Crimping Pin and Socket Contacts
CRIMPING
Crimping may be defined as the art of joining a conductor to a pin or socket contact by controlled compression and displacement of metal.
It has been used for many years.
In a good crimp joint, there is a mutual metal flow causing symmetrical distortion of wire strands and contact material. The mil cross-sectional area is ut slightly reduced and all voids are practically eliminated.
Such a joint is similar to a cold weld. Mechanical strength and good electrical continuity are established. Because of the new environments to which electrical connectors are subjected, there has been a drastic change in thinking relative to the use of precision crimp joints in preference to solder.
CRIMPING CONFIGURATIONS
There are many different types of crimps employed today. These range from the terminal fold-over tab type of crimp to the single indent crimp, the dual indenter crimp, the three indenter crimp, hex crimps, and, finally, the MIL standard four indenter crimp. The four indenter crimp (Fig. 1) provides the most uniform displacement of wire and contact material. The wire strands and the contact material are formed together in a solid mass with little or no reduction of the mil area of the wire strands. A minimum of voids exists and very little extrusion of the wire strands takes place.
The four indenter crimp principle has been used to produce a variety of impressions, the most common being the “bathtub” and “octadent” (also called double indent) (Fig. 2) The octadent configuration has
been chosen by the Military for use in the M22520/1 and /2 tools.
CRIMPING CHARACTERISTICS
Connectors utilizing crimping contacts usually permit the removal of these contacts several times so that modification, circuit changes, or replacement of contacts may be made with little difficulty and with the same quality assurance as in production line assembly. Crimping may be accomplished either with hand tools, power tools, or automated power tools. Repeatability of the crimp operation is characteristic provided precision crimping tools are employed. These tools must be capable of being gaged to insure that proper crimp depths are maintained. Inspection holes in each contact permit quality control personnel to view the wire strand ends thereby assuring that the conductor is properly positioned in the crimp barrel.
MIL-DTL-22520
This specification covers all the requirements for crimping tools used on removable type contacts in electrical connectors.
CRIMP DEPTH DETERMINATION
Having resolved an indenter design, the determination of crimp depth range must be established for each application. There are many factors which contribute to the selection of the proper indenter setting. These are primarily related to contact material and dimensions as well as wire type and size.
The proper crimp depth for a given contact is the one that yields the best mechanical and electrical joint. To determine this setting, many contacts of the same type are crimped though a range of indenter settings from too loose to too tight. The crimped contacts are then subjected to tensile and voltage drop tests.
WIRE PREPARATION
Proper wire preparation also plays an important part in making a good crimp joint. There are two popular methods of wire stripping — mechanical and thermal. During the mechanical stripping process, extreme care must be taken to avoid nicking or removing wire strands, otherwise a loss of tensile strength will result. Conversely, if the insulation is not completely removed, erratic values may be obtained. Heat stripping eliminates the danger of nicking strands. However, depending on the type of insulation being stripped, too much heat can cause actual charring of the insulation or decompose the insulation with the evolution of corrosive gases which react with the conductor platings. There is also a possibility of local annealing of the conductor. Too little heat can deposit an insulation film which can act as a lubricant. Any of these conditions can affect tensile results. Wire preparation is, therefore, another area that requires control if proper tensiles are to be achieved with a wire-contact combination.
Before making a tensile test it is also important that the stripped length of the wire be checked to ensure that the wire extends all the way into the contact wire barrel. During the tensile test it is necessary for the uncrimped end of the wire to be held in such a way that the pull force is evenly distributed to all the strands.
TENSILE TESTING
Tensile testing is a controlled pull test on the crimp joint to determine its mechanical strength. It is a destructive test which usually results in wire breakage in the crimped barrel, the wire pulling out of the crimped barrel, or wire breakage outside of the crimped area. The method and device used to conduct this test have a direct bearing on the results obtained. Per specification, the testing device pulls at the rate of one inch per minute. During the tensile test, the wire is elongating. The breakage or separation point, therefore, is associated with not only the pull force but also the rate of increase of this force.
Tensile curves are plotted for each contact and wire combination. They will usually differ, depending on the type of wire, plating, size of wire, and ariations in contact design and material. A desirable tensile range must be determined for each of these combinations.
MILLIVOLT DROP
Millivolt drop tests are performed across the crimp joint to determine the electrical characteristics. The test current is passed through the contacts and voltage drop is measured from a point on the shoulder of the contact to a point on the wire. Voltage drop values under the maximum allowable indicate a good electrical joint.
