As promised, we are off to the races, untangling the convoluted subject of PCB surface finishes. This is a complicated subject, so bear with me as I try to break it down into bite-sized, manageable portions! This week I will discuss each available surface finish and the pros and cons of each one. Below is my version of SparkNotes for surface finishes:

PCB Surface Finish Comparison Chart

As I mentioned in my previous blog post, many designers working with high speed digital, RF or Microwave applications are using ENIG, ENEPIG and Electrolytic Hard or Soft gold these days. From the chart above, you can see why; these finishes offer many desired properties. However, if wire bonding is required, the field is narrowed to Immersion Silver, ENEPIG, and Electrolytic gold. Here at Transline we are often asked what the difference is between hard and soft gold: quite simply it is the purity of the gold; the purer the gold, the softer the finish. Among all finishes widely available, silver has the best conductivity.

The Tradeoffs

Many engineers and designers report significant signal loss from the nickel that is plated beneath all gold applications. As the desire for greater speeds increase, many are looking for ways to eliminate, or reduce loss due to undesirable resistivity from plating finishes. In the plating process, gold cannot be plated directly over copper for a couple of reasons. First of all, some copper gets chemically dissolved into the plating tanks, tainting the gold purity and the very costly gold bath. Secondly, on the board surface the copper and gold become diffused and mingled not allowing the gold to plate over the top of the copper. Nickel, therefore, is applied over the copper to act as a barrier to prevent these two undesirable effects. It is for this reason that many are looking to ENEPIG (Electroless Nickel, Electroless Palladium, and Immersion Gold). The Palladium is far less resistive, and due to the “skin effect,” the signal travels to the outermost areas of the circuit—through the low resistive Palladium and Gold. The only downfall for this finish is that it is more costly, and may or may not take more time since most PCB suppliers do not have palladium tanks in their facilities, and rely on outside plating services.

With greater frequency, designers are turning to Immersion silver. The drawbacks to Immersion Silver include the inability for fine wire bonding and oxidation. Oxidation can be avoided or delayed by packaging with sulfur-free packaging materials and storing in temperatures less than 95 degrees Fahrenheit. Some mild oxidation can be easily cleaned away. Any unused boards should be resealed for proper storage.

Conclusion

As you can see, plating finishes offer a series of tradeoffs in cost, performance and function. Each designer needs to carefully consider the requirements that are unique to a given project or application. I hope this information helps to clarify what the general pros and cons are of the various surface finishes available.

Next Installment

Next time, I will dig a bit deeper into this subject and discuss the impact of copper smoothness and “skin effect” and how each may help you make decisions about the surface finish. Thanks to all of you who sent me emails and questions on this subject! Keep the comments and questions coming here, or to my email: [email protected]

Further reading:

www.taconic-add.com/pdf/technicalarticles–effectsofsurfacefinish.pdf

www.ddmconsulting.com/Design_Guides/hasl_alt.pdf

— Judy