NASA NEPP

BOK

NDE of Electronic Parts and Packaging

David T. Mih

# JPL

### Background

The purpose of this BOK study is two fold. First is to obtain and report
available Nondestructive Evaluation (NDE) technologies for inspection of
electronic parts and packaging; second is to survey the NDE equipment
throughout all NASA centers that are applicable for this purpose.

### Introduction

For decades, visual inspection and cross-sectional Destructive Physical
Analysis (DPA) were the mainstream of failure analyses in the electronic
parts and packaging field. However, they have become increasingly
difficult as the packaging density improves and multi-layer assembly
prevents a direct line of sight for optical viewing.

In addition, as the size of electronic packages decrease, deformation
and stress by mechanical sectioning make reliable failure analysis
difficult. Moreover, due to the nature of NASA programs, projects
usually do not have the luxury of performing large DPA samplings since
the procedures are destructive to the hardware, and hence are not
desirable, often because of part cost and limited availability.

The following two technologies will be covered by another BOK and hence
are excluded in this study.

SAM Scanning Acoustic Microscope

SLAM Scanning Laser Acoustic Microscope

##### TECHNOLOGY EVALUATION REPORT

The use of flip chips and ball-grid arrays (BGAs), and dense PWBs have
created a demand for inspection methods that do not require circuit
access through rows of pins. Automated optical inspection (AOI) and
metrology techniques such as laser mapping offer some help, but these
techniques require that components and PWB features be visible.

The study has found that the mostly widely used NDE technique is the
X-ray imaging method, including its several derivatives. The other
relatively popular technique is the infrared thermography.

### X-RAY

X-rays are attenuated exponentially by mass or atomic number of matter.
The detectability depends on the spatial resolution of the radiographic
system and the image contrast obtained by the radiographic detector. The
spatial resolution is a function of the detector, which governs the
signal to noise ratio (SNR); and the geometric resolution, which is
related to the image magnification that is governed by source to object
distance and object to detector distance. The geometric unsharpness is
governed by the x-ray source focal spot size, while image contrast is
affected by several factors, such as attenuation coefficient of the
material and anomaly defect, thickness of the anomaly in the direction
of the x-ray beam and intensity of scattered radiation.

## Film X-ray

This method has been around for decades and is still the most commonly
used method in the medical and dental fields. The detector in this case
is film and graininess determines the resolution of the images. The film
is then developed and a negative image is viewed. This method still
yields the best SNR and contrast.

However, the quality of the x-ray image and operator's skill play a
critical role with this method since the inspection involves manually
adjusting contrast and brightness for different regions via adjustments
of voltage and current. This manual method plus the time for film
development are not practical for most electronic inspections.

## Real Time Radiography 

With the development of x-ray detectors and microfocus x-ray sources, a
real time high-resolution inspection can be achieved. The set-up is
shown in Figure 1. The small x-ray beam produced at the anode point
source (4 um) compared with conventional beam dimensions allows
magnification of up to 1000x without loss of image sharpness. A detector
such as an image intensifier coupled to a video camera provides an
instantaneous image on a video monitor.

![](media/image1.jpeg){width="5.002083333333333in"
height="3.5694444444444446in"}

Figure 1. Real Time High Resolution X-ray Imaging System

### 

### Detector Technology

There are two common types of detectors used in today's real time x-ray
imaging systems, analog and digital. The image intensifier is the most
popular analog detector because it's relatively inexpensive, easy to
maintain and provides good contrast since it's essentially a vacuum
tube. The intensifier tube is convex and covered by a thin layer of
cesium iodide forming the cathode of the detector, this layer converts
the incident x-ray into visible light. Other analog detectors include
photo-multipliers and glass-scintillaters coupled with vidicon cameras.

#### The Flat Panel Detector (FPD), Figure 2, has gained popularity recently, this type of detector uses amorphous Si or Se films as radiation sensitive layers. The active imaging areas (11" x 17") are much larger than the image intensifier (6" Dia.), and the thicknesses of the devices are fairly thin to be comparable to the conventional x-ray film cassettes. The spatial resolutions of the devices are similar to conventional image intensifiers, but the noise levels are low. Therefore, high image qualities can be achieved with the new FPD systems. They have been developed so far mostly for medical purposes, but recently have been applied to NDE applications. 