VISUAL INSPECTION
Each contact is inspected under a microscope to make certain the indenture does not crack or tear the base metal, or cause excessive distortion of the contact.
CONTROLLING CRIMP DEPTH
From the tensile curves, a known crimp depth range is established. It is imperative, therefore, that the crimp tool settings be within the established tolerance.
To insure full closure of the tool handles and positive bottoming, it is necessary that tools be cycle controlled. This is accomplished by the use of a precision ratchet device which releases the handles at the positive bottoming position within specification tolerances. This release point and positive bottoming are applicable to all contact sizes.
MEASURING CRIMP DEPTH (GAGING)
Too loose a crimp setting will result in wire pullout and high millivolt drop (high resistance). Too tight a setting will nick the wire strands causing low tensiles and wire breakage within the contact.
Positive bottoming tools can readily be gaged by selecting gage pins dimensioned to the end limits of the known crimp range of a given contact.
AXIAL DEFORMATION
During the crimping process considerable force is applied and material displacement takes place, which may result in axial deformation of the contact. The following factors contribute to axial deformation of contacts:
- Contact material and contact hardness.
- Crimp barrel wall thickness.
- Concentricity of conductor hole to O.D. of crimp barrel.
- If an insulation support is included on the contact, the concentricity of this support (I.D. and O.D.) with respect to the other diameters in the contact.
- Crimp depth - the deeper the crimp the greater the possibility of contact bending.
- Conductor characteristics - conductor hardness, number of strands, size of wire, bunching of strands, the lay of the conductors, plating or the use of solid conductor.
- The condition of the indenters - indenters which are not uniformly dimensioned or aligned or which have extreme variatio in surface Condition can cause contact bending.
- The condition of the rimping tool - a worn crimping tool can contribute to contact bending.
- Method of contact location and support - improperly supporting or positioning the contact in the tool can result in contact bending.
- Method of measuring axial deformation - we have found that this is one of the least understood items relating to the crimp tool specification.
MIL-DTL-22520 is specific in defining and evaluating the axial deformation of contacts. This paragraph allows the following deformation:
Contact Size |
Contact Deformation |
20 & smaller |
.011 TIR |
16 |
.012 TIR |
12 |
.012 TIR |
The TIR allowed includes a maximum of .005 TIR assignable to the contact during its manufacture. (TIR is an abbreviation for Total Indicator Reading and is a measure of the total deviation from a true center line when the item being measured is rotated through 360 °.)
COMPRESSION FORCES
Crimping compression forces are directly related to: A. Indenter Configuration; B. The Amount of Leverage in a Crimping Tool; C. Crimp Depth Required for Satisfactory Results; D. Contact Hardness and Contact-Conductor Combinations.
A. Indenter Configuration
MS drawings are specific as to indenter configuration of the Class I crimping tool. It is possible to change the shape of the indenters to reduce frontal area and thus reduce crimping forces. If the reduction of compression forces was the only factor involved, a knife blade edge on an indenter, or a conical tip shape would be the most desirable configuration. But this would result in cracked contacts, damage to plating, high wire embrittlement because of the concentrated stress of a small crimp area, and would also result in marginal tensile values.
B. The Amount of Leverage in a Crimping Tool
Leverage or linkage systems could be devised to minimize the amount of crimp compression forces. Archimedes’ old adage could apply here wherein he says, “Give me a place to stand and to rest my lever on and I can move the Earth.” From a practical viewpoint, however, the geometry of Class I tools under MIL-T-22520 are specific in tool length and width.
C. Crimp Depth Required for Satisfactory Results
Another way to reduce compression forces is to vary crimp depth. MS drawings are specific in designating crimp depths. It is understandable that the less the indenters indent the lower the compression forces involved. On the other hand, if the tool does not indent as deeply as specified, the possibility exists that sub-marginal or marginal tensile values will result.
D. Contact Hardness and Contact-Conductor Combinations
Contact material is definitely a factor contributing to high compression forces. Some contacts are made of hard material; some contacts have thick walls and some contacts are required to cover a range of conductors, all of which could involve high crimping forces. It is felt that an analysis of these conditions and an attempt to make them compatible with the crimping tool could facilitate the reduction of compression forces.
As can be seen from this brief review of crimping, many factors influence the effectiveness of a crimped joint. However, a good crimping tool compensates for many of these factors by providing proper crimp depths, resulting in termination having high tensile strength, low millivolt drop, and minimum contact deformation. With the use of a well-engineered tool, crimping becomes one of the most reliable methods of wire termination.