The failure of a coaxial cable assembly during operation can be costly, and not just monetarily. The cost of failure could also amount to loss of life, complete mission failure, or the downtime of essential communication, radar, location, or jammer systems. For military, aerospace, and other critical applications, failure of systems that rely on coaxial cable assemblies is unacceptable, and for this reason, assemblies are often required to meet stringent military specifications for use in such applications.
However, the procurement of high-reliability (hi-rel) coaxial cable assemblies has traditionally followed a build-to-print model, which means new designs and programs with equipment under maintenance could potentially require weeks to months for a new order of coaxial cable assemblies to arrive. Because many assemblies are damaged during installation, program delays due to standard shipment times for cable assemblies are common unless a company has a stockpile. It is also challenging to predict a new program’s exact requirements, making it nearly impossible to preemptively order the correct number of assemblies.
This situation poses substantial challenges as technological changes occur very frequently in both commercial and defense systems, and many government initiatives to reduce spending while accelerating the design and procurement cycle. These commercial off-the-shelf (COTS) initiatives, though already valuable and effective in many aspects of the military supply chain, have until recently had little impact on coaxial cable assemblies due to their unique requirements.
Though there are many MIL-SPEC and hi-rel standards and qualifications for coaxial cables, connectors, materials, and assemblies, there is a subset that is more commonly applicable. These standards impose specific requirements on materials, processes, and workmanship to ensure traceability, reliable performance, and the quality of RF test reporting for the specific assemblies being delivered. The following is an overview of the required standards and processes for fabricating and shipping MIL-SPEC and hi-rel coaxial cable assemblies.
Material Control Requirements
MIL-DTL-17, formerly MIL-C-17, is the most comprehensive standard for flexible and semi-rigid cables with solid and semi-solid dielectric cores and one or more inner conductors. The scope of this standard specifies a wide range of requirement classifications including qualification, materials, design and construction, visual and mechanical, operational, marking, weight, and workmanship (Table 1).
MIL-DTL-17 also relies on several other military, government, and industrial standards to supplement the standard in the above requirement classifications (Table 2). Moreover, the standard details methods specific to the standard for military acceptance not otherwise included in other standards.
MIL-DTL-17 includes four main types of inspection requirements: materials inspection, qualification inspection, conformance inspection, and final inspection. The material inspection section of MIL-DTL-17 provides details and reference to standards documentation that is utilized in certifying the materials used in cable assemblies. Materials for jacketing, barrier tape, dielectric, inner conductors, outer conductors, and shielding are all included. The qualification inspection detailed in MIL-DTL-17J (Table 3) requires the use of laboratory facilities acceptable to the government and sample units produced with equipment and procedures that are normally used for production of qualified cables. Failure during qualification inspection can be considered a reason to refuse qualification approval.
Conformance inspection is performed on products for delivery and consists of three inspection groups (A, B, and C), which was illustrated in Table 1. For conformance inspection, sample specimens are taken randomly from the inspection lot of products fabricated during the same production period and with the same materials and processes. During conformance inspection, for Group A and Group B inspections, if defects are found, the inspection lots are screened for similar defects with the defective samples being removed from the lot.
New specimens are then taken from the culled inspection lot, and if defects are found in the second round of sample specimens, the whole inspection lot is rejected. Group C inspections, however, are derived from passing sample units from Group A and Group B inspections. And, if any specimens fail during Group C inspection, the inspection lot is considered to have failed Conformance Inspection.
Prior to delivery, MIL-DTL-17J requires a final inspection of a cable lot, in which continuity, spark test, voltage withstanding, insulation resistance, and out-of-roundness jacket measurements are made. In the case of semi-rigid cable, only continuity and voltage withstanding are tested during final inspection. Selection of sample units from the inspection lot is performed in the same way as conformance inspection. In the case of failure, for anything other than a spark test for non-semi-rigid cable, the failure can be considered cause for refusal of the lot.
The rigorous inspection and requirements of MIL-DTL-17 make it an attractive standard for ensuring the highest quality coaxial cable is used for assemblies. Hence, MIL-DTL-17 coaxial cables are commonly used in aerospace and avionics, interference friend or foe (IFF) radar, satellite communications, electronic countermeasure (ECM), and electronic warfare applications. Many industrial applications such as oil and gas, construction, and chemical systems benefit from MIL-DTL-17 coaxial cable as well because failures in these applications can have catastrophic effects.
