NASA-STD-8739.4_Rev_A.pdf

NASA TECHNICAL

STANDARD

NASA-STD 8739.4A

with Change 2

National Aeronautics and Space Administration

Washington, DC 20546

Approved: 2016-06-30

Superseding NASA-STD-8739.4

With Change 6

WORKMANSHIP STANDARD FOR CRIMPING,

INTERCONNECTING CABLES, HARNESSES, AND

WIRING

Measurement System Identification:

Metric (English)

NASA-STD-8739.4A – 2016-06-30

TABLE OF CONTENTS

DOCUMENT HISTORY LOG

TABLE OF CONTENTS

LIST OF APPENDICES

LIST OF FIGURES

LIST OF TABLES

1. SCOPE

1.1 Purpose

1.2 Applicability

2. APPLICABLE DOCUMENTS

2.1 General

2.2 Government Documents

2.3 Non-Government Documents

2.4 Order of Precedence

3. ACRONYMS AND DEFINITIONS

3.1 Acronyms and Abbreviations

3.2 Definitions

4. REQUIREMENT

4.1 General

5. TRAINING AND CERTIFICATION PROGRAM

5.1 General

6. FACILITIES, EQUIPMENT, MATERIALS, AND PARTS

6.1 Environmental Conditions

6.2 Tool and Equipment Control

6.3 Electrostatic Discharge Control Requirements

6.4 Inspection Optics

6.5 Parts and Materials Selection

6.6 Solvents and Cleaners

7. DESIGN PRACTICES

7.1 General

7.2 Design Considerations

8. INTERCONNECTING CABLE/HARNESS FIXTURING

8.1 General

8.2 Mockup and Wiring Board Design Parameter

8.3 Temporary Identification

8.4 Interconnecting Cable and Harness Protection

9. FORMING WIRES INTO CABLES AND HARNESSES

9.1 General

9.2 Lacing for Trunk, Branches, and Breakouts

9.3 Fabric Braid Sleeving (Prewoven)

9.4 Fabric Braid Directly Woven on Interconnecting Harness or Cable

9.5 Spiral Wrap Sleeving

9.6 Plastic Straps

9.7 Metal Braid Shielding

9.8 Insulation Sleeving/Tubing

9.9 Installation of Heat-Shrinkable Sleeving

9.10 Long Lengths of Shrinkable Sleeving

9.11 Shrinkable Part Requirements and Installation

10. STRIPPING INSULATION FROM CONDUCTORS AND CABLE

10.1 Stripping Round Conductors

10.2 Stripping Jackets Over Shields

10.3 Stripping Flat Conductor Cable

11. CABLE SHIELDING AND SHIELD TERMINATION

11.1 General

11.2 Individual Shield Termination Using Two-Piece Crimp Rings

11.3 Group-Grounding of Individual Shield Terminations

11.4 Large Compression Ring Grounding

11.5 Floating Shield Terminations

11.6 Unshielded Wire Exposure and Total Length of Grounding Wires

12. CRIMP CONNECTIONS

12.1 General

12.2 Examination of Crimp Contact

12.3 Process Controls

13. CONNECTOR ASSEMBLY

13.1 General

13.2 Assembly of Crimp-Type Connectors (Including Terminal Junctions)

13.3 Assembly of Solder-Type Connectors

13.4 Assembly of Connectors with Non-Removable Solder Cups Including Hermetic and

Environmental Types

13.5 Assembly (Torquing) of Adapters and Cable Clamps to Connectors

13.6 Assembly of RF Connectors and Coaxial Contacts

13.7 Process Controls for Two-Piece Crimp Rings and Stub-Type Splicing Devices

13.8 Inspection and Verification Testing

14. HARNESS IDENTIFICATION

14.1 General

15. INTERCONNECTING HARNESS AND CABLE CLEANING

15.1 General

15.2 Cleaning the Harness Assembly

15.3 Cleaning Harness Connectors

15.4 Cleaning Coaxial Connectors (Assembled)

16. INTERCONNECTING HARNESS AND CABLE HANDLING AND

PROTECTION

16.1 General

17. CONNECTOR MATING

17.1 General

17.2 Connector Mating and Demating

17.3 Coaxial Connector Mating

18. TESTING AND INSPECTION

18.1 General

18.2 Testing

18.3 Test Methods

19. SPLICING

19.1 General

19.2 General Information

19.3 Soldered Splices

19.4 Lap Splice

19.5 Lash Splice

19.6 Solder Sleeve

19.7 Soldered Western Union/Lineman Splice

19.8 Solder Ferrule

19.9 Crimped Splices

19.10 Modified Crimp Contact

19.11 Crimp Ferrule Splice

19.12 Butt Splice

19.13 Wire In-Line Junction Devices (Jiffy Junctions)

20. QUALITY ASSURANCE PROVISIONS

20.1 General

20.2 Documentation Verification

20.3 Documentation Authorization

20.4 Verification of Tools, Equipment, and Materials

20.5 Inspection Criteria

LIST OF APPENDICES

WIRE VISUAL AIDS AND ILLUSTRATIONS

CRITICAL PROBLEMS IN COAXIAL CABLE ASSEMBLY

LIST OF FIGURES

Figure 8-1. Line Drawing of Typical Harness Layout Figure 8-2. Typical Harness Board Hardware and Fixtures Figure 9-1. Starting Stitch Figure 9-2. Spot Tie (Typical) Figure 9-3. Closing Stitch and Single Tape—Illustration

Figure 9-4. Alternate Closing Stitch and Single Tape—Illustration Figure 9-5. Running Lockstitch Figure 9-6. Flat Lacing Stitches Figure 9-7. Securing Fabric Braid Sleeving Figure 9-8. Starting Lock

Figure 9-9. Forming Ending Pigtail

Figure 9-10. Braiding at a Breakout or Y Intersection

Figure 9-11. Spiral Wrap Sleeving Figure 9-12. Plastic Strap Orientation

Figure 9-13. Illustration of Shrink Sleeve Installation (Typical) Figure 9-14. Installation of Long Lengths of Sleeving to Achieve Controlled Dimensions

