In-Circuit Tester for Chip-on-Board LED Modules
Initial situation and project task
The development of LED technology is making great progress. In former times, LEDs have primarily been used as indicating devices. Nowadays, they are used for the whole range of lighting technology. In addition to that, more and more automotive manufacturers rely on LED technology. Growing demand for LEDs, increased quality requirements and advanced demand for consistent traceability can only be met by means of automated tester systems.
A special new in-circuit tester has been developed for on-board technology. A 3-stepped testing sequence checks SMD mounting, bonding wires, ESD protection diodes and LEDS. Parallel execution of test steps allows for short handling times. All determined data is processed directly and stored to a database.
Implementation and challenge
In order to generate a scalable, flexible tester system for quasi-mobile use in production facilities, a modular system consisting of 3 main components has been implemented:
In order to achieve short cycle durations, extensive parallel processing is needed, so that the currents of all LEDs on a panel can be measured at the same time.
Operational conditions of the LEDs may not be falsified by temporarily high currents, so that low contact resistance and short connecting lines had to be ensured. Therefore, measurement equipment is placed close to the adaptor and signal conditioning is executed inside the adaptor, operating very close to the device under test.
All components are constructed for high numbers of items and therefore partly over-sized, because faulty modules may not reduce a system’s endurance.
By means of the LabVIEW-based NOFFZ sequencer, an application has been designed, which can be fast and intuitively controlled via a user-friendly GUI on a touch panel. By means of this GUI it is possible to assign testing numbers, order numbers or serial numbers and to configure rights for parameterization and adjustment.
The development of LED technology is making great progress. In former times, LEDs have primarily been used as indicating devices. Nowadays, they are used for the whole range of lighting technology. In addition to that, more and more automotive manufacturers rely on LED technology. Growing demand for LEDs, increased quality requirements and advanced demand for consistent traceability can only be met by means of automated tester systems.
A special new in-circuit tester has been developed for on-board technology. A 3-stepped testing sequence checks SMD mounting, bonding wires, ESD protection diodes and LEDS. Parallel execution of test steps allows for short handling times. All determined data is processed directly and stored to a database.
Implementation and challenge
In order to generate a scalable, flexible tester system for quasi-mobile use in production facilities, a modular system consisting of 3 main components has been implemented:
- 1. Mini 19“Rack, rollaway rack including IPC, PCI ExpressMXI and power supply. It is connected to a PXI-1033 chassis, including PXI 6255, PXI 6513 and PXI 2532.
- 2. Chassis, which integrated as an adaptor-switch-measurement system into an attached chassis. A distribution system provides the chassis with all signals from the tester system. The interface has been equipped cost-efficiently, robust and easy to maintain by means of VG connector bars, so that devices under test can be connected easily
- 3. Device-specific adapter: It contains relays, signal distribution components and device specific signal conditioning with shunts. Because of precise upper contacting it is possible to connect a panel, comprising 32 LED modules at most, with 100 needles at the same time. In addition to that, PXI slots and VG connector bars are implemented. Each adapter has its specific encoding, which ensures automatic access to the suitable testing sequence.
In order to achieve short cycle durations, extensive parallel processing is needed, so that the currents of all LEDs on a panel can be measured at the same time.
Operational conditions of the LEDs may not be falsified by temporarily high currents, so that low contact resistance and short connecting lines had to be ensured. Therefore, measurement equipment is placed close to the adaptor and signal conditioning is executed inside the adaptor, operating very close to the device under test.
All components are constructed for high numbers of items and therefore partly over-sized, because faulty modules may not reduce a system’s endurance.
By means of the LabVIEW-based NOFFZ sequencer, an application has been designed, which can be fast and intuitively controlled via a user-friendly GUI on a touch panel. By means of this GUI it is possible to assign testing numbers, order numbers or serial numbers and to configure rights for parameterization and adjustment.
When a test is conducted, the adaptor is inserted, first (automated adaptor recognition). Afterwards, personnel number, order number and serial number have to be typed in. If this is completed, current consumption of the device under test is determined by a short activation with maximum amplitude. For better accuracy of the measurement, faulty modules are detected and shut down automatically.
After that short activation, input currents are measured and the glowing of the LEDs is executed. Finally, measurement data is exported as CSV or QS-Stat file for further processing and statistical evaluation.
Testing is directly conducted after the wire bonding process. First, SMD mounting and current sources are tested. In a second step, bonding wires, diode polarity and functionality of the ESD protection diodes are checked. In a third and last step, LEDs are checked for ESD damage.
The whole testing sequence is automated and completed in less than three seconds.
Conclusion and future prospects
For easy adaption to further LED products by applying slight changes, the implemented tester system is based on a software and hardware framework. The integrated device adaptor includes a USB interface, so that it is possible to assign measurement data to a specific serial number using a barcode scanner.
The whitening process is an important part of the production process of LEDs for lighting applications. Therefore, it has to be constantly controlled, so that tolerances and colour coordinates are met exactly. The chromatic coordinate has to be measured promptly, so that it the process can be adapted within the limiting values. In this context, requirements towards measurement equipment are really demanding, so that the boundaries of photometry are reached.
A further project regarding the modification of measurement equipment for automated, optical evaluation has already been planned.
Because today’s LEDs have already exceeded the limit of intensity for inspection by the human eye, more and more such optical measurement systems are about to be applied for outgoing good control.
After that short activation, input currents are measured and the glowing of the LEDs is executed. Finally, measurement data is exported as CSV or QS-Stat file for further processing and statistical evaluation.
Testing is directly conducted after the wire bonding process. First, SMD mounting and current sources are tested. In a second step, bonding wires, diode polarity and functionality of the ESD protection diodes are checked. In a third and last step, LEDs are checked for ESD damage.
The whole testing sequence is automated and completed in less than three seconds.
Conclusion and future prospects
For easy adaption to further LED products by applying slight changes, the implemented tester system is based on a software and hardware framework. The integrated device adaptor includes a USB interface, so that it is possible to assign measurement data to a specific serial number using a barcode scanner.
The whitening process is an important part of the production process of LEDs for lighting applications. Therefore, it has to be constantly controlled, so that tolerances and colour coordinates are met exactly. The chromatic coordinate has to be measured promptly, so that it the process can be adapted within the limiting values. In this context, requirements towards measurement equipment are really demanding, so that the boundaries of photometry are reached.
A further project regarding the modification of measurement equipment for automated, optical evaluation has already been planned.
Because today’s LEDs have already exceeded the limit of intensity for inspection by the human eye, more and more such optical measurement systems are about to be applied for outgoing good control.