U.S. patent number RE35,423 [Application Number 08/182,841] was granted by the patent office on 1997-01-14 for method and apparatus for performing automated circuit board solder quality inspections.
This patent grant is currently assigned to ThermoSpectra Corporation. Invention is credited to John Adams, Juan Amoroso, Jr., Paul Axford, Phil Bowles, Mike Juha, Van Nguyen, Charles Preskitt, Ed Ross, Doug Thompson, Paul Turner.
United States Patent |
RE35,423 |
Adams , et al. |
January 14, 1997 |
Method and apparatus for performing automated circuit board solder
quality inspections
Abstract
A method and apparatus for measuring structural characteristics
of a manufactured circuit board containing solder joints by
automated real-time digital X-ray radiographic inspection
techniques. A circuit board under examination is automatically
positioned by a digitally controlled multi-axis positioning system
between an electronic X-ray source and an electronic X-ray imaging
system, X-rays, in a beam of X-rays from the X-ray source, are
directed towards the circuit board. The X-rays are absorbed,
scattered and transmitted through the circuit board. The X-rays
transmitted through the circuit board are directed upon the X-ray
imaging system. The X-ray imaging system converts the transmitted
X-rays into digital images which represent the radiographic density
of the portion of the circuit board under examination. The digital
images are stored within a digital image processor. A computer,
under program control, performs calculational measurements on the
digital images so as to measure the structural characteristics of
the solder joints and components on the circuit board. The
calculational measurements are compared to predetermined standards
corresponding to acceptable quality standards programmed into the
computer. In response to the comparison, the computer provides an
accept/reject decision on the circuit board in addition to
providing manufacturing process control information for correction
of found defects. The questions raised in reexamination request No.
90/002,298, filed Mar. 15, 1991, have been considered and the
results thereof are reflected in this reissue patent which
constitutes the reexamination certificate required by 35 U.S.C. 307
as provided in 37 CRF 1.570(e).
Inventors: |
Adams; John (Escondido, CA),
Amoroso, Jr.; Juan (San Diego, CA), Axford; Paul (La
Jolla, CA), Bowles; Phil (Encinitas, CA), Juha; Mike
(Del Mar, CA), Nguyen; Van (San Diego, CA), Preskitt;
Charles (La Jolla, CA), Ross; Ed (Escondido, CA),
Thompson; Doug (San Diego, CA), Turner; Paul (San Diego,
CA) |
Assignee: |
ThermoSpectra Corporation
(Franklin, MA)
|
Family
ID: |
25260392 |
Appl.
No.: |
08/182,841 |
Filed: |
January 14, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
831997 |
Feb 20, 1986 |
04809308 |
Feb 28, 1989 |
|
|
Current U.S.
Class: |
378/58; 378/901;
378/98.2 |
Current CPC
Class: |
G01N
23/083 (20130101); G01R 31/304 (20130101); G06T
7/0006 (20130101); G01N 23/18 (20130101); G06T
2207/30141 (20130101); G06T 2207/10121 (20130101); G06T
2207/30152 (20130101); Y10S 378/901 (20130101) |
Current International
Class: |
G01N
23/02 (20060101); G01N 23/18 (20060101); G06F
015/46 () |
Field of
Search: |
;378/58,98.2,901 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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52-128190 |
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Oct 1977 |
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JP |
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54-43290 |
|
Nov 1979 |
|
JP |
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59-75140 |
|
Apr 1984 |
|
JP |
|
2143379 |
|
Sep 1986 |
|
GB |
|
Other References
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Tech. Pop. Soc. Manuf. Eng., pp. 1-12, 1985..
|
Primary Examiner: Church; Craig E.
Attorney, Agent or Firm: Banner & Witcoff, Ltd.
Claims
What is claimed is: .[.1. An apparatus for measuring structural
characteristics of selected portions of a circuit board having
components disposed upon and electrically connected thereto at
solder joints, comprising:
X-ray means for providing a beam of X-rays;
multi-axis positioning means for receiving and for selectively
positioning a circuit board, said circuit board having components
coupled thereto at solder joints, within said beam of X-rays;
imaging means for detecting X-rays from said beam of X-rays
transmitted through said circuit board and providing an electronic
image thereof;
processing means for converting said electronic image into a gray
scale coded image; and
computational means for storing a library of measurement algorithms
and predetermined measurement parameters corresponding thereto, for
receiving said gray scale coded image, for analyzing said gray
scale coded image by selecting measurement algorithms from said
library of measurement algorithms with the selected measurement
algorithms corresponding to certain structural characteristics of
said circuit board, performing predetermined computational analysis
on said gray scale coded image based upon each selected measurement
algorithm, and providing a resultant analysis value corresponding
to each computational analysis,
for comparing each resultant analysis value with each corresponding
measurement parameter, and for providing an output corresponding to
the variation of each resultant analysis value from each
corresponding
predetermined measurement parameter..].2. The apparatus of claim
.[.1.]. .Iadd.49 .Iaddend.further comprising filter means for
modifying the energy
spectrum of X-rays in said beam of X-rays. 3. The apparatus of
claim .[.1.]. .Iadd.49 .Iaddend.wherein said imaging means
comprises a solid
state detector. 4. The apparatus of claim .[.1.]. .Iadd.49
.Iaddend.wherein said imaging means comprises:
optical imaging means for providing an optical image corresponding
to the intensity of X-rays transmitted through said circuit board;
and
electronic imaging means for converting said optical image into
a
corresponding electrical image. 5. The apparatus of claim 4 wherein
said optical imaging means comprises a scintillating screen
positioned to
receive X-rays transmitted through said circuit board. 6. The
apparatus of claim 4 wherein said electronic imaging means
comprises:
a video camera;
and reflecting means mounted adjacent said video camera for
reflecting said
optical image from said optical imaging means to said video camera.
7. The apparatus of claim .[.1.]. .Iadd.49 .Iaddend.wherein said
processing means
comprises a high-speed digital gray scale image processor. 8. The
apparatus of claim .[.1.]. .Iadd.49 .Iaddend.wherein said
multi-axis positioning system comprises:
controller means responsive to instruction signals generated by
said computational means for providing predetermined position
signals; and
motion table means responsive to said position signals for moving
said circuit board mounted thereupon in x, y and z orthogonal axis
with
rotation in each axis. 9. The apparatus of claim .[.1.]. .Iadd.49
.Iaddend.wherein said X-ray means comprises an electronic X-ray
source.
The apparatus of claim .[.1.]. .Iadd.49 .Iaddend.wherein said gray
scale coded image generated by said processing means from said
electronic image represents the radiographic density of said
circuit board said beam
of X-rays are transmitted therethrough. 11. The apparatus of claim
.[.1.]. .Iadd.49 .Iaddend.wherein said computational means further
provides an output indication as to whether each resultant analysis
value is within
said corresponding predetermined measurement parameter. 12. The
apparatus of claim .[.1.]. .Iadd.49 .Iaddend.wherein said
computational means provides said output an output signal
indicative of measurement data for the measured structural
characteristics of a manufactured circuit board
under test. 13. The apparatus of claim .[.1.]. .Iadd.49
.Iaddend.wherein said computational means further provides visual
display of said output of measurement data for the measured
structural characteristics of said
circuit board under test. 14. The apparatus of claim .[.1.].
.Iadd.49 .Iaddend.wherein each selected measurement algorithm
corresponds to a certain structural characteristic of a respective
one of an electronic device, a mechanical device, an electrical
component, and a mechanical
component of said circuit board under test. 15. The apparatus of
claim 14 wherein said computational means further provides an
output indication as to whether each resultant analysis value is
within said corresponding predetermined measurement parameter.
.[.16. A method of determining the structural characteristics of a
circuit board having components disposed thereupon and connected
thereto at solder joints, comprising the steps of:
supporting a circuit board having certain structural
characteristics by a multi-axis positioning means adjustable for
optimum exposure of said circuit board to a source beam of
X-rays;
exposing said circuit board to a beam of X-rays having sufficient
energy to penetrate said circuit board;
detecting X-rays transmitted through said circuit board and
providing an electronic image thereof;
converting said electronic image into a gray scale coded image;
providing a library of measurement algorithms;
providing predetermined measurement parameters corresponding to
said measurement algorithms;
selecting at least one measurement algorithm with each selected
measurement algorithm corresponding to a respective one of said
certain structural characteristics;
performing predetermined computational analysis on said gray scale
coded image using each selected measurement algorithm;
providing a resultant analysis value corresponding to each
computational analysis;
comparing each resultant analysis value with each corresponding
predetermined measurement parameter; and
providing an output indicative of the variation of each resultant
analysis
value from each corresponding predetermined measurement
parameter..].17. The method of claim .[.16.]. .Iadd.25
.Iaddend.further comprising the step of providing an output as an
indication of whether each resultant analysis value is within said
corresponding predetermined
measurement parameter. 18. The method of claim .[.16.]. .Iadd.25
.Iaddend.wherein said output includes quantitative data
corresponding to the structural characteristics of at least one
inspected circuit board
component. 19. The method of claim .[.16.]. .Iadd.25
.Iaddend.wherein the step of detecting includes:
disposing a scintillating screen in the path of X-rays passing
through said circuit board, said scintillating screen generating an
optical image of X-rays passing through said circuit board;
viewing said optical image generated by said scintillating screen
with a video imaging system;
providing an electronic signal corresponding to the optical image
observed
by said video imaging system. 20. The method of claim .[.16.].
