Scanning Acoustic Microscopy Training

Scanning Acoustic Microscopy Training This presentation and images are copyrighted by Sonix, Inc. They may not be copied, reproduced, modified, published, uploaded, posted, transmitted, or distributed in any way, without prior written permission from Sonix. 8700 Morrissette Drive Springfield, VA 22152 tel: 703-440 440-02220222 fax: 703-440 440-9512 e-mail: info@sonix.com

This presentation serves as a brief introduction into the theory and operation of scanning acoustic microscopes. 2

Ultrasound Inspection, Using an Acoustic Microscope Ultrasound What does this thing do? Non-Destructive Testing Example Images 3

Ultrasound MEDICAL SONAR University of California Medical Center San Francisco, California What are are Ultrasonic Waves? Ultrasonic waves refer refer to to sound waves above 20 20 khz khz (not (not audible to to the the human ear) ear) 4

Non-Destructive Testing NDT utilizes various non-invasive measurement techniques, such as ultrasonics and radiography to determine the integrity of a component, structure, or material without destroying the usefulness of the item. 5

Where an Acoustic Microscope is utilized. Failure Analysis Reliability Process Control Vendor Qualification Production Quality Control Research 6

Common Applications Plastic encapsulated IC IC packages Flip Chips Bonded Wafers Printed Circuit Boards Capacitors Ceramics Metallic Power Devices/Hybrids Medical Devices Material Characterization 7

Examples Lid seal voids Delamination BGA die attach Die Crack 8

Examples Die Top Delamination Mold compound voids Flip Chip Underfill Voids 9 Die Tilt, B-Scan Die Pad delamination Die Attach Voids

Ultrasound Inspection Theory System Components Transducers 10

Ultrasonic Waves Characteristics of of Ultrasonic Waves Freely Freely propagate through liquids liquids and and solids solids Reflect Reflect at at boundaries of of internal internal flaws flaws and and change change of of material Capable of of being being focused, straight straight transmission University of California Medical Center San Francisco, California Suitable for for Real-Time processing Harmless to to the the human human body body Non-destructive to to material 11

Ultrasonic Inspection Ultrasound A transducer transducer produces produces a high high frequency frequency sound sound wave wave which which interacts interacts with with the the sample. sample. High High frequency frequency sound sound waves waves can can not not propagate propagate through through air. air. Couplant- Couplant-A material material used used to to carry carry the the high high frequency frequency sound sound waves. waves. Water Water is is the the most most common common couplant couplant for for immersion immersion testing. testing. Inspection Modes Pulse Pulse Echo Echo Through Through Transmission Transmission Transducer H 2 O Coupling Receive 12

Scanner The scanner consists of a three axis system, X, Y, and Z. The motor controller directs the movement of these axes. F o c u s Z A x i s X-Axis Y-Axis Step 13 Scan

Transducers High Frequency Short Focus Low Frequency Long Focus 1. 1. Higher Higher resolution resolution 2. 2. Shorter Shorter focal focal lengths lengths 3. 3. Less Less penetration penetration (Thinner (Thinner packages) packages) 1. 1. Lower Lower resolution resolution 2. 2. Longer Longer focal focal lengths lengths 3. 3. Greater Greater penetration penetration (Thicker (Thicker packages) packages) General General rules: rules: Ultra Ultra High High Frequency Frequency (200+ (200+ MHz) MHz) for for flip flip chips chips and and wafers. wafers. High High Frequency Frequency (50-75 (50-75 MHz) MHz) for for thin thin plastic plastic packages. packages. (110MHz-UHF) (110MHz-UHF) for for flip flip chips. chips. Low Low Frequency Frequency (15 (15 MHz) MHz) for for thicker thicker plastic plastic packages. packages. 14

Transducer Beam Profile Depth of Field The purple region is referred to as the focal area or depth of field of the transducer beam. 15

Typical Transducer Selection Sample Application Transducer 16 T/X Receiver PLCC, QFP, PQFP Power Pak BGA Top Capacitors TSOP Flip Chip Underfill Flip Chip Interconnect Bonded Wafer Bonded Wafer 10 MHz w/0.75 focus 15 MHz w/0.5 focus 15 MHz w/0.5 focus 50-75 MHz w/12mm focus 75 MHz w/12mm focus 75 MHz w/12mm focus 110 MHz w/8mm focus UHF w/ 5.9 mm focus 110 MHz w/8mm focus UHF w/ 5.9 mm focus

