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i

NASA/TM—2010–216442

Three-Dimensional Computed Tomography

as a Method for Finding Die Attach 

Voids in Diodes

E.N. Brahm

The University of the South, Sewanee, Tennessee

T.D. Rolin

Marshall Space Flight Center, Marshall Space Flight Center, Alabama

September 2010

National Aeronautics and

Space Administration

Marshall Space Flight Center • MSFC, Alabama 35812
ii

Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076 –1320

443–757–5802

This report is also available in electronic form at

<https://www2.sti.nasa.gov>

Acknowledgments

E.N. Brahm thanks T.D. Rolin for his invaluable guidance and insight into this project as her mentor and also 

NASA Undergraduate Student Research Program (USRP) for making her internship possible. This work 

has been paid in part by the Electrical, Electronic, and Electromechanical Parts, Packaging, 

and Fabrication Branch (ES43).

TRADEMARKS

Trade names and trademarks are used in this report for identification only. This usage does not constitute an official 

endorsement, either expressed or implied, by the National Aeronautics and Space Administration.
iii

TABLE OF CONTENTS

1. INTRODUCTION ............................................................................................................. 1

2. BACKGROUND ON EQUIPMENT ................................................................................. 4

3. PROCEDURE .................................................................................................................... 5

4. DATA AND ANALYSIS .................................................................................................... 8

4.1 Diode 1 ......................................................................................................................... 8

4.2 Diode 2 ......................................................................................................................... 10

4.3 Diode 3 ......................................................................................................................... 13

5. ADDITIONAL OBSERVATIONS ..................................................................................... 18

6. CONCLUSIONS ................................................................................................................ 19

REFERENCES ....................................................................................................................... 21
iv
v

LIST OF FIGURES

 1. The Keyence VHX-600 optical microscope was used to collect this magnified

image of a 1N5550 diode ......................................................................................... 1

 2. The nanome|x 160 NF manufactured by phoenix|x-ray creates x-ray images 

of aerospace components using two-dimensional x-rays and 3DCT data sets .......... 2

 3. The nonmesa 1N5550 diode cross section shows typical construction

of a diode and was photographed with a Leica CTR-6000 metallograph 

with 2.5× objective magnification ............................................................................ 3

 4. This oblique, two-dimensional x-ray of diode 1, part 1N5550, shows the mottled

‘prominent’ die attach (yellow ellipse) that made this diode a good candidate

for analysis ............................................................................................................... 5

 5. The leads, plugs, and glass body are visible in this image of the 3DCT volume

of diode 1. The notch made with the scalpel (blue arrow) and the paint 

on the cathode side plug (yellow arrow) are detectable by x-ray ............................... 6 

 6. The 3DCT scan of diode 1, part number 1N5550, was digitally sectioned 

so that the entire plane of cathode die attach was visible: (a) The unenhanced 

3DCT scan shows the mottled die attach plane, and (b) The highlighted 3DCT 

scan shows the remaining die attach in blue with white lines indicating locations 

of the physical cross-section planes 1 to 3 shown in figures 7–9, respectively ............ 7

 7. The die attach and voiding in physical cross section 1 of diode 1 parallels 

the die attach plane in the 3DCT scan equivalent: (a) The first physical cross 

section of diode 1 was optically imaged at 10× objective magnification in 

bright field, and (b) The 3DCT scan of diode 1 was virtually sectioned 10% 

of the way through the die to represent the portion of the diode remaining 

after the first physical cross section was made. The five very small voids 

on the right half of the cross-section plane (white, yellow, green, blue, 

and purple arrows) and the two very small voids on the left edge (gray 

and black arrows) are present in both the physical cross section and its 

3DCT model ............................................................................................................ 9

 8. The die attach and voiding in the physical cross section 2 of diode 1 parallels 

the die attach plane in the 3DCT scan equivalent: (a) The second physical cross 

section of diode 1 was optically imaged at 10× objective magnification in bright 

field, and (b) The 3DCT scan of diode 1 was virtually sectioned 30% of the way 

through the die to correspond to the second physical cross section. Both the 
vi

LIST OF FIGURES (Continued)

physical cross section and its 3DCT representation have six voids of increasing 

size moving from left to right across the cross-section plane (gray, white, 

yellow, green, blue, and purple arrows). The CT model does not show 

the tiny void on the left (black arrow) ...................................................................... 9

 9. The die attach and voiding in the physical cross section 3 of diode 1 parallels 

the die attach plane in the 3DCT scan equivalent: (a) The third physical 

cross section of diode 1 was optically imaged at 10× objective magnification 

in bright field, and (b) The 3DCT scan of diode 1 was virtually sectioned 

50% through the die to correspond to the third physical cross section. 

