1 Research Objectives Develop non-contact, non-destructive, low cost, fast, accurate, high resolution and automated system for evaluating quality of solder bumps and chips in packaged electronic devices (flip-chips, CSPs, BGAs, MCMs, and chip capacitors and resistors) : Develop non-contact, non-destructive, low cost, fast, accurate, high resolution and automated system for evaluating quality of solder bumps and chips in packaged electronic devices (flip-chips, CSPs, BGAs, MCMs, and chip capacitors and resistors) : –On-line during packaging assembly, and –Off-line during process development for process optimization and failure analysis »Using laser ultrasound and interferometric techniques. Develop vibration analysis model for: Develop vibration analysis model for: –Predicting presence of solder defect(s), and /or device defect(s) by »Predicting vibration modes of devices –Explaining experimental results. –Determining theoretical limits of system sensitivity and resolution. Laser Characterization System for Solder Joint/Bump Interconnects

2 Advanced Electronic Packaged Devices BGA Chip Capacitor Thin Film ResistorFlip Chip

3 Research Issues Solder bump quality is difficult to determine: Solder bump quality is difficult to determine: –Solder bumps on flip-chips,MCMs, BGAs, CSPs, and chip capacitors and resistors are hidden from view –Small bump size/high quantity of connections Existing nondestructive techniques for evaluating device quality Existing nondestructive techniques for evaluating device quality –X-ray / X-ray Laminography –Scanning Acoustic Microscopy

4 Research Issues Limitations of existing techniques Limitations of existing techniques –X-ray / X-ray Laminography: »Limited resolution for high-throughput online systems »Cannot detect small air gaps (i.e. cracked or disbonded joints) »High equipment and operational cost –Scanning Acoustic Microscopy »Requires fluid couplant (could be destructive) »Difficult to image bumps near chip edge »High equipment and operational cost

5 Solution: Laser Ultrasound-Interferometric Inspection System Noncontact and Nondestructive Noncontact and Nondestructive Directly measures strength of chip-substrate bond by exciting structural vibration Directly measures strength of chip-substrate bond by exciting structural vibration High Throughput High Throughput –Fast laser excitation and detection systems –Defects identified with very few measurements –Analysis techniques are simple Relatively low cost Relatively low cost Capable of detecting: Capable of detecting: –Missing solder bumps (balls) –Nonwetted, disbonded or cracked solder bumps –Starved solder –Misaligned or cracked packages

6 System Overview Nd:YAG Pulsed Laser Laser Vibrometer Vision Sensor PC: Control and Analysis X-Y Scanning System Fiber Vacuum Fixtures Test Substrate

7 Ultrasound Generation in Chip Devices Pulsed laser irradiation causes thermal expansion and contraction on specimen surface, generating stress waves within material’s elastic limit, which is source of ultrasound Pulsed laser irradiation causes thermal expansion and contraction on specimen surface, generating stress waves within material’s elastic limit, which is source of ultrasound –Ultrasound induces vibration in chip Laser Source Thermoelastic Expansion

8 Experimental Procedures Component located using vision system Component located using vision system Excitation fiber centered on component Excitation fiber centered on component Detection points scanned and data acquired Detection points scanned and data acquired Defects can be detected if they cause measurable changes to chip’s vibration response Defects can be detected if they cause measurable changes to chip’s vibration response Vibration signatures from defect-free devices are compared with tested devices to identify defects Vibration signatures from defect-free devices are compared with tested devices to identify defects –Signal processing techniques are used to compare vibration responses  Error ratio  Vibration frequency analysis  Pattern Recognition (completed feasibility studies only)

9 Chip Scale Package (CSP) Test Vehicle CSP specifications CSP specifications –7.0 mm square and 0.7 mm thick –I/O connections: 98 –Pitch: 0.5 mm –Bump diameter: 0.3 mm –Organic substrate

10 Chip Scale Package Tests

11 Experimental Results for CSP Excitation and detection locations Excitation and detection locations

12 Experimental Results for CSP Excitation and detection locations Excitation and detection locations

13 Time Domain Vibration Response Good Chip Bad Chip

14 Used to compare two waveforms Used to compare two waveforms f(t) = measured waveform f(t) = measured waveform r(t) = reference waveform r(t) = reference waveform Low error ratios mean two waveforms are very similar Low error ratios mean two waveforms are very similar “Error Ratio” Calculation Error ratio