ABC s Of Acoustics Acoustic Reflections Acoustic Waveforms Image Display 17

Acoustic Properties Material Density LongitudinalWave Acoustic Impedance (g/cm 3 ) Velocity (m/s) (kg/m 2 s) (x10 6 ) Water (20 0 C) 1.00 1483 1.48 Alcohol (20 0 C) 0.79 1168 0.92 Air (20 0 C) 0.00 344 0.00 Silicon 2.33 8600 20.04 Gold 19.3 3240 62.53 Copper 8.90 4700 41.83 Aluminum 2.70 6260 16.90 Epoxy Resin 1.20 2600 3.12 Resin (for IC pkg) 1.72 3930 6.76 Glass (Quartz) 2.70 5570 15.04 Alumina (AL 2 O 3 ) 3.80 10410 39.56 18

Sound Reflection Acoustic Material Properties density (ρ) velocity of sound in material (c) acoustic impedance (Z= ρc) 19

Sound Reflection Whenever a sudden change in acoustic impedance is encountered, like at a material boundary, a portion of sound is reflected and the remainder propagates through the boundary. 20

Reflection vs. Transmission Incident Energy Z= ρc Water Z 1 Plastic Z 2 Z 1 = ρ C where: ρ=1.00 gram/cm 3 C= 1.5 x 10 6 Z 1 = 1.5 x 10 6 Reflected Energy Transmitted Energy Z 2 = ρ C where: ρ =2.00 gram/cm 3 C= 2.00 x 10 6 Z 2 = 4.00 x 10 6 T T T = = = 2 ( Z 1 ) ( Z + Z ) 2 2 ( 1. 5 ) ( 4. 0 + 1. 5 ) ( 3. 0 ) ( 5. 5 ) 1 45% of the sound entering the boundary is reflected. R R R = = = ( Z 2 Z 1 ) ( Z + Z ) ( 4 ( 4 ( 2 ( 5 2. 0. 0. 5. 5 ) ) + 1 1 1. 5. 5 ) ) 21 T =. 55 R =. 45

Reflected Sound Information Measuring the reflected ultrasound can provide: Amplitude Information Polarity Information Time Information 22

ABC s s of Acoustics A-Scan- The raw ultrasonic data. It is the received RF signal from a single point (x,y). B-Scan- A line of A-scans. (Vertical cross-section) 23 C-Scan-Data from a specified depth over the entire scan area. (Horizontal cross-section.

A-SCAN Initial Pulse Transducer Front surface Interface of interest Sample Back surface 24

Ultrasonic Waveforms The Black signal is commonly referred to as the initial pulse or the main bang. This signal occurs at Zero microseconds. The Red signal is commonly referred to as the front surface. This represents the first interface the sound encounters. 1 2 The Green signal would be considered the area of interest. A data gate would be positioned over this signal or group of signals for evaluation. 1 2 The Blue signal is commonly referred to as a back wall echo or back surface. Just as the name implies it is the back or bottom of the sample. 25

A-SCAN 100% + Phase Amplitude %FSH 0% -100% _ Phase Time / Depth A-Scans provide the following information: 1. Amplitude / % of full screen height (FSH) 2. Phase / positive or negative peak 3. Time / Depth 26

C-SCAN IP Front surface Back surface Area of interest The red box (data gate) indicates the depth of information. 27

B-Scan Front surface Signal from indication Back surface The blue line (B-scan gate) represents the depth of information recorded. Front surface 28 Signal from indication Back surface

Inspection Modes Pulse Echo Through Transmission 29

Inspection Modes Pulse-Echo Through Transmission Transmit Transmit & Receive Receive 30 Pulse-Echo Pulse-Echo - - One One Transducer Transducer Through Ultrasound Ultrasound reflected reflected from from the the sample sample is is used. Through Transmission Transmission - - Two Two Transducers Transducers used. Can Can determine determine which which interface interface is is delaminated. Ultrasound Ultrasound transmitted transmitted through through the the delaminated. sample Requires Requires scanning scanning from from both both sides sides to to inspect sample is is used. used. inspect all all interfaces. One One Scan Scan reveals reveals delamination delamination at at all all interfaces. interfaces. Provides Provides images images with with high high degree degree of of spatial interfaces. spatial detail. No No way way to to determine determine which which interface interface is is detail. delaminated. Peak Peak Amplitude, Amplitude, Time Time of of Flight Flight (TOF) (TOF) and delaminated. and Phase Phase Inversion Inversion measurement Less Less spatial spatial resolution resolution than than pulse-echo. pulse-echo. measurement Copyright Commonly Sonix, Commonly used Inc used to to verify verify pulse-echo pulse-echo results. results.

