PCB Design for High Frequencies: Start with the Finish
By: Dave Burrell
Senior Consultant, Product Design
20th December 2017
A Printed Circuit Board (PCB) surface finish is a coating between a component and a bare board PCB. It is applied for two basic reasons: to ensure solderability, and to protect exposed copper circuitry.
Since the early days of Tin/Lead Hot Air Solder Levelling (HASL) finish, there have been many PCB finishes over the years, each with their own advantages and limitations. Cost, technology requirements and legislative demands are only some of the reasons for this growth in choice.
The current, common finishes like Electroless Nickel Immersion Gold (ENIG), Immersion Silver and organics like Organic Solderability Preservative (OSP) provide much better planarity and smoothness for finer pitch devices. An example of such devices would be a Ball Grid Array (BGA), a Quad-Flat No-Leads package (QFN) or a Land Grid Array (LGA).
Changes in RoHS regulations (Restriction of Hazardous Substances) have also made these common finishes more mainstream, making them more accessible over their cheaper counterparts, like OSP and Silver, which tend to be susceptible to shelf life issues.
This difference can be seen more clearly when RF frequencies are introduced. At low RF frequencies, current will typically pass through the copper track of a PCB surface very efficiently. However, as the frequency increases, current tends to pass more on the outer surface/skin of the track, so the plating and its conductive loss becomes of greater significance.
Copper, gold and silver all provide very low resistance and insertion loss; however bare copper is, of course, not suitable as a finish as it will degrade, similarly (but to a lesser extent) to silver.
This leaves us with gold as the most suitable top plating but this has its own unique setback. Gold cannot be put directly onto copper; it needs a barrier layer, provided either by the nickel in ENIG, the silver in ISIG (Immersion Silver/Immersion Gold) or by Palladium in ENIPIG (Electroless Nickel Immersion Palladium Immersion Gold).
At this point, most engineers would opt for nickel in ENIG (the most common solution), but it is very resistive to RF and, as frequency increases, preference moves towards ISIG or ENIPIG. Both of which provide a highly conductive outer skin and, therefore, a better signal path.
As RF frequencies increase to 60 GHz – 80 GHz, the PCB finish has a greater significance to the efficiency and performance of the PCB, becoming a crucial part of the overall design functionality.
In addition, with technologies pushing the boundaries of RF frequencies further in sensors and radar, I predict that these more exotic PCB finishes are going to become more prolific in the future.