Thermal Emission System for Circuit Board Troubleshooting
How It Works
Infrared Thermal Emission
As circuit boards are powered, they emit electromagnetic (EM) energy in a broad spectrum of wavelengths and in an amount that is proportional to their temperature. As the temperature of a board increases, the amount of EM energy it emits also increases. The vast majority of EM energy emitted by circuit boards is in the infrared spectrum. Infrared energy, which is invisible to the human eye, is defined as EM energy with wavelength between 1 and 1000µm.
Optotherm's cameras are able to detect this infrared energy and create two-dimensional images that display the infrared energy eminating from various points on a circuit board. These infrared images represent the temperature distribution over the surface of the board. Optotherm infrared cameras are sensitive to a specific band of the infrared spectrum between 7 and 14µm in wavelength. The majority of infrared energy emitted by circuit boards with temperature below 200°C is within this wavelength band.
Circuit Board Infrared Analysis
The powering and operation of circuit boards involves the flow of electrical current, which produces heat when passing through a circuit having resistance. Since thermal behavior is closely related to current flow, the temperature distribution on an operating package, component, or assembly can be a good, repeatable indication of its operational status. Therefore, infrared images of a circuit board can be analyzed to determine a board's operational status.
Infrared analysis test coverage is a function of the degree of electrical stimulation and power dissipation in the board and in its components. Circuits and components that are not electrically stimulated produce no resistive heat and therefore, cannot be evaluated by infrared analysis. In many cases, simply powering a board exercises the majority of component and circuits. In more complex boards, a diagnostic or functional test may be required to stimulate all areas of a board.
Most electronic packages, components, and assemblies lend themselves to failure analysis by infrared inspection as long as the following conditions are met.
- There is a clear line of site between the camera and board
- Current flows through the components and circuits that are to be tested
- The external surface of components are not composed entirely of bare metal*
*Bare metals (unpainted, unanodized, etc.) have very low emissivity. Emissivity is the efficiency with which a surface emits infrared energy. Therefore, bare metals generally do not emit enough infrared energy to allow accurate measurement using infrared analysis. There are, however, surface treatments that can increase surface emissivity adequately to allow effective infrared analysis. For example, thin Kapton tape can be adhered to BGAs with stainless steel tops to determine the location of an internal short.