Deep Hole Drilling for the Electronics Connector Industry: Coaxial RF Connector Centre Pin Bores, Insulator Bead Mounting, and Hermetic Glass-Sealed Bores

A manufacturer of 40 GHz K Connectors (beryllium copper, 0.711 mm centre pin bore with 0.033 mm concentricity to 1.676 mm and 1.981 mm stepped bores) used a PCD-tipped step gun drill (three steps, polished transitions, Vc = 40 m/min, f = 0.005 mm/rev, oil at 80 bar). CMM measured 0.028 mm TIR. The connector passed return loss testing at 40 GHz.

Electronics Connector Drilling ​

RF connector centre pin bores are the most precisely drilled features in the electronics connector industry. The centre pin bore in a 40 GHz K Connector (Anritsu or equivalent) is 0.711 mm diameter, with a concentricity tolerance of 0.033 mm relative to the adjacent stepped bores (1.676 mm and 1.981 mm) that form the impedance-matching section of the connector. The electrical performance of the connector at 40 GHz — return loss (S11) and insertion loss (S21) — is directly determined by the concentricity of the centre pin bore. A concentricity error of more than 0.033 mm at any of the step transitions causes an impedance mismatch that degrades the return loss by 5-10 dB.

The step drill is a single tool with multiple cutting edges of increasing diameter, with polished transitions between the steps (the radius at each step transition must be less than 0.005 mm to prevent signal reflection at the impedance step). The step drill is custom-ground for each connector type from micrograin carbide with a PCD tip on the smallest diameter step. The drilling parameters: Vc = 30-50 m/min, feed f = 0.003-0.008 mm/rev, high-EP oil at 60-100 bar filtered to 1 micron. The connector body material is beryllium copper (C17200, aged to 40 HRC) or stainless steel 303/316. The insulator bead mounting bores are drilled in Kovar (an iron-nickel-cobalt alloy) for hermetic glass bead sealing. The Kovar bore is gun-drilled and honed to Ra < 0.2 microns to form a hermetic seal with the glass bead fused into the bore at 900-1000 C.

Step Drill Parameters for RF Connector Types ​

The table below compares the step drill geometry for common RF connector standards.

Material Properties and Drilling Characteristics ​

Different connector body materials require tailored drilling approaches.

FAQ ​

Why does concentricity matter for RF connector performance? ​

In a coaxial connector, the centre pin bore concentricity determines the impedance uniformity of the transmission line. A perfectly concentric connector maintains a constant characteristic impedance (typically 50 ohms) through the connector body. Any concentricity error at a step transition creates a localised impedance change, which reflects a portion of the RF signal back toward the source (increasing the return loss, S11). At 40 GHz, a concentricity error of 0.033 mm creates an impedance mismatch of approximately 5 ohms, which reflects 10-15% of the signal power.

What is the function of the polished step transition on the step drill? ​

The polished transition between the step diameters on the step drill creates a smooth, gradual change in bore diameter as the step drill advances through the connector body. A sharp step transition would produce a corresponding sharp change in the bore diameter, which would create a capacitance discontinuity in the transmission line and reflect RF energy. The polished transition radius of less than 0.005 mm distributes the diameter change over a distance that is electrically small compared to the wavelength at 40 GHz (7.5 mm in the dielectric), minimising the reflection.

Why is Kovar used for hermetic glass bead sealing bores? ​

Kovar (an iron-nickel-cobalt alloy, 54% Fe, 29% Ni, 17% Co) has a coefficient of thermal expansion (5.2 x 10^-6 /K) that closely matches borosilicate glass (5.1 x 10^-6 /K) over the temperature range from -60 C to +450 C. When the glass bead is fused into the Kovar bore at 900-1000 C and cooled, the matched expansion rates prevent the glass from cracking or separating from the metal. The Kovar bore must be honed to Ra < 0.2 microns to ensure intimate contact between the glass and the metal surface; any roughness creates leak paths that degrade the hermetic seal (the leak rate requirement is typically < 1 x 10^-8 atm-cc/s He).

How is the step drill custom-ground for each connector type? ​

The step drill is ground from a micrograin carbide blank on a CNC tool and cutter grinder with a resolution of 0.001 mm. The grinding program defines the diameter, length, and transition radius for each step. The PCD tip is brazed onto the smallest diameter step before grinding, and the PCD surface is polished to a mirror finish (Ra < 0.05 microns) on the rake face to minimise friction and chip adhesion. The step drill is inspected on an optical comparator at 50x magnification to verify the step diameters, lengths, and transition radii before use.

What is the return loss acceptance criterion for a 40 GHz connector? ​

The acceptance criterion for a 40 GHz K Connector is return loss (S11) < -20 dB across the full 40 GHz bandwidth, corresponding to a maximum reflected power of 1% and a voltage standing wave ratio (VSWR) of less than 1.22:1. Connectors that pass the return loss test are also tested for insertion loss (S21) with an acceptance criterion of < -1 dB (maximum 20% power loss through the connector pair). The test is performed on a vector network analyser (VNA) with a 40 GHz calibration kit, using the thru-reflect-line (TRL) calibration method.

Data are based on published research and industry experience as of 2026.