.Iadd.25 .Iaddend.wherein the step of performing predetermined
computational analysis further comprises the steps of:
selecting at least one measurement algorithm from a pre-structured
library of measurement algorithms;
controlling the detection of X-rays in accordance with said
algorithms; and
recording each resultant analysis value in an electronic storage
means.
The method of claim .[.16.]. .Iadd.25 .Iaddend.further comprising
the step of providing an output indication whether each resultant
analysis value is within said corresponding predetermined
measurement parameter.
.Iadd.22. A method of determining the structural characteristics of
a circuit board having components disposed thereupon and connected
thereto at solder joints, comprising the steps of:
supporting a circuit board having certain structural
characteristics by a multi-axis positioning means adjustable for
optimum exposure of said circuit board to a source beam of
x-rays;
optically identifying said circuit board;
automatically retrieving an inspection list corresponding to said
circuit board;
exposing said circuit board to a beam of x-rays having sufficient
energy to penetrate said circuit board;
detecting x-rays transmitted through said circuit board and
providing an electronic image thereof;
converting said electronic image into a gray scale coded image;
providing a library of measurement algorithms;
providing determined measurement parameters corresponding to said
measurement algorithms;
selecting at least one measurement algorithm with each selected
measurement algorithm corresponding to a respective one of said
certain structural characteristics;
performing predetermined computational analysis on said gray scale
coded image using each selected measurement algorithm;
providing a resultant analysis value corresponding to each
computational analysis;
comparing each resultant analysis value with each corresponding
predetermined measurement parameter; and
providing an output indicative of the variation of each resultant
analysis value from each corresponding predetermined measurement
parameter.
.Iaddend..Iadd.23. A method as claimed in claim 22, further
comprising the step of selectively positioning said circuit board
in accordance with said inspection list. .Iaddend..Iadd.24. A
method as claimed in claim 22, wherein the step of selecting the at
least one measurement algorithm is
performed in accordance with the inspection list.
.Iaddend..Iadd.25. A method of determining the structural
characteristics of a circuit board having components disposed
thereupon and connected thereto at solder joints, comprising the
steps of:
supporting a circuit board having certain structural
characteristics by a multi-axis positioning means adjustable for
optimum exposure of said circuit board to a source beam of
x-rays;
automatically generating an inspection list corresponding to said
circuit board;
exposing said circuit board to a beam of x-rays having sufficient
energy to penetrate said circuit board;
detecting x-rays transmitted through said circuit board and
providing an electronic image thereof;
converting said electronic image into a gray scale coded image;
providing a library of measurement algorithms;
providing predetermined measurement parameters corresponding to
said measurement algorithms;
selecting, in accordance with said generated inspection list, at
least one measurement algorithm with each selected measurement
algorithm corresponding to a respective one of said certain
structural characteristics;
performing predetermined computational analysis on said gray scale
coded image using each selected measurement algorithm;
providing a resultant analysis value corresponding to each
computational analysis;
comparing each resultant analysis value with each corresponding
predetermined measurement parameter; and
providing an output indicative of the variation of each resultant
analysis value from each corresponding predetermined measurement
parameter.
.Iaddend..Iadd.26. A method as claimed in claim 25 further
comprising the step of evaluating a manufacturing process by
performing statistical analysis on measurement data generated by
executing said at least one measurement algorithm.
.Iaddend..Iadd.27. A method as claimed in claim 25, wherein said
inspection list comprises a sequence of operation for said
positioning means. .Iaddend..Iadd.28. A method as claimed in claim
27, wherein said inspection list further comprises a sequence of
selecting said at least one measurement algorithm.
.Iaddend..Iadd.29. A method as claimed in claim 25, wherein said
inspection list comprises a
classification of said joints. .Iaddend..Iadd.30. A method as
claimed in claim 29, wherein said library comprises algorithms for
a plurality of joint types, and said selection of algorithm is made
in accordance with said classification. .Iaddend..Iadd.31. A method
as claimed in claim 29, wherein said inspection list further
comprises a correction factor associated with a particular one of
said joints for adjusting said selected algorithm.
.Iaddend..Iadd.32. A method as claimed in claim 25, further
comprising the step of identifying the circuit board.
.Iaddend..Iadd.33. A method as claimed in claim 32, wherein the
step of automatically generating said inspection list comprises the
step of retrieving computer-aided design data corresponding to the
identified circuit board. .Iaddend..Iadd.34. A method as claimed in
claim 32, wherein the step of identifying the circuit board
comprises scanning at least a portion of the circuit board.
.Iaddend..Iadd.35. A method as claimed in claim 34, wherein
scanning said circuit board comprises scanning a label
imprinted upon said circuit board. .Iaddend..Iadd.36. A method as
claimed in claim 35, wherein each type of circuit board from a
plurality of circuit board types is imprinted with a different
label. .Iaddend..Iadd.37. A method as claimed in claim 36, wherein
each type of circuit board has a corresponding inspection list.
.Iaddend..Iadd.38. A method as claimed in claim 25, wherein said
inspection list comprises view information and algorithm selection
information. .Iaddend..Iadd.39. A method as claimed in claim 25,
further comprising the steps of:
interpreting the inspection list; and
selectively positioning the circuit board in accordance with the
inspection
list interpretation. .Iaddend..Iadd.40. An apparatus for measuring
structural characteristics of selected portions of a circuit board
having components disposed upon and electrically connected thereto
at solder joints, comprising:
x-ray means for providing a beam of x-rays;
multi-axis positioning means for receiving and for selectively
positioning a circuit board, said circuit board having components
coupled thereto at solder joints, within said beam of x-rays;
imaging means for detecting x-rays from said beam of x-rays
transmitted through said circuit board and providing an electronic
image thereof;
processing means for converting said electronic image into a gray
scale coded image;
means for optically identifying the circuit board;
means for automatically retrieving an inspection list corresponding
to the circuit board; and
computational means for storing a library of measurement algorithms
and predetermined measurement parameters corresponding thereto, for
receiving said gray scale coded image, for analyzing said gray
scale coded image by selecting measurement algorithms from said
library of measurement algorithms with the selected measurement
algorithms corresponding to certain structural characteristics of
said circuit board, performing predetermined computational analysis
on said gray scale coded image based upon each selected measurement
algorithm, and providing a resultant analysis value corresponding
to each computational analysis, for comparing each resultant
analysis value with each corresponding predetermined measurement
parameter, and for providing an output corresponding to the
variation of each resultant analysis value from each corresponding
predetermined measurement parameter. .Iaddend..Iadd.41. An
apparatus as claimed in claim 40, wherein the computational means
automatically selects the algorithms in accordance with the
inspection list. .Iaddend..Iadd.42. An apparatus as claimed in
claim 40, wherein the output from the computational means is
designated as corresponding to the circuit board.
.Iaddend..Iadd.43. An apparatus as claimed in claim 40, wherein the
circuit board is selectively positioned by the positioning means in
accordance with the inspection list. .Iaddend..Iadd.44. An
apparatus as claimed in claim 40, wherein the retrieving means
comprises a computer and
a device containing computer-aided design data. .Iaddend..Iadd.45.
An apparatus as claimed in claim 40, wherein the identifying means
further comprises a display monitor and an input device.
