T-line surface finish

[zappedagain]

Would you expect different performance between this:

MATERIAL: ROGERS R4003, 0.008" THK, HIGH TEMP. FR4, 0.100" THK
PLATING: BOTH SIDES 99.9% Au, 0.5um THICK OVER Ni

and this:

MATERIAL: ROGERS R4003, 0.008" THK, HIGH TEMP. FR4, 0.100" THK
PLATING: BOTH SIDES IMMERSION GOLD

The R4003 copper is 1 oz (35 um) thick. We are trying to explain a steeper than expected frequency response (about 8 dB loss at 25 GHz when 2 dB is expected). The first version listed was designed but never built; the second version was introduced as a cost saving measure and manufactured; we typically only use the first version when we need to wire bond to the finished surface.

Thanks,

John D

 

[Higgler]

If the gold is thin, it's a lossy resistor if it's somewhat isolated from the copper.
I've heard of a similar thing at 32 ghz where original plating of gold was later changed to 5 deposits to gold to build up the thickness. But oxide layers built up in each successive gold application and thereby provided a slight insulation of the gold layers from each other. It led to an extra loss of around 5 dB.

Maybe you had some oxidation of the Cu layer before gold immersion and you ended up with copper, oxide, gold surface finish. Or the gold bonding to the copper had a problem. Current in the outer thin gold layer was akin to current in a resistor.

kch

 

[zappedagain]

We finally figured this one out. The PCB finish was not an issue.

Due to the PCB layout (stack up), the RF connectors had a good contact to ground on the top of the PCB (Rogers), but not on the bottom of the PCB (FR4) as the ground plane on the FR4 material wasn't well connected to the ground plane on the Rogers material. This made the connectors radiate the energy instead of transferring the power onto the coplanar waveguide / microstrip and thus they were lossy.

We ended up notching the FR4 material so the RF connector bonds directly to the ground layer on the Rogers material. A prototype of this showed much better performance.

John D