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Use Direct Plug-in Insulation Displacement Connectors to Streamline Assembly and Lower BOM

By Bill Schweber

Contributed By Digi-Key's North American Editors

Engineers often choose insulation displacement connectors (IDCs) and their associated ribbon cables for multi-position interconnects due to their high contact density, single-step mass termination, and lack of need for wire stripping. Users of IDCs normally assume they comprise a two-part connector arrangement with mating male pins and female receptacle halves. These IDCs are available in a wide array of styles (such as board mount or free-hanging), offered with as few as eight and as many as 50 positions (typically), and are used with multiconductor flat ribbon cable.

While IDCs offer many benefits, designers are always seeking to reduce the direct parts cost, trim the bill of materials (BOM), simplify purchasing and procurement, and streamline the assembly process in a production environment. All of these objectives can be achieved in one step by using a single IDC connector to terminate the flat cable while at the same time eliminating the need for a complementary mating connector.

This innovation, which contradicts “conventional thinking” about IDC connectors, is used in the WR-WST REDFIT IDC SKEDD flat cable IDC connector from Würth Elektronik. It uses the pc board as the mating contact, thus reducing cost, simplifying assembly effort, and removing a line item from the BOM—all without compromising performance. This article discusses the drivers behind such IDC connector innovations before introducing the WR-WST connector and how to take advantage of it.

What are IDCs and why are they needed?

IDCs solve multiple problems by enabling quick and easy termination of a number of signal and power wires along with quick and easy making and breaking of the connection path (Figure 1). IDC-based cable assemblies are widely used in many applications, such as between adjacent pc boards or between a process pc board and a relatively remote user display/keypad assembly. In some designs, the flat cable is even used as a flexible “bus” supporting not only cable end connections, but also connectors placed along the flexible cable so other pc boards can be connected to a common pathway.

Image of IDC cable assemblies use mass terminated crimp-on male and female mating connectorsFigure 1: IDC cable assemblies use mass terminated crimp-on male and female mating connectors along with multi-wire flat ribbon cable such as this 16 wire assembly; the ribbon cable can also be monochrome unlike this rainbow example. (Image source: eBay)

IDC technology was developed over 50 years ago. Since its adoption, it has seen widespread use, growth in the variety of available versions, increased contact density, more contacts per connector, and other improvements. As the name indicates, IDC technology is based on the principle of displacing, or pushing aside, some of the insulation around the cable conductors (wires) and making a direct electrical connection to the copper (Figure 2). There is no need to strip the insulation from the wire, as the sharp metal edges of the contacts pierce the insulation to create a gas-tight connection.

Image of IDC connector top is pushed down to crimp the contacts simultaneouslyFigure 2 (top)

Image of IDC connector assembly with the connector top removedFigure 2 (bottom)

Figure 2: In an IDC connector, the connector top is pushed down to crimp the contacts simultaneously and pierce the insulation of all the wires in the cable (top). Viewing the assembly with the connector top removed (bottom) shows the contacts protruding through the wire insulation. (Image source: Jaycar Electronics Reference Data Sheet)

The development of insulating material that cleanly and neatly cleaves but does not split beyond the piercing zone was one of the many advances that made mass use of IDCs practical. Since the wire is one of many in a flat ribbon arrangement, many such terminations can be made at the same time, so IDC cables are sometimes referred to as “mass termination” connectors. The first IDCs were restricted to solid wire for reliable connections, but technical advances soon extended IDCs to stranded wire as well.

There are many standard available versions of both the male and female mated pairing. These include connectors that solder onto a pc board as well as those that are free-hanging and terminate the end of a cable. In this way, an IDC cable assembly can be connected to a board mounted connector or to another IDC cable.

For example, the Würth 61201023021 is a dual row, 10 position, 2.54 millimeter (mm) pitch, rectangular IDC receptacle (female) connector that is crimped onto the cable (Figure 3). If the cable instead needs a male pin connector, the complementary Würth 61201025821 IDC header connector is available (Figure 4). For both male or female connectors, there are corresponding connectors that mount onto pc boards to complete a cable-to-board path. Note that although these are 10 position connectors, the Würth WR-BHD box header and IDC connector family extends out to 60 position connectors.

Image of Würth Elektronik’s 61201023021 IDCFigure 3: Würth Elektronik’s 61201023021 IDC is a dual row, 10 position rectangular receptacle (female) connector with 2.54 mm pitch. It is designed to be crimped onto a free-hanging flat ribbon cable. (Image source: Würth Elektronik)

Image of Würth 61201025821 IDC header connectorFigure 4: The Würth 61201025821 IDC header connector with male pins is the complement to the Wurth 61201023021 IDC. (Image source: Würth Elektronik)

IDCs and their flat ribbon cables are available in a wide range of positions and options. These include pin spacing (pitch) of 0.050 inches (1.27 mm) or 0.10 inches (2.54 mm), as well as the number of pins. Maximum current capacity rating is usually 1 ampere (A) to 3 A with available wire gauges from thin 30 AWG to heavier 22 AWG. There are also IDCs available for DB-xx connections for the once widely used DB-25, DB-15, and DB-9 sizes (common with RS-232 interfaces).

