WO1999057954A1

WO1999057954A1 - Local-coordinate-based component-position verification

Info

Publication number: WO1999057954A1
Application number: PCT/US1998/009279
Authority: WO
Inventors: Richard Pye; Ronnie K. Riggs; Bruce E. Smyth
Original assignee: Genrad, Inc.
Priority date: 1998-05-06
Filing date: 1998-05-06
Publication date: 1999-11-11
Also published as: AU7370998A

Abstract

A machine-vision system (18 and 22) takes an image of a circuit board (10) and locates in that image the images of different electrical components (12 and 14) by reference to separate sets of local fiducial marks. The system determines from the resultant component locations whether the electrical components are properly positioned.

Description

LOCAL-COORDINATE-BASED COMPONENT- POSITION VERIFICATION

BACKGROUND OF THE INVENTION

The present invention is directed to visual printed-circuit-board testing. It par- ticularly concerns machine- vision detection of proper electrical-component positioning.

Manufacturers of products that employ printed-circuit boards require a high degree of quality from their printed-circuit-board suppliers. Consequently, circuit- board manufacture involves considerable automated testing. Before components are placed on the circuit boards, of course, the component manufacturers perform their own tests. Even if all of the components operate properly, though, the resultant printed-circuit board may not. The reasons vary, but among the most common are missing or misoriented parts, solder bridges, and cold solder joints.

Printed-circuit-board manufacturers employ electrical tests to detect many of these defects. But they have found it profitable also to perform visual tests, which they automate by employing machine-vision equipment. Although such tests typically cannot detect the open circuits symptomatic of poor solder joints, they can detect missing, mispositioned, or misoriented parts. In many cases, an electrical test could find such defects, too. But the vision test often has the advantage that it can be performed earlier in the process, and typically without disrupting it as much. Specifically, visual tests have the advantage that for most purposes they can be performed before the solder-reflow step. Printed-circuit boards are typically "stuffed" with components by pick-and-place machines, which stick components to solder-paste locations on the boards. The solder is then reflowed to make the necessary electrical connections between the components and the board's conductor traces. It is only after this reflow step has occurred that electrical tests can be performed, and repair at that stage tends to be more difficult than it is before solder reflow. Vision tests do not depend on the electrical connections that the reflow step establishes, so they can be per- formed before that step, when repair is still relatively easy. The circuit-board manufacturer therefore has a strong incentive to employ machine-vision inspection.

Such inspection begins with, say, a line-scan camera's forming an image of the printed-circuit board's surface after the components have been attached. From this image, the machine-vision system uses various image-processing techniques to identify image features corresponding to the components of interest and to determine those components' positions on the circuit board.

Machine-vision inspection not only reduces labor cost but also permits some tests to be made visually that humans either cannot perform or can perform only with considerable difficulty. Current-day electrical components and the conductive board traces with which their leads must register are now so small that human-vision verification is impossible, at least with the unaided eye, and is impractical in any event. With the combination of imaging by video cameras and now-conventional image- processing techniques, machine-vision systems can routinely achieve the resolution necessary to make the necessary high-accuracy position determinations.

Unfortunately, it takes more than fine resolution to verify component position reliably. Process variations can actually make the component position required for proper registration vary by as much as the maximum position tolerance for some critical-position components, such as ball-grid-array parts. So machine-vision position verification has not always been reliable even though the vision system's resolution is adequate.

SUMMARY OF THE INVENTION

We have recognized that such systems' reliability can be improved by basing the position-error determinations for different components on their positions with re- spect to different local fiducial marks on the circuit board. By using separate sets of fiducial marks, one can make the positions of detected components with respect to the fiducial marks better indicators of whether those components' leads will register with their respective conductive pads. This is because the use of separate marks makes it possible to make the distance between fiducial marks and the target conductive pads -3-

small for all components and thus to minimize the tracking error that factors such as board warping can cause between fiducial marks and the target conductive pads.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, of which:

Fig. 1 is plan view of a printed-circuit board to be tested;

Fig. 2 is block diagram illustrating the use of a machine- vision system in accordance with the present invention's teachings; and

Fig. 3 is a more-detailed view of a small region of the board, showing indicia placed on the board in accordance with the present invention' s teachings.

DETAD ED DESCRD7TION OF AN ILLUSTRATIVE EMBODIMENT

Fig. 1 is a simplified plan view of a printed-circuit board. Fig. 1 shows only two of the typically large number of electrical components typically mounted on such a board. Reference numerals 12 and 14 identify those two illustrated components. Fig. 1 also omits the conductive traces that interconnect those components' leads with the leads of other components.

When components have been placed on the board 10 — but preferably before the solder has been reflowed to make the requisite electrical connections — a machine- vision system inspects the board to verify that various of its components have been properly placed, as Fig. 2 shows.

The type of machine-vision system typically employed for such applications includes a video (typically line-scan) camera 16 under which the board passes as the camera 16 forms an image used by an image processor 18 to make the determinations described below. The system ordinarily begins with a rough approximation of various board features' positions. For example, there may be some means for detecting board edges in the image, and the processing system can look for various features by reference to the location of those edges. -4-

But a component's position with respect to a feature such as a board edge is not ordinarily a reliable indicator of whether its leads are likely to register with the corresponding board conductors. So it is conventional for an image processor initially to search predetermined neighborhoods with respect to those edges or some other rough reference for fiducial marks disposed at locations such as Fig. I's locations 20 in the board's corners. Once these board fiducial marks have been located, the positions of other features can be referenced to these marks. Conventional approaches base component-position verification on component positions determined in this manner.

