EVALUATION OF LEAD-FREE SOLDER BONDS AND EFFECTS OF THE LEAD-FREE PROCESS ON DEVICES USING ACOUSTIC MICRO IMAGING

摘要

Solder attach has long been established as a method ofrnbonding for various electronic applications. The applicationsrnrange from the attach of heat sinks in relatively large powerrndevices to the very small flip chip interconnects. The qualityrnof the solder bonds is critical. For example large voids in thernbond of a heat sink will prevent proper heat dissipation, andrnan open solder connection in a flip chip interrupts operationrnof the device. Clearly a method is needed to assess thernquality of the bonds. In addition lead free solderrnmanufacturing process changes must be considered,rnspecifically if higher temperatures are needed for reflow ofrnthe different materials. Higher temperatures may have arndeleterious effect on the devices or packaging of the devicesrnnot directly related to the lead free solder bond integrity.rnAcoustic micro imaging (AMI) is one technique that is usedrnfor non-destructive evaluation of solder bonds andrnpackages.rnAcoustic micro imaging uses high frequency ultrasound (5rnto 500 MHz) to image the internal features of samples.rnUnlike x-ray and visual inspection, which are also commonrnmethods for evaluating solder bonds, ultrasound is sensitivernto variations in the elastic properties of materials and isrnparticularly sensitive to locating air gaps (delaminations andrnvoids). This is unique to ultrasound.rnOver the past years much experience has been gainedrnconcerning the acoustic detection of various defect types.rnAlso, in working with the manufacturers of the devices,rninformation has been gained concerning the causes ofrncertain failures and phenomena. However, as new productsrnemerge the manufacturing technology changes. Presentlyrnthere is an effort to convert to lead free soldering methods.rnThere is a need to determine if and how the new materialsrnwill influence the quality and lifetime of the devices. Alsornthe new materials may necessitate changes in the acousticrnanalysis method used to evaluate the devices. With thernevolution of flip chip devices to smaller sizes and/or higherrnIO count the size of the bumps/bonds has becomernincreasingly smaller. This will create the necessity of evenrnhigher resolution in AMI in order to visualize and evaluaternthe small bonds.rnThis paper will present examples of AMI solder bondingrnapplications in microelectronics with particular attention tornlead- free solder bond evaluation, and discuss AMIrndevelopments to meet the challenges presented by therndesign and manufacturing of various components andrnassemblies.