The physical cross section confirms the 3DCT cross section in that both 

have a very small void on the left edge (black arrow), two tiny voids on the left 

(white and yellow arrows), followed by three voids of increasing magnitude 

from left to right (green, blue, and purple arrows) .................................................... 10

10. The 3DCT scan of diode 2, part number 1N5550, was digitally sectioned 

so that the entire plane of die attach was visible: (a) The unenhanced 3DCT 

scan is tilted forward so the glass body and plug on the cathode side are visible, 

and (b) To contrast the die attach from the plug in the 3DCT scan of diode 2, 

the die attach was highlighted blue, and the three cross-section locations 

are shown in white and labeled 1 to 3 ....................................................................... 11

11. The die attach and voiding in the physical cross section 1 of diode 2 parallels 

the die attach seen in the 3DCT scan equivalent: (a) The first physical cross 

section of diode 2 was optically imaged at 10× objective magnification 

in bright field, and (b) The 3DCT scan of diode 2 was virtually sectioned 

25% of the way through the die to correspond to the first physical cross 

section. Both representations have thick, solid die attach in the cross-section 

plane except for a void right of center (yellow arrow) and thin die attach 

with spotty voiding on the right (blue arrow) ........................................................... 12

12. The die attach and voiding in the physical cross section 2 of diode 2 parallels 

the die attach plane in the 3DCT scan equivalent: (a) The second physical 

cross section of diode 2 has a cracked die and was optically imaged at 10×

objective magnification in bright field, and (b) The 3DCT scan of diode 2 

was virtually sectioned 40% through the die to correspond to the second 

physical cross section.The physical cross section and its digital 3DCT model 

both show a void streching across the left side of the plane (yellow arrow) 

except for a small section of die attach present on the left edge (white arrow).

The two tiny voids on the right (green and blue arrows) and the very small 

void near the right edge (purple arrow) also coincide exactly between the two 

representations ......................................................................................................... 12
vii

LIST OF FIGURES (Continued)

13. The die attach and voiding in the physical cross section 3 of diode 2 parallels 

the die attach plane in the 3DCT scan equivalent: (a) The third physical cross 

section of diode 2 has a cracked die and was optically imaged at 10× objective 

magnification in bright field, and (b) The 3DCT scan of diode 2 was virtually 

sectioned 65% of the way through the die to correspond to the third physical 

cross section. Both the physical and the digital representation have a large 

void on the left half of the cross-section plane (yellow arrow) with a tiny piece 

of die attach right of center (green arrow) and a small void near the right edge 

(blue arrow) .............................................................................................................. 13

14. The 3DCT scan of diode 3, part number 1N4942, was digitally sectioned so 

that the entire plane of die attach was visible: (a) The unenhanced 3DCT scan 

shows the cathode die attach in the square of the footprint of the mesa of 

the die, surrounded by the plug and the glass body, and (b) To contrast the die 

attach from the plug in the 3DCT scan of diode 2, the die attach was highlighted 

blue, and the four cross-section locations are shown in white and labeled 1 to 4 ....... 14

15. The die attach and voiding in the physical cross section 1 of diode 3 parallels 

the die attach plane in the 3DCT scan equivalent: (a) The first physical cross 

section of diode 3 has a cracked die and was optically imaged at 10× objective 

magnification in bright field, and (b) The 3DCT scan of diode 3 was virtually 

sectioned 25% of the way through the die to correspond to the first physical 

cross section. Both the physical and the digital representation have voids 

of approximately equal size on the left and right of the center die attach 

(yellow and blue arrows) and a tiny void left of center (green arrow) ....................... 14

16. The die attach and voiding in the physical cross section 2 of diode 3 die 

parallels the die attach plane in the 3DCT scan equivalent: (a) The second 

physical cross section of diode 3 has a cracked die and was optically imaged 

at 10× objective magnification in bright field, and (b) The 3DCT scan of diode 3 

was virtually sectioned 40% of the way through the die to correspond to the 

second physical cross section. The large void on the left edge (yellow arrow), 

tiny void on the right edge (blue arrow), and small void 70% of the way across 

the die attach plane from the left (green arrow) are all seen in both cross sections .... 15

17. The die attach and voiding in the physical cross section 3 of diode 3 parallels 

the die attach plane in the 3DCT scan equivalent: (a) The third physical cross 

section of diode 3 has a cracked die and was optically imaged at 10× objective 

magnification in bright field, and (b) The 3DCT scan of diode 3 was virtually 

sectioned 65% of the way through the die to correspond to the third physical 

cross section. Both representations of the cross section 65% of the way 

through the die show a large void on the left edge (white arrow), a void 
viii

LIST OF FIGURES (Continued)

of the same size afer a small bit of die attach (yellow arrow), two tiny voids 

in the die attach on the right half of the plane (green and blue arrows), 

and a tiny void on the right edge (purple arrow) ...................................................... 15

18. The die attach and voiding in the physical cross section 4 of diode 3 parallels 

the die attach plane in the 3DCT scan equivalent: (a) The fourth physical 

cross section of diode 3 has a cracked die and was optically imaged at 10×

objective magnification in bright field, and (b) The 3DCT scan of diode 3 

was virtually sectioned 85% of the way through the die to correspond 

to the fourth physical cross section. Neither representation of the fourth 

cross section shows any significant die attach ........................................................... 16
ix

LIST OF ACRONYMS

CT computed tomography

EEE electrical, electronic, and electromechanical

3DCT three-dimensional computed tomography
x
1

TECHNICAL MEMORANDUM

THREE-DIMENSIONAL COMPUTED TOMOGRAPHY AS A METHOD 

FOR FINDING DIE ATTACH VOIDS IN DIODES

1. INTRODUCTION

NASA analyzes electrical, electronic, and electromechanical (EEE) parts used in space 

vehicles to understand failure modes of these components. Engineers use this knowledge to avoid 

failures that impact safety, cost, and schedule. Diodes are but one example of such a part that serve 

critical functions throughout typical flight avionics. Figure 1 shows a diode of the type that was 

analyzed in this study. Past research and test data have shown that diodes cannot serve their purpose if there is excessive voiding in the die attach, typically a silver-based material that bonds the 

cathode and anode plugs to the die. One technique that has been used for determining die attach 

integrity is metallographic preparation. This is a time-intensive process whereby mechanical grinding and polishing of the diodes reveal voiding and other anomalies in the die attach. However, 

it is a destructive process and the results are sometimes inconclusive due to observations being 

limited to one plane at a time and the ease of damaging the brittle die in the harsh grinding procedure. A quick, veritable, and nondestructive method for finding voids in the die attach of diodes 

is desirable.

