In this article, you'll learn about ...
- Connector Basics
- Electrical Concerns
- Contact Materials
- Contact Platings
- Mating Gold to Tin
- Connector Ratings
- Thermal Considerations
- The Next Step in Connectors
Just as the standardized Edison screw fitting played a vital role in the widespread adoption of the light bulb, the way in which high-intensity LEDs are integrated into energy-efficient lighting assemblies will impact their long-term reliability, cost and usability.
Since LEDs are essentially point sources of light, they present engineers with an entirely different set of integration issues. High power densities make thermal management a critical part of the fixture design. And, some level of electronic circuitry is required to provide a constant current flow. With few exceptions, LEDs cannot be plugged directly into a normal AC voltage source.
The right connector can help engineers tackle these issues and ensure their LED lighting designs are affordable and easy to use.
An electrical connector consists of a housing and contacts for creating a separable electromechanical interface. This interface can be used for wire-to-wire, board-to-board, or wire-to-board connections.
Each element of the connector—the housing design and material, the contact design and material, and the contact plating—are important to its overall function.
The purpose of the contact is to establish a reliable, yet separable low-resistance connection. There are four characteristics to consider: electrical, mechanical, thermal and environmental.
Electrically, the contact exerts some level of spring force against the wire to ensure a low-resistance connection. Mechanically, that spring force must last the life of the connector. Depending on the application, the spring force must resist vibration and other mechanical disturbances. The interface and bulk contact must be sized to stay cool for the rated current it will carry. To keep the interface resistance low, the contact’s surface must resist the formation of nonconductive films and corrosion.
Spring properties are mainly a function of the base contact material. Plating over the base material guards against corrosion and other environmental deterioration.
The two most fundamental electrical criteria for any connector are voltage and current.
Traditionally, any connector designed to handle less than 1 amp with a temperature rise under 10 C is considered a signal connector. Signal contacts typically have one or two contact interfaces and often incorporate gold plating for the lowest possible interface resistance. Due to tight contact spacing, signal connectors are not well-suited to applications above 48 volts due to voltage separation requirements.
Although signal contacts have an inherently low current rating, higher currents can be carried through these connectors by using multiple contact positions in parallel. However, the manufacturer should be contacted to obtain the appropriate derating factors for the contacts when paralleled. Similarly, higher voltage ratings are sometimes obtained by skip-loading contacts—that is, populating every third or fourth contact position to increase the distance between contacts.
Power contacts can carry more than 1 amp and handle a temperature rise of more than 10 C. Power connectors are typically larger and have fewer contacts, since adequate contact material is needed to carry current without significant joule heating of the contact body. Multiple contact interface points are desirable, since they provide parallel paths that minimize interface resistance and decrease joule heating at the interface. Because power contacts can handle higher voltages, contacts must be spaced further apart to meet dielectric requirements and prevent inadvertent shorts.
The contacts are what make a connector work. Selecting the right material for an application is a compromise between cost, mechanical performance, electrical performance and size constraints. The right material will ensure that adequate force is applied at the interface for the life of the connector.
Most contacts are made from a copper alloy. Brass is the most common material. Brass contacts can be found in both medium-pitch signal and power applications. Brass is a good balance between cost, conductivity and mechanical performance. It is readily available in various strengths to suit many applications. It is prone to environmental corrosion. Ammonia attacks brass vigorously, so plating is strongly recommended for such environments.
Another common contact material is phosphor bronze, which maintains excellent long-term spring properties and can handle higher continuous-use temperatures than brass. Phosphor bronze is excellent for smaller contacts, where its enhanced spring properties can best be leveraged. Numerous alloys are available, providing a broad range of conductivity and mechanical properties. Phosphor bronze contacts are usually found in small- and medium-pitch signal and moderate-power connectors.
A third common contact material is beryllium copper. A premium material, beryllium copper has superior spring properties. As a result, it’s best used for military, aerospace and telecom applications, where minimal size and weight are important. It is significantly more expensive than brass and phosphor bronze, but it can be readily heat-treated to enhance its spring properties.
Other materials are available for special applications. Semi-exotic copper alloys, such as copper-iron or copper-nickel-silicon, provide excellent performance, but are premium materials. Clad materials, consisting of one material bonded atop another, are used in the rarest of instances, due to their cost.
Electrolytic tough pitch copper, combined with a separate spring member made of stainless or plated steel, provides high conductivity. In high-temperature applications, such as electric oven heating elements, steel or steel alloys may be used, although conductivity is drastically reduced compared with copper alloys. The use of ferrous contact materials requires careful design consideration by both the connector designer as well as the product engineer. Inherent corrosion issues require that steel contacts must be robustly plated.
Platings can be broadly categorized into two groups: noble and non-noble materials.
Noble platings use pure gold or a gold alloy as the final plating material. By its nature, gold plating reigns supreme for signal applications, since noble materials do not corrode or oxidize and, therefore, maintain very low resistance interfaces.