Figure 9-15. Sleeving Installation (Typical) Figure 11-1. Terminating Overall Shield in RFI Adapter (Typical) Figure 11-2. Example of Individual Shield Termination Using a Heat Shrinkable Solder Sleeve

Figure 11-3. Individual Shield Termination Using a Two-Piece Crimp Ring

Figure 11-4. Example of Group Grounding of Staggered Shields Figure 11-5. Group-Grounding of Individual Shield Terminations Figure 11-6. Large Compression Ring Grounding (Typical Applications)

Figure 11-7. Floating Shield Termination

Figure 11-8. Conductor Exposure for Individual Shield Termination Types Figure 12-1. Crimp Joint Tensile Failure Categories Figure 13-1. Typical Push Test Tool

Figure 13-2. Application of Retention Tool for Gripping Wire (Typical) Figure 15-1. Visual Examination Inside the Socket Contact for Flux Residue

Figure 19-1. Pre-Tinned Conductors Figure 19-2. Soldered Conductors Figure 19-3. Individual Shield Termination with a Lap Splice and Solder Sleeve Figure 19-4. Sleeving over Soldered Connection

Figure 19-5. Double Sleeving over Soldered Connection Figure 19-6. Pre-Tinned Conductors

Figure 19-7. Lashing of Pre-Tinned Conductors Figure 19-8. Soldered Connection Figure 19-9. Pre-Lash End Type Splice Figure 19-10. Lash End Type Splice Figure 19-11. Soldered Lash Splice

Figure 19-12 Sleeved Lash Splice Figure 19-13. Solder Sleeve Prior to Flow Figure 19-14. Fully Melted Solder Sleeve

Figure 19-15. Western Union/Lineman Splice

Figure 19-16. Initial Wrap for Western Union/Lineman Splice Figure 19-17. Completed Wrap for Western Union/Lineman Splice Figure 19-18. Soldered Western Union/Lineman Splice

Figure 19-19. Solder Ferrule Figure 19-20. Solder Ferrule Figure 19-21. Stripped Wires Prior to Insertion Figure 19-22. Stripped Wire Bundle Prior to Insertion Figure 19-23. Wires Crimped Within Contact

Figure 19-24. Contact Trimmed and Deburred Figure 19-25. Contact Covered With Shrink Sleeving Figure 19-26. End Type Splice Figure 19-28. Butt Splice

Figure 19-29. Butt Splice Prior to Wire Insertion Figure 19-30. Butt Splice Prior to Crimp

Figure 19-31. Properly Crimped Butt Splice Figure 19-32. Butt Splice with Shrink Sleeving

Figure 19-33. Crimped Contact Outside Junction Device Figure 19-34. Crimped Contacts Inserted Into Junction Device Figure A-1

Figure A-2 Figure A-3

Figure A-4 Figure A-5 Figure A-6

Figure A-7

Figure A-8 Figure A-9 Figure A-10

Figure A-11 Figure A-12

Figure A-13 Figure A-14

Figure A-15 Figure A-16 Figure A-17 Figure A-18 Figure A-19

Figure A-20 Figure A-21

Figure A-22 Figure A-23 Figure A-24 Figure A-25 Figure A-26 Figure A-27

Figure A-28

Figure A-29 Figure A-30 Figure A-31

Figure B-1. Illustration of Proper Trimback of Jacket to Isolate it from the Clamping System 100 Figure B-2. Broken Solder Joint Caused by Insufficient Solder Fill Figure B-3. Problem Point for Kynar Stress Relief Sleeving

LIST OF TABLES

Table 7-1. Bend Radii for Completed Interconnecting Cable or Harness Table 9-1. Spot Tie, Plastic Strap, and Stitch Spacing Dimensions

Table 9-2. Distances from Connectors or Connector Accessories to Beginning of Harness Ties 32

Table 9-3. Selection Guide for Use of Polyolefin and Polyvinylidene Fluoride Sleeving [in mm

(inches)] Table 11-1. Shield Termination Control for Group Grounding

Table 11-2. Shield Termination Control (Refer to Figure 11-8) Table 12-1. Crimp Tensile Strength 1

Table 13-1. Pull Force 1 Table 13-2. Contact Retention Test Forces Table 19-1. Crimp Tensile Strength Values for Wire-in-Line Junction (i.e., Jiffy Junction)

Devices Table 19-2. Splice Body Retention Values for Wire-in-Line Junction (i.e., Jiffy Junction)

Devices

CRIMPING, INTERCONNECTING CABLES,

HARNESSES, AND WIRING

1. SCOPE

1.1 Purpose

The purpose of this NASA Technical Standard is to set forth requirements for interconnecting cable and harness assemblies that connect electrical, electronic or electromechanical components.

1.2 Applicability

1.2.1 This standard is approved for use by NASA Headquarters and NASA Centers, including Component Facilities, and Technical and Service Support Centers, and may be cited in contract, program, and other Agency documents as a technical requirement. This Standard may also apply to the Jet Propulsion Laboratory, other contractors, grant recipients, or parties to agreements to the extent specified or referenced in their contracts, grants, or agreements.

1.2.2 This standard applies to critical work, as defined by NPD 8730.5. Critical work is any task that if performed incorrectly or in violation of prescribed requirements poses a credible risk of loss of human life; serious injury; loss of a Class A, B, or C payload (see NPR 8705.4); loss of a Category 1 or Category 2 mission (see NPR 7120.5); or loss of a mission resource valued at greater than $2M (e.g., NASA space flight hardware, Government test or launch facility).

1.2.3 Use of the term “supplier” applies to any entity that is manufacturing hardware in accordance with the requirements herein including NASA Centers and NASA contractors.

2. APPLICABLE DOCUMENTS

2.1 General

The documents listed in this section contain provisions that constitute requirements of this

Standard as cited in the text. Use of more recent issues of cited documents may be authorized by the responsible Technical Authority. The applicable documents are accessible via the NASA

Standards and Technical Assistance Resource Tool at http://standards.nasa.gov or may be obtained directly from the Standards Developing Organizations or other document distributors.

