Punching it Down: Insulation Displacement Connectors

In my misspent youth I found myself doing clinical rotations at a local hospital. My fellow students and I were the lowest of the low on the hospital pecking order, being the ones doing the bulk of the work in the department and paying for the privilege to do so. As such, our locker facilities were somewhat subpar: a corner of a closet behind a door labeled “COMMS”.

In the room was a broken chair and a couple of hooks on the wall for our coats, along with an intriguing (to me) electrical panel. It had a series of rectangular blocks with pins projecting from it. Each block had a thick cable with many pairs of thin, colorful wires fanned out and neatly connected to the left side, and a rats nest of blue and white wires along the right side. We were told not to touch the board. I touched it nonetheless.

I would later learn that these were Type 66 punchdown blocks for the department’s phone system, and I’d end up using quite a few of them over my hacking life. Punchdown connectors were a staple of both private and public telco physical plants for decades, and belong to a class of electrical connections called insulation displacement connections, or IDC. We’ve recently looked at how crimp connections work, and what exactly is going on inside a solder joint. I thought it might be nice to round things out with a little bit about the workings of IDC.

The Need for (More) Speed

As crimp connections were an attempt by the electrical and electronic industry to increase the efficiency of assembly by eliminating the labor-intensive step of soldering connections, so too was insulation displacement developed to save time relative to crimping. As efficient as crimp connections are compared to hand-soldered joints, crimping still takes a fair amount of manual labor in most cases. And even where an assembly process is complicated enough to warrant automatic crimping, there are still multiple steps involved in completing the crimp — stripping insulation, inserting the wire into the crimp connector, and applying crimp pressure, perhaps multiple times. IDCs eliminate the wire preparation steps and result in faster connections with fewer tooling changes, and are much more amenable to mass-termination of conductors.

The first US patent for IDC was issued in 1961 to two inventors working for the Minnesota Mining and Manufacturing Corp. 3M is still big into IDC connectors even now; few of us who have installed a radio or remote starter in a car won’t be familiar with Scotchlock connectors for making taps into a car’s wiring. The original patent illustrations show a striking similarity to the Scotchlocks we still use today, and reveal the basic idea behind all IDCs. A slotted metal blade forms the heart of the IDC, with the slot sized to just under the diameter of the wire to be attached.

In the Slot

US Patent 3,012,219

When an insulated wire is placed in the slot and the appropriate amount of downward pressure is applied, the slot cuts into and moves aside the plastic insulation, exposing the conductor within. As the termination pressure increases, the wire contacts the sides of the slot and begins to deform; in much the way that wire strands inside a crimped connection begin to flow and stretch, so too does the wire in the slot effectively cold weld to the metal contact, forming a gas-tight connection. And like in the crimped connection, the deformation caused by the increased pressure acts to loosen and drive off surface oxidation that would interfere with a clean connection.

Of course this is a generalized case; details of each IDC are particular to the job they’ll be asked to do. Some contact slots are tapered, some are straight; sharpened blades in the slot may be called for in one application while blunt blades work better in others; some slots are spring loaded while others aren’t. And the methods used to terminate these connectors vary wildly as well. A simple Scotchlock up under the dash might need nothing more than a pair of pliers, while tooling for mass-terminating the ribbon cable of an ATA connector will be a more complex press that can spread the force evenly over a long set of contacts.

In the telco space, those 66 blocks in that would-be locker room of yore would have been terminated with a handheld punchdown tool with a type 66 bit. The punchdown tool itself is a spring loaded impact tool; when the hardened steel bit is placed over the contact with the wire loaded in the slot, downward pressure begins both pushing the wire into the contact slot and pushing the bit back into the tool body against spring pressure. When the proper pressure has been applied, the spring triggers a hammer to impact on an anvil, driving the bit down to complete the connection with just the right amount of pressure. Type 66 bits have a sharpened blade on one side of the bit that can trim excess wire as it’s being punched down, or a blunt bit can be used to daisy-chain connections.

IDCs are everywhere these days — automotive wiring harnesses, likely every appliance in your house, and dozens inside most computers. And even though IDC was once strictly reserved for low-voltage connections, chances are good that you’ll start seeing IDC used more and more for residential and commercial mains wiring. The advantages of being able to make a quick, solid, gas-tight electrical connection without multiple tools and manual operations are just too appealing to pass up.

[Header image: By Z22 (Own work) [CC BY-SA 4.0], via Wikimedia Commons]

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