Some industry segments have established specific IDC types so connected devices from different manufacturers can be interchanged. For example, in the personal computer area, the following are common standards:

  • 3.5 inch IDE desktop computer hard disk drives: 2.54 mm pitch, 40 pins, 2 × 20 (2 rows of 20 pins)
  • 2.5 inch IDE notebook computer hard disk drives: 2.00 mm pitch, 44 pins, 2 × 22
  • SCSI 8-bit: 2.54 mm pitch, 50 pins, 2 × 25
  • SCSI 16-bit: 1.27 mm pitch, 68 pins, 2 × 34

For all of the above connectors, the computer manufacturer typically attaches a female IDC connector onto one end of a ribbon cable with a matching male box header or pin header on the computer motherboard. There are also single contact arrangements used with single wires in “punch down” blocks of telephone wiring. A technician can make the connection in the field using a special tool to push an insulated wire between the sharp fork terminals of a single desired contact location.

Going to a one-piece IDC mating

It may seem obvious and even unavoidable that a multi-contact IDC cable assembly needs a mating connector, and that has long been the case. However, there is a newer approach that eliminates the female (receptacle) mating half, and instead uses the pc board as that mating connector (Figure 5).

Image of REDFIT IDC SKEDD connector flat cable IDC family from Würth ElektronikFigure 5: The REDFIT IDC SKEDD connector flat cable IDC family from Würth Elektronik plugs directly into properly sized and plated vias in the pc board, eliminating the need for a matching female IDC. (Image source: Würth Elektronik)

That’s the principle of the REDFIT IDC SKEDD connector flat cable IDC family from Würth Elektronik. This connector is attached to the flat ribbon cable as with any IDC cable, but then plugs directly into properly sized and plated holes or vias (derived from the acronym for “vertical interconnect access”) in the pc board. The result is a reliable connection at lower cost with fewer contact points and fewer assembly steps.

This is a solderless, hand-pluggable, reversible connector, meaning it can be unplugged without special tools, unlike some press or snap-in connectors that cannot be disconnected without a special tool or user contortions (Figure 6). It is based on a 1.27 mm pitch—often called “half pitch”—and is available with four to 20 (even numbers) contacts per connector; the 10-position Würth 490107671012 version exhibits the representative specifications.

Image of Würth Elektronik REDFIT IDC SKEDD connectorFigure 6: The REDFIT IDC SKEDD connector is a solderless, hand-pluggable, reversible IDC, meaning it can be plugged and unplugged without special tools. (Image source: Würth Elektronik)

The REDFIT IDC SKEDD connector targets applications such as consumer electronics, solar power system installations, industrial electronics, and mechanical engineering projects with wiring needs. The vendor guarantees performance for at least 10 mating cycles in the field and 25 at the prototype bench, where environmental conditions are more benign.

This rating is a good match for the many products that will not be disassembled, except perhaps for a one-time repair or upgrade. The contact resistance of the overall system is specified as 10 milliohms (mΩ) with a maximum current capacity and voltage rating of 1 A and 100 volts, respectively. The 28 AWG (1.27 mm) cables can be made off-site or in the factory, as needed.

Material science makes it work

To realize initial and multiple mating cycles, the contact “partners” of the REDFIT IDC SKEDD have to adapt to each other such that no plastic deformation of the metal occurs, as it does in some contact designs. In contrast, the SKEDD contact consists of two arms, connected at their apex; and the flexible fork-type arms remain in the elastic state even when inserted and making connections, ensuring a reversible connection (Figure 7).

Diagram of two arms of the Würth Elektronik SKEDD contactFigure 7: The two arms of the SKEDD contact remain in the elastic state during and after insertion—a key factor in their ability to maintain contact force, and also to be removed and re-inserted. (Image source: Würth Elektronik)

At the end of the insertion process, the spring stiffness of the flexible SKEDD contact reaches its highest value. In this state, the contact’s normal force is high enough to ensure that no signal interruption of more than 1 microsecond (µs) occurs due to mechanical stress.

It may seem that SKEDD contacts and technology are just a sophisticated extension of long used press-fit technology, but this is not at all the case. In the press-fit approach, a solid pin is pressed in the plated through hole. The high frictional force between pin and hole creates a homogenous cold weld between the surfaces, ensuring electrical and mechanical integrity. However, the plated through hole also gets deformed during this press-in process, and removing the solid pin breaks off this connection.

