Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, Italytemperature rise [K]14121086420R=95 mΩR=4,2 mΩ0,00 0,05 0,10 0,15 0,20time [s]8G18G22,252,001,751,501,251,000,750,500,250,00current [A]FR4viaLaser pulseIR-cameraLaser faserLaserFig. 5 Transient temperature behavior of two different damagedviasFig. 7 Setup for IR-thermography measurement using laserexcitationBy means of the very fast IR-camera, sequences of IRimagesof up to 1000 s-1frame rates could be easily recorded.Fig. 5 shows the transient behavior of top pads of differentdamaged nearby vias under current pulse excitation (2 A,50 ms, dotted grey line). The excitation causes on via (8G1,red line) a temperature rise of 13 K. With modern 3D computertomography analysis it was identified that the observedvia has an almost peripheral crack (~350 °).Fig. 7 shows the schematic setup for the IR-thermographymeasurement using laser excitation.With laser pulse heat energy is given on the pad of the viasunder test. The reverse side surface temperature of the pad ismeasured by the IR-camera. Laser and IR-camera are hardwaresynchronized, so that the recording of the IR-camerastarts simultaneously with the beginning of the laser pulse.In order to make more exact statements about the cracksize, a finite element via model with different crack angle issimulated, which is also embedded in organic substrate.Fig. 6 3D computer tomography analysis of via in multilayerboardA correlation between crack angles and temperature riseunder electrical current excitation could be found. With thiswe are able to create a more accurate life time model forelectrical vias.B. LASER EXCITATION ANALYSISIt was shown that the application of IR-thermography usingelectrical excitation is possible. Main disadvantage ofthis method is the need a direct electrical interconnectingline for excitation.For the development of a non destructive and contactlessinvestigation-proceeded the laser excitation with IRthermographydetection was investigated. The idea is tohave a pulsed laser excitation on the reverse side of the PCBsurface and to detect the temperature response on the topside.Fig. 8 Results of FE-simulation temperature response on reverseside excitation by laser pulseFig. 8 shows different temperature response on reverseside of via. If there is no crack corresponding with crack angle0° one can measure a temperature rise of approx. 4.5 K.Due to damaged heat path by crack growth the correspondingresponse decrease down to 1.5 K for nearly cut throughvia.The degree of damage of a via can be determined by thecomparison between the experiments and simulation (Fig. 9)©EDA Publishing/THERMINIC 2008 49ISBN: 978-2-35500-008-9