Issue link: https://iconnect007.uberflip.com/i/721932
September 2016 • SMT Magazine 95 DEFECT FEATURES DETECTED BY ACOUSTIC EMISSION Figure 5 shows one of the most common failure modes (popcorning) as a result of de- lamination, moisture accumulation, and pres- sure release within a plastic package during the board assembly process. Delamination is depen- dent on package construction, package size, die size, lead design, number of leads, and environ- mental stresses, among other influences. Sandor and Agarwal reported a number of anomalies and potential reliability defects in- cluding delamination at die attach, at leads within the mold compound, around the die within the mold compound, on top of the die, and at the backside of the die paddle. The au- thors analyzed the defect anomalies by C-SAM imaging to determine their impact on the reli- ability of PEMs. C-SAM images from the begin- ning of a screening flow were used as a predic- tor of good or poor subsequent electrical per- formance of devices. Images tended to correlate with changes in electrical performance. C-SAM inspection and electrical parametric shifts of de- vices that were subjected to convection reflow were affected less than those equivalent devices exposed to hand soldering and vapor phase re- flow (two zones, preheat and reflow). In an investigation of defect detection for a multilayer ceramic capacitor (MLCC) 8 , it was found that the 50-MHz transducer is more ef- fective in detecting defects during screening by C-SAM than a 30-MHz version. Screening at a higher frequency enabled reducing rejection that was initially discovered during the board level testing. It saved costly rework at the board level even though there was a slight cost in- crease due to additional MLCC rejection. AMI has been used also to analyze flip chip underfill and interconnect bonds since early 2000 when ultra-high frequency transducers are introduced 9 . It was shown that defects such as delamination and void can be detected at each layer and, with 3V (virtual volumetric viewing), the 3D morphology and depth location of the defects can give important information as to the cause of the flaws. Transducers and imaging techniques provided focused access of the ultra- sound beam to the interface of interest (chip/ bump and underfill, or bump and underfill/sub- strate) through any thickness of silicon com- monly encountered. Kessler 10 has shown using color acoustic im- ages, full or partial disband of solder balls and voids in the underfill in a flip-chip assembly. To show such conditions, a flip-chip package was imaged from the top side from the back of the die at the high acoustic frequency of 230 MHz Figure 5: Examples of packages with delamination. The yellow arrows show areas where the existence of delamination can accelerate entry/collection of moisture; the red lines show where the cracks (popcorning) typically occur when the board is exposed to high temperature during assembly 7 .