Figure 1. The Keyence VHX-600 optical microscope was used to collect this magnified

image of a 1N5550 diode.
2

Three-dimensional computed tomography (3DCT) is a promising alternative to metallography because it is a nondestructive and expeditious technique. The most notable advantage of 

three-dimensional x-ray inspection is that it results in a complete picture of the area of concern.1

While the cross sectioning of a diode typically requires many hours, a computed tomography (CT) 

scan of the same diode can be completed in less than an hour. The purpose of this study was to 

verify that CT scans of die attach accurately represent the die attach structure. 

Figure 2 shows the nanome|x 160 NF manufactured by phoenix|x-ray (nanome|x) used to 

x-ray and CT scan the diodes in this study. Verification was accomplished by comparing several 

physical cross sections of three diodes to the corresponding cross-sectional plane in the tomographs 

of those diodes. The three diodes used for this study were manufactured by Unitrode (now Texas 

Instruments). Diodes 1 and 2 have part number 1N5550 and lot date code 8516. Diode 3 has part 

number 1N4942 and lot date code 8413. Unitrode diode construction is typical of other manufacturers’ diodes with the same part number and similar lot date codes. 

Figure 2. The nanome|x 160 NF manufactured by phoenix|x-ray creates x-ray images of aerospace 

components using two-dimensional x-rays and 3DCT data sets.

Figure 3 is an optical bright field image of a cross section of a nonmesa 1N5550 diode taken 

at 2.5× objective showing the typical construction of one of these diodes. Nonmesa diodes are 

characterized by a cathode side that maintains the same dimensions as the anode side. Mesa diodes 

typically have the junction edges ground or laser etched to promote a larger breakdown voltage.
3

Cathode

Anode

Molybdenum

Plugs

Lead

Lead

Die Attach

Glass Body

Silicon Die

Figure 3. The nonmesa 1N5550 diode cross section shows typical construction of a diode 

and was photographed with a Leica CTR-6000 metallograph with 2.5× objective 

magnification.
4

2. BACKGROUND ON EQUIPMENT

Unlike two-dimensional x-ray imaging, 3DCT enables the user to penetrate beneath the 

surface and virtually slice into the target object without physically dissecting it. The digital volume 

model created by the 3DCT scan can be used to view a full interior plane of a diode, including 

substructures like die attach. A traditional x-ray system produces two-dimensional images by collecting the x-rays transmitted through the target object.2 A CT scan creates a three-dimensional 

representation with gray values proportional to the x-ray density of the object using a computer 

algorithm to compile a series of two-dimensional radiographs collected as the object or detector 

is rotated 360 deg.3 The CT images for this Technical Memorandum were created with the phoenix 

datos|x, version 1.2.1.7 (datos|x) software reconstruction of images from the nanome|x. The threedimensional rendering software package VGStudio MAX, version 2.0 (VGStudio MAX) was used 

to view the images and create the digital planes for comparison to the physical cross sections.
5

3. PROCEDURE

The first step in verifying the 3DCT technique was to find diodes that exhibited significant 

spatial variation in the die attach volume. For example, a thick coating of die attach exhibiting 

large voids makes clear the boundary between the die attach and voids in the CT scan, so these 

diodes were chosen for this study. To find a diode with die attach that was suitable for conclusive 

analysis, a large collection of diodes was two-dimensionally x-rayed with the nanome|x to identify 

those with prominent die attach. 

Figure 4 shows the two-dimensional radiograph of diode 1 with a mottled cathode die 

attach plane between the plugs. The mottled appearance suggests a large number of voids, the reason this diode was selected for this study. The two-dimensional radiograph also illustrates that the 

viewing angle must be extremely oblique to just see the die attach only vaguely. The lack of clarity 

is the primary reason that two-dimensional imaging is not preferable as a die attach analysis technique. After screening over 100 diodes, the three selected diodes were notched on the cathode lead 

for digital to physical orientation purposes. The paint was also scratched off the body of the diode 

with a scalpel to facilitate optical microscopic observation. 

Figure 4. This oblique, two-dimensional x-ray of diode 1, part 1N5550, shows the mottled 

‘prominent’ die attach (yellow ellipse) that made this diode a good candidate 

for analysis.

Although these two operations can be considered destructive, they were only implemented 

in this study to ensure the orientation could easily be correlated between physical and digitized 

cross sections and are not necessary procedures. The diode was then positioned as straight as possible in the CT sample holder, which spins the part, and as close to the x-ray source as possible to 
6

obtain the highest resolution image achievable.4 The typical resolution reported by the software 

was 9 μm for each of the three diodes. The x-ray images were then imported into datos|x to create 

the volumetric data file. The subsequent volume file was loaded into VGStudio MAX for threedimensional viewing and image creation. 