In the case of coaxial connectors, the military qualification and conformance specification for both flexible and other coaxial transmission line connectors is MIL-PRF-39012 (currently MILPRF-39012F with Amendment 3). Similarly, as with MIL-DTL-17 covering coaxial cables, MIL-PRF-39012 requires several layers of inspection, including materials qualification, conformance, and periodic inspections (Table 4). The details of these inspections, as well as references to other applicable specifications, are indicated in MIL-PRF-39012. MIL-PRF-39012 describes the required visual, mechanical, and electrical testing necessary to qualify and inspect coaxial cable connectors.
Due to the exacting specifications of MIL-PRF-39012, RF connectors that pass this specification are ideal for use in most high-performance and hi-rel applications, including satellite communications, radar, RF instrumentation and test equipment, indoor/outdoor antennas, CATV, microwave radios, and aerospace. Moreover, the standard covers a wide range of common coaxial connectors such as:
It requires gold-plated, captivated contacts for minimal insertion loss and maximum electrical contact-quality over time and regardless of environment.
MIL-PRF-39012 also requires the manufacturer to maintain quality systems that enable parts covered by the specification to be listed on the Qualified Products List (QPL). Several of the qualification inspection tests Table 5 are performed only during initial qualification of a static design and manufacturing process, while conformance inspection is done prior to delivery, except for some periodic inspection tests. Periodic inspections consist of Group C inspections and are performed at prescribed intervals.
While MIL-PRF-39012 specifies the reliability qualifications for RF connectors, MIL-STD-348 details the exact dimensions and geometries of RF connector interfaces. Specifically, MIL-STD-348 includes the RF connectors and adapters referenced in other MIL-SPEC documents: MIL-DTL-3643, MIL-DTL-3650, MIL-STD-3655, MIL-DTL-25516, MILPRF-31031, MIL-PRF-39012, MIL-PRF-49142, MIL-PRF-55339 and MIL-DTL-83517. The specifications in this standard are deemed U.S. standards for RF connector interfaces and are accepted by all U.S. military services. MIL-STD-348 is an essential standard as it ensures interoperability of RF connectors throughout U.S. military services regardless of manufacturer. As the basis for RF connector design and construction, this standard also provides the foundation for reliable connector performance.
SAE AS23053 (also referred to as M23053, SAE-AMS-DTL-23053 standard and formerly the cancelled MILDTL-23053 standard) is the industry and military standard for heat shrinkable insulation sleeving. This type of insulation sleeving is commonly used for protecting the joint between coaxial cable and connectors, often enhancing the environmental and mechanical ruggedness of a coaxial cable assembly. This specification includes provisions for demonstrating compliance with qualification requirements (see Section 4 and 7.3), in process inspection, and statistical process control inspections.
Coinciding with SAE AS23053, SAE AS5942 establishes the performance requirements for the identification of wire and cable by indirect markings that have been applied to electrical insulating materials including heat shrink sleeving, wraparound labels and “tie-on” tags as well as any other types of material used for indirect marking.
Included in this specification are the processes and methods used to make the mark, which can include impact ink marking, thermal transfer, laser marking, hot stamp, etc. However, SAE AS5942 does not cover direct marking on electrical wire and cable. So, SAE AS5942 covers the heat shrink sleeve commonly installed on coaxial cable assemblies but not the outer insulating jacket of the assembly.
QPL cable and connectors (MIL-STD-790) is a system established by the U.S. Department of Defense (DOD) to simplify and provide assurance to buyers that they are purchasing products that have met the essential qualification requirements of applicable specifications. The QPL and Qualified Products Database (QPD) is a listing that provides approved supplier sources for government activities. To qualify for many military standards, a manufacturer must submit their products to the QPL.
Crimping Tools, Wire Termination General Specification
Superseding MIL-DTL-22520, SAE AS22520 specifies the requirements for inspection gages, accessories, and crimping tools used when connecting removable contacts from coaxial connectors and other electrical connectors, components, and systems. This standard is highly important to ensure interoperability of coaxial connectors and consistent pin placement and geometry within military and industrial standard coaxial connectors.
J-STD Soldering (IPC J-STD-001 and IPC J-STD-006)
Since the cancellation of MIL-STD-2000, J-STD soldering is the industry and military standard for soldering. The J-STDs are run by the Institute for Printed Circuits (IPC), which regularly issues updates to a range of standards. For soldering coaxial cable assemblies, specifically coaxial cable connector parts, J-STD-001 and J-STD-006 are the most applicable to military and aerospace applications. J-STD-001 (currently J-STD-001G), includes methods and verification criteria for producing quality soldered interconnections and assemblies.