.Iaddend..Iadd.46. An apparatus as claimed in claim 40, wherein the
identifying means identifies the circuit board as belonging to a
particular manufacturing lot. .Iaddend..Iadd.47. An apparatus as
claimed in claim 40, wherein the identifying means comprises an
optical scanner. .Iaddend..Iadd.48. An apparatus as claimed in
claim 47, wherein the optical scanner generates a signal for
automatically identifying the circuit board as one of a
plurality of circuit board types. .Iaddend..Iadd.49. An apparatus
for measuring structural characteristics of selected portions of a
circuit board having components disposed upon and electrically
connected thereto at solder joints, comprising:
x-ray means for providing a beam of x-rays;
multi-axis positioning means for receiving and for selectively
positioning a circuit board, said circuit board having components
coupled thereto at solder joints, within said beam of x-rays;
imaging means for detecting x-rays from said beam of x-rays
transmitted through said circuit board and providing an electronic
image thereof;
processing means for converting said electronic image into a gray
scale coded image;
means for automatically generating an inspection list corresponding
to said circuit board; and
computational means for storing a library of measurement algorithms
for a plurality of joint types and predetermined measurement
parameters corresponding thereto, for receiving said gray scale
coded image, for analyzing said gray scale coded image by
interpreting said inspection list, selecting, in accordance with
said inspection list interpretation, measurement algorithms from
said library of measurement algorithms with the selected
measurement algorithms corresponding to certain structural
characteristics of said circuit board, performing predetermined
computational analysis on said gray scale coded image based upon
each selected measurement algorithm, and providing a resultant
analysis value corresponding to each computational analysis, for
comparing each resultant analysis value with each corresponding
predetermined measurement parameter, and for providing an output
corresponding to the variation of each resultant analysis value
from each corresponding predetermined measurement parameter.
.Iaddend..Iadd.50. An apparatus as claimed in claim 49, wherein
said generating means comprises a computer and a device containing
computer-aided design data. .Iaddend..Iadd.51. An apparatus as
claimed in claim 49, further comprising means for identifying the
circuit board. .Iaddend..Iadd.52. An apparatus as claimed in claim
51, wherein a plurality of inspection lists corresponding to a
plurality of circuit board types are resident in a storage device
coupled to said computational
means. .Iaddend..Iadd.53. An apparatus as claimed in claim 52,
wherein said identifying means comprises a scanner for
automatically identifying said circuit board as one of said
plurality of circuit board types. .Iaddend..Iadd.54. An apparatus
as claimed in claim 49, wherein said computational means comprises
an imaging computer having a software module for interpreting said
inspection list. .Iaddend..Iadd.55. An apparatus as claimed in
claim 54, wherein said positioning means selectively positions said
circuit board in accordance with said inspection list.
.Iaddend..Iadd.56. An apparatus as claimed in claim 49, wherein the
inspection list comprises, for each of the joints, a designation of
a set of algorithms from the library of algorithms, and a
designation of a
sequence of applying the set. .Iaddend..Iadd.57. An apparatus as
claimed in claim 49, wherein a plurality of inspection lists
corresponding to a plurality of circuit board types are resident in
a storage device coupled to the computational means.
.Iaddend..Iadd.58. An apparatus as claimed in claim 57, wherein the
computational means comprises an imaging computer having a software
module for interpreting the inspection list corresponding to the
circuit board. .Iaddend..Iadd.59. An apparatus as claimed in claim
49, wherein the library of measurement algorithms comprises
algorithms for a plurality of joint types. .Iaddend.
Description
BACKGROUND OF THE INVENTION
I. Technical Field
The present invention relates to automated circuit board inspection
systems and techniques. More specifically, the present invention
relates to a novel method and apparatus for performing measurements
of the structural characteristics of a manufactured circuit board
having solder joints thereupon by a fully automatic real-time
digital X-ray radiographic inspection techniques.
II. Background Art
In electronic, components are typically mounted upon or inserted
into a circuit board. The electrical contact between the circuit
board and the components is assured by soldering of the component
into permanent position. Thereafter, the electrical integrity of
the circuit board depends upon the mechanical integrity of the
soldering completed during the circuit board assembly. Soldering
processes are well-known and may be reasonably controlled to
correct solder related deficiencies. However, soldering processes
do not always work perfectly with deficiencies such as solder
skips, bridges, insufficient amounts of solder, blow-holes and pin
holes which can occur as a result of variations in materials in the
solder process. Defects, such as those just mentioned, occur
sufficiently often such that it is mandatory to inspect solder
connections to reduce solder connection related failures.
Traditionally, solder quality inspection has been performed
visually merely because of the fact that humans sense more data
visually than with any other of the senses. As a result, previous
inspections standards for solder quality were written in terms of
the external appearance of the solder connection. The objective of
a solder quality inspection is also to insure mechanical integrity
of the solder connection. Since mechanical integrity is dependent
upon the interior structure of the solder connection, visual
inspection techniques are wholly deficient in verifying mechanical
integrity.
The mechanical integrity of a solder connection depends upon the
type of solder alloy used, the solder connection structure (surface
mount versus pin-through-hole) and the presence of an adequate and
uniform volume of solder bonding (or wetting) of the electronic
component to the circuit board. Visual inspection is regarded as a
qualitative test, rather than a quantitative test. In visual
inspections, the external appearance of the solder connection is
used to infer internal structural integrity. Visual inspections are
an accepted solder quality inspection practice used to identify
gross variations in connection structures, such as missing pins,
insufficient solder volume, excess solder or bridging. However,
visual inspection cannot verify the uniformity of the solder within
the connection, and cannot detect defects that are hidden below
components mounted on the circuit board. Solder uniformity has a
critical influence on the strength and durability of the solder
connection. Solder connection strength and uniformity are
particularly important in the connection of surface mount devices
where the devices are held to the circuit board by the solder
connection. It is well-known in the surface mount device art that
solder connections are more susceptible to thermal and mechanical
stress related failures than pin mounted devices. In solder mounted
devices, visually inspected structurally marginal connections, due
to solder non-uniformity, may still provide electrical connection
without the defect being discovered in stress testing. As a result,
the marginal connection or hidden defect is a likely candidate for
a long term failure while under normal mechanical and thermal
stress. With a greater number of surface mount components being
used in circuit boards, visual inspections are proving, in many
cases, to be deficient in detecting structural deficiencies in the
solder connections.
Solder quality visual inspection systems examine the circuit boards
to detect defects such as components missing; components
incorrectly oriented; missing or bent component pins or leads such
that the component does not make a connection; cracked solder
connections; solder bridge between component pins or circuit board
pad; small holes present at the surface of the connection;
insufficient clearance between component pins; excess solder in the
connection; insufficient solder in the connection; solder spurs,
spikes, balls or splashes; poor solder wetting on the board or the
component; a misshaped solder connection which indicates surface
tension problems; component askew pads on the circuit board;
component pins lifted or tilted from the circuit board; component
pins misaligned with circuit board pads; and component pins not
projecting through the circuit board hole. Each of the above
defects indicate conditions that can compromise the electrical and
mechanical integrity of the circuit board.
In many applications, defects are hidden from the human eye or
machine vision inspection systems. An example of such a defect is
in the case of solder porosity or voids. While defects may not be
masked by visual barrier, increasing circuit density may result in
defects which are not readily apparent to the human eye at
production line rates. With machine vision inspection systems,
inspection deficiencies still exist. For example, machine vision
inspection systems would be unavailable for inspecting defects such
as solder balls under a pin grid array.
Typical inspection systems are oriented toward finding defects
rather than avoiding the defects in the production of future items.
The avoidance of defects essentially requires rapid process control
feedback from the inspection system to the production line. Process
control feedback of the defects requires quantitative analysis
feedback of the deficiencies and providing this information to the
production line to control the soldering process. For example,
quantitative quality data such as the excess amount of solder
volume present in a solder connection must be fed back to the
soldering process to reduce the solder used in future units so as
to eliminate the defect. The present human and machine vision
inspection systems lack the ability to provide quantitative data
for feedback control to the process lines for correcting process
deficiencies. With faster production lines, inspection systems must
detect process drift before the production line turns out numerous
defective items. For solder quality process control, the inspection
accuracy and repeatability needed must detect even the smallest
changes in solder connection before they grow to become
defects.
SUMMARY OF THE INVENTION
In the case of solder quality inspections, the use of X-ray
inspection techniques enables the inspection of visually hidden
defects. X-ray imaging and computer-based image processing are well
suited for solder quality inspections. The metallic alloys used in
solder are remarkably opaque to X-rays as compared to the
translucence of the ceramics, epoxies, silicon or copper materials
used in circuit board assemblies. In addition, the ceramics,
epoxies, silicon or copper materials have differing degrees of
translucence so as to permit the distinction between these
materials. As a result, small defects in the solder or the circuit
board are readily identified. The penetrating nature of X-rays is
particularly suited for searching out hidden defects with respect
to solder connections due to contrast between solder and other
circuit board materials and components. X-ray inspection may be
utilized to perform quantitative measurements in quality assurance
inspections of solder connections. The strength of X-ray inspection
is in the ability to display the external and internal structure of
each solder connection. In effect, X-ray inspection images are
three-dimensional, i.e. length, width and thickness with length and
width (or size) being represented by object contrast from
surrounding areas with thickness being represented by the shades of
gray or black. With data corresponding to the size and thickness of
a solder connection, a determination can be made as to the quality
of the solder connection.
