High Power Resistor Design and Test Tips Help Extend Product Lifetimes

The testing resulted in two catastrophic failures, which were identified as fractured beryllium-oxide (BeO) ceramic chips. Such failures can then be correlated to a “root cause” analysis and included in a report that summarizes the cause of the failures. This study helps address materials and processes that could have an impact on failures occurring with present and future component designs. The results have also identified the need for possible design modifications to increase reliability and lifespan.

Digging into Design

During the design phase, the following recommendations apply to high-power resistive devices: reviewing and understanding a customer’s specifications; knowing the maximum operating frequency, the maximum VSWR, and the minimum and maximum operating temperatures at full rated power; knowing pulsed-signal requirements, including pulse width and duty cycle; knowing the power derating specifications; and knowing the power and temperature-cycling requirements.

Selection of materials is critical when designing and developing a high-power resistive component for specific customer requirements. One starting point is deciding which ceramic material can serve as the building-block material to meet a customer’s electrical and thermal design requirements, and whether the design should be fabricated as a thick- or thin-film device. If thin film, should it be based on tantalum-nitrate (TaN) or nickel-chromium (NiCr) films, typically deposited on an alumina, aluminum-nitride, or beryllium-oxide substrate? In addition, if NiCr material is chosen, does the application (such as a high-power pulsed circuit) require any sort of metal reinforcement, such as nickel?

Materials selection also extends to the device flange—it should meet various thermal requirements, such as a coefficient of thermal expansion (CTE) that is well matched to the other materials selected for a hi-rel component. In addition, decisions must be made on the flange plating material, such as inter-metallic materials, and solder or brazing material used to attach a chip component to a flange-mount package. Other materials selection requirements involve whether chip plating will be needed (depending on the solder or brazing requirements) to meet the requirements of a particular application.

Developing a high-power passive component will involve the calculation of an optimum film topology for handling the required power levels while still meeting the RF/microwave performance specifications. It will also necessitate thermal analysis and simulation of the material stack-up per customer requirements using modern computer-aided-engineering (CAE) software simulation tools, as well as simulation of the electrical performance, such as insertion loss, return, loss, and VSWR.

Along with proper materials selection, the choice of manufacturing process should be based on meeting the design goals and customer requirements for a particular component. In addition, test procedures should be formulated in support of characterizing a component for the performance goals set during the design and materials selection stages of a product’s development. It’s crucial that test fixtures be designed for accurate testing of the manufactured component. Also, the fixtures should be able to deliver accurate data consistently throughout the validation process, since this data will determine the success or failure of achieving the design goals. All test data should be recorded, compiled, and incorporated in a validation report that is available for future use.

Validating a Design

For a high-power resistor or termination, two main design considerations can impact the hoped-for long-term reliability of the component: the component being subjected to high temperatures that exceed the limits of the component’s materials, causing reliability issues; and excessively high temperatures being sustained for a long-enough duration that results in damage to the component, or complete failure.

For such a passive component, when power is dissipated, the temperature gradient through the ceramic, solder, flange, and thermal compounds used to construct the component, along with the CTE mismatches among these materials, creates mechanical stresses whenever CTEs are significantly different. The solder attachment of the chip to the flange is subject to fatigue cracking with repeated exposure to stress. A robust design can withstand any fatigue caused by cycling on and off and will not degrade and fail over time.

Material selection is critical for hi-rel components so as to avoid thermally induced stresses that can cause failures. For steady-state operation, a standard material stack-up is normally sufficient. However, when a component must handle power and temperature cycling, solder and flange material selection is critical for high reliability (Fig. 2).

Two cases were used to explore the effects of material selection. In the first case, the material stack-up thermal resistance is 0.1580°C/W with a Δt gradient of +39.5°C at 250 W, resulting in an approximate film temperature of +139°C. This is well below the industry-standard limit of +150° to +180°C. Although the junction temperature (θ_jc) is very good, the CTEs of the materials differ significantly, resulting in mechanical stresses that will eventually lead to component failures. In the second case, the thermal resistance is 0.2036°C/W, which is higher than the first case, with an approximate film temperature of +150.9°C. This is still within the preferred limit, but the CTE of the material stack-up is almost identical to the first case. Nonetheless, the second case’s material stack-up has the advantage of sustaining higher reliability and a longer life span.

Solder voids must be considered during the processing phase when assembling high-power devices. This has been addressed by the semiconductor industry over many years of research and testing. Myriad papers have been written that pertain to the reliability impact of thermal rise within the junction of the semiconductors. A simple rule of thumb: For every 10°C rise in junction temperature, the reliability drops by 50%. Although this rule of thumb is meant for semiconductors, the same design approach and process methods should be practiced for high-power resistors and terminations designed for long lifetimes.

3. Creating a thermal model of a passive resistive component involves studying thermal patterns (a) and understanding differences in thermal flow (b) through different materials in the component.

New designs and validation of current designs should be subjected to thermal-analysis modeling (Figs. 3a and b) and become part of the engineering design practice. Although thermal modeling may not account for some mechanical variances, it can provide a model that is relatively close to the real application. Solder voids are the “invisible killer” for high-power components. Solder voids can create a “thermal bottleneck” or an interruption in the normal thermal conductivity of the resistive film, leading to potentially catastrophic hot spots at high power levels.

4. Voids in component material stack-ups can lead to hot spots and reliability issues when operating at high power levels.

A significant size void directly under a thin film can considerably increase film temperature. The resultant heat flow depends on lateral heat flow through the ceramic to an area where no voids are present in order to dissipate the additional heat. This creates higher thermal resistance and mechanical stress. The mechanical stress and higher temperature could in fact induce a fracture in the ceramic material, causing a catastrophic failure shown in Fig. 4 as opposed to a normal heat flow shown in Fig. 5.

5. Ideally, good columnar heat flow through a passive component provides effective dissipation of heat at high operating power levels and temperatures.