![](media/image2.png){width="3.2291666666666665in"
height="2.0833333333333335in"}

Figure 2. Amorphous Silicon Flat Panel Detector

### Source Technology

In order to view defects in small objects such as solder balls or bond
wires with diameters as small as 25 microns, magnification and
resolution are significant factors. High magnification is almost a
prerequisite for the inspections of microelectronic components, but big
image with low resolution offers little or no value. The image
resolution depends mostly on the dimensions of the x-ray source, or
\"focal spot\". Focal spot size of an x-ray source determines the
sharpness of the image around the perimeters, as shown in Figure 3. For
the majority of x-ray systems currently in use in the electronics
industry, focal spot size ranges up to 8 microns, sometimes described as
"microfocus". A typical microfocus x-ray tube is illustrated in Figure
4. In the past two years, a few x-ray vendors have introduced x-ray
source of less than 1 micron and described as "Nanofocus".

![](media/image3.jpeg){width="6.4944444444444445in"
height="4.047916666666667in"}

###### Figure 3. Magnification vs. Edge Distortion and Source Size

![](media/image4.png){width="3.125in" height="3.8020833333333335in"}

Figure 4. Microfocus X-ray Source *(Courtesy of FeinFocus)*

### 3-D X-ray Systems

In two-dimensional (2-D) x-ray inspection, a source projects downward
normal to the component's surface, therefore it transmits through both
sides and sees all the layers at the same time. Using such a simple
irradiation technique, it is difficult to obtain information about the
depth of the imaged structures. Three dimensional (3-D) techniques such
as tomography, laminography and tomosynthesis examine horizontal slices
of the electronic parts or packaging at desired z-axis heights, and can
therefore analyze several horizontal layers of a component.

**Computed Tomography**

X-ray computed tomography (CT) (Figure 5) has been widely used in the
medical field as well as for NDE in many industrial applications. The
combination of X-ray transmission technique with tomographical
reconstruction allows three-dimensional information about the internal
microstructure. In this case any internal area can be reconstructed as a
set of flat cross sections, which can be used to analyze the two- and
three-dimensional morphological parameters. For X-ray methods, the
contrast in the images is a mixed combination of density and
compositional information.

![](media/image5.png){width="5.15625in" height="1.4583333333333333in"}

Figure 5. Computed Tomography

### Laminography

Because of high absorption and limited access, CT is not always
possible. For example in the case of flat components such as multilayer
printed circuit boards, a reconstruction algorithm may not work due to
beam hardening effects.

Laminographic methods are able to overcome some of these difficulties.
They can be used to visualize the internal details of thin multilayer
structures such as ball grid arrays and printed circuit boards.
Microlaminography allows layer by layer information to be measured in
\'slices\' as small as 10 microns. Questionable solder joints, disbonds
and voids can be detected. These methods yield images of object slices
and allow the determination of the position of the object structures.
Linear translation of the object through the fan beam of an x-ray source
is utilized. Both the x-ray source and the detector remain stationary,
or alternatively the object can remain stationary and the x-ray source
and the detector can be moved synchronously but without relative
movement as shown in Figure 6. During movement, the object is irradiated
by the x-ray at different angles due to the fan beam with a wide angle.
Thus, the elements of the detector successively obtain information of a
given volume element of the object under consecutively changing angles
and these digital projections contain the complete structure information
of all object slices. The projection values then will be sorted and
added to form a digital image.

![](media/image6.png){width="5.364583333333333in" height="2.40625in"}

###### Figure 6. Schematic of a Laminographic System

**Computed Tomosynthesis**

Another variation of laminography is based on relative motion of the
x-ray source, the detector and the object. The x-ray source and the
detector are either moved synchronously in circles or are simply
translated in opposite directions. Due to the correlated motion, the
location of the projected images of points within the object moves also.
Only points from a particular slice, sometimes called the focal slice,
will be projected at the same location onto the detector and the image
will be sharp. Objects above and below the focal slice will be projected
to different locations, and the resultant image will be fuzzy and will
be superimposed as background intensity to the focal slice. This
principle of superimposing projections is called tomosynthesis.

