AUTOMATED VISUAL INSPECTION FOR MANUFACTURING PROCESS
CONTROL WITH APPLICATION TO ELECTRONIC ASSEMBLY
Professor Steven Dubowsky, Room 3-469a
Department of Mechanical Engineering
Massachusetts Institute of Technology
77 Massachusetts Ave. Room 3-469
Cambridge, MA 02139
Tel: (617) 253-2144, FAX: (617) 258-5802
email: dubowsky@mit.edu
EXECUTIVE SUMMARY
The current practice of manual inspection of solder joints in electronic manufacturing is very costly and
not very effective. It has been anticipated that automated inspection systems would be developed for
this application that would yield great productivity gains, coupled with substantial cost savings over
manual inspection. However, the few automated systems for this task that have been placed on the
market have not met these expectations because of characteristic limitations in their underlying
technology. New fundamental technical approaches need to be developed to solve this problem and
produce effective automated inspection systems for this important application.
The problem is being addressed in this work by applying a recently developed optical technique which
uses structured light and computer based vision systems. The technique is called tailored moiré
interferometry. Unlike conventional structured light techniques, this approach does not require large
amounts of computation to yield information about the three dimensional shape of a solder joint. It is
essentially an optical pre-processing of the soldered joint image under structured light to detect poorly
fabricated joints. We hope to show that it is possible to locate common defects associated with a
characteristic change in the three-dimensional shape of the joint, components and/or the solder itself,
using this technology and produce a practical, truly high speed, low cost, reliable automated inspection
method for soldered connections.
MOTIVATION
A typical large company in the multi-billion dollar electronics industry creates hundreds of millions of
solder joints per year. Efficient and reliable inspection of these solder joints is of significant concern
from both quality control and manufacturing cost considerations. Up to 20% of the labor cost associated
with electronic printed circuit board production is from the inspection process. Electronic assembly
inspection has traditionally been performed manually by a skilled inspector using a low power
microscope. Manual visual inspection is slow, tedious, and inconsistent. The electronic industry’s increased use of smaller surface mount devices (SMDs) has significantly increased board space efficiency,
while making proper inspection even more important and more difficult than in the past. While
automated inspection techniques have become available and may represent an improvement over
traditional manual techniques, they also have some disadvantages.
Gull Wing Lead
SMD Body
Solder
Defect
(incomplete fillet)
PC Board
Figure 1. Schematic of Surface Mount Device with Defective Solder Joint.
There are currently several automatic solder joint inspection systems commercially available. The IRT
Corporation uses standard X-ray imaging to inspect solder joint quality [1]. It is effective for components
with hidden leads, but has high initial and maintenance costs, requires leaded solder to be used and can
normally inspect only boards with components on one side. It should be noted that future fabrication
techniques using non-lead materials are currently under development.
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Vanzetti Systems [2] uses a laser to reheat briefly each solder joint. The joint is then classified by using
its resulting infra-red radiation signature. There are concerns about the reheat effect on joint quality,
and reliable identification of joints with such defects as incomplete fillets (see Figure 1) as required by
the proposed Mil-specs governing surface mount solder connections [3]. Another system, manufactured by
RVSI, employs laser triangulation to measure shape of the solder joint [4]. The joint is scanned by a line
of laser light and the resulting line deformations are used to compute the 3D geometry of the joint. This
method requires substantial computational burdens for shape determination and analysis due to the
large amount of data needed for each joint. Inspection times for these methods range from 10 to 45
minutes per board. This long cycle time can be a production bottleneck, especially for mass produced
consumer electronics or computer hardware. As a result of the drawbacks associated with these new
systems, acceptance of them has been slow. The electronics manufacturing industry is still seeking a
fast, effective and economical solder joint inspection technology.
THE RESEARCH
The long term objective of this research program is to develop an automated visual inspection system for
soldered connections which is accurate, low-cost, consistent, reliable and high speed. The work is
focusing on optical processing techniques, that will avoid excessive computational effort. Currently, a
new moiré based structured light method, called tailored moiré interferometry, is being investigated as
the potential basis for an effective, low cost inspection system. In this initial work the inspection of
surface mount solder connections is being taken as the application.
Figure 2a shows a schematic of a SMD gull wing lead with a defect in one of its solder fillets. Figure 2b
shows under inspection by traditional methods such as structured light technique, or laser
triangulation. In this traditional method the deformed shape of all of the lines must be individually
measured at high resolution to obtain the 3D shape of the fillet. This process requires the recording of
a great deal of data and a large amount of computation. On the other hand, tailored moiré
interferometry in Figure 2c, views the deformed pattern through a second pattern, called a viewing
pattern to produce a set of low spatial frequency fringes. The result is a clearly visible signature.
Detecting a signature would require virtually no computational effort from a computer based vision
system. Hence a properly constructed viewing pattern acts like an optical pre-processor to the vision
system. Another advantage of the tailored moiré is that fringes only occur for geometries that vary
from the desired shape. Since the majority of joints inspected are good, very little computation is
needed for these joints. Only when a joint shows a light area must it be further examined and analyzed
to determine the defect. Traditional structured light techniques must perform the same computations
for all joints, good or bad. Because of its optical pre-processing and reduction in complexity, the
tailored moiré method offers the promise of being a basis for a rapid on-line inspection method.
Incomplete Fillet
a. The Defect
b. Traditional Structured Light
c. Tailored Moiré
Figure 2. Comparison of Traditional Structured Light and Tailored Moiré
Past work done at MIT in moiré interferometry has developed the underlying analytical basis [4],
interferometer optimization and design paradigms [5], and contouring of the visible fringes through
special grid patterns [6]. These tools plus further developments will enable us to design the sensitivity
of the interferometer and fringe pattern shape to make determination of solder fillet flaws easily
distinguishable as fringe patterns. We are studying the use of a tailored moiré interferometer to inspect
parts on the scale of SMDs.
REFERENCES
1. Green, M., and Stroebel, T., “Automatic Inspection of Assembled PCBs,” Electronic Packaging and
Production, April 1991, pp 30-33.
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2. Prassad, R.P., Surface Mount Technology, Principles and Practice, Van Nostrand Reinhold, 1989, pp
539-543.
3.
United States Department of Defense, MIL-STD-2000A, Sec 4.23, Surface Mount Solder Connections.
4.
Private Communication.
5. Wander, J.M., The Application of Moire Interferometry to Automated 3-Dimensional Inspection,
MS Thesis, Department of Mechanical Engineering, MIT, 1985.
6. Dubowsky, S., Holly, K., Murray, A.L., and Wander, J.M., “Design Optimization of Moire
Interferometers for Rapid 3-D Manufacturing Inspection,” Proceedings of the 1990 SPIE, Boston MA,
Nov. 5-9, 1990.
7. Reidemeister, E.P., Automated Visual Inspection of General Curvature Surfaces Using Moiré
Interferometry, MS Thesis, Department of Mechanical Engineering, MIT, August 1988.
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