Pure gold, being very soft, is seldom used alone. It’s typically alloyed with cobalt or palladium to increase the hardness of the surface. Contact interface forces can be light with gold (50 grams or less), making it desirable for high-pin count connectors, where the combined mating and unmating forces can be significant.
Since it’s costly, gold plating is very thin. Plating the entire contact is cost-prohibitive. Selective plating, in which gold is applied only to the areas where it will benefit electrical performance, is increasingly common. Another approach is dual plating, in which gold is applied at the separable interface and tin is used for soldered or press-fit connections.
Gold plating is almost always applied over some type of barrier plating, most often nickel, to minimize pore corrosion. While the gold itself does not corrode, corrosion forms through microscopic pores that occur in gold plating when thinly applied. When these pores extend to the base contact material, corrosion can form on the surface of the plating. However, when a nickel underplate is used, the inherent self-passivation properties of nickel limit corrosion forming through the pores. Without the nickel barrier, corrosion will inevitably form and degrade the contact interface.
As one might expect, non-noble platings—typically tin, nickel and silver—are subject to corrosion. Tin and tin alloys dominate as non-noble contact platings.With the right design, in the right environment, tin plating can form a durable and cost-effective interface for most applications.
Non-noble platings are typically applied in a thicker layer to ensure adequate barrier properties. As a result, more force is required to mate and unmate the connector to break through the inevitable nonconductive oxides that will form on the surface.
The exception is silver-based plating. Silver oxides are relatively conductive, malleable and easily displaced. Various anti-oxide coatings can be applied to minimize formation of these oxides, but they cannot be eliminated altogether, so mitigating their effects falls to the experience of the connector designer.
What happens when oxides form on non-noble platings? The biggest degradation mechanism affecting tin and silver is fretting corrosion. This occurs through micro-motions on the contact interface and can be thermally or mechanically induced. These micro-motions cause problems through oxide formation.
To use a medical analogy, each micro-motion opens a fresh “wound” in the plating surface. This wound eventually “scabs over” with a nonconductive oxide layer. As subsequent micro-motions open new wounds and reopen existing ones, the scab layer builds while conductive areas diminish, resulting in increased interface resistance.
In a signal application, this resistance eventually builds to the point where the connector no longer conducts the signal. In a power application, the results are more dramatic. Increased interface resistance results in increased joule heating of the interface and contact. This heat eventually builds to the point where it anneals the contact material and gradually reduces the force exerted at the interface. With decreasing force comes increased resistance, which further increases heating at the interface. Eventually, this vicious cycle will cause the interface to fail catastrophically.
Mating Gold to Tin
Mating a tin-plated connector to a gold-plated connector may sound like a good idea to prevent fretting corrosion, but it’s not. Some tin will transfer to the surface of the gold during the initial mating. This transferred tin forms the nucleus for tin oxide growth on the gold plating. In the end, fretting corrosion remains a possibility despite the premium paid for gold-plating half of the connector.
All is not gloom and doom with non-noble platings. A well-designed connector can mitigate the formation of oxides through two mechanisms. First, it provides an initial wiping motion upon mating to break through oxide films. And second, it applies at least 100 grams of force to minimize micro-motions. The use of contact lubricants and anti-oxide compounds in the manufacturing process also helps prevent fretting corrosion.
The “Tin Commandments,” developed by Tyco Electronics more than 50 years ago, still ring true today:
1. Tin-coated contacts should be mechanically stable in the mated condition.
2. Tin-coated contacts should apply at least 100 grams of force.
3. Tin-coated contacts need lubrication.
4. Tin coating is not recommended for continuous service at high temperatures.
5. The choice of plated, reflowed, hot-air-leveled, or hot-tin-dipped coatings does not strongly affect the electrical performance of contacts coated with tin or a tin alloy.
6. Electroplated tin coatings should be at least 100 microinches thick.
7. Mating tin-coated contacts to gold-coated contacts is not recommended.
8. Sliding or wiping action during contact engagement is recommended with tin-coated contacts.
9. Tin-coated contacts should not be used to make or break current.
10. Tin-coated contacts can be used under dry circuit or low-humidity conditions.
The connector housing provides many important functions. Fundamentally, the housing provides electrical insulation between adjacent contacts and between the contact and the outside world. This insulation is usually stated as the dielectric withstand voltage rating for the connector.
The housing also holds the contacts in one half of the connector in correct relationship with the contacts in the mating half to provide trouble-free mating and unmating. The housing also fixes the spacing between contacts and defines the creep (electrical tracking distance over surfaces) and clearance (linear “line-of-sight” distance) between contacts.
In some instances, the housing provides different cavity configurations to allow a mix of signal and power contacts. In other applications, connector housings are integrated with a larger circuit enclosure, providing an added value to the customer. Lastly, the housing provides some level of environmental protection to the electrical contacts.
