Thick film resistor technology is the ideal choice for many applications. Other resistor technologies may outperform thick film in certain applications but are usually lower density and/or higher cost solutions. Thick film technology has excellent thermal properties and is low inductance. It also offers high resistance density (small size) and is comparatively cheap.
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 for most high volume applications.
High power devices require a larger surface area to dissipate heat or to interface with a heat sink. The TO style (leaded) package is the most common and is available in a number of standard sizes with part numbers (e.g. 1206, 0805, 0402) representing the package dimensions in inches.
Where multiple resistor devices are required on a single substrate the leaded resistor network is the most common solution. Standard devices are available for applications such a voltage dividers and pull up/pull down resistors.
The Need For Custom Resistor Packaging
In some high performance applications using a standard device may not be an option and a custom resistor solution may be required. Where there are dimensional constraints or the need for a particular shape or form factor a custom thick film resistor may be the only solution. Medical applications increasingly require heater resistors with small dimensions that are designed specifically to fit in difficult to access areas.
In high power applications, power dissipation and the removal of heat may be a key issue. Specialist heat sinking may be required or the resistor may need to be designed to take advantage of any cooling air flow.
In general, the amount of heat a resistor may dissipate is directly proportional to its size. Thick film resistors based on standard alumina substrates have excellent thermal properties. A specialist resistor manufacturer may employ various combinations of substrate and resistor materials to deliver the higher performance required for demanding applications.
Custom Thick Film Resistor Package Styles
Custom thick film resistors may be manufactured in a wide range of package styles to fit the application. Various aspect ratios and custom shapes are available as flat substrates. Intricate shapes may be manufactured with cut outs if required to fit around mechanical obstructions or other circuit elements.
Leaded, leadless, custom terminations and true surface mount devices may all be manufactured including flip chip technology based solutions if required. A wide range of encapsulants and protective coatings may be added depending on the application.
A custom thick film resistor solution will almost always be more expensive than a standard product due to economies of scale but this must be measured against failure to achieve optimal system performance and system lifetime management costs.
It is important to choose a custom resistor manufacturer wisely. The major resistor manufacturers tend to focus on volume and their systems, equipment and processes are designed accordingly. If a custom device is not required in sufficient volume their costs can, therefore, be prohibitive.
The reverse is true for a custom resistor manufacturer. They tend to have more flexible manufacturing operations and are set up for low to medium volume manufacture. A custom resistor manufacture may have no capability to manufacture standard devices such as TO style and axial devices preferring instead to concentrate on custom solutions.