Insulation Displacement Connectors (IDCs) have become popular as they are highly economical and a cost effective method for performing wire terminations. No wire or cable preparation required. The design of early or first generation IDCs only allowed for peak application performance over an application range on one or possibly two wire gauges. These designs made a good initial contact but failed to withstand harsh conditions typical in industrial, automotive and appliance applications.
Much research has been done to design an improved IDC, and to analyse the cause of IDC failure. The results of some of this work is summarised here.
The conventional IDCs consist of two fairly rigid contact beams and a wire slot between them. The conductor slot is smaller than the conductor diameter. When a wire is inserted into the IDC slot, the lead-in chamfer cuts and displaces the wire insulation. As the wire is pushed further down in the slot, there is a deformation of both the conductor and the IDC beam. The conductor will have mostly plastic deformation while the contact beams will have elastic deformation (deflect outward) Fig 1.
This deformation is proportional to the 'normal’ (contact) force which acts on the conductor and on the contact beam interface. This normal force is very important to establish and maintain a good gastight connection. Several degrading mechanisms work against this gastight connection. The most important are:
* Stress relaxation
* Movement between conductor and contact which is caused mainly by wire flexing, pulling, and the difference in thermal expansion coefficients of the conductor and the contact material.
All these degrading mechanisms cause normal (contact) force reduction, which depends on the contact spring rate. Fig. 2 shows the spring rate of a conventional IDC contact and the force-deformation curve of different wire sizes.
Fig. 3 shows a PCB mounted, high force, high deflection, low stiffness, torsion IDC before and after wire insertion.
The two front contact beams are pushed against each other with a preset force called preload. It is set by forming the top torsion beams into a curved shape (crown). Fig. 4 shows a cross section of the curved top torsion beams. Preload also can be set by slightly curving the rear torsion beams and the rear base.
When a wire is pushed down against the coined V shaped insulation shear edges, the edges will cut the wire insulation. If the wire is pushed further down, the wire termination slot will open up. This slot can open up to a width larger than the wire diameter without taking permanent set, so it can receive the total full diameter of the stranded wire. At this stage the terminal beams are in a maximum force - maximum deflection position exerting high normal forces on the wire.
When the wire is moved further downward in the wire conduit slot, the high normal forces combined with the downward movement of the wire will rearrange the wire strands into their narrowest shape, and in the case of stranded wire, one single line (if the wire strands are not bundled too tight). The front contact beams will close in on the wire strands and keep them under contact pressure. See Fig. 5.
The ability of the torsion IDC to achieve this large - force, large - deflection characteristic can best be explained by a stress analysis. When an 'F' force (wire) is applied at the centre of the two front contact beams as shown in Fig. 6, the two front contact beams are under bending load. The top beams are under a combined load of torsion and bending load and the rear beams also under torsion and bending load. The deflection at the wire is the sum of the angular deflection of the beams under torsion load and the beam deflection under bending load. Numerical analysis shows that the deflection from torsion load is much larger than the deflection from bending load.
Fig. 7 shows the force - deflection diagram of a conventional IDC, the torsion IDC and the force - deformation curves for different wire sizes. The intersection of curves represents equilibrium points. Since the preload can be set independently, the force - deflection curve can be moved up and down so it intersects the maximum different wire sizes at the satisfactory wire deformation range.
In Fig 7 the torsion IDC with 0 preload provides acceptable connection for 2 wire sizes only. By increasing the preload to the correct levels, the IDC can terminate five or more wire sizes.
Fig. 8 shows conductor sizing or change in the width of the rearranged wire strands. There is only a small change in the normal force because of the low stiffness characteristics of the torsion IDC. Even at zero gap there is enough force to maintain a good gastight contact.
A slightly tapered wire slot improves the connector's reliability. If there is a wire strand movement from vibration or temperature change, the strands will move to the direction of least resistance, which is downward, where the slot is slightly wider (Fig. 9). The lower end of the slot is closed off by the PC board so the wire strands cannot leave the slot. When a wire is inserted into the terminal, the wire insertion force is increased to the maximum while the insulation is cut and the wire opens up the wire slot. From this maximum level, the wire insertion force will rapidly decrease to a minimum while the wire is pushed down in the slot. When the operator pushes the hand wire insertion tool with an increasing force, the wire does not move until the force reaches the termination force level. Then the wire just snaps all the way into the slot - no half way wire insertions. On a conventional IDC terminal, the wire insertion effect is just the opposite. It takes an increasingly large force to push the wire all the way down in the wire slot because the wire gets tighter and tighter.
When there is a wire movement from shock, thermal cycling, or vibration, the wire tends to move out from the slot because it moves in the direction of least resistance. This IDC is not sensitive to the relative motion between the wire and the terminal.
The rear slot on the terminal body works well as a wire strain relief by holding the wire firmly in place. The wire can be pushed in and seated in the strain relief slot by hand. This will keep a section of the wire lined up with the ‘V’ grove of the insertion tool, which terminates the wire by inserting it into the wire slot.
The terminal is designed for a large range of wire sizes and dozens of mating cycles. A single terminal can accommodate a range from 18 to 26 AWG, while the maximum stress in the terminal remains below 70% of yield stress, so no permanent set and very little stress relaxation takes place.
Zierick has developed a highly reliable insulation displacement terminal, which accepts a large range of wire sizes. It is capable of enduring many mating/unmating cycles without terminal fatigue. The wire can be mass terminated, or terminated in the field with a hand tool-which provides a good feed back of the wire ‘snap in’. A year of mass production proved that it provides a reliable termination even in the most demanding automotive application.