cause some of those plated through-holes to fail.

• ENTEC 106^® or organic coatings are thin films of organic substances that protect the copper pads until soldering takes place, where they serve as solder fluxes. An advantage of ENTEC is that it is inexpensive compared to some of the other choices of outer layer surface finishes. The disadvantage with this type of coating is that touching the finish with fingers destroys the coating, rendering the PCB unusable until the coating is stripped and reapplied. As a result, ENTEC and other similar coatings are not suitable for prototype products which are more likely to be handled. It is, however, a suitable finish for production PCBs.

• ENIG (electroless nickel under immersion gold) can be a good coating if applied correctly. “Electroless” and “immersion” indicate that the nickel and gold must be deposited on the copper without the use of electricity. The challenge is that this finish is applied with two complex chemical baths. If the chemistry is not well maintained the result is black pad. This is a material that will cause the solder joints to be weak and fail often as they come out of the soldering operations. Unfortunately, there is no fix for this  problem and the entire assemblies have to be discarded. Another disadvantage with ENIG is that the layer of gold that is applied is so thin that it does not provide long term corrosion protection and the resulting boards have no shelf life.

• ENEPIG (electroless nickel electroless palladium immersion gold) has been around for about 10 years and it has been touted as being a suitable replacement for ENIG. The evidence points to it having similar corrosion exposure and black pad problems. We have seen evidence of both types of failures, so we don’t recommend this finish.

• Immersion tin initially appeared to be a very appealing finish. Since it contains no lead it meets RoHS requirements and it is very easy to apply. But, as noted above, tin grows whiskers that slowly develop into short circuits or leakage paths so we don’t recommend it as a surface finish. Immersion silver is the finish of choice if you can’t afford electroplated gold over electroplated nickel. It has a long shelf life and it is as inexpensive as ENTEC. The only problem with it is that it turns black (tarnishes). However, this is a cosmetic issue not a performance one. In response to this, some OEMs have taken to using ENTEC for everywhere that it is going to be soldered and then using ENIG on the pads that will be exposed after assembly. Thus, the boards look “pretty”, they function as they are supposed to and the customers are happy with their appearance.

Summary

There are a number of outer layer surface finishes available to the industry. Understanding the dynamics, advantages and disadvantages of each is an imperative for ensuring the finish you select is the right one for your particular PCB implementation. Taking these factors into account early on in the design phase will ensure that the entire development process for your product will be seamless through all design, fabrication, and assembly operations.

Would you like to find out more about how Altium can help you with your next PCB design? Talk to an expert at Altium or read this High-Speed PCB Design Guide to get an overview of everything you need to know. To gain further insight into the topics discussed in this article, you can also read the texts in the references below.

References

1. Ritchey, Lee W. and Zasio, John J., “Right The First Time, A Practical Handbook on High-Speed PCB and System Design, Volume 2.”

2. Ritchey, Lee W., “Update on PCB Surface Finishes,” Current Source Newsletter, Volume 1, Issue 4, August 2005