WO2009120951A2

WO2009120951A2 - Automated conformal coating inspection system and methods of use

Info

Publication number: WO2009120951A2
Application number: PCT/US2009/038541
Authority: WO
Inventors: Owen Y. Sit
Original assignee: Nordson Corporation
Priority date: 2008-03-28
Filing date: 2009-03-27
Publication date: 2009-10-01
Also published as: WO2009120951A3

Abstract

A system (10) and method for automated inspection of conformal coatings (12) on printed circuit boards (PCB) (14) using ultraviolet lighting. Such conformal coating (12) includes a tracer that fluoresces in the presence of the UV light. The automated system (10) includes a board holder sub-system (28) adapted to hold the PCB (14) and an ultraviolet (UV) light source (16) that directs UV light onto the conformal coating (12) of the PCB (14). The system (10) further includes a camera (20) having a lens (22) that captures images of the illuminated PCB (14). The system (10) also includes a transport assembly (32, 34) that moves the camera (20) and/or the board holder (28) to aid in inspection of the PCB (14). The camera (20) is adapted to convey the captured images to an image-processing computer (26), which is associated with the camera (20). The image-processing computer (26) is configured for user interface with the system (10) and to interpret the captured image for determining whether the PCB (14) is properly coated.

Description

AUTOMATED CONFORMAL COATING INSPECTION SYSTEM AND METHODS OF USE

Cross Reference to Related Applications

[0001] This application claims the benefit of U.S. Provisional Application No.

61/040,352, filed March 28, 2008, the disclosure of which is hereby incorporated by reference herein in its entirety.

Background

[0002] The present invention relates generally to conformal coating inspection, and more specifically to a system and method for automated inspection of conformal coatings on printed circuit boards (PCB) using ultraviolet lighting.

[0003] A conformal coating is a thin layer of dielectric material that is applied to a populated printed circuit board (PCB) to provide environmental and mechanical protection to the circuitry on the board. The coating protects the circuitry from moisture and containment and prevents short circuit and corrosion of metal conductor and solders joints. The conformal coating also prevents or minimizes the dendritic growth (electrochemical migration) of metal at the solder joints that could later result in short circuit.

[0004] The protection afforded by the conformal coating is only effective if the coating is properly applied. The coating material can be applied using various methods such as brushing, spraying, dipping or for complex electronics board using a specialized robotic machine to apply coating to the selective areas. For electronic circuits with connectors, it is important that these are masked off during the process of applying the conformal coating material. It is also important for the connector pins to be free of the coating material.

[0005] There are a few different types of material resins commonly used for conformal coating purposes, such as acrylic, epoxy, urethane and silicone resins. Most of these are clear to the human eye.

[0006] Current methods of inspection involve a human operator inspecting

PCBs in an ultraviolet (UV) chamber. In this instance, the inspector manually checks individual boards to determine whether the coating is placed correctly around the components, whether it has migrated into "keep out" areas of the boards, and whether the correct amount of coating has been applied. To aid manual inspection of the conformal coating, UV fluorescent tracers can be added to the materiel. The UV tracer allows the coating to fluoresce under UV light. There are numerous limitations to the manual inspection method. For example, the subjectivity of the operator varies from operator to operator. In addition, there is a lack of traceability for the process, not to mention the negative health risks related to the prolong exposure to UV light.

[0007] Other current methods involve automated inspection of PCBs in a chamber illuminated by hundreds of light emitting diodes (LEDs). These LEDs direct various combinations of visible light at the PCBs at a variety of angles. A camera captures images of the PCB, which are subsequently interpreted by an image-processing computer connected to the camera to evaluate the coating on that PCB. While the use of hundreds of visible light emitting diodes is effective, such automated systems could be simplified.

[0008] Finally, many PCBs have connectors mounted at the edges but current systems, automated and manual, do not allow for those edges to be easily inspected to ensure they are free of conformal coatings which can impede pin connectivity.

[0009] Accordingly, it would thus be beneficial to provide an improved system and method for inspection of conformal coatings on printed circuit boards (PCB) that overcomes the aforementioned drawbacks.

Summary

[00010] In one embodiment, an automated system for inspecting a conformal coating on a printed circuit board (PCB) is provided wherein the conformal coating includes a tracer that fluoresces visible light in the presence of UV light. The automated system includes an ultraviolet (UV) light source configured to direct the UV light onto the conformal coating and at least one camera having a lens configured to capture one or more images of the visible light from the tracer in the conformal coating when the tracer fluoresces in the presence of the UV light.