2.2 Government Documents

2.2.1 National Aeronautics and Space Administration

NPD 8730.5 NASA Quality Assurance Program Policy

NPR 7120.5 NASA Space Flight Program and Project Management

Requirements

NPR 8705.4 Risk Classification for NASA Payloads

NASA-STD-8739.6 Implementation Requirements for NASA Workmanship

Standards

2.3 Non-Government Documents

AMS-DTL-23053/5 Insulation Sleeving, Electrical, Heat Shrinkable, Polyolefin, Flexible, Crosslinked

AMS-DTL-23053/6 Insulation Sleeving, Electrical, Heat Shrinkable, Polyolefin, Semi-Rigid, Crosslinked

AMS-DTL-23053/7 Insulation Sleeving, Electrical, Heat Shrinkable, Polyethylene

Terephthalate, Non-Crosslinked

AMS-DTL-23053/8 Insulation Sleeving, Electrical, Heat Shrinkable, Polyvinylidene

Fluoride, Semi-Rigid, Crosslinked

AMS-DTL-23053/11 Insulation Sleeving, Electrical, Heat Shrinkable, Fluorinated

Ethylene Propylene, Non-Crosslinked

AMS-DTL-23053/12 Insulation Sleeving, Electrical, Heat Shrinkable, Polytetrafluoroethylene

J-STD-001F Requirements for Soldered Electrical and Electronic Assemblies

J-STD-001FS Space Applications Electronic Hardware Addendum to J-STD-

001F Requirements for Soldered Electrical and Electronic

Assemblies

2.4 Order of Precedence

This Standard establishes requirements for (enter specific purpose from Scope) but does not supersede nor waive established Agency requirements found in other documentation. Conflicts between this Standard and other requirements documents shall be resolved by the responsible

Technical Authority.

3. ACRONYMS AND DEFINITIONS

3.1 Acronyms and Abbreviations

The following acronyms apply to terms used in this Standard.

AC Alternating Current

ASTM American Society for Testing and Materials

AWG American Wire Gage

CVCM Collected Volatile Condensable Material

DC Direct Current

DWV Dielectric Withstanding Voltage

EMI Electromagnetic Interference

ESD Electrostatic Discharge

ESDS Electrostatic Discharge Sensitive

FEP Fluorinated Ethylene Propylene

GHz Gigahertz

GSFC Goddard Space Flight Center

IR Insulation Resistance

JPL Jet Propulsion Laboratory

NASA National Aeronautic and Space Administration

NASA-STD NASA Standard

NPR NASA Procedural Requirements

OD Outside Diameter

PET Polyethylene Terephthalate

PTFE Polytetrafluoroethylene

PVDF Polyvinylidene Fluoride

RF Radio Frequency

RFI Radio Frequency Interference

RH Relative Humidity

RMS Root Mean Square

SDS Safety Data Sheet

3.2 Definitions

The definitions listed below are in addition to those listed in NASA-STD-8709.22, Safety and

Mission Assurance Acronyms, Abbreviations, and Definitions.

Accessories, Connector: Removable mechanical hardware, such as cable clamps, backshells, and screws, that are parts of the connector assembly in a harness.

Removable mechanical hardware, such as cable clamps, backshells, and screws, that are parts of the connector assembly in a harness.

Adapter: An intermediate device to provide for attaching special accessories or to provide special mounting means.

Backshell: Are a connector accessory that are installed onto the rear connector accessory threads of plug or receptacle connectors to provide mechanical protection to the individual harness wires entering the back of the connector.

Barrel (Contact Wire Barrel): The section of contact that accommodates the stripped conductor.

Birdcaging: The radial expansion of individual strands in a stranded conductor

(bowing outward) that can occur in the exposed portion of the conductor between the insulation strip and termination point.

Braid: A fibrous or metallic group of filaments interwoven to form a protective covering over one or more wires.

Breakout: The separation of a conductor or group of conductors from the main body of wires in a harness.

Bubble Pack: A laminated plastic sheet that is formed with patterned air entrapment

("bubbles"). The bubbles provide excellent cushioning for anything enclosed between layers of the material.

Cable: A shielded single conductor or a combination of conductors insulated from one another (multiple conductor).

Cable, Coaxial: A cable in which an insulated conductor is centered inside another.

The outer conductor is usually a metal braid or metal sheath. Braided cables usually have an outer insulating jacket over the braid. Coaxial cables are used primarily for transmission of RF signals.

Cable, Shielded: One or more insulated conductors covered with a metallic outer covering, usually a metal braid.

Cable Clamp: A mechanical clamp attached to the wire entrance of a connector to support the cable or wire bundle, provide stress relief, and absorb vibration and shock.

Certification: The act of verifying and documenting that personnel have completed required training, have demonstrated specified proficiency, and have met other specified requirements.

Cold Flow: Movement of insulation (e.g., Teflon) caused by pressure.

Conductor: A lead or wire, solid, stranded, or printed wiring path serving as an electrical connection.

Connector, Body: The main portion of a connector to which contacts and other accessories are attached.

Connector, Grommet: An elastomeric seal used on the cable side of a connector body to seal the connector against contamination and to provide stress relief.

Connector, Insert: The part of a connector that holds the contacts in position and electrically insulates them from each other and the connector body.

Contact: The conductive element in a connector or other terminal device that mates with a corresponding element for the purpose of transferring electrical energy.

Contact, Crimp: A contact whose crimp barrel is a hollow cylinder that accepts the conductor. After a conductor has been inserted, a tool is used to crimp the contact metal firmly onto the conductor.

Contact, Insertable/Removable: A contact that can be mechanically joined to or removed from an insert. Usually, special tools are used to insert (lock) the contact into place or to remove it.

Contact, Pin: Male-type contact designed to slip inside a socket contact.

Contact Retention: The axial load in either direction that a contact can withstand without being dislodged from its normal position within an insert or body.

Contact, Socket: A female-type contact designed to slip over a pin contact.

Contaminant: An impurity or foreign substance present in a material that affects one or more properties of the material. A contaminant may be either ionic or nonionic. An ionic, or polar compound, forms free ions when dissolved in water, making the water a more conductive path. A nonionic substance does not form free ions, nor increase the water's conductivity. Ionic contaminants are usually processing residue such as flux activators, finger prints, and etching or plating salts.