Figure 5 is the three-dimensional view of diode 1 in VGStudio MAX after the gray values 

of air and other similar density materials were set to transparent. The image shows the leads on 

the left and right, the notch in the cathode lead, the dark glass body in the center, and the plugs 

between the leads and glass body. The dark area on the cathode side plug is due to the green paint 

that is used to mark the diode when it has passed screening. The clipping tool in the VGStudio 

MAX software was used to digitally section into the diode until the entire plane of the cathode die 

attach was visible. Figure 6(a) shows the square die attach plane, surrounded by the circular plug 

that is enclosed by the dark circular glass body in the digitally sliced 3DCT scan of diode 1.

Figure 5. The leads, plugs, and glass body are visible in this image of the 3DCT volume 

of diode 1. The notch made with the scalpel (blue arrow) and the paint on the 

cathode side plug (yellow arrow) are detectable by x-ray.

After digital volumetric modeling, the diode was prepared for mechanical grinding and 

polishing. The first step in this process was to place the sample in ≈40 mL of epoxy formed from 

the mixture of five parts Buehler epoxide resin to one part Buehler epoxide hardener by weight. 

The diode leads were mounted in plastic specimen support clips to keep the body of the diode 

as level as possible in the epoxy. Using the notch in the lead as a fiducial, the diode was positioned 

so that each cross-sectional plane could be easily mapped back to the digital planes created with 

CT. The support assembly was placed in a SamplKups® mold cylinder and covered with the epoxy. 

To remove air bubbles formed while mixing the epoxy, the mold cylinder was placed in a 1×10–2

torr (1.3 P) vacuum chamber for 5 min, then allowed to cure overnight without heat or pressure.

The potted diode was removed from the mold cylinder and labeled with the diode number 

and a mark indicating the location of the cathode for orientation purposes by etching the hardened 
7

Figure 6. The 3DCT scan of diode 1, part number 1N5550, was digitally sectioned so that 

the entire plane of cathode die attach was visible: (a) The unenhanced 3DCT scan 

shows the mottled die attach plane, and (b) The highlighted 3DCT scan shows the 

remaining die attach in blue with white lines indicating locations of the physical 

cross-section planes 1 to 3 shown in figures 7–9, respectively.

epoxy with an electric engraver. To obtain a cross section, the potted diode was abrasively cut on an 

8-in-diameter Buehler EcoMet® 3 variable-speed grinder-polisher (EcoMet) until the desired depth 

into the die was reached. The cross-sectioning regimen consisted of a progression of grinding steps 

starting with 320-, 400-, and 800-grit, 8-in Buehler Carbimet® paper discs or Allied High Tech 

Products, Inc. (Allied) silicon carbide paper discs at 420 rpm. A different EcoMet was used for each 

grit size to avoid cross contamination. Rough polishing was accomplished using 6-grit diamond 

suspension on an Allied Gold Label polishing cloth at 900 rpm followed by 1-grit colloidal silica on 

an Allied White Label polishing cloth at 900 rpm. Final polishing consisted of 0.05-grit colloidal 

silica on an Allied Final Red C polishing cloth at 500 rpm. A typical polished cross-sectional plane 

is illustrated in figure 3. 

The diode cross sections were photographed using a Leica CTR6000 metallograph under 

5×, 10×, and 20× objective magnification in bright field view. The entire die did not fit in the field 

of view for the 10× (diodes 1 and 2 (1N5550) only) and 20× objective magnifications, so pictures 

were taken of the die and die attach in overlapping sections. The 10× segments for diodes 1 and 2 

were then merged to create a panorama of the entire die using the PhotomergeTM Panorama feature 

of the Adobe® Photoshop Elements, version 5.0 program. The potted diode was sectioned and 

photographed several more times, each time grinding deeper into the diode to obtain additional 

reference planes. Images of each cross-sectional plane were then matched to the two-dimensional 

digital plane of the cathode die attach from the volumetric data of that diode. Comparisons were 

made between the digital and physical representations of die attach and voiding.
8

4. DATA AND ANALYSIS

The accuracy of the 3DCT scan in representing the presence and voiding of the cathode 

die attach in diodes was verified by ensuring that the die attach appearance in the physical cross 

sections confirmed the die attach appearance in the 3DCT scan. After several physical cross sections were obtained, the exact position of each physical cross-section plane in the CT scan was 

determined by matching the locations and sizes of the voids appearing in the cross sections to the 

locations and sizes of the voids appearing in a line in the die attach plane of the CT scan. To verify 

these cross-section positions, cross-section lines were drawn on the printed CT scan die attach 

planes to ensure that the cross-section planes were parallel. Next, it was verified that the ratios 

between the lengths of the die attach and the distances between several pronounced voids were 

consistent between the printed pictures of the cross sections and the CT scan. The 5× optical magnification pictures were used for 1N5550 diodes in these tests because the entire die could be viewed 

in one frame. The 10× optical magnification pictures were used for the smaller 1N4942 diode in 

these tests because the entire die was visible in a single frame. 