[00011] In another embodiment, a method is provided for automated inspection of conformal coatings on printed circuit boards (PCB) wherein the conformal coating includes a tracer that fluoresces visible light in the presence of ultraviolet (UV) light. The method includes illuminating the conformal coating with the UV light, capturing an image of the visible light fluoresced from the tracer in the conformal coating, analyzing intensities of the visible light in the captured image fluoresced from the tracer in the conformal coating, and in response to the analysis of the intensities, determining whether the PCB is properly coated by the conformal coating.

Brief Description of the Drawings

[00012] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the invention.

[00013] Fig. 1 is a block diagram of an automated conformal coating inspection system in accordance with an embodiment of the invention;

[00014] Fig. 2 is an enlarged diagrammatic view of an alternate embodiment of the lighting and camera sub-system of the automated conformal coating inspection system of Fig. 1;

[00015] Fig. 3 is an elevational view of an alternate embodiment of the board holder sub-system of the automated conformal coating inspection system of Fig. 1; and

[00016] Fig. 4 is an enlarged diagrammatic view of an alternate embodiment of the board holder sub-system of the automated conformal coating inspection system of Fig. 1.

Detailed Description of Specific Embodiments

[00017] Fig. 1 depicts an embodiment of an automated system 10 for inspecting a conformal coating 12 on a printed circuit board (PCB) 14. The system 10 includes a lighting sub-system 15 that has an ultraviolet (UV) light source 16 that directs UV light onto the conformal coating 12, which includes a tracer that fluoresces in the presence of the UV light. The lighting sub-system 15 also can include an optional white light source 17 that directs white light onto the conformal coating 12 of the PCB 14. The system 10 further includes a camera sub-system 18, which includes a camera 20 having a lens 22 (Fig. 2) positioned above the PCB 14 to capture one or more images of the illuminated PCB 14 when light is emitted onto the PCB 14. The captured images are conveyed to an image-processing computer 26 for determining whether the PCB 14 is properly coated, as further explained below.

[00018] The system 10 also includes a board holder sub-system 28 having support pins 30 (Fig. 3) that holds the PCB 14 during inspection. In an alternate embodiment, the board holder sub-system 28 defines an automated conveyor system (Fig. 4) that includes a conveyable platform 29 configured to hold one or more PCBs 28 about one or more edges. A transport assembly, which is also part of the automated system 10, includes X-axis and Y-axis motors 32 and 34, which are connected to and move the camera 20 and board holder sub-system 28, respectively, to aid in inspection of the PCB 14. It should be understood that the transport assembly, alternatively, can include an XY- axis motor (not shown) which is connected to only the board holder subsystem 28 or only the camera 20. In addition, the camera 20 may be connected to the Y-axis motor 34 and the board holder sub-system 28 connected to the X-axis motor 32. Other options are contemplated including incorporation of Z-axis movement, as desired.

[00019] The image -processing computer 26 is associated with the camera 20 as well as a motion controller 38 for providing operating instructions to the X and Y motors 32, 34. The image-processing computer 26 is configured for user interface with the system 10 and interprets the captured images for determining whether the PCB 14 is properly coated. In particular, the image-processing computer 26 includes one or more software programs capable of executing machine vision algorithms for measuring the thickness and/or coverage of the conformal coating 12 based upon the intensity of the fluoresced tracer as depicted in the captured images. User interfacing may be accomplished via a computer monitor, keyboard, and/or mouse, depicted generally as numeral 40, which are cooperatively associated with the image -processing computer 26.

[00020] One such system 10 that may be adapted to perform automated inspection of conformal coatings using UV light is the B Series Benchtop Automated Optical inspection system, such as the YTV B 3 AOI Benchtop Automated Optical inspection system, or the F Series inline Automated Optical inspection system, available from YesTech Inc. of Carlsbad, California. Certain of the sub-systems 15, 18, 28 and operation of system 10 are further described next.

[00021] Lighting and Camera Sub-System [00022] The lighting sub-system 15 is able to illuminate the board 14 in either white light or UV lighting under computer control. One or more UV light or white light sources 16, 17 may be utilized. The white light produces a natural image of the board 14 whereas the UV light triggers the fluorescence tracer in the conformal coating 12, making it glow or fluoresce. The brightness or intensity of "the glow" is proportional to the thickness of the conformal coating 12. By measuring the intensity of the glow, the coverage and thickness of the coating 12 can be determined by the computer software. The white light is used in fiducial alignment, barcode reading, or optical character recognition (OCR). Barcode reading or OCR is desirable to provide traceability for the process.