Crimp: The physical compression (deformation) of a contact barrel around a conductor to make an electrical and mechanical connection to the conductor.

Crimping: A method of mechanically compressing or securing a terminal, splice, or contact to a conductor.

Drain Wire: A wire that runs linearly along a foil shield wire or cable and is used to make contact with the shield. Grounding of foil shields is done with drain wires.

Electromagnetic Interference: The unwanted intrusion of electromagnetic radiation energy whose frequency spectrum extends from subsonic frequency to X-rays.

Ferrule: A short metal tube used to make crimp connections to shielded or coaxial cables.

Fillet: A smooth concave buildup of material between two surfaces; e.g., a fillet of solder between a conductor and a solder terminal.

Grommet: An insulator that covers sharp edges of holes through panels and partitions to protect wire insulation from cut-through damage.

Harness: One or more insulated wires or cables, with or without helical twist; with or without common covering, jacket, or braid; with or without breakouts; assembled with two or more electrical termination devices; and so arranged that as a unit it can be assembled and handled as one assembly.

Insertion Tool: A device used to install contacts into a contact cavity in a connector insert.

Interfacial Seal: A sealing of mated connectors over the whole area of the interface to provide sealing around each contact.

Jacket: The outermost layer of insulating material of a cable or harness.

Joint: A termination.

Mate: The joining of two connectors.

Molding: The sealing of a connector backshell area or a cable breakout with a compound or material that excludes moisture and provides stress relief. The material is injected into molds that control its configuration.

Outgassing: The release of a volatile part(s) from a substance when placed in a vacuum environment.

Radio Frequency: The frequency spectrum from 15 kHz to 100 GHz. Cables are seldom used above 18 GHz.

Radio Frequency Interference: Electromagnetic radiation in the radio frequency spectrum from 15 kHz to 100 GHz.

Repair: The action on a nonconforming product to make it acceptable for the intended use.

Rework: The action on a nonconforming product to make it conform to the requirements.

Sealing Plug: A plug that is inserted to fill an unoccupied contact aperture in a connector. Its function is to seal an unoccupied aperture in the assembly, especially in environmental connectors.

Shielded Cable: Cable surrounded by a metallic covering intended to minimize the effects of electrical crosstalk interference or signal radiation.

Shielding: The metal covering surrounding one or more conductors in a circuit to prevent interference or signal radiation.

Solder: A nonferrous, fusible metallic alloy used to join metallic surfaces.

Soldering: The process of joining clean metallic surfaces through the use of solder without direct fusion of the base metals.

Solder Sleeve: A heat-shrinkable solder termination device with meltable sealing preforms at ends.

Splice: The joining of two or more conductors to each other.

Spacecraft: Devices, manned or unmanned, which are designed to be placed into a suborbital trajectory, an orbit about the earth, or into a trajectory to another celestial body.

Strain Relief: A connector device that prevents the disturbance of the contact and cable terminations.

Stranded Conductor: A conductor composed of a group of smaller wires.

Stress Relief: The formed portion of a conductor that provides sufficient length to minimize stress between terminations.

Strip: To remove insulation from a conductor.

Supplier: In-house NASA, NASA contractors, and subtier contractors.

Tang (Connector Backshell): A backshell tang is a tapering metal projection (straight, 45°, or 90° to the axis of the connector) designed to accommodate cable-tie attachments. The cable-ties grip and hold harness wires exiting from the connector, thus providing stress relief for the wires.

Tines: Tines are the members of a contact retention system that capture or "lock" removable crimp contacts into the contact cavities.

Wicking: A flow of molten solder, flux, or cleaning solution by capillary action.

Wire: A single metallic conductor of solid, stranded, or tinsel construction, designed to carry currents in an electrical circuit. It may be bare or insulated.

Wire Dress: The arrangement of wires and laced harnesses in an orderly manner.

4. REQUIREMENT

4.1 General

4.1.1 When there is a conflict between the requirements of NASA-STD-8739.6 and those herein, the requirements in NASA-STD-8739.6 shall take precedence.

4.1.2 A program shall be established to assure continuing process capability.

4.1.3 Special controls shall be developed for process parameters and equipment settings that influence product compliance with critical performance and quality requirements.

5. TRAINING AND CERTIFICATION PROGRAM

5.1 General

This section has been superseded by NASA-STD-8739.6. Refer to NASA-STD-8739.6, Chapter

5 and Appendix A therein for applicable training and certification requirements.

6. FACILITIES, EQUIPMENT, MATERIALS, AND PARTS

6.1 Environmental Conditions

6.1.1 The environmental control requirements of NASA-STD-8739.6 shall apply.

6.1.2 Lighting. Light intensity shall be no less than 1077 lumens per square meter (lm/m2)

(100 foot-candles) on the surface where cabling and wiring are being assembled, inspected, or tested.

6.2 Tool and Equipment Control

6.2.1 The tool and equipment selection, use and control requirements of NASA-STD-8739.6 shall apply. See NASA-STD-8739.6 for metrology and calibration requirements.

6.2.2 The supplier’s process documentation for tool and equipment use and control shall be available for review and approval prior to processing mission hardware.

6.2.3 The following requirements apply for wire strippers:

a. Either precision mechanical tools or thermal strippers shall be used for insulation stripping.

b. Thermal strippers shall have variable temperature control.

c. The tools shall not nick, ring, gouge, or stretch conductors or remove plating so that the base metal shows. Superficial scraping of conductors is acceptable providing conductor base material is not exposed.

d. The cutting edges of wire trimming tools shall be maintained sharp and free from nicks and indentations.

6.3 Electrostatic Discharge Control Requirements

When Electrostatic Discharge (ESD) control is required during workmanship operations defined herein, ESD Control shall be in accordance with Chapter 7 of NASA-STD-8739.6.

6.4 Inspection Optics

6.4.1 The inspection optics shall be in accordance with Chapter 6 of NASA-STD-8739.6.

6.4.2 Visual inspection shall be aided by magnification between 4X and 10X.

6.5 Parts and Materials Selection

6.5.1 Specifications used to order connectors may require special instructions to ensure grommets and gaskets are a non-outgassing type. When connectors cannot be special-ordered to exclude grommets and gaskets which do not meet project outgassing requirements in their as-delivered condition, instructions shall be provided to the Workmanship operator for removing or replacing those connector subcomponents.