15 Baseline Comparison of Two Reference Chips NANA

16 Misaligned CSP Compared to Reference NANA

17 CSP with Missing Solder Ball and Reference NANA

18 Missing Solder Ball – Refined Detection

19 Flip-chip on Ceramic Board Test Vehicle Data Acquisition Scanning Pattern Data Acquisition Scanning Pattern AB C D Interferometer Detection Point Input Fiber Position Solder Ball Flip-chip on Ceramic Board Chip Size: * 2.281mm Solder Ball Size: 0.33mm Pitch Size: 0.67mm

20 Solder Bumps of Flip-chip Samples

21 Signals from Flip-chip Samples Signals at Detection Point A Signals at Detection Point B Signals at Detection Point C Signals at Detection Point D A B CD Bad Chip 1 Bad Chip 2

22 Error Ratio Values of Flip-chip Samples Error Ratio of Good Chip 2 Error Ratio of Bad Chip 1 Error Ratio of Bad Chip 2 Note: Good Chip 1 is chosen as reference A B CD Bad Chip 1 Bad Chip 2

23 Detecting on Top of Each Solder Bump Error Ratio of Good Chip 2 Error Ratio of Bad Chip 1 Note: Error Ratio can also be used to localize a defect

24 Experimental Results Compared with Modeling Results 1 st Mode 2 nd Mode 3 rd Mode 4 th Mode Good Chip 1 (Reference) Modeling Experiment504627N/A784 % Error Bad Chip 1 Modeling Experiment438570N/A776 % Error

25 Test Summary System has been used to detect: System has been used to detect: –Misaligned CSPs –Missing solder balls (flip chips and CSPs) –Cracked/damaged flip chips –Flex cracks in MLCCs Significant sources of error: Significant sources of error: –Alignment of excitation laser spot to samples –Precision errors in detection point scanning

26 Conclusions Developed Laser Ultrasound and Interferometric System for Interconnect Inspection Developed Laser Ultrasound and Interferometric System for Interconnect Inspection Developed the following data analysis methods: Developed the following data analysis methods: –Time domain Error Ratio based method –Frequency domain based method or a device with few solder bumps, frequency and/or Error Ratio based analysis is used. When many bumps are present, Error Ratio based analysis will always produce good results. For a device with few solder bumps, frequency and/or Error Ratio based analysis is used. When many bumps are present, Error Ratio based analysis will always produce good results. Developing defect pattern recognition method that uses probabilistic neural network Developing defect pattern recognition method that uses probabilistic neural network Developed FEM based on vibration modal analysis Developed FEM based on vibration modal analysis Long-term goal: Develop prototype system Long-term goal: Develop prototype system

27 Conclusions (cont.) System limitations: System limitations: –Defect must affect the vibration response to be detected –Manufacturing variations ( solder bump size, bond pad location, etc.) could mask tiny defects System sensitivity / resolution is mainly determined by: System sensitivity / resolution is mainly determined by: –Number of detection points –System noise –Laser power –Precise positioning of laser incident and detection points on same spots on all devices –Laser wavelength because different materials absorb different wavelengths System sensitivity and resolution have not been tested to their limits because of limited number of tests performed. However, for samples tested, system is able to detect a missing solder ball in flip chip, which has a diameter of 120µm and pitch size of 457µm.

28 Conclusions (cont.) Current System Throughput Estimation For One Detection Point Current System Throughput Estimation For One Detection Point »Laser pulse rate: 20 pulses/sec (0.05s/pulse) »Number of signal averages: 32 or 64 →Laser excitation/Signal averaging time subtotal: 1.6s ~3.2s »Motion stage travel speed: 35mm/sec »Travel distance between detection points: 0.5mm~3mm →Stage travel time subtotal: 0.015~0.085s →Data analysis time: ≈ 0.1s/pt. → Total time per point varies from 1.72 seconds to 3.39 seconds → Total time per point varies from 1.72 seconds to 3.39 seconds

29 Impact of Research Discussion is under way to commercialize Laser Ultrasound- Interferometric System Discussion is under way to commercialize Laser Ultrasound- Interferometric System System can: System can: – Increase yield of flip-chips, BGAs, chip scales, MCMs, chip capacitors and resitors, and other surface mount components. – Be used in production line for on-line inspection for quality assurance. – Be used off-line for process optimization during process development and failure analysis. Increase in packaged product yield and throughput will save packaging industry millions of dollars per year. Increase in packaged product yield and throughput will save packaging industry millions of dollars per year.

30 Ongoing Research Efforts System improvements System improvements –Higher precision alignments of laser to device –More control of excitation energy characteristics –Large substrate capacity –More automation / higher throughput Finite element modeling Finite element modeling –Predict vibration modes of devices –Determine theoretical limits of system sensitivity and resolution Test more complex devices Test more complex devices Test variety of devices Test variety of devices