Evolving High-Density Contact Designs
By Bob Hult, Bishop & Associates Inc.

The design of a separable electrical contact seems like a fairly simple task. Pressing two conductors together can complete a circuit, but like almost everything else, there is much more to it. It’s quite a challenge to create and maintain a low-resistance connection that remains stable after many mating cycles—wear factors such as a corrosive environment, shock, or vibration not withstanding—and still be capable of conducting large currents with minimal loss. Large connectors that contain hundreds of contacts must offer low-insertion and extraction forces, while connectors utilized in high-speed circuits add the requirement for controlled impedance and minimal skew.

The venerable pin-and-socket contact system is only one of literally dozens of contact designs that have been developed over the years to satisfy a vast array of diverse applications.

Traditional contact styles now include leaf, bifurcated bellows, shepherds crook, box, beam-on-blade/pad, tab and receptacle, and tuning fork. Contacts that internally derive their normal forces at the interface are fabricated from a variety of copper alloys that balance spring characteristics with conductivity. Other contact systems may use plastic springs molded into the housing or external stainless steel springs to ensure adequate force at the interface. Zero insertion force connectors, such as pin grid arrays, use lever action in the housing to push the pin grid array (PGA) pins against stationary contact springs to generate normal forces after the contacts are mated. Hermaphroditic contacts that mate with themselves eliminate any confusion about male or female contact gender. Hyperboloid contacts offer a unique combination of high reliability and low insertion forces. The wide range of interface applications has spawned these many configurations that often address specific applications or environments.

Demands by system designers for higher density/pin counts and power capacity on smaller centerlines, continues to drive new contact development. Traditional stamped-and-formed manufacturing technology has made tremendous advances, but may be reaching practical limits, therefore opening the door to non-traditional interface alternatives.

Connector systems that use compression contacts are proliferating to address high-density, high pin count, and low-profile stacking applications. These would include device-to-PCB, board-to-board mezzanine, and flex circuit-to-board applications.

The Cinch CIN::APSE contact was one of the first z-axis compressive interfaces to enter the market. It consists of a bundle of gold-plated wires to form a compressive button that is loaded into an hourglass-shaped molded wafer cavity. The contact is allowed to float to compensate for expansion and surface mismatch. Profiles—as low as 0.8mm and centerlines to 1mm—are possible.

Neoconix, based in Sunnyvale, California, offers a unique interposer system that consists of a high-density matrix of spring contacts integrated into a two-sided wafer.

Interposers with up to 3,000 contacts on as small as 0.8mm pitch can be created.

 

These gold-plated metallic contacts provide a very low resistance connection with a high working range of up to 6 mils per side. The solid substrate prevents overstressing the contact beyond its elastic range. These connectors are capable of up to 10,000 mating cycles.

 

Both single- and dual-beam configurations are available. Since the wafers are fabricated using conventional printed circuit board technology, custom configurations can be quickly produced on standard grids with no tooling charges.

Amphenol InterCon Systems offers the cLGA land grid array (LGA) socket system, which is based on a C-shaped beryllium copper spring contact in a molded plastic carrier. As the two mating surfaces come together, the spring compresses, creating normal force as well as a wiping action on the mating pads.



Formed contact areas ensure dual redundant points of contact for reliability.

Available contact grids include 1.0mm and 1.27mm centerlines.

The initial applications were developed for LGAs, but the technology has been expanded to address board-to-board stacking, as well as flex circuit-to-board applications.

 

Connectors that utilize metallic contacts can be designed to provide intrinsic spring characteristics with very low contact resistance. Another design, developed by LK Technology Group LLC, utilizes a unique braided wire design.

The sheared ends of the wires, at top and bottom, are plated to provide redundant high-pressure points of contact.

A matrix of contacts is assembled into a wafer to form an interposer with centerlines as small as 0.3mm. Test data indicates that these contacts are capable of establishing reliable interfaces of less than 20 milliohms, with approximately 10 grams of normal force in ball grid array (BGA) or LGA applications. The system can perform in circuits operating at up to 40 GHz, with durability to 1mm mating cycles.

 

Pure metallic contacts offer low resistance but require assembly into defined centerlines. One-piece z-axis compressive connection systems that are integrated into a polymer assembly can minimize alignment mismatch while easing the assembly process.

The Zebra connector from Fujipoly consists of alternating layers of conductive carbon and insulating silicon rubber. Layers can be thin enough to provide multiple conducting layers per pad and eliminate the need to align the connector to the pad centerlines. These elastomeric connectors are commonly used in LCD displays, as they are limited to very low current applications.

 

Elastomeric connectors, manufactured by Shin-Etsu, consist of rows of gold-plated brass wires in a silicone rubber carrier. The vertical wires deflect as they are compressed between conductive pads, establishing high-pressure points of contact. These connectors are typically used in board-to-board and component-to-board applications.

 

Paricon Technologies Corporation has developed their Pariposer high-density interface, which consists of plated spheres that have been magnetically aligned within a sheet of silicon rubber and then cured.

When compressed between parallel contact pads, they form short 6- to 10-milliohm vertical columns, with excellent isolation between contacts.

This connection system is able to maintain necessary normal forces over long periods of time and elevated temperature. The thermal conductivity of this material can also contribute to lowering critical junction temperatures. These interconnects are often focused on test and burn-in requirements, but can be adapted to stacked board applications.

 

Colorado Springs-based PITek.US manufactures another unique interconnect that is based on the use of very hard particles, which have been plated and electrodeposited as bumps on conductive pads. When pressed between mating surfaces, these particles deform and penetrate oxide surface films to establish gastight connections.

 

Reliable connections can be created with as little as 10 grams of force per contact. This low level of force is enough to drive the sharp edges of these particles into the mating surface and provide a contact resistance of 3 to 5 milliohms.

 

The connection established between a device and PCB can be as short as 0.001 inch, providing outstanding high-speed electrical performance. Since the connection occurs at the asperities of the pads and particles, wiping action is not required which allows durability of 1.5mm mating cycles.

 

Bishop & Associates Comments

• High-density/high-speed circuits are demanding miniature, high-pin-count, separable interconnects.

• New semiconductor devices must be tested that open new opportunities for high-density test and burn-in sockets, which can deliver excellent high-speed performance over a one-million mating cycle life time.

• Traditional stamped-and-formed contact systems may be reaching practical manufacturing limitations.

• Compressive interconnects can provide exceptionally short connections between parallel surfaces on centerlines as small as 0.3mm.

• Evolving technologies that utilize formed contacts integrated into a wafer interposer can be used to provide high-density connection in PCB-to-device, board-to-board, and board-to-flex-film applications.

• Several of these technologies incorporate metallic contacts to minimize contact and bulk resistance, while others use plated particles to achieve connection.

• The performance of a compressive connector is dependent on the ability of an external clamping mechanism to maintain adequate registration and pressure across the interface.

• Research continues on contacts that utilize conductive nanomaterials that may form the basis for the next generation of high-density interconnects.

Robert Hult
Director of Product Technology, Bishop & Associates, Inc.
Robert Hult has been in the connector industry for more than 36 years. Hult began his career as a sales engineer for Amphenol. He joined AMP in 1972 and served in several management positions through 1996. In 1997, Hult joined Foxconn as group marketing manager for Intel, based in Chandler, Arizona, U.S.A. Prior to joining Bishop & Associates, Hult was the regional application engineering manager for Tyco Electronics.

Hult graduated in 1968 from Bradley University with a Bachelor of Science degree in electronics technology and a minor in business.