6.5.2 Materials used shall be subjected to NASA approval.

6.5.3 In order to satisfy the crimp contact pull and crimp ring testing requirements in paragraph 12.3 and 13.7, respectively, for connector types which are sold without sufficient spare contacts or crimp rings, suppliers will need to purchase extra contacts or crimp rings. The number of extra parts will be determined by the number of shifts or production periods in which a given type of contact or crimp ring is used. Lot traceability for contacts obtained for crimp testing shall be maintained between the contacts used in the connector and the contacts used for pull testing.

6.5.4 For nickel-plated wire, or silver-plated wire in applications above 150º C, nickel-plated crimp barrels are preferred.

6.5.5 Materials not conforming or not required for the operations involved for the processes described herein shall be removed from the work area or tagged non-usable.

6.6 Solvents and Cleaners

6.6.1 The requirements of 6.7 of NASA-STD-8739.6 apply for the selection of solvents.

7. DESIGN PRACTICES

7.1 General

7.1.1 Harness design shall make provision for all Project performance requirements as they apply to each harness section as well as the following:

a. Installation stress and final lay (e.g., ease of bending, flexibility in twisting, stress relief).

b. Electrical isolation.

c. Ability to fit into confined spaces or near or around mechanical edges.

d. Thermal management.

e. Current and voltage derating.

f. Voltage drop.

g. Repairability and reliability (e.g. redundancy, not using connector contact locations, spare pin requirements, arc tracking).

h. Identification of connectors and cable and harness segments.

7.1.2 Precautions shall be taken to prevent the mismating of connectors, caused by interchanging or by reversing, through one of the following techniques:

a. Use of constraints that locate similar connectors built into interconnecting cables and harnesses so they cannot be interchanged.

b. Selection of different sizes for connectors to be located adjacent to each other.

c. Polarization or dissimilar keying of adjacent, similar connectors.

d. Ensure clarity in marking and coding connectors.

e. Use of confidence loop circuits to check out proper mated positions.

7.2 Design Considerations

The following considerations shall be taken in any interconnecting cable or harness design and incorporated into the design as applicable:

7.2.1 Properties of wire insulation, electrical contacts, lacing tape, braid sleeving, plastic strap, wrap sleeving, and plastic tubing (processibility, flammability, arc tracking resistance, vacuum stability, resistance to heat, cold flow, etc.) shall be appropriate for the application.

7.2.2 Plastic straps should have metal tangs that lock securely into the "ribbed" portion of the straps when appropriate for the application.

7.2.3 Tin-plated parts (e.g., terminals, crimp barrels, etc.) shall be fused or alloyed with tin-lead plating. See NASA-STD-8739.6 for lead-free (Pb-free) control requirements.

7.2.4 Methods for identifying cables, connectors, and wires shall be provided.

7.2.5 Design features shall address radio frequency interference/electromagnetic interference

(RFI/EMI) shielding performance requirements.

7.2.6 High Voltage (HV) circuits carrying potentials in excess of 200VACRMS, or 300VDC through environments that support arcing shall be terminated in single-contact, high voltage connectors.

7.2.7 Electrical wiring of redundant systems, redundant subsystems, or redundant major elements of subsystems shall not be routed in the same bundle or through the same connector with wiring of the other system, subsystem, or subsystem element.

7.2.8 American wire gage (AWG) 24 wire size and larger is preferred for conductors in interconnecting cable and harness assemblies, including coaxial or triaxial cables.

7.2.9 High strength copper alloy shall be used for AWG 24 and smaller conductors.

7.2.10 Torque values applicable for connectors, backshells, and other hardware shall be defined.

7.2.11 Harness designs and routing shall provide support and protection to harnesses and cables so that they are protected from abrasion, cold flow, cut through, vibration, chafing, flexing, and sharp edges.

7.2.12 Designs shall minimize splicing.

7.2.13 Semi-rigid coaxial cable shall be preconditioned using thermal cycling prior to preparation for connectorization. This procedure should also be considered for longer runs of flexible coaxial cable.

7.2.14 Line voltages shall be confined to connectors with sockets to preclude exposing voltage points when connectors are disconnected.

7.2.15 Specifying the use of sealing plugs and unused contacts in environmental connectors.

7.2.16 When necessary to support electrical performance or mechanical flexibility around routing angles, wire dress and lay configurations shall be specified in the manufacturing instructions. Fabricate cables containing discrete wires in one or more layers by winding the wires together uniformly.

7.2.17 Whether successive twisted layers are twisted contrahelically or unidirectionally is optional. Winding shall prevent the introduction of residual twist into individual conductors.

7.2.18 The length of lay for each twisted layer shall be between 8 and 16 times the outer diameter of the harness.

7.2.19 The bend radius data given in Table 7-1 shall apply for bending that occurs in the installed interconnecting harness or cable.

7.2.20 Wires exiting from connectors shall be stress relieved.

7.2.21 The use of solder sleeves (where wire insulation temperatures permit), hand soldering, or crimp rings are acceptable for terminating individual shields.

7.2.22 Concern for placement of power and ground lines in contact assignments for system safety.

7.2.23 Materials for potting connectors suited for applications.

7.2.24 Selection of metal braid shielding should be sized as appropriate per application.

7.2.25 Splicing. Damaged or broken conductors shall not be spliced.

7.2.26 The following requirements apply for crimped connections.

a. Crimped contacts shall be used with stranded wire only.

b. Solid wire shall not be used with crimped contacts.

c. Solid, tinned wire shall not be used in crimped contacts.

7.2.27 Associated materials, parts, and hardware shall be selected to enable the cable and harness to meet performance requirements.

7.2.28 Provision shall be made for stress relief in wires entering connectors from harnesses

(e.g. connector backshells, encapsulation, and stress relief boots).

7.2.29 Support of wiring, wire bundles, and harnesses shall be used to minimize shock and vibration induced stress.

7.2.30 Excessive flexing or pressure on the harness over sharp or rough edges shall be prevented.

7.2.31 Harness and cable protection shall be added in areas where sharp or rough edges are present and abrasion could occur.