4.1 Diode 1

Figure 6(a) shows the cathode die attach plane in the virtually sectioned 3DCT scan of 

diode 1, part number 1N5550. The die attach is the porous square in the center with the voids 

appearing darker than the remaining die attach, the result of using a special lighting algorithm 

provided in the VGStudio MAX software. The die does not appear in the 3DCT scan due to its 

much lower x-ray density. To facilitate finding the exact locations in the 3DCT scan that corresponded to the physical cross sections, Jasc® Paint Shop Pro, version 8.10 was used to highlight 

the cathode die attach in blue. Figure 6(b) shows the highlighted CT scan of diode 1. The white 

lines in the image correspond to the three physical cross-section planes made of diode 1. The physical cross sections, locations designated 1 to 3 in figure 6(b), are shown with their 3DCT counterparts in figures 7–9, respectively. 

The cathode die attach is on the top, mesa side of the die in each physical cross section. 

The cathode side glass body, cathode plug, and die attach are depicted in the sliced digital volume. 

The physical and digital cross sections are oriented so the left and right correspond as shown. 

Figure 7 depicts cross section 1 of diode 1, in which ≈10% of the die has been ground away. 

The five small, dark voids in the die attach on the right half of the physical cross-section plane 

and the two tiny voids on the left edge of the plane in figure 7(a) correspond to the five small, dark 

voids on the right and two tiny voids on the left edge of the digital 3DCT model of the physical 

cross-section plane in figure 7(b). 

Figure 8 depicts cross section 2 of diode 1, in which ≈30% of the die has been ground away. 

Both figures 8(a) and (b) depict six voids of increasing size from left to right, separated by very 
9

Figure 7. The die attach and voiding in physical cross section 1 of diode 1 parallels the die attach 

plane in the 3DCT scan equivalent: (a) The first physical cross section of diode 1 was 

optically imaged at 10× objective magnification in bright field, and (b) The 3DCT scan 

of diode 1 was virtually sectioned 10% of the way through the die to represent the portion of the diode remaining after the first physical cross section was made. The five 

very small voids on the right half of the cross-section plane (white, yellow, green, blue, 

and purple arrows) and the two very small voids on the left edge (gray and black 

arrows) are present in both the physical cross section and its 3DCT model.

Figure 8. The die attach and voiding in the physical cross section 2 of diode 1 parallels the die 

attach plane in the 3DCT scan equivalent: (a) The second physical cross section of 

diode 1 was optically imaged at 10× objective magnification in bright field, and (b) 

The 3DCT scan of diode 1 was virtually sectioned 30% of the way through the die to 

correspond to the second physical cross section. Both the physical cross section and 

its 3DCT representation have six voids of increasing size moving from left to right 

across the cross-section plane (gray, white, yellow, green, blue, and purple arrows). 

The CT model does not show the tiny void on the left (black arrow).
10

Figure 9. The die attach and voiding in the physical cross section 3 of diode 1 parallels the die 

attach plane in the 3DCT scan equivalent: (a) The third physical cross section of 

diode 1 was optically imaged at 10× objective magnification in bright field, and 

(b) The 3DCT scan of diode 1 was virtually sectioned 50% through the die to correspond to the third physical cross section. The physical cross section confirms the 

3DCT cross section in that both have a very small void on the left edge (black arrow), 

two tiny voids on the left (white and yellow arrows), followed by three voids of 

increasing size from left to right (green, blue, and purple arrows).

small bits of die attach and ending with a small piece of die attach on the right edge. However, the 

physical cross section in figure 8(a) shows a tiny void to the right of the first void on the left edge 

that does not appear in the 3DCT version of the section. This discrepancy is due to ‘pull-out’ as the 

silicide is visible, bonded to the cathode plug where the die attach is missing. 

Figure 9 depicts cross section 3 of diode 1, in which ≈50% of the die has been ground away. 

Figures 9(a) and (b) show a very small void on the left edge followed by five voids of increasing size 

from left to right with the most distinct die attach present on the right half of the plane. Figure 9 

illustrates that a 3DCT scan can also differentiate between thick and thin die attach. The die attach 

is thin on the left of the plane in both representations. Thin die attach in the 3DCT scan is not as 

emphasized as thicker die attach and often appears mottled, possibly leading to missed correlation 

between the physical and digital cross sections when highlighting with grayscales. The die attach 

in the physical cross sections and the digital die attach in the 3DCT scan for diode 1 were in nearly 

exact congruence for each section. 

4.2 Diode 2

Figure 10 shows the cathode die attach plane in the virtually sectioned 3DCT scan of 

diode 2, part number 1N5550. This particular diode is viewed at an angle so that the lighting algorithm in VGStudio MAX makes the die attach more distinct. Figure 10(a) shows the unenhanced 

die attach plane. Figure 10(b) shows the die attach colored blue with white lines across the die 

attach area corresponding to the three physical cross sections made of diode 2. The cross sections, 

locations designated 1 to 3 in figure 10(b), are shown with their 3DCT counterparts in figures 11–

13, respectively. 
11

Figure 10. The 3DCT scan of diode 2, part number 1N5550, was digitally sectioned so that 

the entire plane of die attach was visible: (a) The unenhanced 3DCT scan is tilted 

forward so the glass body and plug on the cathode side are visible, and (b) To contrast the die attach from the plug in the 3DCT scan of diode 2, the die attach was 

highlighted blue, and the three cross-section locations are shown in white and 

labeled 1 to 3.

Figure 11 depicts cross section 1 obtained from diode 2, in which ≈25% of the die has been 

ground away. Notice that both the physical cross section in figure 11(a) and the digital cross section 

in the 3DCT scan in 11(b) have solid die attach in the cross-section plane except for a small void 

slightly right of center and several tiny voids on the far right of the die attach plane. In the 3DCT 

scan, the tiny voids on the right appear as a dappled area in the die attach. 

