ALMA Memo 585 Loss of Gold Plated Waveguides at 210-280 GHz

The loss at 210-280 GHz of room temperature waveguides plated with BDT 200 bright gold and Pur-A-Gold 125 soft gold is within ~10% of the theoretical loss based on the DC conductivity of pure gold and the classical skin effect. This is not the case for some other types of gold plating meeting the same ASTM (or MIL) specs but in which a trace of nickel is used as a brightener. Introduction Following the recent replacement of the NRAOCDL bright-gold plating solution used for the ALMA Band 6 SIS mixers, it was noticed that an initial darkening of the surface occurred on immersion of the brass parts in the bath. Although the final plating had good adhesion and appeared to be of high quality, we were disconcerted by the change in behavior of the bath and decided to measure the microwave loss of plated waveguide samples. Our concern was that the manufacturer might have made a subtle change in the composition of the solution which would not affect its ASTM specification but could increase the microwave loss. Recently, we had observed that the nickel brightener used in some commercial gold plating baths could cause significantly increased microwave loss, even though the plating met the appropriate ASTM (or MIL) specs. Plating Processes The NRAO-CDL plating lab uses two gold plating processes, one for hard, bright, gold based on the Enthone BDT 200 bath [1] which uses an arsenic brightener, and the other for soft, wirebondable gold based on the Enthone Pur-A-Gold 125 bath [1]. According to the manufacturer's data sheets, BDT 200 meets ASTM B 488-01 Type I (purity > 99.7%) Code C (hardness 130-200 KH25), while Pur-A-Gold 125 meets ASTM B 488-01 Type III (purity > 99.9% Code A (hardness # 90 KH25). The test waveguides were plated with 75 μ-in of gold, which is about ten skin depths in the frequency band of measurement. Fig. 1. The waveguide pattern in the test pieces. The waveguide is WR-3.7 (0.037" x 0.0185"). The total waveguide length is 10.000 inches.