7.2.32 The use of splices in a harness design should be minimized as much as possible. The simplest and most reliable wiring design is one that results in the routing of a dedicated, continuous, and unbroken conductor from point to point. The following shall be incorporated into any splice design:

7.2.33 Shrink sleeving over the splice area shall be protected from cold flow and abrasion.

8.1.2 Permanent bends and offsets shall be built into harnesses so that the final wire dress will not be under continuous stress and tension after installation.

8.1.3 Connector backshells shall accommodate bends and offsets in wire harnesses, as appropriate, to avoid continuous stress.

8.1.4 The layout shall be designed to limit the amount of bending, pulling, and other handling a harness will receive during installation.

8.2 Mockup and Wiring Board Design Parameter

8.2.1 A full-sized, three-dimensional (3-D) form layout fixture shall be provided for all complex interconnecting cables and harnesses to ensure proper routing, wire lengths, connector configurations, support requirements, and access requirements of the wiring harnesses. The form layout fixture may be limited to partial installations which contain the complex wiring harnesses.

Typical harness board layout and typical hardware and fixtures are shown in Figures 8-1 and 8-2, respectively.

8.3 Temporary Identification

8.3.1 Temporary identification markers may be used for in-process identification requirements. All temporary markers shall be removed from completed cabling and harnessing and shall not leave a contaminating residue.

8.4 Interconnecting Cable and Harness Protection

8.4.1 The supplier shall establish and implement procedures to protect interconnecting cables and harnesses from damage and degradation.

8.4.2 Connectors not being actively assembled shall be individually protected by wrapping them in bubble pack or other physical covering.

8.4.3 At the end of the work shift, protective covering shall be spread over the harnesses in fabrication.

8.4.4 Harnesses not in active fabrication (those in temporary storage) shall be covered by protective covering.

Figure 8-2. Typical Harness Board Hardware and Fixtures

9. FORMING WIRES INTO CABLES AND HARNESSES

9.1 General

9.1.1 Wiring shall be assembled in interconnecting cables or harnesses as described herein.

9.1.2 Fabrication methods and assembly techniques that assure the production of high quality interconnecting cables and harnesses shall be used.

9.2 Lacing for Trunk, Branches, and Breakouts

9.2.1 When engineering documents specify the use and type of lacing, the following requirements shall apply.

9.2.2 Lacing tie-ends shall be trimmed.

9.2.3 When knots are staked (see Figure 9-7), the necessary compounds, as well as any special design requirements, shall be specified.

9.2.4 The following requirements apply for the starting stitch:

a. Harnesses laced with single tape shall initially be tied with a starting stitch.

b. Single-tape starting stitches shall be the same as a spot tie with a running end or as shown in Figure 9-1 View A.

c. Starting stitches for double lacing shall be as shown in Figure 9-1 View B.

d. Starting stitches shall not place stress on wire terminations.

9.2.5 Spot Ties. Spot ties shall consist of a clove hitch followed by a square knot as shown in

Figure 9-2 or other non-slip knots. See Table 9-1 for spot tie spacing.

9.2.6 The following requirements apply for the closing stitch:

a. Single or double lacing tape shall be terminated with a closing stitch as shown in Figure

9-3.

b. Lacing shall be terminated at every major breakout or branch and at the extremity of the harness (Major breakouts normally contain a large percentage of the wire volume, such as 25 to 30 percent or more).

c. The stitching shall terminate close to the extremity of the harness but shall not stress the wire terminations.

d. Closing and starting stitches at branches and breakouts shall be next to the breakout. An alternate closing stitch method is shown in Figure 9-4.

e. Single or small multiple breakouts of two or three wires need not have closing and starting stitches, but may have running lockstitches on each side (Figure 9-5).

Figure 9-3. Closing Stitch and Single Tape—Illustration

Figure 9-4. Alternate Closing Stitch and Single Tape—Illustration

Figure 9-5. Running Lockstitch

9.2.7 Running Lockstitch. Continuous lacing shall be achieved using running lockstitches as shown in Figure 9-5.

9.3.2 Braided sleeving shall be snugly dressed down over the wire bundle.

9.3.3 Continuous braid sleeving shall be secured at the ends by plastic cable straps, spot ties, clamps, heat shrinkable sleeving, or a potting material.

9.3.4 When secured, the covering shall not slide freely.

9.3.5 Sleeving shall be trimmed and terminated at a breakout but shall not be punctured or slit to provide openings for breakout.

9.3.6 After installation, braided sleeving shall be secured or treated in one of the following ways to eliminate fraying or unraveling.

Figure 9-7. Securing Fabric Braid Sleeving

9.3.7 Braided sleeving may be secured by a spot tie or plastic cable strap. The end of the braid shall be tucked under and secured with a spot tie or plastic strap (Figure 9-7). The end of the braid may be secured by connector clamps, other hardware, or potting.

9.3.8 The ends of glass braids may be bonded using an adhesive. Excessively frayed glass braid material shall be trimmed away prior to application of the adhesive.

9.3.9 When the adhesive used to bond the ends of glass braid is dry, the braid shall be secured by spot tying or other means so that it does not slip on the wire bundle.

9.3.10 Polyamide (nylon) braids (for use on ground service equipment) may have their ends sealed by use of a "hot knife" or similar instrument.

9.4 Fabric Braid Directly Woven on Interconnecting Harness or Cable

9.4.1 Fabric braids woven directly on interconnecting harnesses or cables may be loose or tight, as necessary to produce the degree of flexibility required. The braid shall be smooth and shall provide coverage that leaves no gaps through which the wires can be seen.

9.4.2 No frayed ends of fabric braids shall be visible.

9.4.3 All pigtails shall be secured.

9.4.4 Braids applied tightly shall not terminate so close to connectors that they stress wires attached to solder cups.

9.4.5 Spot ties, plastic straps, lacing, and other temporary holding means shall be removed from wire bundles prior to braid application.

9.4.6 Flat tapes may be left under braid if the tape has a low profile. Typical braiding techniques are shown in Figures 9-8, 9-9, and 9-10.

