Hello, I am measuring an a PCB in the over to see insertion loss behavior.
I plugged 1.85 sma connectors on both ends of the trace and started measuring s-params up to 130C.
at first test it ran good i took out the smas and plugged in another PCB and ran test again up to 130C.
on the 4th test I got S-param show me there is open circuit although everything got cooled down and I recalibrated the VNA between each test.
I got into conclusion that the heat ruins the dielectic material in the connector but later I found out there is no dielectic material in the1.85 connector.
What is happening in this situation?
Thanks.

 

The PCB, connector, and fixture all expand differently. At 130 °C, a small shift can reduce or completely lose center-pin contact.

 

few questions:
1. what is the name of this air dielectic connector ? its not sma.
2.how much time i need to wait before recalbrating ,after vna cable hold 125C

 

Hello, I am measuring an a PCB in the over to see insertion loss behavior.
I plugged 1.85 sma connectors on both ends of the trace and started measuring s-params up to 130C.
at first test it ran good i took out the smas and plugged in another PCB and ran test again up to 130C.
on the 4th test I got S-param show me there is open circuit although everything got cooled down and I recalibrated the VNA between each test.
I got into conclusion that the heat ruins the dielectic material in the connector but later I found out there is no dielectic material in the1.85 connector.
What is happening in this situation?
Thanks.

The SMA dielectric is unlikely the problem. A 1.85 mm connector is designed for high-temperature RF testing, so the issue is more likely mechanical or contact-related after repeated thermal cycling.

Possible causes:

Connector contact degradation: Heat cycles can reduce spring force or deform the center pin, causing an intermittent open.
Launch/interface movement: Thermal expansion differences between connector, PCB, copper, and solder can create tiny gaps.
Solder fatigue: Repeated cycling may cause micro-cracks at the center pin or ground joints.
Connector wear: Frequent mating cycles can damage the sensitive 1.85 mm interface.

Check continuity after cooling, inspect the launch under a microscope, and test with a known-good connector/cable. If the issue follows the connector, replace it; if it follows the PCB, inspect the launch or stack-up.

 

The more intresting phenomena i see is as following.
When i use an old connector i have the insertion loss getting bigger as temperature rises .
However if i put brand new sma then the insertion loss in the 25C is equal to the incertion loss of 125C the insertion loss is cyclic. Why is that ?