Remember the Copper When Choosing PCBs
Achieving reliability and high performance from a PCB material is not simply about selecting the optimum dielectric material for an application—the copper on that material also plays a critical role. The quality of the copper on a PCB, including its surface roughness, and how the copper is joined with the dielectric material, can go a long way towards determining the performance possible with that PCB material, particularly at RF and microwave frequencies.
Copper provides the signal pathways on a PCB, but those circuit patterns must be formed with precision and that starts with the quality of the copper and how well it is attached to the PCB’s dielectric material. The melding of copper and dielectric is critical to the performance and reliability of a high-frequency PCB, since the way those two materials combine can influence performance over different operating conditions. For example, copper has a coefficient of thermal expansion (CTE) of about 17 ppm/°C, which is a measure of the amount of expansion and contraction that the material undergoes with changes in temperature. Ideally, a PCB’s dielectric material has an in-plane CTE closely matched to that of copper so that both materials will expand and contract similarly with changes in temperature.
While much has been posted in previous blogs concerning PCB dielectric materials, a circuit board’s copper is also critical to reaching and maintaining desired performance levels. Two types of commercial copper foils are typically used with PCBs: electrodeposited (ED) and rolled-annealed (RA) copper. The copper foils are formed in different ways and given different types of treatments to improve and preserve adhesion to various circuit dielectric materials. For example, ED copper is formed using a copper-sulfate solution, by electrolytic deposition onto a slowly rotating polished stainless-steel drum, and removing the copper in a continuous coil. The side against the drum provides the smoother finish. RA copper foils come from successively passing an ingot of solid copper through a rolling mill.
The different types of copper lend different qualities to PCB material. In quickly comparing circuit materials with ED and RA copper types, for example, PCBs with ED copper have shown good success when used in applications where mechanical stress may be critical, while circuit materials with RA copper are a good fit for applications where thermal shock might be a concern. PCBs with ED copper are very robust mechanically, although rapid thermal cycling can cause thermal stress cracks in narrow conductors formed with ED copper. Over similarly challenging thermal cycling conditions, PCBs with RA copper have shown improved resistance to conductor cracking from thermal stress.
Both copper foil types used for PCBs are generally treated with a passivation process that leaves a protective layer on the copper that is typically only a few Angstroms thick. This passivation treatment layer, which protects the copper from tarnishing for a certain period (as long as six months), can be easily removed by means of simple exposure to a weak acid, such as 10% hydrochloric (HCl) acid or sulfuric acid (H2SO4), which does no harm to the PCB material. Still, it is important to realize that this protective passivation treatment should not be ignored, since at higher temperatures, such as those used to process polytetrafluoroethylene (PTFE) laminates, modifications can occur to the passivation treatment layer. These modifications, which include conversion to different alloys, can make the passivation treatment much more difficult to remove than with a simple light acid bath. A light micro-etch treatment may even be needed to remove a passivation layer converted by exposure to high temperatures prior to circuit fabrication.
The two types of PCB copper can have very different grain structures, with grain structures also offering insight into the copper surface profile. ED copper foils used with PCB materials typically are available with coarse, moderate and fine grain structures. The copper with coarse grain structures provide the roughest copper surfaces while the copper with fine grain structures also have the smoothest copper surfaces. The surface roughness of a PCB copper foil can also be affected by additional treatments given to the copper to typically improve adhesion and compatibility with a target dielectric material.
These different copper surface profiles can have advantages and disadvantages. A copper foil with rougher surface profile typically enables a stronger bond between the copper foil and the dielectric laminate than a copper foil with a smoother surface profile. But the rougher copper surface profile also typically exhibits worse insertion-loss performance and greater phase variations than a PCB with otherwise similar materials but smoother copper surface profile, possibly due to the increased inductance of the rougher copper surface profile. PCB materials with smoother copper surface profiles tend to enable improved etch definition for fine-featured, high-frequency circuits.
At higher frequencies (and smaller wavelengths), smaller portions of the copper conductor surface in a PCB are used for transmission of signals, so that the effects of rougher copper surface profiles can be much more significant at higher frequencies. For thinner circuits in particular, increases in copper conductor surface roughness for a PCB material have been linked to increases in phase constant or effective dielectric constant for some PCB designs. This can cause complications when attempting to use computer models to predict the performance of circuits fabricated on those materials, if not accounting for the effects of rough copper surface profiles.
To aid circuit designers, Rogers PCB materials are available with different types of copper, including ED and RA copper, with different weights or thicknesses, such as 1 oz. (35 μm), 0.5 oz. (18 μm), and 0.25 oz. (8.5 μm) copper. These different types of copper are also available with different surface roughness profiles, to enable a designer to meet different application needs, such as with the rugged mechanical capabilities of circuit materials with ED copper or the flexible thermal-cycling capabilities of circuit materials with RA copper. Understanding these differences in copper foils for PCBs can help guide the process of selecting the best circuit material for a particular application. More details on those differences can be found by downloading a free PDF copy of the white paper, “Copper Foils for High Frequency Materials,” from the Rogers site at: http://www.rogerscorp.com/search/index.aspx?q=Copper%20Foils.
Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.