Figure 9-8. Starting Lock

Figure 9-9. Forming Ending Pigtail

Figure 9-11. Spiral Wrap Sleeving

9.6 Plastic Straps

9.6.1 Installed straps shall be locked to prevent them from loosening or opening.

9.6.2 Straps shall be placed on both sides of a breakout; otherwise, spacing between straps shall be as required by Table 9-1.

9.6.3 The "ribbed" side of a strap shall always be placed against the wires.

9.6.4 Straps shall be tightened so that they do not slide back and forth on the assembly;

however, they shall not be so tight as to cause noticeable indentation or distortion of the wires in the harness. Proper strap orientation is shown in Figure 9-12.

9.6.5 Plastic straps are usually installed by tooling. Tooling shall be tension-controlled to meet the strap-tightening requirements previously stated.

9.6.6 Surplus strap ends shall be trimmed flush at the back end of the strap head (this is done automatically by most tooling).

Figure 9-12. Plastic Strap Orientation

9.7 Metal Braid Shielding

9.7.1 Metal braid shielding can either be woven directly over a core or obtained in prebraided form and installed by sliding it over the wire bundle. To prevent potential damage (cold flow) of the underlying wire insulation, a separator (e.g., a tape) shall be applied over the wire bundle to give the wire continuous protection.

9.7.2 Prewoven braiding shall be tightened down to contact the wire bundle.

9.7.3 Braids shall be terminated as specified by the engineering documentation.

9.7.4 Prewoven metallic braid shall be cleaned in a suitable solvent to remove contamination prior to installation over the harness (see Chapter 15).

9.8 Insulation Sleeving/Tubing

9.8.1 Insulation sleeving shall be installed on all terminations that are not otherwise insulated or potted, except those at ground potential (e.g., overall shield and coaxial cable terminations).

9.8.2 Insulation sleeving shall be installed to meet the dimensional requirements of the applicable drawing or specification.

9.8.3 The following requirements apply to heat-shrinkable sleeving:

a. The heat-shrinkable sleeving selected shall be larger than the maximum diameter of the object being covered, and after shrinking it shall provide a tight fit over the object in the area of maximum diameter. This will cause the sleeving to have a tight mechanical grip on the item it covers. A guide for selecting sleeving sizes is given in Table 9-3.

b. There shall be no partially or incompletely shrunken areas.

c. Sleeving may have a slight surface crazing, but it shall be free of cracks, punctures, and charred or burned areas.

9.8.4 Tubing. Tubing shall be secured by spot ties or otherwise restrained to prevent them from sliding back and forth over the wire bundle.

9.8.5 Insulation tubing shall extend beyond the wire or cable insulation or jacket, respectively, a minimum of two times (2X) the diameter of the larger of the two. See 19.2.6 for this requirement as applied to splices.

9.9.2 The degree of heat used and the exposure time will depend on the size and type of sleeving. A typical shield termination is shown in Figure 9-13. Sleeving coverage dimensions are given in this figure. If a tight fit is required for the minimum diameter, additional sleeving may be required.

Note: CAUTION: EXTREME CARE SHOULD BE EXERCISED TO ASSURE THAT THE

AMOUNT OF HEAT APPLIED IS NOT DETRIMENTAL TO THE OBJECT BEING

COVERED. WHEN OTHER TEMPERATURE-SENSITIVE MATERIALS AND PARTS

ARE IN THE VICINITY, THEY SHOULD BE PROTECTED FROM EXPOSURE TO

DIRECT HEAT.

Figure 9-13. Illustration of Shrink Sleeve Installation (Typical)

9.10 Long Lengths of Shrinkable Sleeving

9.10.1 Long lengths of shrinkable sleeving installed over interconnecting harnesses and cables shall provide protective coverage of the designated area without leaving residual stress in the material.

9.10.2 The sleeving shall be in contact with the interconnecting harness or cable along its length. One method of controlling endwise shrinkage (reduction in length) of the sleeving during installation is shown in Figure 9-14.

Note: Medium to long lengths of harnesses/cables with shrink sleeving are extremely difficult to bend and coil without damage.

Figure 9-14. Installation of Long Lengths of Sleeving to Achieve Controlled Dimensions

9.11 Shrinkable Part Requirements and Installation

9.11.1 Shrinkable parts shall meet the following requirements (see Figure 9-15 for typical sleeving installation):

a. They shall be uniformly shrunk to assure a tight fit where required.

b. The part shall be positioned according to drawing or specification requirements.

c. The shrinkable sleeving shall firmly grip the item over which it has been installed.

d. The shrinkable part shall be free of cracks, punctures, blisters, and burned areas.

9.11.2 When shrinkable parts are installed, the gripping section of the part is shrunk first (For example, the gripping-end section of a connector backshell boot is shrunk over the connector before the smaller cable-gripping end is heated).

Figure 9-15. Sleeving Installation (Typical)

10. STRIPPING INSULATION FROM CONDUCTORS AND CABLE

10.1 Stripping Round Conductors

10.1.1 Insulated wires shall be prepared in accordance with the following requirements:

a. After removal of the insulation segment, the remaining conductor insulation shall not exhibit any damage such as nicks, cuts, crushing, or charring.

b. Conductors with damaged insulation shall not be used.

c. Scuffing from mechanical stripping or slight discoloration from thermal stripping is acceptable.

d. After removal of the conductor insulation, the conductor shall not be nicked, cut, or scraped to the point that base metal is exposed.

e. Conductors that were reduced in the cross-sectional area shall not be used.

f. The lay of wire strands shall be restored as nearly as possible to the original lay if disturbed.

g. The conductor shall be cleaned following restoration to the original lay.

10.1.2 The following requirements apply to tinning of conductors for solder cups:

a. The portion of stranded wires that will eventually become a part of the finished solder connection shall be coated with tin-lead solder and cleaned prior to attachment. Additional flux may be used.

b. The flux shall be applied so that it does not flow under the insulation except for traces carried by solder wicking.

c. Flux shall be removed with cleaning solvent applied so that its flow under the conductor insulation is minimal.

d. Wire strands shall be discernible after tinning.

e. Solder wicking shall not extend to a portion of the wire which is required to remain flexible.

f. Solder build-up or icicles within the tinned wire area shall not affect subsequent assembly steps.

g. The length of untinned strands measured from the end of the tinned portion to the beginning of the wire insulation shall not be greater than 1 wire diameter.