[00023] The UV light emitted from the UV light source 16 includes any desired

UV wavelength that corresponds to the wavelength at which the tracer fluoresces. In one example, the tracer fluoresces at about 365 nm with the wavelength of the UV light thus, being about 365 nm. Any suitable tracer or dye that fluoresces under UV light may be utilized in the conformal coating 12 of the PCB 14. For example, the dyed fluoropolymers disclosed in U.S. Patent No. 6,894,105, which is incorporated by reference herein in its entirety, may be utilized. In another example, the functional dyes of U.S. Patent Application Publication No. 2005/0240020, which is incorporated by reference herein in its entirety, may be utilized.

[00024] The conformal coatings 12 are protective coatings that conform to the surface of the PCB 14. Properly applied conformal coatings 12 can increase the working life of the PCB 14 by protecting its components and the board itself. Conformal coatings 12 can, for example, provide a barrier to moisture, chemicals, dust, fungus, ultraviolet light, and ozone, as well as act as a stress-relieving shock and vibration absorber. Various materials such as, for example, polyurethanes, acrylics, epoxies, and silicones are commonly used for conformal coatings 12. The selection of a conformal coating 12 is generally based upon desired performance and processing requirements for a specific application. The tracer or dye may be incorporated into the coating 12 by means and methods known to those skilled in the art.

[00025] The camera 20 is generally depicted in Fig. 1 as being situated above the

PCB 14 so that the lens 22 may be oriented directly at a confronting surface 42 of the PCB 14 to capture one or more images of the illuminated PCB 14. The camera 20 alternatively may be may be angled, e.g., at a 45° angle, relative to the confronting surface 42 of the PCB 14 so that the lens 22 captures one or more edge images of an outer edge 44 of the illuminated PCB 14 as compared to merely images of the confronting surface 42 of the PCB 14. In one example, as shown in Fig. 2, the camera 20 with lens 22 may be angled or positioned perpendicular to the confronting surface 42 of the PCB 14. And, in conjunction with angled mirror 48, which is positioned to allow the lens 22 to view the surface 42 of the PCB 14, and a y-axis motor (not shown), the camera 20, in this embodiment, can be manipulated to capture images of both the surface 42 and outer edges 44 of the PCB 14. In addition, while only one camera 20 is generally depicted in Figs. 1 and 2, it should be understood that more than one camera may be provided in the system 10.

[00026] Board Holder Sub-System

[00027] The board holder sub-system 28 holds the PCBs 14 for inspection. As shown in Fig. 3, the board holder sub-system 28 supports the PCB 14 from below using support pins 30 having magnetic bases 52. Distal ends (not shown) of each pin 50 are configured to securely situate the PCB 14. For example, the distal end may include a male portion that cooperates with a corresponding female portion in the PCB 14 so as to securely seat the PCB 14. The holder sub-system 28 also allows for the placement of optional mirrors 54 adjacent the seated PCB 14. These mirrors 54 allow the top down viewing camera 20 of Fig. 1 to view the outer edges 44 of the board 14. Again, many boards 14 have connectors mounted at the edges 44, these mirrors 54 allow the system 10 to inspect those connectors to ensure they are free of conformal coating 12 that can impede the pins connectivity.

[00028] In an alternate embodiment, as shown in Fig. 4, the board holder subsystem 28 defines an automated conveyor system that includes conveyable platform 29 configured to hold one or more PCBs 28 about one or more edges. The conveyable platform 29 can seamlessly transport the PCBs 28 from one or more stations (not shown) prior to and/or subsequent to inspection thereof. Pneumatically driven support pins (not shown) with magnetic bases may be alternatively, or optionally, provided to hold or assist in holding the PCBs 28.

[00029] Setting-Up The Inspection Program

[00030] The inspection program starts with having a "good board". A good board is a board with a conformal coating that has been manually inspected to be free of defects. The steps for set-up are as follows. First, arrange the support pins and the optional mirrors for the board holder sub-system, and then place the good board onto the support pins for viewing by the camera. Alternately, a good board held on the conveyable platform is situated for viewing by the camera. Next, start the application software and under the guidance of the software define the perimeters of the software. For example, define a pair of fiducials (or alignment marks) that will be used to determine the location of the subsequent PCBs, define the "keep-out" areas where the conformal coating should not be found, and define the "coverage" areas where the conformal coating should be found. Finally, start the learning process of the software. During the learning process, the software learns the brightness or intensity of all of the areas on the surface (and optionally outer edges) of the board under UV light and white light. Thresholds are calculated based on the brightness obtained for that area on the board. At the end of the learning process, three types of areas can be defined on the board: (1) Type A: Areas where the coating must be found, (2) Type B: Areas where the coating must not be found, and (3) Type C: Areas where the coating is irrelevant, i.e., may or may not be found.