Figure 12 depicts cross section 2 of diode 2, in which ≈40% of the die has been ground away. 

Figure 12(a), the physical cross section, and 12(b), the 3DCT representation of the cross section, 

show a voided left half of the die attach plane except for a small bit of die attach on the left edge 

of the plane. There is also a tiny void just to the right of the large void, a tiny void approximately 

three-fourths across the plane from the left, and very small void near the right edge in both 

representations. 

Figure 13 depicts cross section 3 of diode 2, in which ≈65% of the die has been ground away. 

The left half of the die attach plane is voided except for a small piece of die attach on the left edge 

in both the physical cross section in figure 13(a) and the 3DCT cross section in 13(b). Figure 13(a) 

and (b) both show thick die attach to the right of the large void followed by a small void on the 

right side of the die attach. The 3DCT and physical cross section die attach were in nearly perfect 

agreement for each slice of diode 2.
12

Figure 11. The die attach and voiding in the physical cross section 1 of diode 2 parallels the die 

attach seen in the 3DCT scan equivalent: (a) The first physical cross section of diode 2 

was optically imaged at 10× objective magnification in bright field, and (b) The 3DCT 

scan of diode 2 was virtually sectioned 25% of the way through the die to correspond 

to the first physical cross section. Both representations have thick, solid die attach 

in the cross-section plane except for a void right of center (yellow arrow) and thin die 

attach with spotty voiding on the right (blue arrow).

Figure 12. The die attach and voiding in the physical cross section 2 of diode 2 parallels the die 

attach plane in the 3DCT scan equivalent: (a) The second physical cross section of 

diode 2 has a cracked die and was optically imaged at 10× objective magnification in 

bright field, and (b) The 3DCT scan of diode 2 was virtually sectioned 40% through 

the die to correspond to the second physical cross section.The physical cross section 

and its digital 3DCT model both show a void streching across the left side of the 

plane (yellow arrow) except for a small section of die attach present on the left edge 

(white arrow). The two tiny voids on the right (green and blue arrows) and the very 

small void near the right edge (purple arrow) also coincide exactly between the two 

representations.
13

Figure 13. The die attach and voiding in the physical cross section 3 of diode 2 parallels the die 

attach plane in the 3DCT scan equivalent: (a) The third physical cross section of 

diode 2 has a cracked die and was optically imaged at 10× objective magnification 

in bright field, and (b) The 3DCT scan of diode 2 was virtually sectioned 65% of the 

way through the die to correspond to the third physical cross section. Both the physical and the digital representation have a large void on the left half of the cross-section 

plane (yellow arrow) with a tiny piece of die attach right of center (green arrow) and 

a small void near the right edge (blue arrow).

4.3 Diode 3

The die and glass body of diode 3 were fractured upon removal from the mold cylinder, 

so the grinding progression used for diode 3 was augmented to avoid exacerbating the condition. 

Mechanical grinding began with 600-grit, then 800-grit, followed by rough and final polishing as 

described in section 3. Starting with a smaller grit ensured milder abrasive cutting which, in turn, 

minimized die damage that can lead to extensive die attach pull-out. 

Figure 14(a) shows the cathode die attach plane in the 3DCT scan of diode 3, part number 

1N4942. Whereas diodes 1 and 2 had relatively solid die attach along the edges and large voids 

in the center, diode 3 is voided around the edges with the die attach centrally concentrated. The 

edges of the die attach in diode 3 also appear more marked in the 3DCT volumetric model than in 

the former diodes, so there is little to no areas of mottled, thin die attach. Figure 14(b) shows the 

enhanced CT scan of diode 1. The die attach is highlighted in blue to delineate the precise boundary of the die attach. White lines across the die attach area correspond to the locations of the four 

physical cross sections made from diode 3, designated 1 to 4 and pictured in figures 15–18, respectively. The 10× objective magnification, bright field metallograph pictures of the four cross sections 

and their 3DCT counterparts are shown in figures 15–18. Diodes of the 1N4942 variety are significantly smaller than 1N5550 diodes. The maximum width for a Unitrode 1N4942 diode is 2.16 mm, 

while the maximum width for a Unitrode 1N5550 diode is 3.68 mm. This is why the 3DCT images 

of diode 3 are pixelated when displayed at the same size as diodes 1 and 2. 
14

Figure 14. The 3DCT scan of diode 3, part number 1N4942, was digitally sectioned so that the 

entire plane of die attach was visible: (a) The unenhanced 3DCT scan shows the cathode die attach in the square of the footprint of the mesa of the die, surrounded by 

the plug and the glass body, and (b) To contrast the die attach from the plug in the 

3DCT scan of diode 2, the die attach was highlighted blue, and the four cross-section 

locations are shown in white and labeled 1 to 4.

Figure 15. The die attach and voiding in the physical cross section 1 of diode 3 parallels the die 

attach plane in the 3DCT scan equivalent: (a) The first physical cross section of 

diode 3 has a cracked die and was optically imaged at 10× objective magnification 

in bright field, and (b) The 3DCT scan of diode 3 was virtually sectioned 25% of 

the way through the die to correspond to the first physical cross section. Both the 

physical and the digital representation have voids of approximately equal size on 

the left and right of the center die attach (yellow and blue arrows) and a tiny void 

left of center (green arrow).
15

Figure 16. The die attach and voiding in the physical cross section 2 of diode 3 die parallels 

the die attach plane in the 3DCT scan equivalent: (a) The second physical cross 

section of diode 3 has a cracked die and was optically imaged at 10× objective 

magnification in bright field, and (b) The 3DCT scan of diode 3 was virtually sectioned 40% of the way through the die to correspond to the second physical cross 

section. The large void on the left edge (yellow arrow), tiny void on the right edge 

(blue arrow), and small void 70% of the way across the die attach plane from the left 

(green arrow) are all seen in both cross sections.