10.1.3 Insulation Irregularity. Mechanical or thermal stripped insulation irregularity is acceptable if it does not exceed 1/4 of the outside diameter of the wire, including the insulation.

10.1.4 Insulation Clearance. The following requirements apply:

a. Soldered Connections.

(1) The insulation shall not be imbedded in the solder joint.

(2) The contour of the conductor shall not be obscured at the termination end of the insulation.

(3) The insulation clearance shall be less than two wire diameters, including insulation, but in no case shall permit shorting between adjacent non-electrically common conductors.

(4) Insulation clearance shall be referenced from the first point of contact of the conductor to the terminal.

b. Crimped Connections.

(1) The minimum insulation clearance for all crimped connections shall be 0.25mm

(0.01in.).

(2) The maximum insulation clearance for conductors 20AWG and smaller shall be

0.75mm (0.03in.).

(3) The maximum insulation clearance for conductors 18AWG and larger shall be

1.25mm (0.05in.).

10.2 Stripping Jackets Over Shields

10.2.1 Jackets over shields may be stripped by either thermal or mechanical means.

10.2.2 Nicked shield strands shall not exceed 10 percent of the total number of strands.

10.2.3 There shall be no severed strands in the shield.

10.3 Stripping Flat Conductor Cable

10.3.1 Flat conductor cable shall be stripped by mechanical means.

10.3.2 Conductors shall not be nicked, gouged, necked down, or severed during stripping.

10.3.3 After removal of the insulation segment, the remaining conductor insulation shall not exhibit any damage such as nicks, cuts, or crushing. Scuffing from mechanical stripping is acceptable.

11. CABLE SHIELDING AND SHIELD TERMINATION

11.1 General

11.1.1 Interconnecting cables and harnesses shall be designed and constructed to minimize electromagnetic couplings between wires within the assembly that are sensitive to induced interference. The following cable styles are listed in order of increasing RFI/EMI control:

twisted pair, shielded wire, single-braid coaxial cable, double-braid coaxial cable, triaxial cable, twisted-shielded quadrax cable. For harnesses, grounded copper braid is the most effective RF shielding.

11.1.2 On RF signal cables, both the inner conductor and outer conductor braid shall be electrically continuous.

11.1.3 Cable and harness shields shall be mechanically terminated (e.g. crimp rings, compression band).

11.1.4 Cable and harness shields shall be electrically grounded in accordance with the manufacturing instructions (e.g. pigtail, conductive backshell).

11.2.3 The grounding wire detachment force from the crimped sleeve shall meet the requirements of paragraph 13.7.

Figure 11-3. Individual Shield Termination Using a Two-Piece Crimp Ring

11.3 Group-Grounding of Individual Shield Terminations

11.3.1 When grounding wires of individual cable shields are grounded to one point, they shall be spliced to a common grounding wire.

11.3.2 No more than four conductors plus a drain wire shall be terminated in one splice (see

Figure 11-4 and Figure 11-5).

11.3.3 For ordinary RFI/EMI protection, the shield shall be terminated within 100mm (4 inches) of the center conductor termination for the x-distance, and the combined length of shield grounding wires shall not exceed 190mm (7.5 inches) for the y-distance. For interference sensitive circuits, preferred lengths are 20 and 115mm (.75 inches and 4.5 inches). See Table 11-

1 for the x and y distances. When this does not provide adequate isolation, RFI/EMI connector backshells may be necessary.

Figure 11-5. Group-Grounding of Individual Shield Terminations

11.4 Large Compression Ring Grounding

11.4.1 Individual shields and overall shields may be grounded using large compression rings.

Shields should be dressed across the inner collector ring either forward towards the connector

(Figure 11-6B) or backwards away from the connector (Figure 11-6C). Grounding wires shall exit the front or rear of the compression ring as required by design, and shall be formed in accordance with Figure 11-6A.

11.4.2 Grounding exiting the side of the compression ring away from the rear of the connector shall be dressed beneath the compression ring when terminating to a connector pin. The outer compression ring shall be crimped securely over the shields and inner collector ring.

11.4.3 The grounding wire detachment force from the crimped sleeve shall meet the requirements of paragraph 13.7.

Figure 11-6. Large Compression Ring Grounding (Typical Applications)

11.5 Floating Shield Terminations

11.5.1 The outer jacket shall be trimmed to expose the shielding braid.

11.5.2 The braid shall be folded back over the outer jacket as illustrated in Figure 11-7.

11.5.3 Combing the braid is optional. Heat-shrinkable sleeving shall be installed over the trim point.

Figure 11-8. Conductor Exposure for Individual Shield Termination Types

12. CRIMP CONNECTIONS

12.1 General

12.1.1 Crimped connections shall be inspected 100 percent in order to verify compliance with this standard.

12.1.2 Insulation clearances shall be as specified in paragraph 10.1.4.b.

12.2 Examination of Crimp Contact

12.2.1 For manual crimping operations prior to wire insertion, the assembler shall examine the crimp contacts for the conditions in 12.2.2.

12.2.2 Contacts that show the following conditions shall be prohibited:

a. Cracks in the plating or base metal.

b. Tarnishing or discoloration of the plating.

c. Plating removal or flaking.

d. Out-of-roundness of the wire well entrance.

e. Exposed base metal.

12.3 Process Controls

12.3.1 Tool and Equipment Control.

a. Full-Cycle Ratcheting. Crimp tools shall contain a full-cycle ratcheting mechanism which shall prevent the indenters from releasing before the crimp cycle has been completed.

b. Number of Indenters. Each crimp tool shall have four or more indenter blades

(preferably double-indenter blades).

c. Calibration Adjustments. The calibration adjustments shall be accessible only when the tool is disassembled.

d. Calibration adjustments shall be made only by the manufacturer of the tool or by a calibration laboratory.

e. Sealing of Adjustable Tools. All adjustable crimp tools shall be set, sealed or locked, and verified prior to use.

12.3.2…