Inspection Modes Pulse-Echo Front Surface Back surface 1 2 Transmit & Receive 1 Front Surface Air Gap Air Gap 2 31

Inspection Modes Through Transmission 1 1 2 3 Transmit 2 Receive 3 32

Focusing Sound 33

Too Close 1 Focusing the Transducer Too Far Focused Too Close Focused 2 3 2 1 Too Far 3 34 Focusing an ultrasonic transducer is similar to focusing an optical microscope. When optimum focus is reached the signal will reach a maximum peak. (See the A-scans images to the left)

De-focused-- too close Water path Correct focus 28% 1. Note the time in microseconds of the signal at the different focus locations. (Red arrow) De-focused-- too far 85% 33% 2. Also note the amplitude of the signal. (white box) When the signal is not in focus the amplitude is lower compared to that of correct focus. 35 *The ultrasound is focused on the surface of the penny.

Focusing Sound De-Focused - Too Close Focused on Die De-Focused - Too Far Away Amplitude = 42% Time =10.5 us Amplitude = 82% Time = 14.5 us Amplitude = 55% Time = 18.5 us 36

Practical Application Digital Oscilloscope Front Surface Follower Data Gates 37

Digital Oscilloscope Initial pulse 1st Echo 2nd Echo 3rd Echo 38 The 1st set of echoes is the area of interest, gate placement will be on this group. Multiple Echoes

Gates Gates Gates are are used used to to collect collect information information at at desired desired interfaces interfaces within within the the sample. sample. The The gate gate is is placed placed over over the the signal signal or or signals signals of of interest. interest. The The absolute absolute value value of of the the highest highest amplitude amplitude signal signal which which breaks breaks the the gate gate threshold threshold within within the the gated gated region region is is recorded. recorded. (Figure (Figure 1) 1) If If no no signal signal breaks breaks the the gate gate threshold threshold no no data data is is recorded. recorded. (Figure (Figure 2) 2) Signal Signal amplitude amplitude can can be be increased increased or or decreased decreased by by adjusting adjusting gain. gain. 1 2 Gate Threshold Highest Amplitude signal No data recorded 39

Practical Application Image Data Peak Amplitude Time of Flight (TOF) Phase Inversion 40

Peak Amplitude Peak amplitude imaging is used when defects result in changes in the amount or strength of ultrasound reflected. It is the most common type of imaging technique. 41

Peak Amplitude Peak Amplitude 100 78 75 50 X1 25 0 Signal height is measured in absolute value for Peak Amplitude images. 42 Amplitude Copyright 78% Sonix, Inc

Time Of Flight Time of Flight (TOF) imaging works by measuring changes in the time it take sound to reflect off a particular interface. Most commonly used to measure die tilting. 43

Time Of Flight 1 X2 X1 2 Time of Flight images provide a relative depth within a sample. Structures which appear white or light gray are closer to the surface of the sample. Structures which appear darker shades of gray or black are deeper within sample. 44 The peak signal for location 1 occurs at 14.2 microseconds (light gray) while the peak signal for location 2 occurs at 14.6 microseconds (dark gray).

Peak Amplitude vs. TOF Peak Amplitude Time of Flight X2 X2 X1 X1 Amplitude =73% Time =14.2 microseconds Amplitude =67% Time =14.6 microseconds 45

Phase Inversion Phase Inversion imaging is used when defects cause changes in polarity (phase) of the signal. Most commonly used for top and back side imaging of plastic encapsulated devices. Do not use phase inversion imaging for flip chip, bonded wafer or die attach imaging. 46

Phase Inversion Normal Phase Inverted 47

X Phase Gate X RED Yellow Sonix Sonix uses uses a proprietary proprietary algorithm algorithm to to detect detect phase phase inversion. inversion. This This method method is is independent independent of of signal signal amplitude amplitude as as long long as as the the signal signal is is not not saturated saturated (100% (100% screen screen height). height). X 48

Image Comparison & Correlation Through Transmission Peak Amplitude Image of Die Top Phase Inversion Image of Die Top Peak Amplitude Image of Die Attach 49