Suppose, for instance, that component 12's nominal x position with respect to a coordinate system established by reference to the main board fiducial marks is X_c. This position (together with the appropriate y position and orientation angle) is what is required to make a given one of the component's leads register with the corresponding nominal solder-pad position X_p. If machine-vision image analysis reveals the component's actual position to be x_c, then the component's -position error Δx would con- ventionally be considered to be x_c - X_c. This is the error conventionally used as the basis for determining whether the component position is within tolerances. If that position is not within tolerances, the system produces an electrical signal indicative of this fact. Such a signal may cause a display such as Fig. 2's display 22 to produce an image of the board with the mispositioned component's position highlighted. (The error value may be used for other purposes, too. Even if it is within tolerances, it may alone or with measurements from other boards be a basis on which a decision to adjust the process can be made.)

Conventional processes base their position determinations on such fiducial marks because the etching process that lays down the board's conductive traces is or- dinarily the same as the one that produces the fiducial marks, so those marks' positions tend to track the target conductor-pad positions better than, say, board-edge positions do. The positions of detected components with respect to these fiducial marks are therefore better indicators of whether those components' leads will register with their respective conductive pads than, say, their positions with respect to other position ref- erences would be. Even so, a component position determined in this manner is not always as reliable an indicator as is sometimes necessary, and we have found a way of making the position determination a better indicator. To increase reliability, we base error determinations on the components' positions with respect to different fiducial marks for different sets of components. We typically have the vision system search for local fiducial marks in predetermined neighborhoods defined by reference to the main board fiducial marks. We also search for a component of interest in a neighborhood whose position is defined either by reference to the main fiducial marks' positions or by reference to those of the local fiducial marks.

In either case, we base our position-error determination on the local fiducial marks' positions. The resultant improvement flows from the fact that board warping and similar factors cause fiducial marks' tracking of conductor-pad positions to be imperfect, and component-position tolerances can be tight enough that the resultant variation in conductor-pad positions with respect to common fiducial marks is signifi- cant. By providing different fiducial marks for different components or component groups, though, we can place the marks relatively close to respective components. This minimizes the absolute tracking error between a fiducial mark and the conductive pad with which a lead of the associated component needs to register.

Specifically, variation in the pad's position with respect to the main board fi- ducial marks is largely reflected in the departure of the local marks' measured x positions x_/from their nominal x positions X_f. So a better indicator of whether the component's leads will likely register properly is an error measurement Ax' = δx_c - δx, where δx_c ≡ x_c -X_c and δx_/≡ x_/- Xf. Stated differently, the difference between a component's measured and nominal positions in the frame of reference of the local fiducial marks is a more-useful error measurement than their difference in a more-global reference frame. So whereas system 18 determines component 12' s position error by reference to local fiducial marks 24, it determines that of Fig. I's component 14 by reference to different local marks (not shown) located closer to that component.

For the local fiducial marks, we prefer to employ dedicated marks that, al- though produced in the same etching process as the conductive traces, are separate -6-

from them and not part of the board's functional circuitry. This leaves us the freedom to design the marks in such a manner as to maximize the vision system's ability to recognize and locate them. But the present invention's broader teachings can also be practiced in embodiments that instead employ existing board features as the local fidu- cial marks. For example, Fig. 3's vias 26 can be used in place of marks 24 as component 12's local fiducial marks. When such features are used, it is sometimes difficult for the vision system to distinguish an intended via from others, since the vias is not typically designed specifically for vision-system recognition. So it is often preferable in that situation to select a plurality of such vias as the local fiducial mark, i.e., to have the vision system search for the pattern that a cluster of such vias makes.

It is apparent that the present invention can be practiced in a wide range of embodiments. It therefore conduct a significant advance in the art.

What is claimed is:

Claims

1 1. For determining whether an electrical component is properly located on a cir-

2 cuit board, a method comprising the steps of:

3 A. employing a machine- vision system to form an image of the circuit

4 board;

5 B. locating in the board image global-fiducial images of a first set of global

6 fiducial marks on the circuit board;

7 C. locating, in a neighborhood of the circuit board image defined by refer-

8 ence to the positions of the global-fiducial images, local-fiducial images

9 of a local set of fiducial marks on the circuit board; ιo D. determining the location, in the board image, of a component image of li the electrical component with respect to the local-fiducial images;

12 E. generating an indication of whether the location of the component im- i3 age with respect to the local-fiducial images thus determined is within ι predetermined tolerances of a predetermined nominal location.

1 2. A method as defined in claim 1 wherein:

2 A) the printed-circuit board includes conductive traces that are so posi-

3 tioned as to register with leads of components properly mounted on the

4 board; and

5 B) the local set of fiducial marks are separate from the board's conductive

6 traces and so positioned as not to register with leads of components

7 properly mounted on the board.

1 3. For determining whether electrical components are properly located on a cir-

2 cuit board, a method comprising the steps of:

3 A. employing a machine-vision system to form an image of the circuit

4 board;

5 B . locating in the board image first local-fiducial images of a first set of lo-

6 cal fiducial marks on the circuit board; -8-

C. locating in the board image second local-fiducial images of a second set of local fiducial marks on the circuit board separate from the first set of local fiducial marks; D. determining the location, in the board image, of a first component im- age of a first electrical component with respect to the first local fiducial images; E. determining the location, in the board image, of a second component image of a second electrical component with respect to the second local fiducial images; F. generating an indication of whether the locations thus determined are within predetermined tolerances of a predetermined nominal component locations.