Aerospace-grade SMA: CTE Analysis from −65 °C to +165 °C

Brass, stainless steel, beryllium copper, and PTFE expand at different rates. We analyze how thermal expansion and plating stability affect SMA performance across the aerospace temperature range.

Aerospace and high-reliability applications often require SMA connectors qualified from −65 °C to +165 °C. Temperature cycling is not just about surviving the range; it is about keeping impedance, VSWR, and contact resistance stable as different materials expand and contract at different rates.

1. Coefficient of thermal expansion (CTE)

PTFE expands much more than metal. If the dielectric support is too long or not captured well, thermal cycling can shift the center conductor position, changing impedance and VSWR.

2. Plating stability

Gold plating is preferred for high-reliability applications because it does not oxidize and maintains low contact resistance across temperature extremes. Nickel under gold adds hardness and wear resistance but can introduce slight magnetic loss at microwave frequencies. For aerospace, a typical choice is 1.27 µm gold over 1.27 µm nickel on the body, with thicker gold on mating surfaces.

3. Design choices for wide temperature

• Short dielectric support: Minimizes PTFE expansion effects on impedance.
• Beryllium-copper center contact: Maintains spring force after repeated temperature cycles.
• Stainless-steel body: Better CTE match to aluminum panels and better corrosion resistance than brass.
• Captured insulator: Prevents axial movement of the dielectric during temperature swings.

4. Qualification tests

A typical aerospace qualification sequence includes thermal shock (−65 °C ↔ +165 °C), random vibration per MIL-STD-202, and contact-resistance measurements before and after. A well-designed connector shows contact-resistance change below 1 mΩ and VSWR shift below 0.02 after cycling.

For salt-spray comparison of gold vs nickel plating, see Gold vs Nickel Salt-Spray Test.