The present invention is a fully automated X-ray solder quality
inspection system and a method for performing solder quality
inspections utilizing X-rays.
In summary, the present invention is a method and apparatus for
measuring structural characteristics of a manufactured circuit
board containing solder joints by automated real-time digital X-ray
radiographic inspection techniques. A circuit board under
examination is automatically positioned by a digitally controlled
multi-axis positioning system between an electronic X-ray source
and an electronic X-ray imaging system. X-rays, in a beam of X-rays
from the X-ray source, are directed towards the circuit board. The
X-rays are absorbed, scattered and transmitted through the circuit
board. The X-rays transmitted through the circuit board are
directed upon the X-ray imaging system. The X-ray imaging system
converts the transmitted X-rays into digital images which represent
the radiographic density of the portion of the circuit board under
examination. The digital images are stored within a digital image
processor. A computer, under program control, performs
calculational measurements on the digital images so as to measure
the structural characteristics of the solder joints and components
on the circuit board. The calculational measurements are compared
to predetermined standards corresponding to acceptable quality
standards programmed into the computer. In response to the
comparison, the computer provides an accept/reject decision on the
circuit board in addition to providing manufacturing process
control information for correction of found defects.
It is an object of the present invention to provide a novel and
improved fully automated real-time X-ray radiographic solder
quality inspection system and method for measuring the structural
characteristics of solder joints on circuit boards.
It is yet another object of the present invention to provide a
method and apparatus for performing X-ray radiographic inspections
of circuit board solder connections and providing decisions based
on preprogrammed instructions for the acceptance or rejection of a
circuit board under test while providing data feedback to a solder
process production line.
It is a further object of the present invention to provide a method
and apparatus for performing X-ray solder quality inspections
utilizing a motion processor controlled multi-axis positioning
system for permitting the collection of multiple view X-ray imaging
data and providing calculational measurements upon the multiple
view image data by computer under preprogrammed instructions for
determining structural defects, including solder quality defects,
in manufactured circuit boards.
It is still a further object of the present invention to eliminate
preprogramming by having the inspection machine accept circuit
board specification from another computer and automatically devise
the sequence of motions and tests required to inspect the
particular circuit board type .
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, features and advantages of the present
invention will be more fully apparent from the detailed description
set forth below taken in conjunction with the drawings in which
like referenced characters identify corresponding throughout and
wherein:
FIG. 1 is a perspective view of an X-ray inspection system of the
present invention;
FIG. 2 is a block diagram of the major components of the system of
FIG. 1;
FIG. 3 is a flow chart of the operation of the system FIGS. 1 and
2;
FIG. 4 is a block diagram of the inspection list program; and
FIG. 5 depicts the imaging of the selected solder connection
defects.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is an automated real-time circuit board
solder quality inspection system which uses digital X-ray
radiographic imaging techniques and a rule-based defect recognition
system. Referring to FIGS. 1 and 2, FIG. 1 illustrates a
perspective view of the major component layout while FIG. 2
illustrates in block diagram form the major components of the
system. In FIGS. 1 and 2, the automated X-ray circuit board solder
quality inspection system 10 includes a control system 12 and an
imaging system 14. Control system 12 is comprised of three major
systems, control system computer 16, multi-axis positioning system
18, and safety system 20. Imaging system 14 is comprised of three
major systems, camera system 22, X-ray system 24 and image
processing/defect recognition system 26.
In control system 12, control system computer 16 is a digital
computer which has computer peripherals 28 associated therewith.
Computer peripherals 28 include such equipment as data storage
system 28a, printer 28b, display monitor 28c, and keyboard 28d and
an interface with the multi-axis positioning system 18 and safety
system 20 along with an interface to the image processing/defect
recognition system 26 and x-ray 24. Also included are interfaces to
external computers and/or robots.
Also included in control system 12 is multi-axisw positioning
system 18. Multi-axis positioning system includes an x-y
positioning table 30 which permits movement of a circuit board
mounted therein in a horizontal plane. The x-y positioning table 30
is mounted upon a rotation table 32 which permits 360 degrees
rotation of the x-y positioning table 30. Rotation table 32 and x-y
positioning table 30 and translator 38 may be generically defined
as a motion table which is mounted upon a tilt beam 34 which
permits tilting of the motion table in an angled plane to the
horizontal plane. A z movement system 36 permits movement of the
motion table assembly in a vertical direction. A translator 38 is
utilized to move a circuit board mounted upon x-y positioning table
30 to and form a position adjacent to an exterior wall of cabinet
40 at the load/unload door 42 to a central position within cabinet
40. A motion controller and components 44 receives the control
signals from the control system computer 16 and provides the
appropriate electromechanical movement within the multi-axis
positioning system.
A safety system 20 includes axis limits switches 46, safety door
interlocks 48, cabinet X-ray shielding 50, and radiation monitoring
system 52. Axis limit switches 46, safety door interlocks 48, and
radiation monitoring system 52 provide status information signals
with respect to the multi-axis positioning system 18, the position
of the load/unload door 42, and the level of radiation at selected
locations within and about the X-ray inspection system. Safety
system 20 is provided for both operator safety and equipment
failure protection.
In imaging system 14, camera system 22 is mounted within cabinet 40
beneath the motion table assembly. Camera system 22 includes a
fluorescent or scintillating screen 54 mounted in the upper wall of
lead shielded camera box 56. Mounted within camera box 56 is mirror
60, lens 62, and low-light-level video camera 64.
X-ray system 24 is also mounted within cabinet 40 and is comprised
of an electronic X-ray source 66 and X-ray spectrum filter 67
mounted within a X-ray source cabinet 68. Cabinet 68 includes an
electrically actuated mechanical X-ray source cabinet shutter 70
mounted at a lower wall of cabinet 68. Source 66 generates a beam
of X-rays that exit through an opening in the lower wall of cabinet
68. X-ray spectrum filter 67 modifies the X-ray energy spectrum in
such a way that adjusts the sensitivity of the system to the
component under inspection. X-ray source door 70 when in the closed
position cuts off the beam of X-rays emanating from cabinet 68 by
covering the opening.
Included with imaging system 14 is image processing/defect
recognition system 26. Image processing/defect recognition system
also includes a digital image processor 72 and an imaging computer
74. The image processor 72 includes at least three image memories,
also called frame buffers. System 26 includes peripheral devices 76
such as an image display monitor 76a, streaming tape drive or
optical disk 76b, flexible disk drives 76c, hard disk 76d, image
display monitor 76e, keyboard 76f, and joystick controller 76g.
Referring to FIG. 1, the components of FIG. 2 are illustrated in
their structural cooperation. Inspection system 10 is controlled by
control system computer 16 which is mounted within main cabinet 40
but external to the main cabinet X-ray shielding 50 located within
main cabinet 40. Control system computer 16 controls the operation
of the multi-axis processing system 18 and additional devices such
as the load/unload door 42 and the X-ray source door 70. Control
system computer 16 is also responsible for reporting results of a
board test through BOARD ACCEPTED and BOARD REJECTED and status
lights (not shown). Control system computer 16 includes a data
storage system such as hard disk 28a mounted within computer 16.
Data storage system 28a is used for storing circuit board
inspection programs and inspection result data. Also associated
with computer 16 is a printer 28b which prints defect tags for
individual boards and summary reports for board lots. Computer 16,
also includes, mounted in an operator console cabinet 80 a display
monitor 28c which displays control status information and messages
regarding system operation and a keyboard 28d or any other input
means which may be included to provide operator inspection control
of a circuit board. Imaging computer 74 and computer 16 are
interfaced with the other so as to communicate over a common bus.
Computer 16 also continuously monitors various sensors so as to
detect system faults from indicators such as are included within
safety system 20.
An electronic X-ray source 66, which generates a continuous beam of
X-rays of an energy level of about 160 Kev, is mounted within lead
shielded X-ray source cabinet 68. To provide a continuous and
stable source X-ray level, X-ray source 66 operates with an anode
current of 0.2 mA. One type of X-ray source is disclosed in U.S.
Pat. No. 4,521,902. Source cabinet 68 is positioned in an upper
portion of cabinet 40 and includes a source X-ray door 70 which
permits, when open, a beam of X-rays, which may be collimated to
improve image quality to project downwardly and outwardly through
an X-ray source door opening in cabinet 68.
Mounted beneath cabinet 68 is the motion table which is comprised
of x-y positioning table 30, rotation table 32, and translator 38.
