Manufacturing

Crimping

Q: How do I choose the correct crimp die size for a coaxial cable?

The die size is set by the connector manufacturer for a specific cable, not chosen freely. Crimp tools use a hex die measured across the flats after compression. For example, RG-58 with a standard BNC crimp uses a 0.213 inch (5.41 mm) hex for the ferrule and a 0.068 inch hex for the center pin, while RG-142 and RG-400 use a 0.255 inch ferrule hex. Always match the die callout printed on the connector datasheet to the matching cavity in the tool. An oversized die under-crimps and lets the ferrule pull off; an undersized die over-crimps, cracks the dielectric, and degrades return loss. Reputable tools use a ratcheting mechanism that will not release until full compression is reached, which removes operator variability.

Q: What pull-out force should a properly crimped RF connector withstand?

For miniature coaxial connectors on RG-58 and RG-174 class cable, a correctly crimped ferrule should survive a straight axial pull of 40 to 90 N (roughly 9 to 20 lbf) without the connector separating from the cable, per MIL-STD-202 Method 211 and IEC 60352-2 guidance. Larger N-type and TNC connectors on RG-213 are typically specified at 110 to 220 N. The pull-out strength comes from the ferrule biting into the braid and the cold flow of metal into the knurled connector body. If a sample pulls off below spec, the usual causes are an oversized die, missing braid under the ferrule, or a worn tool. Production lines verify pull-out on periodic sample coupons rather than every part.

Q: Why does crimping change the return loss of a coaxial connector?

A crimp alters the local geometry of the transmission line, and any change in conductor diameter or dielectric spacing creates a small impedance discontinuity. Over-crimping compresses the dielectric, raising capacitance and dropping the local impedance below 50 ohms; under-crimping leaves an air gap that raises impedance. Either produces a reflection that shows up as degraded return loss, especially above 6 GHz where the crimp length becomes an appreciable fraction of a wavelength. A well-controlled crimp on a precision connector holds return loss better than 25 dB to 18 GHz. This is why mmWave assemblies above 18 GHz generally favor captivated or solder-clamp center contacts over crimped pins, reserving crimping for the outer ferrule.

/krimp-ing/

A cold-forming termination process in which a precision die permanently deforms a metal ferrule over the braid and a contact onto the center conductor, bonding an RF coaxial connector to its cable without solder or heat. The deformed metal undergoes cold flow that creates a gas-tight, low-resistance joint and a controlled mechanical grip. Compared with soldering, crimping is faster, far more repeatable in production, and immune to flux contamination, which is why it dominates high-volume cable-assembly lines from RG-174 patch leads to RG-213 feeders. A correctly tuned hex die holds contact resistance under 1 mΩ and preserves better than 25 dB return loss to 18 GHz.

Category: Manufacturing

Pull-out force: 40 to 220 N

Contact R: < 1 mΩ

How a Crimp Terminates a Coaxial Cable

A coaxial crimp termination is really two crimps performed in sequence. First the center pin is crimped onto the exposed center conductor using a small four-indent or hex cavity, forming the inner electrical path. Then a tubular ferrule is slid over the exposed braid and the connector body and compressed with a larger hex die, simultaneously clamping the braid for shielding continuity and gripping the cable jacket for mechanical strain relief. Because the process is purely mechanical, there is no heat-affected zone, no flux residue, and no risk of wicking solder up the braid and stiffening the cable, all of which are failure modes that plague hand soldering.

The joint integrity comes from cold flow. When the die compresses the soft copper or brass ferrule past its yield point, the metal flows plastically into the knurls or serrations of the connector body and into the interstices of the braid strands. The result is a large area of intimate metal-to-metal contact with the oxide layers fractured and excluded, which is what makes the joint gas-tight and stable over thousands of thermal cycles. The compression ratio, typically 15 to 25 percent reduction in ferrule outer diameter, is engineered so the metal yields without cracking the dielectric or shearing the braid.

Tooling discipline is the whole game. Production crews use ratcheting crimp tools that physically cannot release until the die reaches full closure, and the hex die size is dictated by the connector datasheet for one specific cable type. Mixing a die intended for RG-58 onto RG-142 will either under-crimp and let the connector pull off or over-crimp and crush the dielectric, ruining the impedance match.

Crimp Force and Compression Geometry

Compression Ratio:
CR = (D_initial − D_crimped) / D_initial × 100% (typ. 15 to 25%)

Hex Across-Flats vs. Diameter:
AF = D_crimped × (√3 / 2) ≈ 0.866 × D_crimped

Crimp Joint Resistance (parallel asperities):
R_c ≈ ρ / (2a) + ρ_film / (πa²) with R_c < 1 mΩ target

Required Crimp Force (approx.):
F ≈ σ_y × A_contact × k (k ≈ 2 to 3 for confined upset)

Where D = ferrule diameter, ρ = bulk resistivity, a = effective contact-spot radius, σ_y = ferrule yield strength, A_contact = crimped contact area, k = constraint factor. A 5.41 mm hex on annealed copper needs roughly 4 to 8 kN of press force.

Termination Method Comparison

Method	Contact Resistance	Pull-out Force	Cycle Time	Frequency Limit	Best Use
Hex crimp	< 1 mΩ	40 to 220 N	5 to 15 s	~18 GHz	High-volume cable assembly
Solder/clamp	< 0.5 mΩ	90 to 250 N	30 to 90 s	> 40 GHz	Precision and mmWave
Clamp (non-solder)	1 to 3 mΩ	110 to 200 N	60 to 120 s	~12 GHz	Field-installable, reusable
Compression (push-on)	2 to 5 mΩ	60 to 130 N	10 to 20 s	~3 GHz	CATV, fast field install
Twist-on	5 to 20 mΩ	20 to 60 N	5 s	~1 GHz	Low-cost consumer only

Common Questions

Frequently Asked Questions

How do I choose the correct crimp die size for a coaxial cable?

Match the die callout on the connector datasheet to the matching tool cavity; it is dictated by the connector and cable, not chosen freely. RG-58 BNC uses a 0.213 in (5.41 mm) ferrule hex and a 0.068 in center-pin hex, while RG-142 and RG-400 use a 0.255 in ferrule hex. An oversized die under-crimps and lets the ferrule pull off; an undersized die crushes the dielectric and degrades return loss. Ratcheting tools enforce full closure to remove operator variability.

What pull-out force should a properly crimped RF connector withstand?

Miniature connectors on RG-58 and RG-174 should survive a 40 to 90 N (9 to 20 lbf) axial pull without separating, per MIL-STD-202 Method 211 and IEC 60352-2. Larger N-type and TNC on RG-213 run 110 to 220 N. The grip comes from the ferrule biting the braid and cold flow into the body knurls. Below-spec pulls usually mean an oversized die, missing braid under the ferrule, or a worn tool; lines verify on periodic sample coupons.

Why does crimping change the return loss of a coaxial connector?

A crimp alters local conductor and dielectric geometry, creating a small impedance discontinuity. Over-crimping compresses the dielectric and drops impedance below 50 Ω; under-crimping leaves an air gap that raises it. Either reflects energy, worsening return loss above 6 GHz where the crimp is an appreciable fraction of a wavelength. A controlled crimp holds better than 25 dB to 18 GHz, which is why mmWave assemblies favor captivated or solder contacts above 18 GHz.

Related Terms

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