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Crimp-on connectors: speed phase matching of cable assemblies for sophisticated electronics.


In the microwave business, says Bill Budynas, development engineer for Anaren Corp., Syracuse, N.Y., "meeting dimensional and phase-matching requirements of 32 semi-rigid cable assemblies is more formidable because of the vagaries of soldering connectors needed on each cable assembly.

"Our situation improved when we switched to crimp-on connectors for termination of semi-rigid cable. Realiability has increased, manufacturing costs are lower, and yield has increased from less than 20% to over 95%."

Anaren now uses crimp-on SMA connectors from AMP Inc., Harrisburg, Pa.

Connectors terminate interconnect cable assemblies used in Anaren's Digital Electronic Support Measures (DESM) receiver, an electronic system requiring ultra-tight tolerance for phase matching of RF cables.

In the DESM, semi-rigid cables may have 11 or 12 bends. Phase matching is required between multiple cable assemblies, and the entire system must be able to survive in a vibration and shock-filled environment.

Semi-rigid cables used in the DESM have a steel-core, silver-plated center conductor encased in Teflon dielectric.

The dielectric has solid copper shielding surrounding it, drawn over the dielectric and center conductor to form the completed cable. Two sizes of SMA connectors are required, for the 0.141 inch OD (outside diameter) and 0.086 inch OD assemblies.

Complex Of Cables

The DESM has 350 complex, individually assembled cables. Multiple sets of 32 cables are phase matched to within 2 degrees of signal phase, and within one-thousandth of an inch tolerance over a 14-inch cable length.

Gene Augusto, Anaren systems design engineer, recalls, "We made up about 40 soldered cable assemblies with 0.086 SMA male connectors attached on one end only. Because of phase mismatch, the project engineers rejected all 40."

In a subsequent prototype run, only 20 of 110 were acceptable phase-matched assemblies.

It was evident that variations created during the soldering process were causing out-of-phase rejects. Also, soldered connections wee susceptible to fracture failures.

The dielectric of the cable and a connector may creep, or grow, during heating. Dielectric growth cannot be accurately predicted.

Crimp-Ons Pass Test

Tests pitted the crimp-on SMA connectors against soldered SMA connectors. Ten cable assemblies with solder connectors and 10 with crimpons were installed on opposite sides of a large power divider. All were torqued to manufacturers' specifications.

Cables were subjected to strain during 120 hours of vibration testing. After 30 hours, cables were checked. Then for 90 hours all the SMA connectors in the test were over-torqued, as often happens in actual service.

Within the first few hours of vibration testing, the solder connectors showed cracks. After 30 hours, all solder connections exhibited some type of failure. By the end of 60 hours, all solder connections failed beyond the point where they could be tested.

But every crimp-on connector survived the entire 120 hours. No phase mismatch resulted after vibration testing of the crimp-on connectors.

Budynas obtained sample quantities of crimp-ons for 0.086 OD and 0.141 OD cables for prototyping.

"The process is much simpler--cut the cable to length, put on the connector, and trim," he says. "With this process, the plus or minus 2 degrees phase-match tolerance and one-thousandth inch length tolerance held in the first 32 of 40 assemblies we made."

The crimp process is parellel to the centerline of the cable, forcing a factory-assembled grip ring into the center body, trapping the shield of the coaxial cable. Minimum reduction is transferred to the internal diameter of the cable shield, preserving original cable performance characteristics.

Savings With Crimp-Ons

Significant cost savings are possible because:

* Termination is reduced from minutes to seconds.

* Skill level required to produce good termination is reduced.

* Scrap rates are reduced.

* Better overall consistency resulted from elimination of solder head distortion of the cable and/or the connector.

Crimp connectors have proven out in the Anaren DESM. Anaren has shipped 50 systems, which include over 17,000 cable assemblies. The production yield is over 95% on the 17,000 cable assemblies. No in-service failures have been reported.
COPYRIGHT 1990 Nelson Publishing
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Publication:Communications News
Date:Feb 1, 1990
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