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A Look Inside
Automated X-Ray Inspection
by Rick Roth, Lectronix, and Don Miller, YESTech
AXI is an effective technology for finding manufacturing defects in
electronics assembly operations.
Manufacturers of advanced electronics products know that simultaneously
producing a cost-competitive product and meeting or exceeding their customers’
quality expectations are vital to success. For these manufacturers, automated
X-ray inspection (AXI) is becoming increasingly popular because, like its
counterpart automated optical inspection (AOI), AXI is a noninvasive inspection
solution.
X-ray images of solder joints can be automatically analyzed to detect structural
defects such as insufficient solder, voiding, shorts, and opens that typically
account for approximately 90% of the total defects on a complicated board. But
unlike AOI, X-ray imaging is not hindered by ball grid array (BGA) devices,
component shields, heat-sinks, and high-density double-sided boards.
This key advantage of finding hidden defects makes AXI the logical choice for
inspecting complicated boards with BGAs, CGAs, CSPs, or components under RF
shields. The increasing popularity of array-style packaging is impacting a very
large number of boards today.
Many cell phones and wireless communications products are placing RF shields
over unsoldered components at pick-and-place, using the reflow processes to
solder them to the board.
X-ray inspection is the best way to detect solder defects that are obscured by
these shields.
In-line inspection during surface-mount assembly is most valuable when used to
locate defects close to the fault source. This strategy enables quick detection,
correction, and enhanced process control. Inspection cycle time also is critical
for in-line AXI systems and must be fast enough to do the job within the allowed
time while accurately verifying correct assembly and identifying errors.
AXI can be useful at many stages of the assembly process, but time and resource
constraints usually limit most products to a single X-ray inspection. For that
reason, it should be implemented where it will provide the maximum benefit to
the process.
Since automatic analysis of finished solder joints is AXI’s strength, most
systems are placed after the solder process, whether wave or reflow. At this
point, all solder joints on the board are present and can be covered in a single
test. Also, by waiting until the completion of the assembly process, any other
defects such as damaged or missing components will be detected (Figure 1).
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Figure 1. Screen Showing AXI of a Leaded Device |
Adoption of AXI has gained momentum as component sizes shrink, boards become
smaller with denser placements, and cycle times decrease. All of these factors
cause a higher probability of errors occurring, such as missing, misaligned, or
incorrectly soldered components (Figure 2).
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Figure 2. Screen Showing AXI of a BGA Device |
The key to further adoption is for AXI to meet the demanding standards and
lower-cost models of most PCB assembly operations. AXI system suppliers must
offer equipment with a level of technical sophistication, quality, and
reproducibility that will:
• Accurately detect and identify assembly and solder defects.
• Minimize false calls.
• Offer efficient operation in manufacturing environments. This includes keeping
pace with the line rate and ease of use.
Types of Systems
There are two principal types of AXI systems:
2-D or Transmission X-Ray
With this technique, X-rays are generated at a fixed point source, pass through
the PCB assembly, and form an image on an electronic detector. The image is
converted into a digital image and transferred to a computer where the analysis
takes place.
Transmission X-ray is widely used for single-sided boards in automotive and
other high-reliability applications. Advanced image processing software now is
available to distinguish topside components from those on the bottom side for
inspection of double-sided boards. This is the most common form of X-ray
inspection for electronic assemblies.
3-D X-Ray
This technology provides clear images of single layers or slices of the board to
facilitate inspection of double-sided boards. The laminographic 3-D technique
requires the X-ray source and detector to move in a circular pattern 180 degrees
out of phase. Only features in one plane are in focus, and components and solder
joints not in the plane are effectively blurred out.
The tomosynthesis technique creates 3-D images by combining multiple
transmission images taken from different angles. Both techniques are commonly
used today in X-ray inspection applications for more complicated electronic
assemblies.
The proven results of AXI make it one of the most effective technologies for
finding manufacturing defects in electronics assembly operations. Its acceptance
is increasing, and it is successfully being used in ever-differing manufacturing
environments.
Use of AXI systems will result in lower rework costs, better process control,
and improvement of the assembly process to further reduce defect rates. The
ultimate benefits are fewer field failures and improved profitability.
An Application
Adherence to quality standards is all in a day’s work for a leading
Michigan-based supplier of automotive integrated electronics technology,
software, and product design. Lectronix serves OEMs and ODMs in the automotive,
communications, and control industries. In conjunction with developing highly
reliable products that survive demanding environmental conditions, the company
adheres to stringent test and inspection guidelines backed by ISO 9001:2000 and
ISO/TS 16949:2002 certifications.
