Rugged gold- and copper-plated bellows solve contact problems for flexible circuitry in military, industrial, and medical applications.
By Paul Hazlitt
For designers of specialty electronics, electrodeposited metal bellows are an often-overlooked solution to compound design problems. Properly engineered, electrodeposited bellows can provide resilient contacts with greater reliability and flexibility than common spring contacts (see Fig. 1). They can also blend high electrical conductivity with long-life, low-contact forces, and mechanical repeatability. Gold-plated electrodeposited nickel bellows carry up to 4 A without excessive heating.
Alternatively, electrodeposited copper bellows provide similar advantages with up to 10-A current capacity. With either composition, electrodeposited-bellows contacts open design opportunities to make consistent connections with fewer parts and less force. However, using bellows contacts cost-effectively requires special insight into their fabrication and the electronic design process.
Since the 1960s, the same electrodeposited-bellows contacts developed for research laboratories have been used in industrial, military, and other applications. Compared with spring-loaded pogo pins, wire-mesh cylinders or pads, and bent spring-lever contacts, electrodeposited bellows remain a premium-priced solution to difficult design requirements. They nevertheless provide powerful design advantages in many specialized applications.
One example application ideal for bellows contacts is a microchip tester requiring a contact probe with extra-low spring forces to make consistent electrical contact with test pieces yet protect delicate microcircuits. Other special applications for bellows contacts are laser electronics that must maintain continuity between circuit boards that may expand, contract, or twist with changing temperature, or battlefield electronics that require ultra-reliable, long-life pushbutton switches sealed against a dusty, dirty, and wet environment.
Low force
In electrodeposition, the metal is not liquid. Rather, the metal exists in a compound, such as copper sulfate or nickel sulfate. When electric current is applied, the metal ion breaks the chemical bond with sulfate at the atomic level and joins with the aluminum on a precision-machined mandrel. No melting is involved. The electrodepositing process builds a nickel or copper bellows of precise thickness and diameter on the aluminum mandrel. Dissolving the mandrel leaves a strong, convoluted, flexible tube as small as 0.89 mm in diameter with walls 0.008-mm thick.
Electrodepositing forms exceptionally small parts with low mass-both important advantages in miniaturized electronics. By comparison, common pogo pins must be relatively large for practical assembly. Equally important, electrodepositing consolidates the resilient bellows, custom contact surface, and terminations in a single piece. Unlike three-piece pogo pins, bellows connectors have no separate parts to complicate assembly or wear out. The closed end of the precision electroform faithfully replicates a round, flat, or pointed contact geometry to suit the application. The termination end of the bellows can be left open for a soldered post or electroformed with an integral shank or flange.
The electroforming nickel used in bellows is well suited to sturdy, spring-action contacts. The nickel yields 125,000-psi-minimum tensile strength, 110,000-psi-minimum yield strength, and 270-Vickers hardness. The combination of strength and ductility makes for extremely flexible bellows with long fatigue life. The electrodeposited parts preserve all the desirable mechanical properties of the metal in almost any shape.
Electrodeposited nickel bellows have a 24-carat gold coating that is 0.00005-in. (1.27-µm) thick, which facilitates low dc resistance and minimal insertion loss. The coating can be applied externally or inside and out, creating a uniform, conductive skin that cannot delaminate as the bellows compress and extend. While ductile nickel gives a properly designed bellows contact virtually infinite life, gold plating enhances corrosion resistance and gives the contact a 4-A current-carrying capacity. For higher-current applications, soft copper bellows handle up to 10 A.
Although copper has a higher current capacity, the mechanical tradeoff is that electrodeposited copper has half the elastic limit of electrodeposited nickel. Electrodeposited copper takes a set (in other words, is permanently deformed) with half the travel of nickel, so it makes a less durable contact.
Spring rate
With either metal, extra-thin walls enable electrodeposited bellows to compress and recover their length with exceptionally low spring rate. An electrodeposited nickel bellows 1 mm in diameter, for example, provides just 3 grams of spring force at mid-compression. In contrast, piston-like pogo pins of comparable diameter and stroke have several times the spring rate. Due to the limitations of common metal stamping, even simple bent-lever contacts generate relatively high spring forces. Electrodepositing makes bellows walls just one-tenth the thickness of ordinary stampings and one-quarter the thickness of hydroformed bellows. The sharp radii of electrodeposited bellows segments can also be made just three times the wall thickness, smaller than radii produced by mechanical forming.
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In an electronic contact array or in applications involving delicate devices, the low spring force of electrodeposited bellows can dramatically reduce the force required to make contact. The automated microchip tester, for example, uses a plunging probe to test each delicate semiconductor. The spring-action bellows on the probe limits contact force to 28 g (1.1 oz).
With the appropriate design travel and wall thickness, the dynamic properties of electrodeposited nickel bellows can also provide an essentially infinite life of 1 ¥ 108 cycles. Unlike spring- or mesh-type contacts that take a set under sustained compression, the spring force of the bellows stays consistent over the life of the part.
In the laser, contacts between stacked circuit boards need consistently low spring force to maintain continuity without deforming delicate parts should the boards shift or warp. Bellows sandwiched between the boards maintain contact despite thermal expansion and do not take a set despite prolonged compression. The low mass of the electrodeposited bellows also makes the entire electronic assembly less sensitive to vibration.
Seal and contact
Seamless, nonporous electrodeposited bellows will not allow dust, dirt, or moisture to contaminate precision contacts. They also have greater chemical and temperature resistance than the elastomeric boots or bushings commonly used to seal pushbuttons. Gold-plated nickel bellows, for example, are viable for use at temperatures from cryogenic to 350°F.
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Metal bellows contacts can stand up to sterilizing chemicals in medical electronics. They can also seal out harsh operating environments in industrial and military applications. In one piece of ruggedized battlefield electronics, bellows contacts within elastomeric boots provide fail-safe pushbutton seals should the elastomer fatigue and break. The electrodeposited bellows are formed with a terminating flange captured by a pressure fitting.
With knowledgeable technical support, electronic designers can use electrodeposited bellows contacts to meet prototype requirements and short deadlines without high tooling costs. In production applications, one-piece electrodeposited bellows contacts can replace stamped or machined contacts, and pay off in reduced parts count for easier assembly and greater reliability. In addition, electroformed contacts can consolidate special shapes, properties, and functions that can be combined in no other practical way. For example, a self-aligning flexible bellows contact minimizes the effects of shock and vibration in dynamic applications.
The paired miniature bellows contacts are electroformed with a convex tip and concave receptacle to self-align and compensate for angular and parallel offset. Standard sizes range from 0.037- to 0.125-in. outside diameter to accommodate a maximum misalignment of 21° to 36°. Used in critical assemblies, the self-aligning electroformed bellows also give designers a less costly alternative to tight-tolerance assembly components (see Fig. 2).
The material and manufacturing process determine the properties of electrodeposited bellows contacts. Successful, cost-effective applications require a close working partnership with an experienced electroformer. To optimize the bellows contact design, the electroform supplier must know current capacity, the available size envelope, the deflection range, and the operating environment. With those specifications, the metal is selected and the bellows diameter, wall thickness, and compression stroke are tailored for best mechanical and electrical performance. In many ways, electrodeposited bellows contacts are flexible tools for electronic designers. They can be a cost-saving means to maintain sure contact in difficult and changing applications.
PAUL HAZLITT is director of engineering at Servometer, 501 Little Falls Rd., Cedar Grove, NJ 07009. Tel: 973-785-4630; email: paulh@servometer.com.
