Cameras scan diodes for proper markings

Top and bottom surfaces of different types of diodes are visually inspected by two cameras for lead defects and correct laser-etched characters.

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Top and bottom surfaces of different types of diodes are visually inspected by two cameras for lead defects and correct laser-etched characters.

By Keith C. Hill

A major manufacturer of semiconductor devices for use in power-management applications has realized substantial improvements in quality and throughput for a surface-mount-diode production line after installing a dual-camera, digital vision-inspection system. The principal benefits include faster inspection times, better yields of quality diodes, and reduced labor costs.

Previously, diode inspection was done manually after the diodes were mounted onto a tape-and-reel carrier. In that method, only the diode markings could be inspected. Replacing poorly marked diodes required that the carrier tape be lifted for diode removal and a properly marked diode inserted as a replacement. The current system automatically rejects flawed diodes, ensuring that only acceptable diodes are mounted onto the tape.

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FIGURE 1. Diode inspection begins when two diodes are removed from vacuum cups on a conveyor belt by a pick-and-place machine and loaded into vacuum pockets on the rotary indexing table at Station 1. The table indexes 90° clockwise to Station 2, where a laser etches the logo and product type on the top side of the diodes. Another 90° clockwise indexing step moves the pocket-borne diodes into the top camera's field of view at Station 3, where the laser-etched marks are inspected. Prism apparatus is required to reduce the camera's apparent field of view to deliver sufficient image resolution. Two prisms were needed because the vision system had to be adapted to the space constraints imposed by the existing handling equipment. At Station 4, the diodes are picked and placed for further tests, such as diode-lead inspection.

Furthermore, in manual inspection surface-mount diodes with defective leads often were not discovered until after they had been assembled onto a board and its associated equipment malfunctioned. Now, the diodes are detected and rejected before board assembly, saving time, product, and cost.

Imaging-system elements were integrated by Simac Masic & TSS bv (Reading, Berkshire, UK) into a large automated diode-handling apparatus provided by Sillner Industries GmbH & Co. KG (Regensburg, Germany). The complete diode-inspection system was initially installed at a facility of General Semiconductor Inc. (County Cork, Ireland), where it was operated for about 20 months and inspected some 200 varieties of diodes. It has since been moved to a facility in the Republic of China. (General Semiconductor was recently acquired by Vishay Intertechnology Inc. (Malvern, PA), a manufacturer of passive electronic components.)

The surface-mount diodes are approximately 5 X 3 mm. Their bodies are made of black plastic, with a metal terminal attached to each of the 3-mm ends. The terminals are folded underneath the diode. A logo, anode bar (on some devices), product type (two or three characters), and date code (one character each for year and month) are laser-etched into the top surface of the diode bodies on the production line for identification by the Sillner machine. Then, both the top and bottom surfaces of the diodes are visually inspected for defects and to determine that the laser-etched information is legible and correct.

Step by step
During production-line operation, the Sillner system feeds loose diodes onto a long conveyor belt that holds a large number of vacuum cups in a single line. A pick-and-place machine takes two diodes at a time from the cups and loads them onto a rotary index table at Station 1 (see Fig. 1). The rotary table then indexes clockwise by 90° to laser-marker Station 2, where the two diodes are marked with appropriate characters by the laser etcher. Next, the table indexes clockwise another 90° to digital vision-inspection Station 3. This station contains one of two cameras connected to a PPT Vision Inc. (Minneapolis, MN) Scout DSL system. Camera one is mounted above the rotary table and inspects the laser identification marks on both diodes (see Fig. 2). The second camera inspects the diode leads for defects at the next inspection stage. These 640 X 480-pixel progressive-scan cameras can provide up to 4000 full-frames/min.

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FIGURE 2. Image of top side of typical diodes is shown on the vision system monitor. Top system camera checks for proper laser etching of the logo and the characters that designate the device type.
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After another 90° clockwise indexing step to Station 4, the diodes are again picked up and placed for further assembly and inspection steps, such as the camera-two lead-inspection station, and reject stations for electrical-test and vision-inspection failures. The last step in the assembly process is placement of the acceptable diodes onto a tape-and-reel carrier. The rotary inspection system indexes 180 times/min. Because each index step moves two diodes, the production-line throughput is 360 diodes/min. The diodes are stationary for approximately 200 ms.

The first or top camera inspects the top surface of the diodes for accurate and legible laser markings, surface scratches, bad body fill, and dirty body. Typical laser-etch defects include no mark, light mark, illegible mark, double marks, or incorrectly positioned mark. In addition, all identification characters must be etched within the diode body boundary and at least 70% of the height of the anode bar must be visible.