The level of environmental protection afforded by the housing is defined by the internationally recognized Ingress Protection (IP) rating system. Each electrical connector is given a two-digit IP rating. The first digit refers to protection from physical objects; the second deals with moisture. Most unsealed connectors have an inherent rating of IP20.
Preventing physical touching of the electrical contacts is also a key function of the housing and is particularly important in high-voltage applications. At a minimum, the housing prevents handling damage to the contacts themselves, which may occur during assembly or installation of the finished product. When dealing with elevated voltages, safety standards require design features to prevent accidental touching of the electrical contact. Most often, the contacts are recessed and shrouded as a mechanical barrier to contact.
The housing must be robust enough to handle the application’s environmental conditions. The connector must withstand both application temperature ranges and processing temperatures, such as those experienced during reflow soldering. If outdoor use is anticipated, the housing should resist ultraviolet radiation. If a connector will be subjected to mechanical shock and impacts, the housing should be made from a tough polymer or even a metal shell.
If active latching is required, specify a polymer that allows latch flexibility while still meeting other mechanical and electrical requirements. Even chemical exposure should be considered. For example, if the connector will be used in a gasoline pump, the housing must withstand continued exposure to volatile hydrocarbons without becoming brittle.
When selecting the best connector for an application, basic electrical, mechanical, and environmental performance requirements must considered.
Electrically, the connector must be compatible with the current levels and voltages of the application. Continuous voltage and current should be considered, as well as transient and surge conditions that may occur over the life of the product.
Mechanical considerations can cover a range of features. As with all electronics, the trend is toward miniaturization, so connectors are packing more contacts in a smaller area. As a result, the physical size of the connector relative to the application needs to be considered. Mating direction and wire dress also need to be considered.
How the connectors are held together after mating can also affect the selection process. Some connectors rely on active mechanical latches, others use small detents, and some rely simply on friction to hold them together. Beyond obvious form factor issues, the connector must be evaluated for how much it can withstand mechanical abuse, such as vibration and shock.
Although it’s typically not an issue in lighting applications, shielding from electromagnetic noise may be necessary. Most connector suppliers offer both shielded and unshielded versions of the same connector. The connector must be designed specifically to terminate the cable’s shield, since it is practically impossible to convert a nonshielded connector to a shielded one. The key to a reliable, effective shield is to provide a 360-degree termination that maintains a low-impedance path from the cable through the connector to ground.
Signal applications typically do not require thermal management. Power applications do. Power contacts have a current rating that indicates the maximum current that the contact can continuously carry. This rating is usually based on a 30 C temperature rise in the contact and is based on measurements of a single contact. When multiple power contacts are used in a housing, the allowable current is derated to allow heat dissipation. When evaluating a connector, consult its specifications to determine the suitable current de-rating factor.
In high-intensity LED applications, the LEDs themselves generate enough heat to require thermal management all by themselves. This is typically accomplished by a heat sink and sometimes forced-air cooling.
LEDs perform better when operating at lower temperatures. The challenge of integrating thermal management into a lighting design is an excellent example of why engineers must consider interconnections early in the design process. Thermal management poses unique packaging challenges, since most high-intensity LEDs are small and produced as surface-mount devices. Integrating the connector in amongst the LEDs, circuit boards, optics and thermal devices is quite challenging if left to the end of the design process. Considering the connector early in the design process provides for a more tightly integrated, optimized solution that can make assembly and repair more efficient.
Beyond the normal electromechanical aspects, connectors must also address the special needs of solid-state lighting. These include higher operating temperatures and the ability to provide housings in specific colors so the connector blends into the visible parts of the lighting designer’s fixture. Furthermore, it’s often desirable to use board-mounted connectors with softened edges to minimize shadowing and the possibility of occluding the light output of low-profile, surface-mount LEDs.
The Next Step in Connectors
Most LED lighting designs incorporate one or more LEDs onto a circuit board, usually a metal clad one for thermal conductivity. This assembly is then integrated into the fixture.
But that’s not the only way to do it. An engineer could combine a heat sink into the electromechanical design of the connector to create a thermoelectric connector that mates directly to the LED.
An example is the high-intensity LED holder recently introduced by Tyco Electronics. This small, low-profile holder has snap-together contacts for both electrical and thermal connections directly with the LED. The holder includes a contact carrier and a retention clip to secure the LED to the carrier. The module-to-cable interface is a standard, two-position, post-and-receptacle connection that facilitates plug-and-play operation.
Without the need for solder, thermal adhesives or metal-clad circuit boards, assembly is simple and cost-effective. The holder also expands the mounting options for LEDs beyond the planar constraints of circuit boards. As an added benefit, replacing a faulty LED or changing colors is straightforward. The consumer simply needs to detach the retention clip, remove the LED, and replace it with a new one.