J-STD-006 (currently J-STD-006B) with Amendments 1 & 2, prescribes nomenclature, requirements and test methods for electronic grade solder alloys, fluxed and non-fluxed bar, ribbon, and powder solders for electronic soldering applications, and for “special” electronic grade solders. Specifically, the J-STDs apply to the types of solder, and methods of use, when attaching the center pins, outer conductor, and shielding of coaxial cable to the coaxial connectors.
Given the rugged environments to which military-grade coaxial assemblies are subjected, anything less than high-quality soldered connections could result in faults of the electrical or mechanical performance of an assembly’s pin and/or outer conductor/shielding connection. Crimp/pinch-type coaxial connectors are also more susceptible to corrosion and failures from pulling or flexure than J-STD soldered connections.
To satisfy DoD requirements, as well as other aerospace and avionics national agencies, manufacturers must implement lot traceability. This lets a manufacturer trace all components of a part back to their original source. This is a valuable qualification because in the event of part failure, component traceability makes it possible to determine the root cause of failure and instead of scrapping the entire assembly, the failing component source or its manufacturing process can be modified.
With extremely complex systems, such as modern military and aerospace systems, lot traceability is essential to diagnosing and preventing failures. What this means for RF coaxial cable assemblies is that each component of the connection, cable, jacketing, shielding, insulating heat shrink, etc. can be individually traced back to its source in the event of assembly failure.
100% Testing and Test Reporting
Though datasheets can be effective tools to gauge the nominal, minimum, and maximum performance of a coaxial cable assembly, datasheets do not provide information on a given cable’s actual performance. Having test reports for each coaxial cable assembly provides a proof-of-performance, quality assurance, and helps with traceability in the event of assembly failure. This is only possible if every coaxial cable assembly is tested prior to delivery. Though conformance inspection is required by many military standards, having actual test reports for assemblies, including RF performance test plots, provides an additional assurance that every coaxial cable assembly meets, or exceeds, military and industrial standards.
IPC/WHMA-A-620 Workmanship Criteria
The IPC/Wire Harness Manufacturer’s Association WHMA-A-620 standard is the only industry-consensus standard for determining acceptable workmanship quality for cable and cable harness assemblies. This standard details practices and requirements for materials, methods, tests, and acceptance criteria for cable assemblies. Section 1.17 (1-5) of IPC/WHMA-A-620 specifically deals with workmanship/handling of cable assemblies and harnesses and focuses on methods that ensure optimal quality while reducing defects during manufacturing.
Hi-Rel Coaxial Assembly Features
Whether for military, aerospace, communications, industrial, mobile, laboratory, or scientific use, reliable cable assemblies are becoming increasingly valued for applications where the cost of cable assembly failure and maintenance are excessive. Though the applications requiring hi-rel coaxial cable assemblies are diverse, there are several common features that are desirable for most of these applications. These features include commercial availability, enhanced shielding, durable jacketing material, a wide operating temperature range, temperature conditioning of cables, sealed connectors, superior assembly, and test reporting.
Many common coaxial cable assemblies use a single shield as an outer conductor, and for flexible coaxial cables, this shield is often of braided metal wires which may be coated. If any amount of stress or pressure is placed on single-shielded cable assemblies, the physical deformation of braided shielding may cause substantial electrical performance variations and even possible failure. Hence, the use of double-shielded coaxial cables is preferred, especially for applications in harsh installation and operating environments, to help ensure reliable electrical performance. Moreover, double shielding a coaxial cable can prevent signal leakage and external interference in the presence of high-powered electromagnetic energy and RF signals.
For example, in an equipment cabinet with transmitters and receivers, a variety of relatively high-powered RF signals at various frequencies are likely to be conducted by coaxial cable assemblies. If improper shielding is used, signals may leak from a cable assembly and cause interference (or at least to the noise floor) with other transmitters and receivers or other radio components. Conversely, poorly shielded cable assemblies may only marginally attenuate outside interfering signals, allowing for potential receiver overload or ingression of interfering signals.
There are several common jacket materials and methods of assembly for Hi-Rel coaxial cable assemblies. The three main materials are Polytetrafluoroethylene (PTFE), Perfluoro alkoxy (PFA), and Fluorinated Ethylene Propylene (FEP). These fluoroplastic materials offer many similar benefits for use as coaxial cable jackets. However, there are a few areas where FEP provides advantages over PTFE or PFA. Namely, FEP presents a lower melting point and curing temperature, allowing these assembly jackets to be more easily manufactured.
FEP also exhibits similar corrosion resistance to PTFE and PFA, but exhibits enhanced detergent resistance. Both PTFE and FEP have better weather resistance compared to PFA. All three fluoroplastics have similar dielectric constants, but FEP demonstrates superior dielectric strength compared to PTFE. Moreover, FEP has a much higher dissipation factor than both PTFE and PFA and an arc resistance comparable to PTFE.