Figure 17. The die attach and voiding in the physical cross section 3 of diode 3 parallels the die 

attach plane in the 3DCT scan equivalent: (a) The third physical cross section of 

diode 3 has a cracked die and was optically imaged at 10× objective magnification 

in bright field, and (b) The 3DCT scan of diode 3 was virtually sectioned 65% of the 

way through the die to correspond to the third physical cross section. Both representations of the cross section 65% of the way through the die show a large void on the 

left edge (white arrow), a void of the same size afer a small bit of die attach (yellow 

arrow), two tiny voids in the die attach on the right half of the plane (green and blue 

arrows), and a tiny void on the right edge (purple arrow).
16

Figure 18. The die attach and voiding in the physical cross section 4 of diode 3 parallels the die 

attach plane in the 3DCT scan equivalent: (a) The fourth physical cross section of 

diode 3 has a cracked die and was optically imaged at 10× objective magnification 

in bright field, and (b) The 3DCT scan of diode 3 was virtually sectioned 85% of the 

way through the die to correspond to the fourth physical cross section. Neither representation of the fourth cross section shows any significant die attach.

Figure 15 depicts cross section 1 of diode 3, in which ≈25% of the die has been ground away. 

The 3DCT cross section in figure 15(b) parallels the physical cross section in 15(a) in that both 

show die attach only in the center of the plane and a tiny void left of center in the die attach. The 

voids on the left and right edges are of approximately equal size in both depictions. 

Figure 16 depicts cross section 2 of diode 3, in which ≈40% of the die has been ground away. 

There are three voids in both the physical and the digital representation of the cross section in figure 16(a) and (b), respectively. There is a large void across the left third of the die attach plane, 

a small void in the right third of the plane, and a tiny void on the right edge. The small void may 

be difficult to see in the 3DCT cross section in figure 16(b) because the cross section cuts through 

the bottom of the void, so most of the void has been sliced away. In the 3DCT cross sections, it is 

difficult to distinguish the voiding on the edge of the slice because the contrast between the voids 

and die attach is not as clear as in the physical cross sections. One must evaluate the entire die 

attach plane in the CT scan to draw hard conclusions about small voids. The voids must be significant enough in size to be clearly visible in the 3DCT scan. Very small voids, especially those smaller 

than the CT scan resolution of 9 mm, are almost indistinguishable from the pixelation and variance 

in gray values in the CT scan. This tiny void and other voids that are cut through at the bottom are 

easier to see in the full plane 3DCT depictions in figure 14 where the entire void is visible. 

Figure 17 depicts cross section 3 of diode 3, in which ≈65% of the die has been ground away. 

Both representations of cross section 3 agree that the left half of the die attach plane is voided 

except for a tiny patch of die attach present a quarter of the way across the plane. There are two 

tiny voids in the die attach on the right of the plane on the left and in the center of the remaining 

die attach. Both physical and digital cross sections also show a tiny void on the right edge. 
17

Figure 18 depicts cross section 4 of diode 3, in which ≈85% of the die has been ground 

away. There is no significant presence of die attach at this depth. The location of cross section 4 

of diode 3 was determined by equating the scale between the metallograph pictures and the CT 

view of the die attach plane that was established by the previous cross sections. Die attach voiding 

in the 3DCT scan and the physical cross sections matched well for diode 3.
18

5. ADDITIONAL OBSERVATIONS

Figures 12, 13, and 15–18 show a fractured die. The die of diode 2 (figs. 12 and 13) was fractured during the abrasive grinding, one of the potential consequences of this harsh technique. The 

die of diode 3 was fractured during its removal from the SamplKups® mold cylinder. Elimination 

of fractures is sometimes outside the control of the operator because fractures are exacerbated by 

the voiding in the die attach.

Pull-out is a consequence of abrasive grinding that can lead an operator to incorrect die 

attach conclusions. The die attach is softer than most of the other materials in the diode, so the 

abrasive grinding cuts away at the die attach faster. This can lead to chunks of die attach coming 

out and leaving a hole that resembles a void in the die attach plane. To differentiate between pullout and voiding of the die attach in the physical cross section, the plug was checked at the location 

of the missing die attach for the presence of silicides. The presence of silicides without die attach 

indicates pull-out as seen in the second section of missing die attach in figure 8(a). Because the 

other cross sections in this study, including those with fractured dies, had no significant pull-out, 

one can conclude that the physical cross sections match their digital CT equivalents.