When using press-fit technology with flexible rather than solid pins, the plated through hole remains intact but the pin material itself gets deformed. Although it is possible to remove the flexible pin since the mechanical connection is not as strong as it is with a solid pin, the pin itself is damaged and can’t be used a second time.

In contrast, the elastic mating and absence of plastic deformation of both the contact and the plated through via of the SKEDD approach ensures the mating pair can be reconnected without degradation, while its four-way contact enhances reliability (Figure 8).

Diagram of Würth Elektronik SKEDD contact does not deform itselfFigure 8: Unlike press-fit technology with solid or flexible pins that deform the via or pin, respectively, when inserted, the SKEDD contact does not deform itself or the via; this is critical to ensuring viable re-insertion of the connector and its contacts. (Image source: Würth Elektronik)

Interconnection advantages and opportunities

In a typical application of a conventional IDC cable, a cable with connectors on both ends mates with corresponding board mounted connectors to join adjacent pc boards. Using the REDFIT IDC SKEDD connector, designers obtain multiple advantages:

  • It eliminates the need for the mating board connector; that’s two parts saved if the REDFIT IDC SKEDD connector is used at both ends.
  • Since the REDFIT IDC SKEDD connector can be inserted into the top or bottom of a board, there is more flexibility in the routing of the flat ribbon cable and relative board spacing and orientation, and it allows direct, shorter connections (Figure 9).
  • Finally, there is a small weight and materials savings. This may not be a factor in some designs, but it is a significant one in many.

Image of Würth Elektronik REDFIT IDC SKEDD connectorFigure 9: Since the REDFIT IDC SKEDD connector can be inserted on either side of the pc board, it offers more options in routing the cable and placement of connected boards. (Image source: Würth Elektronik)

The plastic housing of the REDFIT IDC SKEDD connector is an important part of the connector performance. The housing has two guide pins of slightly different diameters at opposing corners to ensure correct mating orientation and to prevent reverse polarity errors in assembly (Figure 10). The housing material is a plastic that is resistant to high temperatures (rated -25⁰C to +105⁰C), meets the UL94 V-0 flammability standard, has superior chemical resistance, and has a low coefficient of thermal expansion to minimize failures due to thermal cycling.

Image of guide pins with slightly dissimilar diametersFigure 10: The guide pins with slightly dissimilar diameters at opposite corners of the connector housing prevent insertion of the connector in the reverse position. (Image source: Würth Elektronik)

To prevent intermittent contacts due to vibration, in addition to the four contact points at each pin, there are two small friction locks on each plastic board guide that function as a passive locking system. The result is a tool-free termination that self-locks upon insertion and will maintain a viable connection until disconnected by hand.

Board fabrication specifications are fully defined, easily met

Proper fabrication of the pc board vias is necessary for successful and effective use of the REDFIT IDC SKEDD connector. This is not an issue, however, as the critical requirements are commensurate with today’s standard pc board specifications. The via fabrication does not require special steps or additional pc board tolerance precision beyond what is normally required.

The dimensional drawing calls out the pc board layout for the entire connector and drill hole dimension, as well as associated tolerances for the finished vias in a production board (Figure 11). Note that for prototype test and debugging where more insertion/removal cycles are often required, a wider tolerance drill pattern is offered.

Diagram of Würth Elektronik REDFIT IDC SKEDD connectorFigure 11: The specified dimensions and tolerances for the production pc board hole layout and sizing to accept the REDFIT IDC SKEDD connector, as well as tolerances, are commensurate with modern board fabrication standards (left); the modified drill pattern for debugging applications has slightly looser tolerances and allows for more insertion/removal cycles (right). (Image source: Würth Elektronik)

Conclusion

The two-piece IDC cable assembly and system has been used for many years, and is proven to be a valuable, widely used multiwire interconnect approach. Now, the one-piece REDFIT IDC SKEDD connector flat cable IDC family from Würth Elektronik offers an alternative that eliminates the need for the mating female IDC receptacle. Instead, its hand insertable/removable male pin connector plugs directly into the pc board. By doing so, it reduces cost, trims the BOM, eliminates another source of potential intermittent contact errors, and offers additional options for cable routing and board arrangement.

Related Digi-Key Video and Training Modules

Würth Elektronik, REDFIT IDC SKEDD Connector Solutions

Würth Elektronik, REDFIT IDC–SKEDD Connector from Würth Elektronik eiSos

References

  1. Jaycar Electronics Reference Data Sheet” (Internet Archive Wayback Machine)
  2. Insulation-displacement connector” (Wikipedia)

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Bill Schweber

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.

At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these.

Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls.

He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.

About this publisher

Digi-Key's North American Editors