### Infrared Thermographic Inspection

Infrared Thermography or Radiometry, is a nondestructive and non-contact
method for observing and measuring thermal energy from materials in all
phases; solid, liquid and gas. To observe gases and thin films which
appear transparent to short-wave (3-5µm) or medium wave (8-12µm) IR
camera's, special filters can be used to selectively narrow the incoming
radiation at a specified waveband of interest. For instance; carbon
dioxide, an atmospheric gas, releases IR radiation in the 9-10.5µm
wavelength, so a filter that attenuates the electromagnetic radiation
below and above that waveband will allow the gas emissions to
effectively be seen by the IR camera. However; Silicon, Silicon Nitride
and Gallium Arsenide, semiconductor substrate materials, appear
transparent in the waveband of \~1µm to 15µm, a waveband were most
infrared detectors are designed, so heat is virtually non detectable
from these materials using a conventional IR camera. One solution is to
use a near IR (0.8 to 1.2µm) camera to measure radiation from these
types of materials. An infrared (IR) thermographic inspection system
(Figure 7) can be used to obtain hot spots in the printed wiring board
where local heating could be caused by electrical shorts.

![](media/image7.png){width="4.135416666666667in"
height="2.7916666666666665in"}

Figure 7. Thermographic Imaging System

***[Case Studies and Application Illustrations]{.underline}***

**X Ray**

In the electronics industry, inspection systems using x-rays have proven
particularly adept at examining hidden solder connections in PWBs, and
provide a practical way to inspect the connections between ball-grid
arrays (BGA) and a PWB, as well as other defects.

[Open connections]{.underline}. Open connections or cracks appear as
light areas where a good connection would show solid dark images.
(Figure 8)

![](media/image8.png){width="4.309027777777778in" height="2.9625in"}

Figure 8. Open Connection

[Solder bridges and voids**.**]{.underline} Solder bridges between
contacts or short circuits usually result from excess solder or
misplaced solder. Bridges also can occur after rework of a BGA board,
because of the difficulty of re-screening solder paste on the reworked
area. Bridges show up in an x-ray image as dark connections, while voids
appear as light spots within the dark ball area in the x-ray image
(Figure 9). Entrapment of voids can result from improper re-flow
temperature that prevents flux from escaping or from matter picked up by
the solder paste from an improperly cleaned PCB.

![](media/image9.png){width="3.6777777777777776in"
height="2.770138888888889in"}

Figure 9. Bridges and Voids

[Misregistration.]{.underline} Misregistration occurs when PWB pads,
solder balls, solder paste, or the PCB solder mask are not properly
aligned, or "registered". An x-ray image of misregistered components
(Figure 10) will show slight offset from PWB contacts.

![](media/image10.png){width="3.6777777777777776in"
height="2.7645833333333334in"}

Figure 10. Misregistration or Alignment Offset

**Summary**

X-ray inspection is not a single technique but a family of techniques,
each of which will perform more or less well in particular situations.
Achieving success among the x-ray alternatives depends on a PCB\'s
topology, technology, application, and other factors.

X-ray imaging falls into two broad categories: 2-D and 3-D.
Two-dimensional (2-D), or transmission, x-ray inspection involves a
source shining down perpendicular to the board\'s surface, thereby
\"seeing\" both sides at the same time. Three-dimensional (3-D)
techniques such as tomography, laminography and tomosynthesis examine
horizontal slices of the board at desired z-axis heights, and can
therefore analyze several horizontal layers of a PCB in a single
inspection pass.

CT allows nondestructive imaging of object slices, but with the
restriction that the objects have to be irradiated from the full angular
region.

Laminography can inspect in adjustable layer-by-layer steps from 10um to
2.5 mm. A maximum of 2.56 mm /256 layers can be scanned in one cycle,
with maximum specimen thickness of 10 mm that can be visualized by
several scans.

2-D x-ray offers shorter inspection times than 3-D because 3-D needs to
construct multiple images---one per \"slice\" through a PCB. Therefore
2-D is recommended for real-time analysis in time critical situations.