Operating Temperature Range
Automotive and industrial operational temperature ranges are substantial, but often inadequate for military and aerospace applications. Many materials and assembly methods are unable to maintain specified performance at the wide operating temperature ranges required by military specifications. Exposure to harsh environments with limited cooling capability often in cabinets containing heat-producing RF electronics is a common occurrence for hi-rel coaxial cable assemblies.
As a result, cable assembly components and manufacturing methods designed to ensure higher than average operating temperatures can help to acheive reliable performance over a long lifetime and reduced risk of degradation due to environmental stresses. Operational temperature ranges for electronics and electrical components are typically as follows. Actual operating temperature range depends on the cable type:
- Commercial: 0° C to 85° C
- Industrial: −40° C to 100° C
- Automotive: −40° C to 125° C
- Extended: −40° C to 125° C
- Military: −55° C to 125° C
Thermally pre-conditioning cables in a temperature chamber prior to assembly serves to increase the electrical and mechanical stability of the cables over time. An industrial temperature chamber is programmed to cycle precut cables multiple times from cold to hot temperature extremes to “pre-age” and stabilize cables prior to cable assembly construction.
Seal gaskets, though not electrical in nature, ensure the long-term operational reliability of coaxial cable assemblies. These gaskets help to prevent environmental intrusion from moisture, chemicals and debris that in the best case increase insertion loss and in the worst case result in cable destruction. Silicone rubber gaskets are a standard for sealing gaskets and tend to provide years of maintenance-free environmental protection.
Commercial-off-the-shelf (COTS) components are increasingly sought after by military and government agencies to reduce acquisition time, mitigate the high cost of custom ordered components, and develop a more competitive purchasing process. However, COTS components must still meet rigorous MIL-SPEC and industry standards to be viable for military or government use. Moreover, even if MIL-SPEC cables and connectors are used, the assembly process must also meet a variety of other military standards. Although there are suppliers of coaxial cable assemblies who design products specifically to meet military and government standards, their assemblies are rarely available outside of a lengthy quote and ordering process.
The RF industry has until relatively recently been a provider for military, aerospace, government, scientific, SATCOM, and large-scale communication network applications. Hence, the model for manufacturing and supplying RF components, devices, and assemblies has traditionally been a build-to-print or quote-based ordering system. This is largely how the acquisition of coaxial cable assemblies has occurred. The main focus of this method has been on fulfilling 100% of the desired features, with assemblies being customized for particular applications.
Making a Coaxial Cable Assembly that Meets MIL-SPEC
Meeting MIL-SPEC qualifications and inspections is no simple task. This process is double challenged when it attempts to realize orders for military standard cables within hours of an order being placed. Pasternack has made this possible with a standardized system of quality manufacturing steps, along with in-situ tests/inspections designed to ensure component- and assembly-level compliance with all relevant military and industry standards. This capability, especially at volume, is rare among coaxial cable assembly manufacturers, as most manufacturers are dedicated to fulfilling orders with planned lead times negotiated during an already lengthy ordering process.
Over the course of a 36-step process, of which five steps are dedicated to incoming material flow and 31 are involved with order processing, Pasternack technicians and machines will process, assemble, and verify every critical mechanical and electrical property of a coaxial cable assembly.
This includes ensuring the pin and connector attachments are held to J-STD-001 class 3 soldering standards, the crimping is done according to SAE AS2250 standards, the marking meets SAE AS5941 standards, and the overall workmanship is in accord with IPC/WHMA-A-620 class 3 standards. Moreover, each coaxial cable assembly is 100% tested for connector gaging of the pin and insulator position, insertion loss, VSWR, dielectric withstanding voltage (DWV), and is further visually inspected for workmanship, configuration, and marking. Additionally, Pasternack performs sample testing for length and mass.
As military and aerospace systems continue to electrify, there is an increased need for hi-rel coaxial cable and connector assemblies of all shapes and sizes. With interconnect being one of the last steps in system design, and even in production, any delay or errors with interconnect orders can lead to unfortunate and costly setbacks in meeting contract deadlines or fielding viable prototypes necessary for contract wins.
The traditional sourcing process for these cables has involved a lengthy process of quotation, inspection, certification, production, and shipment that can require weeks or even months for cable assemblies to arrive. Design changes or errors may result in even longer delays to a program. If expected delays are unacceptable, a special waiver is sometimes requested to substitute non-certified cable assemblies, creating reliability and traceability concerns.