For purposes of clarity in FIG. 1, tilt beam 34 and z movement
system 36 are not shown in structural form and may be implemented
in many forms by one skilled in the art. When a circuit board 78 is
mounted on the motion table and is undergoing examination, the beam
of X-rays is projected towards a portion of circuit board 78 and an
opening in the motion table. The X-rays transmitted through the
circuit board 78 are directed towards lead shielded camera box 56
which is mounted at the bottom of cabinet 40 within shielding 50.
Mounted in an upper panel of box 56 is fluorescent or scintillating
screen 54. Mounted beneath screen 54 is an aluminized front surface
mirror 60 mounted at a 45 degree angle to a horizontal plane. Also
mounted within box 56 is low-light level video camera 64 which has
a lens 62 directed between camera 64 and mirror 60.
In the operation of the inspection system, the translator is
positioned adjacent to the load/unload door 42 for receiving,
within a fixture thereupon (not shown) adapted for holding a
particular circuit board type, circuit board 37. Also mounted upon
translator 38 is step wedge 39 which is a section of varying
thicknesses of stainless steel. Step wedge 39 is utilized in the
image processing as a known reference (a known density) by which
gray levels of an acquired image may be referenced so as to account
for drift in the X-ray source output. The step wedge as a reference
may also be used to calibrate the X-ray source to a specific
output. Upon initialization of the system by the operator, such as
by control pushbutton switches (not shown) or the closure of
load/unload door 42, the translator 38 is moved to position the
circuit board directly beneath the X-ray source door opening of
cabinet 68.
The inspection routine is directed by image processing/defect
recognition system 26 which includes a main inspection program for
computer 74. Programmed into imaging computer 74 through the
peripheral devices or by downloading from another computer system
is data such as view location, device type-, pin number, etc. along
with the selection of image inspection algorithms from a library of
algorithms stored within imaging computer. Computer 74 instructs
control system computer 16 to move the motion table during the
inspection period and open X-ray source door 70 during the
inspection and closing a X-ray source door 70 upon completion of
the inspection. When the X-ray source door is opened, a beam of
X-rays is projected towards an area of the circuit board ranging in
size up to about two and one half inches square.
The circuit board is moved about by the motion table under control
system computer control 16 to reach a selected view for inspection.
The motion table may be moved through a horizontal plane in an x-y
direction along with being rotated, tilted, or moved in a vertical
direction toward or away from the X-ray source (so as to provide an
imaging zoom feature).
A beam of X-rays projected upon the circuit board results in some
photons scattering about the board within cabinet 40 which are
absorbed by cabinet shielding 50. Another portion of the X-ray beam
is absorbed by the circuit board and solder connection. Yet another
portion of the X-ray beam is transmitted through the circuit board
where it impacts upon fluorescent or scintillating screen 56 which
is positioned in-line with the X-ray beam.
The X-rays impacting upon the fluorescent or scintillating screen
are converted into a visible light image. The visible light image
of the X-ray shadow image created by the transmission of the X-ray
beam through the circuit board is reflected by a flat planar mirror
mounted at an angle 45 degrees to the horizontal. The X-ray shadow
image appearing at fluorescent or scintillating screen 54 is
reflected at a 90 degree angle through lens 62 and into camera 64.
The use of a mirror in the system enables the camera to remain
outside of the X-ray beam. The analog output of video camera 64 is
provided to image processing/defect recognition system 26.
During the imaging of the circuit board, the analog output of the
camera may be displayed upon an image display monitor 76a as a
512.times.480 pixel image for operator viewing. System 26 includes
a high-speed gray scale image processor which digitizes each image
pixel into an eight-bit code which corresponds to one of 256 shades
of gray. In the gray scale, 0-255, the darker, or denser, the
material absorbing the X-ray in the form of a shadow, the lower the
gray scale number. Lighter areas in which the X-rays are
transmitted through the circuit board, by absorbing less X-rays,
have higher gray level numbers.
Image processing/defect recognition system 26 imaging computer 74
performs a multiframe average of the digitized image and stores it
within an image (frame buffer) memory. Imaging computer 74 performs
computational measurement upon the image during the movement of the
motion table to a new view position or the load/unload position
adjacent to the load/unload door. During the movement of the board
to the unload position under the control of control system computer
16, imaging computer 74 processes all previously computed
measurements by performing analysis on the image measurement data
and directs the control system computer 16 to store and output the
results.
FIG. 3 illustrates a flow diagram of the operation of the
inspection system. At the beginning of an inspection cycle, the
X-ray source door is closed with the translator in the load/unload
position adjacent the load/unload door. The control system computer
begins an inspection cycle after a circuit board has been loaded
onto the motion table and the operator has initialized the
inspection cycle. The control system computer instructs the
translator to position the circuit board beneath the closed X-ray
source door. After the load/unload door has been closed by the
operator, the control system computer sends to the image
processing/defect recognition system identifying information as to
the type of circuit board that is to be inspected. The control
system computer may typically obtain this information, such as
circuit board type and lot, from a bar code imprinted on the
circuit board and read by a bar code scanner. Alternatively this
information may be provided by an operator through the operator's
keyboard or by a message from an external computer.
The imaging computer uses this information to select portions of a
main inspection program that are applicable to the type of circuit
board to be inspected. Each circuit board has a circuit board
specific inspection list associated therewith. Each inspection list
contains view information used by the main software module,
Inspection List Interpreter (ILI), within the imaging computer for
instructing the control system computer in motion control activity.
The specific inspection list is also used by another software
module, Image Measurement Module (IMM), within the imaging computer
for taking measurements on the image. Other software modules within
the imaging computer are a Results Interpreter Module (RIM), a
Blackboard Interface Module (BRIM), and a Blackboard. Each circuit
board type will be inspected with different views and different
sets of measurement and analysis routines. Hence, each circuit
board type has a specific IMM, RIM, BRIM, and Blackboard
structure.
The primary responsibility of the IMM is to take measurements from
the image data and place the measurements on the Blackboard. To
perform these measurements the IMM contains pre-defined measurement
routines or algorithms specified by the inspection list for each
joint. These algorithms are selected from a library of algorithms
which includes algorithms for all joint types.
The primary responsibility of the RIM is to analyze the measurement
data placed on the Blackboard by the IMM. The RIM contains
pre-defined analysis routines that correspond to the measurements
specified in the inspection list for each joint. Based on the
analysis of the measurements, the RIM places on the Blackboard the
results of the measurement analysis as to which pins were defective
and the type of defect.
The BRIM is responsible for providing the defect data placed on the
Blackboard by the RIM to the control system computer for reporting
of the defect to the operator.
The Blackboard is merely a temporary data storage medium by which
the IMM, RIM and BRIM store and retrieve data in communicating with
another module.
Returning to the inspection system operation, when both have
acknowledged to the other that each is ready to begin the
inspection, the imaging computer instructs the control system
computer to move the motion table to the first view position. Since
a view position table associated with each circuit board type is
contained within a control system computer memory, the imaging
computer need send only a "move to position" command. The view
position table, created previously for the particular board type,
contains the axis values for each defined view. As a result, when
the control system computer receives a "move to position" command,
it uses the values from a local view position table stored therein
to provide control signals to the motion controller.
Upon completion of the move to the first view position, the imaging
computer instructs the control system computer to open the X-ray
source door to permit the beam of X-rays to be directed through the
circuit board to the camera system. Also upon completion of the
move to the first view position, the imaging computer, based upon
the programmed instruction list, determines whether there is
another view position following the present view position or
whether the present position is the last view position that an
image is to be acquired and measured. This data is transferred to
the control system computer for controlling the motion table's next
move to either a new view position or the load/unload position.
The next event in the inspection cycle is the taking of a
multiframe average of the X-ray image of the first view provided by
the image processor. The averaging of the frame image is taken on
all views. The average frame image data are stored within the
imaging system computer in a memory, frame buffer one. During the
averaging of the frame image data on the first view position image,
no measurements on the averaged frame image data are performed.
After the frame average is taken, the imaging computer instructs
the system control computer to close the X-ray source door so as
the cut off the X-ray beam directed towards the circuit board.
In the condition there is another view to be taken, the imaging
computer instructs the control system computer to move the motion
table to the next view position. Simultaneously, the imaging
computer transfers the previous view averaged frame image data from
frame buffer one to another memory, frame buffer three.
Upon completion of the moving of the motion table to the new view
position and the transfer of the average frame image data to frame
buffer three, the imaging computer again determines whether or not
the present position is the last view position. Should there be
another view position, the X-ray source door is opened and averaged
frame image is computed and stored into frame buffer one.