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FIGURE 3. Bottom surface images of these diodes taken by the bottom camera show scratches on the body of the right diode, as well as shaded areas on the leads, which may indicate improper plating.
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The second camera inspects the bottom surface for cracks, twisted and missing leads, too long or not fully formed leads, poor or excessive plating, chips in body (>0.6 X 0.6 mm), and overall length, including leads. Some diodes use nickel-plated leads, and the plating is occasionally deposited in an irregular or patchy manner. Irregularities in deposition appear as different gray scales in the images (see Fig. 3). The vision system also looks for excessive solder on the leads if the leads have been tinned. Excessive solder shows up as blobs on the sides of the leads.

The diodes produced in the Ireland facility encompassed approximately 200 device types, all having the same body shape and size but with different markings. Changeover from one diode type to another occurred as often as 12 times a day. Therefore, the vision system had to accommodate a simple software-program conversion for the different markings as well as a quick changeover between diode types.

The changeover for electrical test and marking parameters is selected using a keyboard connected to the controller. During operation, the appropriate inspection program is selected automatically using the machine's programmable logic controller.

The vision system provides a pass/fail signal to the controller so that a faulty diode can be rejected. It also categorizes failure modes, presents these data on the system monitor, and stores data for subsequent off-line analysis. The vision system also triggers an alarm if the fail rate exceeds certain limits.

Key elements
Simac Masic selected a two-camera DSL Scout system for the diode-inspection application. However, because of space constraints for camera mounting imposed by the existing system, the company designed and added two glass prisms to the top camera. These glass prisms are located between the first camera and the rotary index table pockets and function like a periscope.

This system setup change was required to ensure adequate image resolution because the two diodes are positioned 15 mm apart in the rotary table pockets. A field of view (FOV) that wide would not yield sufficient resolution with one camera viewing both diodes. However, the prism setup reduces the apparent distance between the two diodes to 5 mm—a reduced FOV that enables the top camera to deliver the required resolution.

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FIGURE 4. PPT Vision Program Manager software creates an application program for diode inspection. It allows icons to be clicked and dragged from the toolbox and assembled into a flow chart for inspection. Each icon represents a different image-processing tool, including OCV to check laser markings, line gauge to make measurements, and blob-analysis tools to evaluate surface defects such as chips and scratches.
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Simac Masic used PPT Vision Program Manager (VPM) software to create an application program for the diode-inspection task. VPM software allows icons in the toolbox to be clicked and dragged from the toolbox and assembled into a flow chart for the diode-inspection task (see Fig. 4). Each icon represents a different image-processing tool, including optical character verification to check laser markings, a line gauge to make measurements, and blob-analysis tools to evaluate surface defects.

The top camera inspects two diodes at a time with a pixel resolution of approximately 13 µm. This resolution is required to reliably detect print defects. The inspection time for two diodes is approximately 160 ms.

The bottom surface of the diodes is inspected with the second camera looking upward as the diodes are held in the vacuum cups of the conveyor belt adjacent to the rotary table. Here again, two diodes are inspected simultaneously. Pixel resolution is approximately 32 µm, with a FOV of 20.5 X 15.4 mm, which is adequate to detect 0.6 X 0.6-mm defects, provided there is enough contrast. Inspection time is approximately 60 ms for the two diodes. The diodes must be stationary for approximately 17 ms, during which time the bottom camera acquires full-resolution images. Processing is done during the remainder of the inspection cycle.

A built-in Pentium-based PC operates the PPT Vision DSL Scout vision-inspection system and communicates with the Sillner PLC, which, in turn, controls the overall production line via a digital, parallel, input/output channel. Image processing is done using a DSP-based board and the VPM icon-based, Windows-compatible software. Technicians use a touchscreen or trackball to run a 15-in. color SVGA monitor that has 1024 X 768-pixel resolution.

The vision system incorporates an inspection program for one device type of each of the three font sizes Vishay/GSI uses. For other product codes, it was necessary to teach the system 26 alphabetic and 10 numeric characters. This was done during installation and required a set of diodes marked so that all characters are available. Inspection of a new diode required that the product code be entered via the vision-system keyboard.

Pass and fail results, including the categories of failures, are displayed on an operator panel and also are logged to the system hard disk (or serial port) for subsequent off-line analysis. The vision system provides an output signal to signify a failed part. Movement of failed parts is tracked by the system PLC so that they drop into the appropriate reject bins.

KEITH C. HILL is UK manager at Simac Masic & TSS bv in Reading, Berkshire, UK.

Company Information
PPT Vision Inc.
Minneapolis, MN 55344

Sillner Industries GmbH & Co. KG
93057 Regensburg, Germany

Simac Masic & TSS bv
Reading, Berkshire RG8 7BP, UK

Vishay Intertechnology Inc.
Malvern, PA 19355

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