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Verifying polymeric LED displays

Optical inspection and electrical test combined in one instrument
Verifying polymeric LED displays

IPTE has developed an automated tester for displays based on polymeric LEDs, by which visual inspection and electrical test are combined in one instrument.

Organic LEDs (OLED) and as one variant polymer LEDs (PolyLEDs) are not more flexible than the familiar classic semiconductor LEDs, but also considerably more economical. These PolyLEDs are – as the other LEDs – active components. They operate by applying a voltage to a thin film of light emitting polymer. PolyLEDs can be made in virtually any kind of display form, the same way as their near relatives do, making them an interesting alternative to passive liquid crystal displays which need backlighting. Both the physical and the functional characteristics are similar; the electrical difference is that LCDs are voltage-driven while PolyLEDs are current-driven. This fact must be taken into consideration for the concept of the tester. The system overhead becomes particularly high with color displays, because then considerably more control circuits are needed than is the case with black-and-white.

This tester was developed for polymeric displays as are commonly used in telecom equipment. The displays are using dot matrices, and are currently being tested both as simple components without drive electronics, and as complete functional modules with their connections, electronic controls and connectors. Two different instrument categories were developed: cell and module tester. Philips is both the manufacturer of PolyLEDs and contractor for this tester.
Modular design concept
The modular test concept achieves a high level of flexibility in a wide variety of displays. The test fixture and camera unit are product-specific and hence rapidly interchangeable. The instrument consists of a combination of signal generators, PXI modules for matrix triggering, electrical test modules and the energy supply. Software standards (TestStand, LabView, IMAQ) ensure rapid programming of the procedures and parameters as well as a quick retooling of the tester when another display type is examined. The selection of a program takes place automatically with barcode reader.
The fixture is designed for displays with a maximum size of up to 150 x 150mm, and is made of solid stainless steel, due to the required high-precision contacting. These contacts presented an engineering challenge since the PolyLED displays are produced with a contact clearance of 0.2mm, and there are up to 320 terminations on a dis-play that need to be contacted then by 640 pins. This high pin number is required to ensure perfect contacts of all I/Os for both electrical and optical test. The functional tester is based on an 18-slot PXI mainframe (three height units), and for contacting and signal triggering own-developed modules are in use. The mainframe transmits signals almost without cross-talk and distortion-free. The scanner is made of several 96-contact relay cards from IPTE (up to six line bus).
Governing criteria for optical inspection
The measurement unit for the optical tests includes up to three cameras. Two cameras provide the imaging for the orientation of the display, using fiducials. The third, high-resolution camera with a corrected lens carries out the inspection. The PolyLED displays put special requirements on the optical inspection as a result of three features: first, erratic black spots can occur in the dis-play substrate, which could have a distractingeffect upon the overall image. Second, the dis-play is to be checked for uniform luminance across its entire surface and, third, each individual pixel is measured and inspected to determine its basic brightness and to detect any cross-talk caused by neighboring and active luminous spot. In addition, the usual pattern tests are also carried out.
During each test, the signals required for triggering the display originate from a generator especially developed for this purpose, which simultaneously assumes the function of the driver unit. The analysis of the images takes place with the help of NI’s IMAQ vision software, which was modified to suit the requirements of display tests. A subsequent subjective evaluation of the display by the operator remains indispensable, despite all the finesses contained in the tests. Defects such as, for example, blowouts on the edge of the glass or other damage to the display surface require a trained staff to be securely recognized. After this, the operator can add this information to the inspection results. Reportedly, a number of testers have already been installed in Europe, and soon volume production of the PolyLED display shall be carried out in Asia.
ZUSAMMENFASSUNG
Bald schon dürften passiven Flüssigkristallanzeigen durch Displays auf Basis von aktiven Polymer-LEDs ernsthafte Rivalität erwachsen, denn sie benötigen kein Backlighting. Nun gibt es die ersten optischen/ elektrischen Tester für diese in den kommenden zwölf Monaten am Markt verfügbaren Anzeigemodule.
RÉSUMÉ
Les écrans à base de LED polymères actives ne vont pas tarder à inquiéter sérieusement les écrans à cristaux liquides passifs car ils ne nécessitent pas de rétro-éclairage. Les premiers testeurs optiques/ électriques pour ces modules d’affichage qui seront disponibles dans les douze prochains mois sont maintenant proposés.
SOMMARIO
Gli indicatori passivi a cristalli liquidi troveranno ben presto forte concorrenza nei display basati su LED a polimeri attivi, in quanto questi ultimi non hanno bisogno di retroilluminazione. Sono ora disponibili i primi tester per questi moduli di indicazione, che saranno disponibili sul mercato nel corso dei prossimi dodici mesi.
Current Issue
Titelbild EPP EUROPE Electronics Production and Test 11
Issue
11.2023
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