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PCBD-Oct2015

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54 The PCB Design Magazine • October 2015 is at the edges where the addition of the plated finish can increase the conductor loss, which will increase the insertion loss. One experiment showed the difference in insertion loss of a microstrip transmission line circuit using bare copper and ENIG, but with different substrate thickness. It was found that the thinner circuits had a larger difference in insertion loss when comparing the circuits with bare copper to circuits with ENIG plated finish, as shown in Figure 1. The material used in Figure 1 is RO4003C, a low-loss high-frequency circuit material. The dissipation factor for this material is 0.0027 when tested at 10 GHz in a clamped stripline test. If this same study used an FR-4 material (Df ≈ 0.020), the outcome would be very differ- ent, mainly due to the fact the FR-4 has about four times more insertion loss than RO4003C at 10 GHz and that additional loss is due to the dielectric loss. As an example, the same circuit using 20 mil FR-4 when tested at 10 GHz using bare copper has about 0.8 dB/inch insertion loss and Figure 1 shows the 20 mil RO4003C lami- nate with bare copper has an insertion loss of about 0.2 dB/inch at 10 GHz. The high dielec- tric loss of FR-4 makes the conductor loss much less significant, so the addition of a lossy plated finish will not be as obvious on FR-4 as it would on a low loss material. As part of this plated finish study it was found that several finishes do not have much impact on insertion loss, when compared to a microstrip circuit using bare copper. Organic Figure 1: results of experiment showing difference in insertion loss of microstrip transmission line circuit using bare copper and enig, with different substrate thickness. lightning speed laminates IMPACT OF FINAL PLATED FINISH ON PCB LOSS

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