Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyB. Laser-proof boxIf the thermal excitation has to be very localized and in ashort period of time laser beams are the method of choice.To be free in arrangement of laser fiber to specimen we havedesigned a laser proof box (Fig. 2). So we can handle a classIV-laser under condition of class 1b laser.A. ELECTRICAL EXCITATION ANALYSISFig.3 shows the schematic setup for the IR-thermographymeasurement using electrical excitation. A short pulsed currentthrough the vias causes a heat up in the vias. The transientthermal responses at the top surface of the excited viaswere on-line monitored by an IR-camera.IR-CamCurrent pulseViaII=2A; t=50msFR4IFig. 2 Laser-proof box with integrated xyz-position systemFig.3 Setup for IR-thermography measurement using electricalexcitationThe box contains a xyz-positioning system with 12.5 µmresolution in each axis. A variation of the distance betweenthe laser fiber end and the specimen the beam spot diameterfrom 0.5 mm up to 65 mm with 200 mm distance to objectcan easily adjusted. Using a collimator we are independentof distance with spot diameter of 5 mm.The electrical resistance of a vias rises with the increasingcrack. Due to the different resistance values different temperaturesat vias will develop, which can be detected on thepads surface with the IR-camera.III.PULSE IR-THERMOGRAPHY AS NON-DESTRUCTIVEFAILURE ANALYSIS METHODA common analytic method for via testing (e.g. life-timeestimation) is based on electrical resistance measurementswhere the vias are connected in series (e.g. daisy chain structure).The disadvantages of this method are that a failed viain the chain is first detectable if the via is almost torn due toincreasing horizontal crack angle growth in the cylindricalvia solid. Conventionally destructive method like cross sectioningor nowadays 3D computer tomography as nondestructivemethod are used to identify cracks in a failed via.These methods are very time and cost expensive and show incase of the cross sectioning only a small fraction of thecrack. The planned large scale crack testing after e.g. a manufacturingprocess of a PCB is not available.As a non-destructive failure analytic method the pulsedIR- thermography method was investigated using electricaland laser excitation.Fig. 4 IR-image after electrical current pulse (2 A, 50 ms)Fig. 4 shows the surface temperature of two different damagedvias. The used high precision resistance measurementsystem detects 4.2 milli ohms at the left via and 95 milliohms at the right one. The increasing resistance of the rightone is causes by an higher joule energy dissipation (1) whichis followd by an increasing temperature in crack region. Dueto heat conduction in the via cylinder to the board surface atemperature rise can be detected by the IR-camera.P2= R ⋅ I(1)Where, P is the joule energy dissipation, R the electricalresistance of via and I the electrical current.©EDA Publishing/THERMINIC 2008 48ISBN: 978-2-35500-008-9