Simultaneously, measurements are taken on the averaged frame image
data stored in frame buffer three. Upon completion of the
acquisition and storage of frame average into frame buffer one, the
X-ray source door is closed and the motion table is moved to the
next view position. Upon completion of both the frame average into
frame buffer one and the measurements on frame buffer three, the
previous averaged frame image data stored in frame buffer one is
transferred to frame buffer three. Should another view be
programmed into the inspection list, the sequence of just described
events is repeated. It should be noted that by opening the X-ray
source door only during the acquisition of the frame averaged
image, minimal exposure of the circuit board to the X-ray beam is
achieved.
However, should there not be a view following the present view, the
inspection cycle goes into a QUIT mode. In the QUIT mode, the
present averaged frame image data is stored into frame buffer one
while measurements are made on the data stored in frame buffer
three. Upon completing the computation of the averages of the
present frame image data and storage into frame buffer one, the
motion table is moved to the load/unload position for unloading of
the circuit board.
Upon completion of both the frame average into frame buffer one and
the measurements taken on the averaged frame image data stored in
frame buffer three, the averaged frame image from the last view is
transferred from frame buffer one into frame buffer three. Once the
last view data is in frame buffer three measurements are taken upon
the data. Upon completion of the measurements on the last view
data, the imaging computer performs a defect analysis on the data
collected from all prior measurements. The results of the analysis
are prepared and transferred to the control system computer for
reporting. If any defects were found, a defect tag is printed
through the printer associated with the control system computer.
The defect tag indicates the location of the defect and the defect
type. If no defects were found on the circuit board then no
printout is provided. The system may be provided with "BOARD
REJECT" and "BOARD ACCEPT" lamps which indicate the status of the
board upon completion of the test. The control system computer
keeps two counts for statistical purposes along with all the defect
records for each lot of boards. The counts are a running tabulation
of the number of circuit boards "accepted" and "Inspected". Once
the motion table is returned to the load/unload position and the
circuit board has been unloaded, the system is ready to begin
another inspection cycle.
Still referring to FIG. 3, if after the imaging computer instructs
the control system computer to move the motion table to the first
view position and it is determined that no further view positions
are required, the X-ray source door is opened, an averaged frame
image is taken and stored in frame buffer one. Upon storage of the
average frame image data in frame buffer one, the X-ray source door
closes and the motion table is moved to the load/unload position
for subsequent unloading of the circuit board. Simultaneously, the
averaged frame image data is transferred to frame buffer three
where measurements are taken on the averaged frame image data. Upon
completion of the measurements, the imaging computer performs an
analysis of all of the measurements in the view so as to detect
defects. The results of the defect analysis are transferred to the
control system computer for generating the appropriate report and
operator status indications. After the reports have been generated
and the circuit board removed from the motion table, the inspection
cycle is completed.
An inspection list is associated with each type of circuit board to
be tested. The inspection list is generated either by an operator
who generates the inspection list in accordance with the inspection
parameter requirements or inspection data may be downloaded from a
computer-aided design (CAD) system and the inspection list is
generated automatically by the computer. An inspection list for
each type of board to be inspected is stored within a storage
medium in the imaging computer with the corresponding view position
coordinate list stored within the control system computer.
FIG. 4 illustrates an exemplary flow chart of a typical inspection
list. The inspection list is comprised of a single view or a series
of views and, as illustrated in FIG. 4, included view numbers 1
through M. Each view number contains inspection instruction data
for acquiring and analyzing image data. The view number is an
integer which labels a view subtree wherein the view subtree
contains the axis values of the motion table. The constituents of
the axis values are the x, y, field of view or zoom axis, tilt and
rotate coordinates. The view subtree is essentially structured as a
sublist of the view number.
In the view subtree the x view center is the x axis displacement
from a home or zero position on the motion table. The selected x
coordinate is positioned in the center of the field of view. The y
view center is the y axis displacement from the home position on
the motion table. The selected y coordinate is also positioned in
the center of the field of view.
The field of view is the horizontal length of a plane perpendicular
to the field of view or zoom axis which is projected on to the
image display monitor as measured from the left edge to the right
edge of the monitor screen.
The tilt value is the absolute tilt angle value measured in 1/10th
of degrees ranging from 0 to 45 degrees in a horizontal plane. The
rotate coordinate is the absolute rotate angle measured in 1/10th
degrees of rotation ranging from 0 to 360 degrees from an initial 0
degree angle.
The field of view index is an integer assigned to a specific field
of view value. This is used to select parameters depending upon the
field of view for inspections. A tilt index is an integer assigned
to a specific tilt value and is used to select parameters depending
upon the tilt for inspections.
Gain and offset values are used for enabling the best image for
each view to be acquired. The gain determines the amount of
contrast that the video camera displays on the image display
monitor. By increasing the gain, the image appears lighter. By
decreasing the gain, the image appears darker. Offset is the amount
of brightness that the video camera displays on the image display
monitor. An increase in offset makes the image appear darker. A
decease in offset makes the image appear lighter.
Associated with each view there are a series of joint numbers 1
through N with each joint being given an integer identifying
number. The joint number is an index to the pin file associated
with the inspection list which contains information about the pin.
An actual joint typically consists of a bottom pad, barrel and top
pad with a pin running through the middle of the pads and barrel.
However, a joint in the inspection list is simply an area that is
to be inspected so that it may be either a solder joint, a
capacitor, a calibration position, a device, or any other area on
the circuit board.
Each joint member within a view includes a joint subtree which
contains information about the joint. This information includes the
joint type, device, name, pin number, x and y screen location,
measurements algorithms, joint orientation and correction
factors.
A joint type is entered into the sublist as an integer which
represents the classification of the joint. This information is
used to select inspection parameters which are dependent upon the
joint characteristics, such as pad size. The device name is also
included using the manufacturer's character abbreviation, which
typically stands for the type of electrical component. A pin number
is assigned as an integer value associated with the order of the
pins on the device. The device name and pin number are useful
information when a defect is discovered so that the control system
computer will provide a specific component name and pin number in
the defect tag printout.
The x screen location is defined as the horizontal displacement of
the cursor from the original upper left hand corner of the image
display monitor. This location is measured in pixels from 0 to 511
going across the image monitor from left to right. The y screen
location is the vertical displacement of the cursor from the
original upper left hand corner of the image monitor. This location
is measured in pixels from 0 to 479 going downward in the image
display monitor.
The joint orientation is an integer value which represents the
direction of a pin with respect to the image monitor. This value is
used in the selection of inspection parameters used which are
dependent upon the joint orientation. For example the three o'clock
position on the monitor may be assigned an integer value 1 with the
six o'clock position on the monitor corresponding to 90 degree
rotation and is assigned the integer value 2. Continuing in a
clockwise notation system, the nine o'clock position corresponds to
180 degree rotation with the assigned integer value 3 and the
twelve o'clock position corresponding to a 270 degree rotation with
an assigned integer value of 4.
Most important in the joint sublist are the measurement algorithms
associated with each joint number. An integer identifies the
algorithm from a library of algorithms (discussed later) which are
used to perform image measurement and defect analysis. The results
of the defect analysis are used to flag defects found in the image.
For purposes herein, the algorithms numbered for joint number 1 as
being algorithm numbers x.sub.i through x.sub.j. A correction
factor may be included as an integer value which would provide
"local joint effects" information to an algorithm. It would allow
an algorithm to be adjusted on a per joint basis. For example, an
integer value would be used to inform the algorithm that a
structure blocks the view of a joint.
In the algorithm used to determine a defect, the imaging computer
uses a "rule-based" approach with a set of rules defining what a
good feature and a bad feature are for parameters automatically
measured by the system. The rules define what an acceptable solder
connection is and what constitutes a connection that is defective.
For each type of joint a set of parameters or thresholds for
various solder-joint characteristics are defined. These parameters
may be quickly adjusted by the user, although such adjustments are
often not needed. Thickness, density and shape of the solder
connections are determined by using the measurement algorithms and
comparing the results of the measurements to pre-established
criteria. Utilizing the measurement algorithms and analysis
algorithms, a library of inspection routines for standard
electronic component packages and configurations may easily be
created. While some components may require certain measurement and
comparison algorithms, others may not. Certain component packages
are susceptible to known solder connection defects such that the
algorithms may be readily chosen and placed appropriately into the
inspection list.
FIG. 5 represents three imaged circuit board defects with FIG. 5a
illustrating insufficient solder in a through-hole beneath a
pin-grid array. In FIG. 5a barrel solder connections 92, 94, 96,
98, 100, 102, and 104 are illustrated. In this defect condition,
exemplified by barrel solder connection 104, a cavity exists in the
barrel solder connection. The setup requirements for providing the
best possible imaging and recognition of this defect requires a
large tilt in the motion table. In the measurement algorithm, the
minimum and maximum gray values, of a gray level pixel population 2
or more, are calculated. The average gray level in the barrel is
calculated and normalized against a step wedge imaged gray level.
