Modern microprocessors run hot and gaining an insight into how these chips dissipate heat both to the air and the PCB to which they are attached is the subject of serious interest to the industry.
One analytical tool is the IR camera. Observing an assembled PCB’s surface temperature provides a clear indication of any potential problems. An example of an overheated component detected using an infrared thermal imaging camera is shown in Figure 1.
In this type of application the key performance factors required are the IR camera’s sensitivity and its ability to operate effectively at fast frame rate. High sensitivity allows the camera to measure temperatures with greater accuracy and the fast frame rate allows analysis of the temporal behaviour of the package.
Studying heat dissipation at the chip level requires observation of very small regions of interest, typically with a resolution of the order of 10 micrometres. Measurements at this resolution can be made only by using an IR camera with a high quality microscopic lens. That brings demanding optical design challenges since the limiting factor for spatial resolution is the diffraction limit. The dependence of the diffraction limit on the wavelength is given by the Airy disk diameter which is equal to 2.44 x l x F#, where l is the wavelength of interest and F# is the aperture of the lens. The Airy disk diameter indicates the limit above which a single pixel will be dependent on its neighbours.
Making silicon opaque
It is also important to consider that at the 3 to 5 µm IR waveband typically used for thermal scanning, unexpected results can be seen when observing silicon chips due to the semi-transparency of the material at this wavelength. Because of this, the radiation measured from the camera can come not only from the surface of the chip, but also internally. This can be confusing when trying to pinpoint the source of hotspots.
The solution is to observe in the 8 to 12 µm range, which makes silicon appear opaque, and allows exclusive observation of the surface. The trade-off is that the Airy disk diameter increases by a factor of two, lowering the resolution and accuracy as a result.
To offer a practical solution to these demands, Cedip has developed a set of microscopic lenses specifically designed for the electronics microscopy applications that offer spatial resolution of 10 and 20 µm.
An example of temperature measurement taken with a Cedip MWIR camera and a G3 (10 µm) resolution lens is shown in Figure 2.
The bonding structure can be seen, demonstrating the benefits of higher resolution.
There is a growing range of applications for IR cameras in the electronics industry, from simple inspection of PCBs for GO-NO GO testing to microscopic examination of wafers.
Optical performance, spectral response, sensitivity and frame rate are the key parameters for such instruments.
Further information – Dmitri Ishchenko works for Applied Infrared Sensing (03) 9556 5451