 Lectronix was an early adopter of several advanced manufacturing techniques that
helped to meet high quality standards. In 1999, the company installed both AOI
and AXI to verify part placement and solder-joint integrity. This inspection
strategy helped the company ensure quality in all phases of production,
resulting in high yields even on the most complex assembly runs.
In early 2005, Lectronix decided to replace its outdated inspection systems. In
addition to the equipment becoming dated, obtaining replacement parts was nearly
impossible. The company was facing extensive downtime, and the current equipment
setup time for new programs was taking several hours to as much as three days.
Programming was difficult and required a specially trained employee to define
all rules and create proprietary Lectronix parts libraries.
The company began looking at new equipment from some of the traditional
manufacturers of automated X-ray systems as well as systems from YESTech, a
supplier new to the company. Lectronix discovered that the YTX-6000 AXI System
offered capabilities comparable to those of machines from other vendors at a
significant cost savings.
In addition, comparable defect detection was possible with much less programming
than required by competing machines. Also, the YESTech SPC software package
proved helpful when converting manufacturing data to more of a preventative
application tool.
The significance of the
new AXI system cannot be overstated. In the past, the company was unable to
fully implement AXI on many assemblies for two reasons.
First, the time to program the AXI system prohibited its use in many cases,
especially for short runs in which programming
time would create
production bottlenecks. Second, many times board design would not allow adequate
AXI coverage due to component shadowing, requiring additional time to create
further custom part libraries if even possible.
The
YESTech YTX-6000 addressed these issues and allowed Lectronix to quickly and
easily further implement automated X-ray inspection across all product
assemblies.
The recent setup of a new, very complex board demonstrated a large cost savings.
The process required about one hour or less including programming of AXI for all
components, not just BGAs. The old inspection equipment would have taken about
16 hours of programming time.
The programming efforts were shortened in part because the YTX-6000 does not
require special programming languages. The new equipment was programmed quickly
and easily, which translated to substantial operating cost savings.
The new system’s reporting features provided detailed information, not only for
quality monitoring and reporting, but also for on-the-line repair. Lectronix
repair technicians can call up the board image and see exactly where the suspect
item is located.
The SMT process has benefited as well. A noticeable increase in yields has been
achieved by analyzing the data collected by the AXI system.
Three-Step Inspection Approach
The equipment also fits in well with Lectronix’s tiered inspection procedure,
which is a highly effective component in the quality program.
Prototype
Test and inspection methods including both AXI and AOI are used whenever
possible for prototypes. The combination of methods quickly gets prototypes into
the engineer’s hands.
First Production Run
Quality is maintained from the first board off the line on the first production
run. The standard procedure is to use AOI and AXI to verify the correct build of
the first five boards of all production-run setups and at the beginning of each
shift. This confirms the setup and that all SMT processes are in control.
Production
AXI allows Lectronix to constantly monitor the SMT process. Each board produced
has a specific detailed inspection plan. Operators can shut down the production
line if any of the inspection results get out of range. This constant feedback
loop improves the process and prevents costly rework from being required. Any
errors that occur are pinpointed so that they can be fixed immediately.
The Results
The ultimate benefit of AXI is seen as reduced field failures. Inspection and
test processes improve the reliability of the product in the field. Because AXI
can detect marginal solder joints, something not detectible by traditional
in-circuit test, customers experience fewer product returns, resulting in lower
warranty cost and lower overall product cost.
Lectronix was looking for value as well as enhanced capabilities from the new
inspection equipment. For several weeks, products were tested on the previous
system as well as on the YESTech machine and the results compared. In finding
defects, the YTX-6000 was equal to or better than the previous equipment. This
result allowed the company to confidently integrate the new machine as the
primary inspection equipment without risk of decreasing yields.
About the Authors
Rick Roth has been the senior vice president of operations for Lectronix since
its inception in 2002. Over his 18-year career in the electronics industry, he
has held management positions in procurement, manufacturing, and operations
management as well as cofounding an electronics design and manufacturing company
in 1995. Lectronix, 5858 Enterprise Dr., Lansing, MI 48911, 517-492-1900,
e-mail: roth@lectronix.biz
Don Miller is president and cofounder of YESTech. He has 27 years of experience
in semiconductor and PCB inspection with tenure as vice president of sales.
YESTech, 1317 Calle Avanzado, San Clemente, CA 92673, 949-361-2714, e-mail:
sales@yestechinc.com |