If the difference between the minimum and maximum gray levels is
greater than a predetermined threshold programmed into the machine
or the average gray level exceeds a second threshold than a defect
exists. In theory, a large difference between the minimum and
maximum gray level values in the barrel indicates porosity, voids
or blow-holes. A high average alone indicates a more uniform lack
of solder, many voids, blow-holes or no solder at all.
FIG. 5b illustrates a surface mount device (SMD) where bridging,
solder forming an unintended conductive path between solder joints
and devices has occurred. The motion table is typically setup to
detect solder bridging in a SMD with a 1-inch field of view and no
tilt angle. This defect is illustrated in FIG. 5b where pads 110,
112 and 114 are shown. There exists in FIB. 5b a solder bridge 116
between pads 110 and 112. For the inspection, a 1-inch field of
view and no tilt angle are the typical set up requirements. Two
tasks are typically performed to verify whether a defect is
present. Given the centroid of the joint, the normalized black
count between joints (A) is greater than a threshold then a bridge
exists. The second task is checks for the presence of an edge
between joints and if an edge is found then a bridge exists.
FIG. 5c illustrates a J-leaded surface mount device (SMD) wherein
the misalignment of the pins or leads 120 and 122 are respectively
offset from the solder pads 124 and 126. If the leads 120 and 122
were properly aligned they would be respectively superimposed over
pads 124 and 126 in the image. As a result, there would be no
separation in the image of the leads and pads. In this defect
condition a component is typically askewed so that the leads are
not centered on the pads. The algorithm for measuring the defect
condition uses a given centroid of the pin with the centroid of the
pad being computed. The shift in centroid of the pin and pad are
measured (A). The total shift over the row of pins is next computed
(B). Next the number of pins offset in a row are computed (C). A
pin is offset if (A) is greater than a first threshold. A device is
offset if (B) is greater than a second threshold or (C) is greater
than a third threshold.
A solder bridge in typical throughhole solder connections is
defined as solder that has formed an unintended conductive path
between solder joints is considered a bridge defect. Spikes/icicles
(not shown) are non-conductive paths of solder extending beyond the
pad. This defect may occur on the top or bottom pad or around the
lead and always extends beyond the pad. To detect a solder bridge
or spike/icicle a circular profile of pixel data around the joint
pad is gathered, excluding any known device interference. The data
is filtered using a median filter. A defect exists if the
differential along the profile varies more than a rule-based
threshold.
In identifying the particular type of defect, the average gray
level of the joint (J.sub.a) is calculated. The angle of the defect
from the position in the profile is next calculated. This is
accomplished by scanning outwardly from the joint pad, at the found
angle, by following the path of the lowest gray level until the
gray level rises above a threshold or a fixed distance is covered.
The path length is recorded as the defect length (D.sub.1). The
algorithm next calculates the average gray level of the defect
(D.sub.a) centered at the end of the path with the same area
dimensions as used to calculate the joint average (J.sub.a). From
the point along the path where the lowest gray level was found, the
algorithm scans plus or minus 90 degrees so as to form an arc from
the joint center and following the path of the lowest gray level
until the gray level rises above a threshold or a fixed distance is
covered. The sum of the path is recorded as the defect width
(D.sub.w). If the defect length (D.sub.1) is greater than a
threshold and the average gray level of the defect minus the
average gray level of the joint (D.sub.a -J.sub.a) is less than a
threshold then the defect is a bridge. Otherwise, if the defect
length (D.sub.1) is greater than the minimum threshold and the
defect width (D.sub.w) is greater than a minimum threshold, then
the defect is a spike/icicle. Since bridges, spikes/icicles will
appear as protrusions emanating from the pad edge, a bridge will
appear as a protrusion with a length close to that of the distance
between the pins on the board with the gray level of the protrusion
end being similar to the joint gray level. The rest of the
protrusions will be considered spikes/icicles as long as the length
and width meet a minimum criteria.
Solder ball defects typically appear spherical and may appear
anywhere on the circuit board. Under the assumption the suspect
solder ball has been found by previous algorithms the 0.sup.th,
1.sup.st and 2.sup.nd moments of the solder are calculated to
obtain the length of the major and minor axes. If the ratio of the
major and minor axes is within a tolerance of 1.0, the solder is
considered somewhat symmetrical. Next the spherical area of the
object is calculated by assuming the diameter (D) is the average of
its major and minor axes by using formula .pi.(D/2).sup.2. The
ratio of this area to the 0.sup.th moment is used in obtaining the
spherocity (S). If the spherocity is within a tolerance of 1.0 the
object is considered a solder ball. The setup for detecting this
type of defect typically requires a large field of view without any
tilt.
Another defect which may occur in a typical solder connection is
excess solder in the bend radius where solder extends into the
stress release bend of a horizontally mounted component. This
defect may occur in all axial lead components at the bend radius.
The setup requirements of the motion table typically require a 1
inch field of view, or less, with a 30 degree or greater tilt angle
with no rotation. The theory behind this measurement and comparison
algorithm is that solder in the bend radius will also appear on the
back side of the bend radius. This defect can be described as a
lump of solder on the back of the lead which will appear as a
change in a bend angle of the lead. Instead of bending in toward
the component body, the lump will make a slight bend away. In the
algorithm, a search of the lead outside edge away from the
component body is conducted so as to find the bend. The angle of
the lead is then recorded. The angle is projected past the bend and
the image is sampled for solder in the area where the lead would
be. The defect exists if solder is found at the projected
angle.
Another defect is where a bent or missing lead occurs such that the
component lead has been bent so that the lead does not enter the
hole, or the lead is completely missing. This algorithm is used in
inspecting throughhole components on the insertion side of the
board. Typical motion table setup requirements are a large field of
view with no tilt. The theory in inspection for this defect is that
the lead, having a lower density than solder, increases the
standard deviation of the solder barrel. In determining the defect,
the standard deviation of the barrel is calculated and if the
standard deviation is below a threshold then a defect is determined
to exist.
The surface mount defect known as a bent lead occurs when a lead is
bent to one side within the plane of the device. In measuring the
defect, a 1-inch field of view is required with no tilt angle. The
measurement algorithm is performed by computing the centroid of the
pin and the pad. The shift in the centroids of the pin and pad (A)
are next measured. Next, the average shift over the row of pins (B)
is calculated. A lead is bent if the absolute value of the average
shift over the row of pins subtracted from the shift in centroids
of the pin and pad is greater than a threshold, i.e. ABS (A-B)>
threshold.
A cold pad defect is typically a "cold solder joint" which is a
phrase generally describing a poor quality joint. This defect may
show signs of dewetting, voids, cracks, or an unusual solder
distribution. This defect may occur in through-hole devices on the
top or bottom of the circuit board. The motion table setup requires
a minimum field of view with a tilt sufficient to displace the
upper and lower pads. The measurement algorithm utilizes the given
pad centers and two limiting angles for pad examination. The radial
symmetry factor of the joints are calculated by calculating the
standard deviation along two arcs (A and B) such that the MAX
(A,B)=C. If C is greater than a threshold then the defect
exists.
Dewetting on the lead defect is a failure of the solder to
completely stick to the lead and usually the solder is slightly
pulled away from the lead. This defect typically occurs at the top
and bottom of leads in throughhole components. The motion table
setup typically requires a small field of view with a 30 degree or
greater tilt angle, with multiple rotated views necessary to
examine the entire lead circumference. Dewetting will typically
appear as a blackish halo around the lead which indicates a higher
density of solder. Instead of flowing evenly around the fillet, the
solder has gathered into sections of higher and lower density. To
determine whether dewetting has occurred, the barrel average gray
level (B.sub.a) is calculated. The average gray levels are recorded
in three locations when scanning from the barrel up the lead toward
the component. The position of the highest differential in gray
levels of the white-to-black transition for each of the three scans
is recorded. The average gray level of the arc defined by the three
points of higher differentials of gray level is then computed as
the value (A.sub.a). If A.sub.a -B.sub.a > threshold, then the
dewetting defect exists.