Mistakes in accenting the die attach in blue in the 3DCT scan and in slicing the digital diode 

in VGStudio MAX are possible sources of error for incongruities between the physical cross sections and the 3DCT scans. The die attach in the 3DCT scan was colored blue through an imprecise 

combination of selecting the die attach by gray values and by hand in Photoshop. Therefore, there 

are likely small errors in the highlighting, especially for small voids and where the die attach is thin 

and indeterminate. The sectioning tool in the volume rendering software, VGStudio MAX, is inexact and presents difficulties when attempting precise clipping. The result is that CT cross-section 

planes may not correspond exactly to their physical counterparts. The notch cut into the diode 

is effective for general physical to digital orientation, but there could be slight differences between 

the two. The small size of the diode also amplifies any small errors in the placement of the lines 

indicating the locations of the cross sections in the CT scans in figures 6, 10, and 14.

There is another technique available to study die attach voiding, the scribe and break technique (MIL-STD-750E, Method 2101.1, 5.2).5 This technique is typically used to complement 

cross sectioning, and it requires scribing the glass packaging and fracturing the die intentionally. 

Unfortunately, this technique does not work well sometimes because the glass packaging may cause 

the silicon to fracture in undesired locations. This can complicate interpretation of the results and, 

like cross sectioning, the technique renders the diode unusable.
19

6. CONCLUSIONS

Cross sectioning is not an optimal method for examining die attach in diodes because it is 

time consuming, is a destructive technique, and can yield questionable results. It takes several hours 

to days to sample enough cross sections to compile an extensive picture of the die attach in a diode, 

and even then, slices of the die attach plane cannot provide a comprehensive assessment of the 

voiding. Additionally, the brittle silicon die is highly susceptible to cracking, and voids in the die 

attach increase the possibility of pull-out during the abrasive sectioning. This renders failure analysis conclusions questionable in many instances. Slightly angled or uneven polished cross-section 

planes render it nearly impossible to capture a sharp picture in high magnification due to the low 

focal length of most high-magnification optical objectives. Potting and sectioning the diode in the 

cross-section process destroys it.

Despite the errors discussed in the previous section, results from this study show that 3DCT 

scanning is a superior method for failure analysis of the diode die attach as compared to cross 

sectioning in terms of time efficiency, retention of diodes, and definitiveness of evaluation. A CT 

scan yields a complete portrayal of the die attach in less than an hour, and because the technique 

is nondestructive, the diode is still operable after testing. Because the diode is intact after the scan, 

3DCT can be repeated if additional tests on the diode need to be performed. Die attach voiding 

is clearly visible in the virtual volume model, and, unlike in cross sectioning, the voids cannot be 

attributed to pull-out. The thickness of the die attach is also discernable from the CT scan. The 

three-dimensional scan is preferred over the two-dimensional x-ray because the plugs on either side 

of the die obstruct the view of the die attach and the picture is less distinct and less detailed in two 

dimensions.
20
21

REFERENCES

1. Maur, F.: “X-ray Inspection for Electronic Packaging Latest Developments,” Electronic Packaging Technology Proceedings, ICEPT 2003, Fifth International Conference on Electronic Packaging Technology, pp. 235–239, 2003.

2. “Basics of Tomography,” European Synchrotron Radiation Facility, 2008, <http://www.esrf.eu/

UsersAndScience/Experiments/Imaging/ID22/Applications/Imaging/TomoBasics>, 

cited July 8, 2010.

3. Kropas-Hughes, C.V.; and Neel, S.T.: “Basics of Computed Tomography,” American Society 

for Nondestructive Testing, Back to Basics, 2000, <http://www.asnt.org/publications/materialseval/basics/may00basics/may00basics.htm>, cited July 8, 2010.

4. Pacheco, M.; and Goyal, D.: “X-ray Computed Tomography for Non-Destructive Failure 

Analysis in Microelectronics,”Reliability Physics Symposium (IRPS) Proceedings, 2010 IEEE 

International, Garden Grove (Anaheim), CA, pp. 252–258, 2010. 

5. MIL-STD-750E, Method 2101.1, 5.2.1, Department of Defense Test Method Standard, Test 

Methods for Semiconductor Devices, November 20, 2006.
22

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Three-Dimensional Computed Tomography as a Method

for Finding Die Attach Voids in Diodes

E.N. Brahm* and T.D. Rolin

George C. Marshall Space Flight Center

Marshall Space Flight Center, AL 35812

National Aeronautics and Space Administration

Washington, DC 20546–0001

Unclassified-Unlimited

Subject Category 33

Availability: NASA CASI (443–757–5802)

Prepared by the Space Systems Department, Engineering Directorate

*The University of the South, Sewanee, TN—NASA Undergraduate USRP Intern

M–1291

Technical Memorandum

NASA/TM—2010–216442

radiography, computed tomography, electrical, electronic, electromechanical (EEE) parts, die attach, 

cross section, nondestructive evaluation

01–09–2010

UU 32

NASA

U U U

NASA analyzes electrical, electronic, and electromechanical (EEE) parts used in space vehicles to understand failure modes of these components. The diode is an EEE part critical to NASA missions that can fail due to excessive 

voiding in the die attach. Metallography, one established method for studying the die attach, is a time-intensive, 

destructive, and equivocal process whereby mechanical grinding of the diodes is performed to reveal voiding in the 

die attach. Problems such as die attach ‘pull-out’ tend to complicate results and can lead to erroneous conclusions. 

The objective of this study is to determine if three-dimensional computed tomography (3DCT), a nondestructive 

technique, is a viable alternative to metallography for detecting die attach voiding. The die attach voiding in twodimensional planes created from 3DCT scans was compared to several physical cross sections of the same diode to 

determine if the 3DCT scan accurately recreates die attach volumetric variability.

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