Connector Pin
Geometry and the Mating Interface
The connector pin is one half of a contact pair: a solid male pin on one connector and a slotted female socket (with spring fingers) on the other. When the two halves mate, the pin slides into the socket and the fingers grip it with a defined normal force, establishing a low-resistance electrical joint that must behave, electrically, as a seamless continuation of the center conductor. Because RF current at microwave frequencies flows in the skin of the conductor, the surface finish, roundness, and concentricity of the pin matter as much as its bulk material. A scratched, bent, or out-of-round pin disturbs the local field and radiates energy as a reflection rather than passing it cleanly to the next stage.
The single most controlled dimension is pin depth, the axial position of the pin tip relative to the connector's mating reference plane. The pin is deliberately recessed a small amount behind that plane so two mated connectors never bottom out against each other. If a pin protrudes past the reference plane it will push the mating socket back and can crush or fracture the supporting dielectric bead; if it is recessed too far an air gap opens in the dielectric and contact resistance climbs. Both faults appear as degraded return loss and, in metrology connectors, as non-repeatable measurements. Pin depth is checked with a dedicated pin-depth gauge before a precision connector is trusted on a calibration standard.
Material choice ties directly to mechanical life. Female socket fingers and many male pins are made from C17200 beryllium copper because its spring temper lets the contact flex and return to shape after repeated insertions while keeping good conductivity. A thin gold flash over a nickel barrier layer gives a stable, oxidation-free surface so contact resistance stays low and consistent over the connector's rated mating cycles, which range from a few hundred for commercial SMA up to several thousand for precision interfaces.
Pin Depth and Impedance Equations
Z0 = (59.96 / √εr) × ln(D / d) Ω
50 Ω ratio in air (εr ≈ 1):
D / d ≈ 2.30 → for D = 3.5 mm, pin d ≈ 1.52 mm
Reflection from a pin-junction discontinuity:
Γ = (Zstep − Z0) / (Zstep + Z0) → RL = −20·log10|Γ| dB
Where εr = dielectric constant, D = outer-conductor inner diameter, d = pin (center) diameter, Γ = reflection coefficient, RL = return loss. A center-conductor diameter error of 0.05 mm in a 3.5 mm air line shifts Z0 by roughly 2 Ω (dZ0/dd ≈ −59.96/d), enough to drop return loss several dB at millimeter-wave frequencies.
Center Pin Across Common Connector Series
| Connector Series | Male Pin Diameter (mating) | Freq Max | Pin / Socket Material | Typical Pin Depth |
|---|---|---|---|---|
| N-Type | 1.60 mm (0.063 in) | 18 GHz | BeCu, Au over Ni | Recess set by gauge |
| SMA | 0.91 mm (0.036 in) | 18 GHz | BeCu / brass, Au plated | 0.000 to 0.010 in recess |
| 3.5 mm | 0.91 mm (0.036 in) | 26.5 GHz | BeCu, Au over Ni | 0.000 to 0.003 in recess |
| 2.92 mm (K) | 0.91 mm (0.036 in) | 40 GHz | BeCu, Au over Ni | 0.000 to 0.003 in recess |
| 1.85 mm (V) | 0.51 mm (0.020 in) | 67 GHz | BeCu, Au over Ni | 0.000 to 0.002 in recess |
| 1.0 mm (W) | 0.25 mm (0.010 in) | 110 GHz | BeCu, Au over Ni | 0.000 to 0.001 in recess |
Note that the male mating pin diameter is not the same as the air-line center-conductor diameter used in the impedance formula above. SMA, 3.5 mm, and 2.92 mm share a common 0.91 mm (0.036 in) mating pin, which is why they intermate, even though SMA is PTFE-filled while 3.5 mm and 2.92 mm are air-dielectric. The larger center-conductor diameter inside the dielectric-supported body of the connector (for example 1.52 mm in a 3.5 mm air line) is what sets the characteristic impedance; the slimmer pin tip is what physically engages the socket fingers at the reference plane.
Frequently Asked Questions
What is connector pin depth and why does it matter?
Pin depth is the axial distance between the connector's mating reference plane and the pin tip, set as a slight recession so mated connectors never bottom out. For SMA the male pin sits roughly 0.000 to 0.010 in (0 to 0.25 mm) behind the reference plane. Protrusion can crush the mating socket and dielectric; excess recession opens an air gap and raises contact resistance, degrading return loss. It is verified with a pin-depth gauge before any precision use.
Why are RF connector pins made from beryllium copper?
C17200 beryllium copper (about 1.8 to 2.0% Be) combines good conductivity (about 22% IACS heat-treated) with the spring temper, fatigue resistance, and yield strength a flexing contact needs. Female socket fingers must spring back after each insertion, and BeCu holds those properties far better than brass or phosphor bronze. The pin is then gold plated over a nickel barrier for low, oxidation-free contact resistance. Brass pins appear in low-cost commercial parts; precision connectors use BeCu.
How does pin diameter set the 50-ohm impedance?
In an air line, Z0 = 59.96 × ln(D/d), so a 50 Ω line needs D/d ≈ 2.30. In a 3.5 mm air line the 3.5 mm outer bore and a 1.52 mm center conductor meet that ratio (separate from the 0.91 mm mating pin tip). A change in pin diameter, an off-center pin, or a step at the pin-to-socket junction creates a local impedance discontinuity that reflects energy. That is why precision pins hold tolerances of a few ten-thousandths of an inch and why bent or worn pins measurably hurt return loss.