Dewetting on the pad is a defect characterized by a jagged edge
occurring at the circumference of the solder on the pad and may be
found at top and bottom pad areas for through-hole components. In
imaging this type of defect a small field of view with a 30 degree
or greater tilt angle and multiple rotated views are necessary to
examine the pad. Dewetting will appear as a white halo around the
barrel which indicates a lower solder density. Instead of flowing
evenly around the fillet, the solder has pulled away from the
barrel thereby leaving a gap. This gap lowers the density so as to
produce the white halo around the barrel. In measuring the defect,
the barrel average gray level (B.sub.a) is computed. The barrel is
scanned out towards the pad edge at three locations with the
average gray level recorded. The position of the highest
differential in gray levels of the black-to-white transition for
each of the three scans is recorded. The average gray level of the
arc defined by the position of the highest differential in gray
levels recorded is computed as value (A.sub.a). If A.sub.a -B.sub.a
> threshold, then the dewetting on the pad defect exists.
Excess solder on the bottom lead is characterized by solder
obscuring the end of the lead on the bottom side of the circuit
board and typically occurs at circuit board bottom side pad area.
The imaging for this type of defect typically requires a 1-inch
field of view or less with a 30 degree or greater tilt angle. In
the measurement of this type of defect, when the lightest gray
level minus the barrel gray level is large the lead tip is
clinched. This could mean that the pin is covered with solder or
the lead was not clinched. Testing the difference between the pad
gray level with the barrel gray level will indicate if the solder
does cover the lead tip. When the solder covers the lead tip it
also covers the pad more than normal. This covering of the pad with
more than normal solder lowers the difference in gray level between
the barrel and pad. In performing the measurement, the gray levels
from the center of the barrel to the end of the bottom pad are
sampled with the lightest gray level average (G.sub.1) being
recorded. The average gray level at the barrel center (B.sub.a) and
the average of the bottom pad gray level (P.sub.a) are recorded. If
the average of the bottom pad gray level subtracted from the
lightest average gray level (G.sub.1 -B.sub.a) is less than a
threshold, and the average gray level of the barrel center
subtracted from the average gray level of the bottom pad (P.sub.a
-B.sub.a) is less than a threshold, then the defect exists.
Excess solder on the top of the lead is characterized by solder on
the lead surface above the top fillet. This defect typically occurs
in the soldering of dual in-line packages, single in-line packages
and through-hole devices. The imaging setup requirement for this
type measurement is typically a 1-inch field of view at a 30-45
degree tilt angle. The measurement of the image data utilizes a
given center position of the barrel wherein the average gray level
of the barrel (B.sub.a) is calculated. The data on the lead is
extracted wherein between the knee and pad, the following are
calculated: maximum standard deviation of the rows (R.sub.s),
maximum standard deviation of the columns (C.sub.s), and the
minimum gray level of population 2 or more (M.sub.1). If the
maximum standard deviation of the rows (R.sub.s) or the maximum
standard deviation of the columns (C.sub.s) is greater than a
threshold, then small solder globs exist on the lead. If the
average gray level of the barrel subtracted from the minimum gray
level (M.sub.1 -B.sub.a)<threshold, then the lead is covered
with solder.
Excess solder on the pads occurs when solder extends beyond the
edge of the pad and is considered a solder defect. This defect may
occur in individual or paired top and bottom pads. In the
measurement of the image for the defect, the approximate joint
center location is given. If the measured diameter of the pad is
greater than a threshold then a defect exists.
Insufficient lead clearance occurs when a clinched lead protrudes
towards another lead so that the clearance between the two leads is
less than a specified amount. This defect occurs only on the bottom
side of through-hole components. The typical image measurement
setup requires a large field of view with no tilt. In theory, the
distance from the end of a lead to any other object can be defined
from the nontilted position. Although the actual clearance may be
larger due to the depth not being measured, the lead has the
potential of being within the measured clearance if bent. In
performing a measurement on the solder connection, a circular
profile of pixel data around the joint pad is gathered, excluding
any known device interference. The data is filtered using a median
filter. A potential defect exists if the differential along the
profile varies more than a threshold level. For potential defects,
the angle of the defect from the position in the profile is
calculated. The measurements are continued by scanning outwardly
from the joint pad at the found angle, so as to follow the path of
the lowest gray level until the gray level rises above a threshold,
or a fixed distance is covered. Next, the measurements are taken by
continuing to scan from the end out, straight and to both sides,
for a distance equal to the specified clearance. If contact with a
second lead is found then insufficient clearance exists.
In surface mount devices a defect may occur when the amount of
solder volume between the pin and pad is insufficient. This test
typically requires a 1-inch field of view with no tilt. In
measuring the image, the approximate joint center location is given
wherein the average gray level of a window on the joint is
computed. If the average gray level is smaller than a threshold,
then there is insufficient solder at the connection.
In all through-hole components insufficient solder at the top or
bottom fillet occurs when solder fall-back into the barrel is more
than a specified amount. Typically this measurement requires a
field of view dependent upon the barrel dimensions with a large
tilt angle. If a normal amount of solder exists on the pad, the
fall-back into the barrel is acceptable. The fall-back is measured
by testing whether solder is present at the maximum acceptable
fall-back location. If the solder does not extend from one side of
the barrel to the other, then the fall-back extends beyond this
point and the joint is defective. In measuring the image, the
average gray level of the pad is computed and recorded. If this
computation shows a normal or more than normal amount of solder,
then there is no defect. However, should this not be the case, the
location where the barrel and pad meet is computed and recorded. At
the point where fall-back becomes unacceptable, the width of the
solder in the barrel is measured by utilizing the gray scale level.
If the width is less than the barrel diameter, then a defect
exists.
A lefted pin defect occurs when a pin is lifted up from the pad
area which results in no bonding between the pin and pad. However,
solder may still be present on the pin and pad. This defect
typically occurs on surface mounted devices and flat pack
components. The image measurement typically requires a 1-inch field
of view with no tilt. In measuring the image the center locations
on a row of joints are given. Inspection windows are placed about
each pin and pad area so as to calculate the average gray level and
the black counts for various thresholds. The results are then
compared with thresholds to determine if a defect exists.
Through-hole component misorientation occurs when a component is
improperly inserted into the circuit board. This defect typically
requires a large field of view with no tilt to perform the image
measurement. In theory the internal/external structure of a
component varies enough so that when misoriented, the image is
significantly different than when oriented properly. In performing
the measurement on the image, the average gray level of a section
of the component is calculated and normalized to a step wedge. If
the average minus the expected average, corresponding to a properly
aligned component, is not within a tolerance limit, then a defect
exists.
A missing component is another typical defect which occurs when a
component is absent from the circuit board. This measurement
typically uses a large field of view without tilt. The density of
the component will decrease the average gray level at the expected
position on the board. For devices with low X-ray density, the
gradient may yield a stronger signature. In the measurement, the
expected location and size of the component is given. The average
gray level of a section of a component that is significantly
different in signal from the background is calculated. This signal
is normalized with a step wedge and/or local background. If the
average minus the expected average is not within a predetermined
tolerance, then a defect exists. For low-density components an
additional test may be required. This test requires that the
maximum gradient across the expected component edges, where the
solder joints are located, be measured. If this value is less than
a threshold, then a defect exists.
A splash in an open area is a typical defect which occurs when
amorphic solder globs are stuck to the circuit board in random
locations. The defect may occur at all open areas on the circuit
board. Measurement of the image for this type of defect requires a
large field of view without a tilt. In measuring the image the
average (A) and standard deviation (S.sub.d) gray levels of the
test zone are computed. The average gray level is normalized with
calibration data (N.sub.a). If the expected average subtracted from
the calibration data (N.sub.a -A.sub.b) is greater than a threshold
or the expected standard deviation subtracted from the measured
standard deviation (S.sub.d -S.sub.de) is greater than a threshold,
then a defect exists.
A blow-hole or a void is typically defined as a cavity on the top
or bottom fillet which may occur on all throughhole component
solder connections. The image measurement typically requires a
field of view dependent upon the barrel dimensions with a large
tilt angle. To cover all areas of the pad this test requires
multiple views at different rotations. Voids and blow-holes
typically cause small areas of low-density gray levels in the
fillet image. In measuring the image, the minimum and maximum gray
levels, of population 2 or more, of the fillet are calculated. If
the difference between the minimum and maximum gray levels is less
than a threshold then no defect exists. However, should the two
gray levels difference exceed a threshold, the area of the
potential defect is reexamined. The area of the potential defect is
measured by using the black count at a gray level threshold set to
a defined level above the minimum gray level. If this area is less
than a threshold for a void/blow-hole, then a defect exists.
Upon reading of the previous description of the preferred
embodiment, and person skilled in the art will readily understand
how to make or use the present invention. Various modifications to
these embodiments will be readily apparent to those skilled in the
art, and the generic principles defined herein may be applied to
other embodiments without the use of the inventive faculty. Thus,
the present invention is not intended to be limited to the
embodiment shown herein, but is to be accorded the widest scope
consistent with the principles and novel features disclosed
herein.
* * * * *