SMT molded resistors

Many different options exist for high-voltage, high-power applications

BY KORY SCHROEDER
Stackpole Electronics
Raleigh, NC
http://www.seielect.com

As little as 5 years ago, it would have been difficult to find a reliable 3-W SMT resistor, regardless of technology or packaging. Today, many different options are readily available.

SMT molded packages

SMT molded packages are truly the future of the power electronics industry. Compliant terminations, the ability to withstand high temperatures, and the ability to accept many different types of resistive elements mean that this type of package should be at the top of the list for consideration by power design engineers. Whether their design is driven by electrical performance, environmental and long-term reliability, or cost, SMT molded resistors can provide the perfect solution.

There are many SMT resistor options available for high-power applications.

The resistor technology and package choices are determined by the parameters and performance features that are most important to a design engineer, the product, and the overall direction of the EE’s company. In most cases, these requirements steer the engineering community to the SMT molded wirewound or a variation thereof.

Before discussing the details of the latest improvements in the SMT wirewound world, it should be pointed out that other technologies can be, and are, used as the internal resistor. These technologies have their advantages and disadvantages.

Competing technologies

Metal-film and metal-oxide elements offer lower inductance and are usually less expensive, but can’t approach the overall electrical and environmental robustness or pulse handling of a wirewound element. Thick-film elements are much less expensive, are low in inductance, and are impervious to moisture corrosion, but also can’t match pulse withstanding, electrical or environmental robustness ,and fall short of the precision capabilities of the standard wirewound element.

Carbon-comp elements provide excellent pulse-energy handling and no inductance but are much more expensive, have longer lead times, and have very poor precision and environmental stability. In addition, none of these other technologies are capable of the high-temperature operation offered by wirewound technology.

Resistance lows and highs

For low-resistance current-sensing, a design engineer can choose between flat ceramic chips and stamped, welded, or formed metal elements, as well as molded packages with compliant metal terminations.

For higher values, packaging options are a bit more limited, but the technology options broaden considerably. Among the technology choices for a 3-W SMT resistor are metal film, metal oxide, carbon comp, thick film, and — the most prevalent technology for this type of requirement — wirewounds.

Wirewounds

Wirewounds can be found in SMT packages in varying dimensions (especially height) starting typically with 0.5-W sizes around the size of a 2010 chip up to 5 W. Low-profile packages normally use either a flat metal ribbon as the resistance element (low values only) or a film chip as the element.

The benefits of the molded package and the large metal compliant terminations improve power handling of these resistive elements by 30% to 100%, depending on the size and element involved. This is due to the fact that the molded package suspends the resistive element off of the board, and to the efficiency of the compliant terminations in getting the heat generated by the element away from the part and into the board or the ambient air.

Film-chip devices

Film-chip elements can be chosen for high pulse withstanding, high voltage, or fusibility to tailor the characteristics of the part to suit the application need. For SMT molded packages with standard heights, the wirewound element can be tailored in many ways, some of which have only recently been discovered.

For example, there are occasions when the higher inductance of a wirewound element is desired. In those cases, the wire alloy and thickness is chosen such that the given nominal value will require a maximum number of turns to achieve it.

More often though, either low inductance or noninductive versions are needed. Low inductance can be manufactured by reversing the above philosophy and choosing the alloy/thickness combination so that only a few turns are needed to realize the requested value.

Noninductive devices use an Ayrton Perry method of winding two elements on a single part, each in the opposite direction of the other, thus canceling much of the inductive field normally observed. However, these characteristics are nothing new.

Handling very high voltages

What is more challenging is designing and building a wirewound resistor to survive a very high voltage, and high energy pulse of extremely short duration (30 ms, for example), but that fuses quickly and gracefully when exposed to continuous severe overload. Fusing gracefully is typically defined by opening without catching a piece of cheesecloth draped over the part on fire.

This type of requirement is common in products that have some type of human interface, such as large household appliances. To build a wirewound fulfilling this requirement involves the careful choice of materials, specialized processing, and extremely tight process control. The margin for error is so small that only the largest and most experienced manufacturers are capable of supplying these specialized parts.

High-power devices

Another trend regularly seen in the world of SMT wirewounds is the ability to achieve higher power ratings within a given package size. For example, a chip size of 4730 originally has been the default size for a 2-W power rating.

Using special alloys, advanced welding techniques, and through a stabilization process, this same part can withstand 3, 4, and even 5 W. This ability to handle extreme levels of power and high temperature is the main feature that wirewounds provide over other technologies.

SMT wirewounds are typically rated for temperatures of 275C, meaning if you can keep the part soldered to the board and stay within the power derating curve limits, then it will continue to operate reliably for many field hours.

However, there are applications that require high power ratings of 1 W and above, but don’t require ability to handle high temperatures or extreme pulse handling that a wirewound element provides. For those applications, it is possible to use a film chip as the resistance element.

The film-chip element is not only very cost effective, it can also provide many hours of reliable operation, and has good overall tolerance and TCR characteristics. Another inherent feature that using a film chip provides is the ability to fuse.

Finally, the compliant terminations found in the typical SMT molded package virtually eliminate field failures due to solder joint and part cracking; this is a key feature for any application that experiences regular temperature fluctuations where the long-term reliability of a ceramic chip the size of a 2010 or larger would be suspect. ■