Introduction

Design Concept

Wire Loading Characteristics

Test Methodology

Conclusions

In many cases multi-wire planar cable is used to provide very cost effective mass termination. However, discrete multi-wire cables are also used in cost reduction efforts since the cable preparation and contact insertion steps are eliminated. These types of applications provide rapid assembly of high density harnesses at reduced cost. IDC harnesses have been found to produce low defect rates during assembly and excellent performance in service.

The advantages of this technology are low applied cost and high reliability. One disadvantage is the restriction on connector geometry. Usually a rectangular shape with a double row of contacts provides the optimum form factor for this system. In addition, cable strain relief is required to assure success in the field as motion at the wire/terminal interface can create instability. Very often dual slots, and in some cases wire insulation grips, are needed in applications where high mechanical l stresses occur.

Design Concept

The essential difference between a crimp and an IDC contact is the way in which wire deformation is achieved. With crimps, the pre- stripped wire and terminal are severely deformed under high pressure crimping dies to break through oxides and achieve metal to metal contact. This involves plastically deforming the terminal and axially extruding the wire by applying a relatively high force per contact. Usually cold welding is produced at the asperity level while very little elastic energy is stored in the terminal system. The critical dimensions for crimped contacts are the tolerances on crimp heights achieved with the crimping tool (as shown in Figure 1 below). This requires careful set up and continuous monitoring to maintain crimp height quality as a function of time.

In contrast, much lower forces are needed for IDC terminations. In this case, the insulated wire is pressed into a slot that is designed to displace the insulation and remove oxides by deforming the wire with shear forces that produce localized plastic deformation. This is done in one motion and provides a gas tight high pressure interface between the wire and terminal. A robust IDC system is designed to store substantial energy in the terminal as the latter acts as a spring member during and after termination.

In IDC terminations, slot width and insertion depth are important. The slot width dimension is easily controlled to tenths of mils in the blanking process. In addition, wire insertion is accomplished with a tool that provides easy control of the insertion depth. As insertion depth tolerances are typically several mils, termination quality can be monitored by visual examination. This is relatively easy to accommodate in a production environment and therefore offers an additional advantage over crimping.

Performance Characteristics

Crimps work well in the field because metal to metal contact is generated during crimping and a small amount of elastic energy is stored in the wire due to axial extrusion. As time goes on, if the crimp joint remains in a mechanically stable condition, additional diffusion welding can improve the interface. However, stress relaxation and creep in the terminal/wire system are processes which tend to degrade mechanical stability. Thus, depending on the mechanical design, the latter processes may eventually cause degradation. If the interface exhibits marginal strength initially and is weakened due to vibration and/or stress relaxation, then mechanical instability may limit field life.

The mechanical stability of IDC terminations depend on the spring properties of the terminal and loading conditions of the wire. This is relativity easy to control from a design point of view. In addition, external strain relief of the cable protects against movement at the wire terminal interface. In the case of solid wire, with proper strain relief, the IDC termination will perform as well or better than crimps because of the inherently greater mechanical stability. This is due to the amount of elastically stored energy in the deflected terminal which maintains a high pressure interface. Typically, for small wire sizes such AWG 26, the terminal is designed to provide several pounds of force at the interface and several mils of elastic deflection. In the case of larger wires such as AWG 20, the forces could go as high as 15 to 20 pounds.

With regard to stranded wire, the mechanical stability of the strand bundle plays a significant role in performance. There are two factors that effect performance. First; since the strand bundle is under compressive load, there is a tendency towards lower contact forces as the bundle relaxes in the slot due to mechanical disturbance, stress relaxation and creep. The level of potential relaxation depends on the type of stranded wire used. The number and lay (or twist) of the strands, the conductor top coating (plating) and the type of insulation play a role in mechanical stability.

For a given insulation type, unplated high count stranded wire with little or no lay is the most difficult to reliably terminate; while, overcoat seven strand cable is the easiest and often performs as well as solid wire. Second; since contact is made to a limited number of strands ( typically four out of seven strands ), the conductivity between strands effects the overall conductivity. The latter can be optimized if the wire is tin coated. In the stranded wire case it is apparent a well designed strain relief that tightly grips the wire insulation is important. Sometimes additional ( or redundant ) IDC slots provide the necessary mechanical stability. With the proper amount of deflection ( compliancy ) in the terminal and an effective strain relief, mechanical stability can be optimized in stranded wire IDC terminations.

Wire Loading Characteristics

Since each type of wire represents a unique set of parameters, it is necessary to evaluate the loading characteristics in each case to determine the design criteria for terminating a specific type of wire. The loading characteristics of solid or stranded wire can be measured in the laboratory with a force gage that is fixtured to simulate a slot for a given lead-in geometry. The results are used to determine the loading requirements for the terminal (as shown in Figure 2 below). The wire loading characteristic can be superimposed on the force deflection curve of a given design. It should be noted, the ramp angle, transition radius and material thickness significantly effect the loading characteristics of a given wire.

In following this analysis the design objective is to provide a terminal that crosses the wire curve in a predetermined design zone. The design zone for a given geometry is determined by inspecting the wire interface region after insertion in the simulation fixture. By definition, the design zone is the region of the loading curve where the insulation is displaced and the conductor is effectively deformed to establish high pressure metal to metal contact. In the case of stranded wire, the design zone typically represents the most mechanically stable region of the loading curve where good contact is made to as many strands as possible without severely damaging individual strands.

Test Methodology

The mechanical stability of the IDC interface is of primary importance in field performance. Consequently, vibration, mechanical and thermal shock, and temperature/humidity cycling are important stresses to consider in testing. laboratory tests which accelerate these stresses to produce simulated field aging should be seriously considered in product qualification testing. During this type of test program, the change in termination resistance should be monitored as the primary performance characteristic. A simple failure criteria of 10 Rc can be used to judge performance ( ten times the minimum constriction resistance of the Wire/terminal interface ).

Conclusions

When one considers the underlying principles of IDC terminations, it becomes apparent this technology can perform as well as crimped contacts in many applications. In addition, this can be accomplished at a reduced applied cost. This is a desirable situation which prompts one to seriously consider IDC technology in applications where it can be used in the harness assembly operation. Many applications provide opportunities for the use of IDC techniques to maintain performance at reduced cost.