Customer Solutions
PXI-Based Vision System Detects Flaws in Plastic Film
Author(s):
Howard Foster, MicroCraft Corporation
Industry:
Industrial Controls/ Devices/ Systems
Product:
LabVIEW, Motion Control, PXI/CompactPCI, Vision
The Challenge:
Developing a vision and motion system to detect, count, and categorize micron-scale defects in a sample of plastic film.
The Solution:
Creating an automated motion and vision system using PXI for both the computer system and hardware.
Since its first development of plastics, uses for this versatile material have made it suitable for countless applications, especially in the computer industry where plastic sheeting is highly valued in the creation of printed circuit boards (PCBs).
A PCB is made by applying thin wires of metal, often in multiple layers, to a substrate. With advancing technologies, the wires that are laid out are only tens of microns in width, therefore, exacting production is extremely critical. One method of production is to first lay out the circuit design onto a piece of specialized plastic using lasers to etch lines in the desired pattern. The plastic then adheres to the substrate and is coated in a solution that deposits the metal into the etched lines. One difficulty in this procedure is that defects can appear in the plastic sheet used for the circuit layout. Even 10 to 100 micron-sized defects can break or short connections on the boards.
Finding and Categorizing Defects in Plastic Sheets
MicroCraft was challenged by a manufacturer of plastic sheeting to come up with an inspection system to find and categorize micron-sized defects. The system needed to be as automated as possible, with the ability to implement current testing procedures, and also capable of cataloging images and statistical information about the defects to a searchable database.
The client’s method for detecting defects required an operator to evaluate an image projected onto a wall with an overhead projector. The operator would circle defects on the sheet and then carry the sheet to a microscope with a film camera attached to photograph the defect. Next, the operator estimated the defect area, length, and width based on other calibrated pictures. Finally, the operator made a handwritten log entry for all the information. The operator then placed the picture placed in a “picture book,” in a section for the defects category. This was a time-consuming and tiring process, which was done sporadically because of the cost-prohibitive assignment of having an operator perform this task. No more than 10 samples could be completed in an 8-hour work shift.
Integrated PXI, Vision, and Motion Save Time and Money
MicroCraft used a vision and motion system to perform the client’s original test procedure, implementing a cross-polarized light field. Here, the defects show up as bright spots due to the distortion of the aligned polymer chains in the material. First, we selected a set of cameras with the appropriate resolution and magnification for a two-step analysis. The company divided cameras into two categories – large and small fields of view (LFOV and SFOV, respectively), and further defined them as having low magnification (LFOV: 0.3X) and high magnification (SFOV: 6.4X). The LFOV camera is used for the initial inspection of the film to find defects using the cross-polarized fields. Operators use the SFOV camera for a secondary evaluation of the defects and to capture images and measurements for data storage.
To accommodate the client’s sample size (8 in. diameter), we covered a backlight table with a polarizing sheet, placing it on a 2-axis linear rail system to control the lighting and motion of the sample. We tied motion, vision, and data collection together with a National Instruments PXI-based system. Using a PXI-7324 motion board and two PXI IMAQ boards (the 1407 and 1422), we developed a system that exceeded client needs, thereby saving time, man-hours, and ultimately money.
Vision Builder Helps MicroCraft Pinpoint Resolution
After the operator places and orients a sample for correct polarization, the system automatically scans the sixteen, 2 in. square “quadrants” of a sample. Using the LFOV camera, the operator looks for suspect defects. Then, using vision analysis VIs to determine the center of the suspect, the PXI system commands the table to move such that each suspect is in the field of view for high magnification. To make correct location assignments to the suspects, we used several image analysis VIs to counter common lens problems, such as pin-cushioning. Next, the company used motion analysis VIs to create an algorithm that properly positioned the suspect under the SFOV camera.
When the operator locates a suspect using the SFOV camera, a live image appears. The operator can reject the suspect, if it is a dust speck, for example, or classify the defect and make calibrated measurements for area, length, and width. Each operator also controls the “bump” in the table to better orient the defect in the live image.
We achieved resolution with the SFOV camera through calculations using NI Vision Builder and a USAF resolution target at 1.17 microns/pixel. With this resolution, the system can easily detect and quantify defects in the client’s required 10-100 micron range.
If the suspect is a defect worth cataloging, the operator can name and classify the defect and “snap” an image of it. The system then saves the image to a database along with measurements performed using several of the powerful IMAQ Vision VIs, including Edge Detection, Fill Hole, Threshold, Set Simple Calibration, Complex Particle, and Complex Measure. Using NI Vision Builder, we drastically cut development time, facilitating quick image analysis algorithm building and easy calibration determination.
The motion and vision toolkits, with their well developed function and analysis VIs, greatly reduced system development time. At ~15 minutes per sample, operators can test up to 30 samples per shift. The client thus saves enough time and money such that they now can dedicate an employee to regular testing of all similar materials they produce. The client considers the return on their investment in MicroCraft and National Instruments highly valuable.
For more information, contact:
Howard Foster
MicroCraft Corporation
3209-154 Gresham Lake Rd.
Raleigh, NC 27615
Phone: 919-872-2272
Fax: 919-872-5822
E-mail: howard.foster@microcraftcorp.com
