To develop new and novel thermal NDT methods for the detection of defects specified in the defect catalogue. To develop and design a new and novel thermography system that can provide real-time images of modern IC assemblies and detect misalignment and poor morphology as well as cracks in wires, interconnects (ball and pad) and to integrate the system for in-line operation with other NDT techniques. Also to develop automated algorithms for defect recognition.
Background
All objects that generate heat in the electromagnetic spectrum also generate infrared radiation. Infrared radiation (IR) has a longer wavelength then visible light, and as such infrared radiation cannot be seen but can be detected. IR from an object is converted using a suitable IR detector and displayed as a colour image. In this project both active thermography and passive thermography are being investigated
- Active thermography yields information of the internal component and board structures. I.e. delaminations, disbonds and other anomalies present below the surface of the board and in the components themselves.
- Passive thermography gives the ability to obtain and compare thermal signatures for a given PCB. A short circuit will exhibit a thermal radiation signature that is different from its surroundings. Therefore, PCB circuit faults can be pin-pointed by the identification of "hot spots" within the circuit.
The most common mode of active thermography is pulsed thermography. In this technique, also known as transient thermography, the surface under investigation is pulse heated (time period of heating varying from milliseconds for short pulse or flash thermography to a few seconds for long pulse thermography) by one or more powerful lamps and the resulting thermal transient at the surface is monitored using an infrared camera. The duration of the heating pulse depends on the thermal and physical properties of the materials, as well as its thickness. So, the heat flow into the sample is altered in the presence of a subsurface defect or feature, creating temperature contrast at the surface and as a result can be detected. This is demonstrated in figure 2 where three BGAs are thermally imaged and show the chip die within their packaging's. The materials of the die and the packaging have different thermal diffusivity giving rise to different temperature contrasts.
Fig.1 1st time derivative image acquired at 0.551 seconds revealing delaminations within the board. Frame rate of 60Hz.
Fig.2 Initial image revealing three chip die within their packagings. Image acquired 0.267sec after the initial flash.
Image Processing
By mathematically processing the images it is also possible to show the images of the first and second derivative. In many cases, features that cannot be seen in the original averaged images can be revealed in the first or second derivatives. The first derivative representation is made in the time domain, not the spatial domains, i.e. the first derivative is the changes of the pixel values along the time axis. Likewise the second derivatives representation is made in the time domain, not in the spatial domains. The second derivative is the changes of the changes of the pixel values along the time axis.
Fig.3(a) Averaged Image. Extracted from the image shown in figure 2.
Fig.3(b) 1st Derivative
Fig.3(c) 2nd Derivative. In the bottom IC, the visualisation of an irregular shaped die indicates possible delamination
In passive thermography there is no application of an external heating source. However, in order to generate a heat signature there is a requirement to power the PCB. Passive thermography will give us information on the temperature distribution of the board and all the components i.e. in figure 4, temperature is equated to the different colours with an accuracy of 2% of absolute in degrees Celsius. This will enable information on the potential component failures and board failures at the surface and is very fast in terms of analysis comparable to optical Inspection. In theory powering the PCB could be a destructive process. However, as we are only looking to compare thermal signatures, it is possible to exploit passive thermography by inspecting the PCB at lower power, and only momentarily powering the board to prevent any destructive short circuits.
Fig.3 Thermography Setup (In this case used in passive thermography mode)
In-line Automation
The passive thermography system is currently being automated for in-line inspection. A linear actuator will be used to mechanically place the power probes onto the PCB to allow momentary "power-up" of the PCB. The software will compare predefined user thresholds at different positions on the thermal signature to determine a PCB is defective.
Fig.4 Hot underside of an IC PCB-component failure
Fig.5 Hot component on PCB (component side)
Fig.6 555 timer IC with its output shorted by five resistors
Fig.7 Proposed Passive Thermography Setup
