A range of thick film power resistor package options is available. These include leaded, leadless, true surface mount devices, custom terminations, and flip-chip technology-based solutions.
Manufacturers use a range of encapsulants and protective coatings to protect packages from mechanical and environmental damage. The choice of package and coating ultimately depends on the application.
Power Resistor Packaging Challenges
In high power applications, power dissipation and the removal of heat is an issue. Heat must be dissipated to prevent damage to the resistor materials without impacting on any nearby components.
Thick film power resistors can reach temperatures that exceed safe operation within seconds. It is important to keep the operating temperature range within maximum specified limits – typically 50 – 60⁰C.
At the resistor component level, the classic solution is to use materials with superior thermal properties and sufficient mass to dissipate heat. However, this solution can compromise component packaging density. While a small package size is desirable, this must not compromise the heat dissipation of the resistive element.
At the system level, using a combination of heat sinks, forced air cooling, water cooling and heat pipes, can keep the operating temperature range within specified limits. When heatsinks are used in isolation they require excellent thermal bonds between the resistor element and heatsink to avoid thermal fractures of the resistor.
Standard Thick Film Resistor Package Styles
Standard thick film devices are available in leadless surface mount, axial (through-hole) and MELF (Metal Electrode Leadless Face) style packages. When multiple resistor devices are required on a single substrate, the leaded resistor network is the most common solution.
High power resistor devices require a larger surface area to dissipate heat or interface with a heat sink. The TO style (leaded) package is the most common. It is available in many standard sizes with part numbers (e.g. 1206, 0805, 0402) representing the package dimensions in inches.
Custom Thick Film Power Resistor Package Options
In some high-performance applications using a standard device may not be an option. Where there are dimensional constraints or a particular shape or form factor requirement, a custom resistor package may be the only solution.
A custom thick film power resistor package will often be more expensive than a standard product due to economies of scale. But this must be measured against failure to achieve optimal system performance.
It is important to choose a custom resistor manufacturer wisely. The major resistor manufacturers tend to focus on volume. Hence they design their systems, equipment and processes accordingly. If a custom device is not required in high volume, their costs can be prohibitive.
The reverse is true for a custom thick film power resistor manufacturer. They tend to have more flexible manufacturing operations and are set up for low to medium volume manufacturing. They may have no capability to manufacture standard devices. Preferring instead to concentrate on custom thick film resistor packages.