[00031] The brightness threshold established during the learning process is utilized to determine if an area passes or fails the conformal coating inspection. In particular, for type A or "coverage" areas, the brightness must be greater than the threshold; for type B or "keep out" areas, the brightness must be lower than the threshold; and for type C areas, no inspection needs to be performed and, thus, the threshold does not matter. In other words, the status for type C areas will always be "pass".

[00032] Steps for Automated Board Inspection

[00033] Once the learning process has been accomplished via the good board and the brightness thresholds established, automated inspection of conformal coatings of the PCBs can occur. The steps of operation are as follows. First, the operator can load the PCB into the system, i.e., manually seats the PCB on the support pins for viewing by the camera, or a PCB is transported on the conveyable platform for viewing by the camera. Then, using the user interface for the image-processing computer, the automated inspection process is started. Next, the system performs fiducial alignment to locate the board's position using white light. After which time, the system appropriately positions the camera and/or board for image capture. One or more images of the board are captured by the camera under UV lighting. Then, utilizing the computer software, the brightness value of the area is compared with the thresholds established during the learning process. A pass or fail status can be assigned to each area of the board. At the end of the inspection process, the operator has a chance to reevaluate all the areas that failed the inspection. A defect report may be generated with a graphical map showing the area(s) that failed the inspection. Then, the operator can remove the board from the system, and place it in either a pass or fail bin. Finally, these steps are repeated for the next PCB.

[00034] Accordingly, an improved system 10 and method for inspection of conformal coatings 12 on printed circuit boards (PCB) 14 is provided that overcomes the drawbacks of current automated and manual inspection systems and processes. In, particular, the automated system and method of the present invention provides fast, reliable, consistent and repeatable inspection results. In addition, the inspection process has good traceability. For example, the board's serial number, its inspection result, and the login information of the operator running the inspection may be desirably tracked. Also, human exposure to UV light and coating chemicals is minimized.

[00035] While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Thus, the invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant' s general inventive concept.

What is claimed is:

Claims

1. An automated system for inspecting a conformal coating on a printed circuit board (PCB), the conformal coating including a tracer that fluoresces visible light in the presence of UV light, the system comprising:

an ultraviolet (UV) light source configured to direct the UV light onto the conformal coating; and

at least one camera having a lens configured to capture one or more images of the visible light from the tracer in the conformal coating when the tracer fluoresces in the presence of the UV light.

2. The system of claim 1 further comprising:

a board holder sub-system adapted to hold the PCB;

a transport assembly configured to move at least one of the camera or the board holder to aid in inspection of the PCB; and

an image -processing computer associated with the camera, the camera being adapted to convey the images to the image-processing computer, the image-processing computer configured for user interface with the system and to interpret the captured images for determining whether the PCB is properly coated.

3. The system of claim 1 wherein the UV light source is configured to emit UV light at a wavelength of about 365 nm.

4. The system of claim 1 further comprising:

a white light source configured to direct white light onto the conformal coating of the PCB.

5. The system of claim 1 further including at least one mirror that is positioned relative to the PCB, the at least one mirror configured to assist with determining whether an outer edge of the PCB is properly coated.

6. The system of claim 1 wherein the camera is movable to an angled relationship with a confronting surface of the PCB for capturing an edge image of the conformal coating on an outer edge of the illuminated PCB.

7. A method for automated inspection of conformal coatings on printed circuit boards (PCB), the conformal coating including a tracer that fluoresces visible light in the presence of ultraviolet (UV) light, the method comprising:

a) illuminating the conformal coating with the UV light;

b) capturing an image of the visible light fluoresced from the tracer in the conformal coating;

c) analyzing intensities of the visible light in the captured image fluoresced from the tracer in the conformal coating; and

d) in response to the analysis of the intensities, determining whether the PCB is properly coated by the conformal coating.

8. The method of claim 7 further comprising:

e) repeating a) through d) for additional PCBs having conformal coatings including tracers that fluoresce visible light when illuminated by the UV light.

9. The method of claim 7 wherein c) analyzing the intensities of the visible light comprises:

c) analyzing the intensities of the visible light originating from an outer edge of the conformal coating to determine whether an outer edge of the PCB is properly coated.

10. The method of claim 9 wherein c) analyzing the intensities of the visible light comprises:

c) analyzing the intensities of the visible light with the image-processing computer.

11. The method of claim 7 further comprising:

e) conveying the captured image to an image-processing computer.

12. The method of claim 11 further comprising:

f) defining parameters for the image -processing computer for analyzing the intensities of the visible light in the captured image and fluoresced from the tracer in the conformal coating to determine whether the PCB is properly coated.

13. The method of claim 12 wherein the parameters are defined before the conformal coating is illuminated with the UV light.

14. The method of claim 7 wherein the UV light includes a wavelength of about 365 nm.