Eye-of-the-needle and canoe-compliant-terminal designs each have pros and cons for solderless connector assembly onto PCBs.
By John Fili
Exclusion of lead from tin-lead solder can cause problems in manufacturing and product reliability. These problems are due to whisker growth and damage to components from exposure to elevated tin only or high tin-alloy soldering temperatures. However, the challenge of lead-free assembly may not be as difficult as originally thought. Many component and connector makers are turning to suppliers of electronic contact-pin terminals for compliant terminals as a solderless, pressfit solution. Indeed, solder-free, pressfit, compliant-terminal technology has been commonly used in non-automotive applications for decades, and is quickly gaining popularity in the automotive sector (see sidebar).
Compliant section basics
A terminal with a compliant section is installed into a hole that has been drilled into or through the printed circuit board (PCB). The insides of this hole are metal-plated to establish contact between the layers of the PCB and the terminal (see Fig. 1). As its name implies, a compliant section’s compliancy feature allows the part to comply with or adjust to the size and shape of the plated through-hole (PTH) in the PCB. The feature maintains an electrical and mechanical connection without soldering. The compliancy allows devices to experience thermal changes and vibration without loss of contact.
There are many designs for compliant sections that have been produced successfully. Two very different designs are of particular interest, each with its own advantages and drawbacks: the eye-of-the-needle (EON) and canoe compliant terminal (CCT).
The EON is the defacto standard compliant design and is commonly available in strip-stamped form. For a buyer, this near-commodity status is advantageous. This product can be stamped on any high-speed precision press with the standard feeding equipment offered by the press maker. Tooling designs for this product are well established also. In addition, the author knows of no patent restrictions on this design.
So why not use this design for all applications? While the wide acceptance and availability of this design is certainly positive, there are a few disadvantages to consider. First, the contact with the PCB is low; only about 40% of the available surface. Second, EON terminals use opposed spring beams to maintain force against the inside walls of the PTH (see Fig. 1). As a result, the pressure is concentrated and unidirectional, normally along the row of terminals in the connector. The build-up of force in one direction can warp and stretch the PCB (see Fig. 2). Together with the small surface area, this can damage fragile PCB layers.
 FIGURE 2. The compliant sections in a series of eye-of-the-needle compliant terminals (top) compress the row of PCB holes in a lateral direction (bottom). |
Typically, EON terminals are cut from a flat strip or ribbon of metal that is passed through a progressive die. The individual terminals are usually carried on the pin tips through the die, as well as through the plating process and into assembly (see Fig. 3). When the carrier is removed, the tip of the terminal remains unplated in the final assembly. For some products, this unplated tip is another disadvantage.
 FIGURE 3. Eye-of-the-needle compliant terminals stamped from strip are carried on the pin tips during stamping and plating. A push shoulder is used as an assembly aid. |
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Canoe compliant terminal
The canoe compliant terminal (CCT) is formed out of wire instead of being cut out from strip. The expertise required to make this product is less common than that required for the strip-cut process. Equipment is also more specialized; while a standard high-speed press can be used, the feeding system required for wire is not standard. For these reasons, CCT has not been commoditized. However, due to more efficient use of materials - 5% trim for wire versus more than 60% for strip - cost is generally competitive when terminals are made from wire.
The CCT, which uses a curved leaf spring to generate the compliant force, offers certain design improvements that may be important. The contact area is much larger, about 60% of the available PTH. Also, the force exerted on the PCB is multi-directional, lowering the potential for damage and distortion of the PCB.
The normal carrying method through plating and into assembly for wire-formed terminals is by use of a separate piece of strip material called a bandolier. During plating, the terminal receives less coverage in the bandolier area and some small contact points are left unplated. However, the placement of the bandolier is movable and can be designed to carry the terminals in a less critical area.
Connector assembly
Because the EON compliant section design is manufactured by being cut out of strip stock and the CCT is formed from wire, the assembly details are affected. Manufacturing of finished connectors requires that the strip of terminals be separated into individual conductors and assembled into a pre-molded plastic body or placed in a mold and molded into the component. The main consideration for EON versus CCT assembly at this stage is the type and location of the carrier and how it will be removed from the assembly. Separation of EON terminals from the pin-tip carrier requires either cutting or a bend-and-break operation. This operation can introduce burrs or chips into the assembly. Alternately, CCT parts are held in individual pressfit slots in a bandolier. They are pressed off the bandolier without cutting or breaking material. A properly designed and manufactured bandolier will allow the terminals to be pressed off cleanly without burrs or chips.
Assembling a finished compliant connector onto a PCB is a pressfit operation. It requires that the components be well supported and aligned. A mechanical force is necessary to push the compliant terminals into their PCB holes. Because EON is cut out of strip, a push shoulder can be added to the part to aid in assembly. This allows the connector to be pressed onto the PCB by applying pressure to the body of the connector, not the terminals. In some cases, this reduces the cost of assembly.
In contrast, a CCT part made from square or round wire does not normally have a push shoulder. Assembly of this product onto the PCB requires pressing on the terminals themselves if the connector is pre-molded. However, if the connector is insert-molded, it may be possible to assemble the connector to the PCB with pressure on the connector body. Note that a specialized assembly tool is needed for the pre-molded connector to support and apply pressure directly to the terminals.
Innovative producers continue to bring new products into this dynamic market. A recent new product development is an EON terminal made from wire and carried on a removable “bandolier” (see Fig. 4). This new product will offer the efficient material use of wire, the improved plating coverage of a bandolier carrier, and the widely accepted EON compliant in one product. The manufacturing process to produce this product in high volume has been developed and product testing is ongoing.
 FIGURE 4. Canoe-style compliant sections can be carried on a detachable bandolier (left), while a new eye-of-the-needle compliant section design is made from wire (right). |
Component makers in the automotive market, as well as other markets, face a tough problem in removing lead from their products. While for some the solution lies in lead-free solders, another way is available that avoids the risks of high temperature and tin-whisker growth. That way is solder-free assembly using compliant technology. When component makers are able to assemble their products without soldering, many other benefits are realized beyond the elimination of the hazardous substance, lead. There is no flux residue to be cleaned, another area of environmental concern and legislative action. And, the failure modes of the solder process such as bridging, cold spots, and solder splash are no longer present. In summary, compliant pressfit terminals are an effective way to eliminate lead from connector and PCB assemblies. They use less assembly steps, and therefore simplify the component assembly process, while reducing environmental concerns and costs. Established designs that meet ANSI and IEC standards are readily available, and innovative new products are under development to meet customer needs.
JOHN FILI is vice president of manufacturing and operations president of Solid State Stamping, 43350 Business Park Drive, Temecula, CA 92590; Tel: (951) 676-6100; Email: jfilif@sss-i.com.
The Lead-free Motivation
Lead-free electronics assembly is a top priority for automotive connector and component makers worldwide. Motivation to remove lead from electronic equipment stems from two sources. First, specific legislation such as Directive 2000/53/EC of the European Parliament attempts to minimize the impact of end of life vehicles on the environment. The Commission’s statements direct the removal of all lead from vehicles with some few exceptions by July of 2006. Readers may find detailed information concerning this legislation at the European Union website (http://europa.eu.int/eur-lex/en/). Second, vehicle producers certainly recognize that consumers are increasingly interested in making environmentally conscious choices where possible. They therefore seek to differentiate their products as smart and responsible choices.
