PCB007 Magazine


Issue link: https://iconnect007.uberflip.com/i/1259453

Contents of this Issue


Page 55 of 87

56 PCB007 MAGAZINE I JUNE 2020 Skin Effect Unlike DC or AC current that flows through the whole conductor trace, RF current does not penetrate deeply into electrical conduc- tors but tends to flow along their surfaces; this is known as the skin effect. Signal loss in the conductive copper layer is directly related to the phenomenon of "skin effect." Skin depth is the depth of the conductor the RF current uses. Basically, less of the conductor is used as the frequency increases Figure 1. Two phenomena that have a direct impact on insertion loss due to the "skin effect" are the degree of copper roughness (Figure 1) and the nature of the surface finish used. Surface finishes that contain electroless nickel on the surface, like electroless nickel immersion gold (ENIG) and electroless nickel electroless palla- dium immersion gold (ENEPIG), show greater insertion loss due to the resistive properties of the electroless nickel as compared to cop- per. Newer finishes like electroless palladium immersion gold (EPIG) and immersion gold electroless palladium immersion gold (IGE- PIG) are the preferred finishes for minimum insertion loss in high-frequency applications. Copper Surface Roughness Copper surface roughening is purposely do- ne to enhance adhesion of the conductor to the dielectric in multilayer structures. Roughening is accomplished by chemical or mechanical means, creating anchoring sites for the resin. This has worked well for non-RF current appli- cations, and it would also work well for RF sig- nals propagating at lower frequencies. Howev- er, as the frequency increases closer to 10 GHz or above, the skin depth is reduced. When the skin depth is equal to or less than the copper surface roughness (Figure 1), then the rough- ness will result in increased resistivity of the trace and will impact the conductor loss and the phase angle response of the circuit. As signal frequency increases, electrical sig- nals increasingly run closer to the copper con- ductor surface. This leads to increased resis- tance and transmission loss. Circuits using copper with a rougher surface will have more conductor loss than circuits us- ing copper with a smoother surface. More spe- cifically, the copper surface at the substrate- copper interface is the concern for surface roughness in relation to conductor loss. Recent developments in enhancing adhesion in multi- layer boards go beyond the standard roughen- ing created by black and brown oxide. Today, most inner layers rely on chemical etching to micro-roughen the traces for maxi- mum bonding. However, micro-roughening is not the answer to minimizing conductor signal loss. Chemical bonding is becoming the choice for adhesion for traces carrying high-frequency RF signals; it is also very effective on smooth copper surfaces. One chemical bonding system offered today is a combination of immersion tin, followed by a treatment with a silane coupling agent. The treatment is usually carried out in hori- zontal conveyorized equipment and results in excellent adhesion between conductor and di- Figure 1: Copper conductor skin effect.

Articles in this issue

Archives of this issue

view archives of PCB007 Magazine - PCB007-June2020