In the case of thermal inspection for microelectronics, software is the
key issue. As most of the thermal applications rely on the detection of
anomalies in the expected temperature pattern, there might be effects of
artifacts (e.g. reflections from the environment, air turbulences,
inhomogeneous infrared emission coefficient) that give rise to false
alarms or, even worse, hide real and critical anomalies. Unlike
structural thermal inspection , most of the contrast in an unprocessed
infrared image of a circuit is not due to a temperature differential;
rather the differences in the emissivity create the most contrast. A
good software program with the proper algorithms must be used to measure
and compensate for the emissivity of the material being tested in order
to generate an accurate, calibrated thermal image. Otherwise misleading
material thermal signal will result. Custom made focusing objective
lenses are also required since semiconductor materials and related
circuits emit low contrast thermal signals in addition to their small
sizes.

**Conclusion**

As with all test and inspection issues, no ideal solution exists.
Neither 2-D nor 3-D x-ray techniques can address all inspection needs.
Solder opens on ball-grid arrays caused by lack of coplanarity on the
board pad are visible only by 3D systems via a board slice, because on
2-D systems the open is hidden by the shadow of the solder ball itself.
3-D x-ray systems can quantitatively measure the volume of the solder
joint, determining heel length, fillet height, and other
characteristics. 2-D x-ray offers shorter inspection times than 3-D
because 3-D methods take multiple slices or views and then reconstruct
the images.

**NASA NDE EQUIPMENT LIST**

(For Microelectronic Inspection Only)

+-----------------------+-----------+----------------------------------+
| **Equipment Type**    | *         | **Contact Information**          |
|                       | *Center** |                                  |
+-----------------------+-----------+----------------------------------+
| Micro-focus X-ray     | JPL       | *David Mih (818) 354-9218*       |
| Imager **^1^**        |           |                                  |
|                       |           | *david.t.mih@jpl.nasa.gov*       |
+-----------------------+-----------+----------------------------------+
| Infrared Microscope   | JPL       | *Ron Ruiz (818) 354-2184*        |
| **^2^**               |           |                                  |
|                       |           | *Ronald.P.Ruiz@jpl.nasa.gov*     |
+-----------------------+-----------+----------------------------------+
| Digital               | GRC       | *Richard Rauser (216) 433-5018*  |
| Rad                   |           |                                  |
| iography/Laminography |           | *Don Roth (216) 433-6016*        |
+-----------------------+-----------+----------------------------------+
| Inner View X-ray      | SSC       | *William St. Cyr ((228)          |
| System                |           | 688-1134*                        |
|                       |           |                                  |
|                       |           | *William.W.St_Cyr@nasa.gov*      |
+-----------------------+-----------+----------------------------------+

1.  Practically every NASA center has a film x-ray system. LaRC has a CT
    system, WSTF has a real time x-ray imager but with a regular sized
    source. However, none of them are suitable for electronic inspection
    either due to a large x-ray source or large tomographic slices,
    since they are designed for large structural inspections. All of the
    above lack the resolution required for microelectronic inspection.

*2* MSFC, KSC and LaRC possess thermal imagers; but again, they are
designed for large composite structure inspection rather than for
microelectronics application.

**Manufacturers of X-ray Equipment**

Agilent Technologies
<http://we.home.agilent.com/USeng/nav/-536887030.0/pc.html>

Dage [[http://www.dageinc.com/]{.underline}](http://www.dage-group.com/)

Glenbrook Technologies
[http://www.glenbrooktech.com](http://www.glenbrooktech.com/)

Faxitron Wheeling [http://www.faxitron.com](http://www.faxitron.com/)

Feinfocus <http://www.feinfocus.com/>

Lixi, Downers Grove, IL http://www.lixi.com/

Nicolet Imaging Systems <http://www.nicimg.com/>

Operations Technology, Incorporated http://www.optek.net/

Phoenix X-ray <http://www.phoenix-xray.com/>

Photon Dynamics, Electronics Division, Aliso Viejo, CA 92656Formerly CR
Technology

SkyScan/MicroPhotonics <http://www.skyscan.be/>

Teradyne, Inc. http://www.teradyne.com/axi/xstation.html

Viscom <http://www.viscom.de/>

VJ Electronix http://www.vjelectronix.com/vjebeta/index.asp

X-Tek Systems <http://www.xtek.co.uk/>