Customer Solutions

Reliable and Flexible Automated Testing of Cell Phone LCD Assembly Using NI LabVIEW

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The Challenge:

The Solution:

Automated LCD Assembly Test Station

Developing a Test System with PXI

We developed a highly reliable and flexible test system using the NI PXI platform. The NI LabVIEW application performs various functional tests – visual inspection, audibility test, current and voltage measurements, speaker excitation, etc. The main test of the system is the visual inspection of the LCD where the digital I/O module (NI PCI-6508) generates various patterns on the LCD screen, a Pulnix Camera acquires the images with an image acquisition card (NI PCI-1411), and the images are tested for the patterns generated using NI Display Test called from LabVIEW. If a test unit passed the visual inspection and other tests, the unit is sent for final assembly while a failed module is labeled with a serial number to track and the test it failed and is sent back to the assembly line. The test system was built in less than two weeks, with the help of LabVIEW, NI Display Test, and the flexible PXI platform, and the test system tested more than 25,000 units in its first week of operation meeting their demand and justifying the customer’s decision to go with National Instruments tools and with two more test systems.

Testing Needed for Large EMS Company

The client is one of the worlds’ largest EMS (electronic manufacturing service) companies – manufactures and assembles LCD modules for one of the leading cell phone manufacturers in the world. Each and every LCD assembly that the client manufactures has to be tested before it is shipped to their end customer. The LCD assembly consists of various functional units – LCD screen, backlight, speaker, microphone, battery – each requiring a different type of test to be performed on it. They had different manual test stations for testing different parts of the LCD assembly. The LCD assembly required delicate handling and the different test stations required loading and unloading the assembly many times which was not advisable. The total test time for the assembly was also very high and the system also suffered the problems of any common manual test system - accuracy, consistency and reliability.

The client had to test a new model of LCD assembly for which they had to establish a test system that required higher rates of testing and more reliability and traceability than its existing manual testing, and the new line had to be set up within two weeks as they had to meet their customers demand. Hence, the client wanted a single test station that would test all the functional parts of the system, fully automated, highly reliable, and meet their production requirement by lowering the test time.

They expected the functional tester to meet the following requirements:

• Perform visual inspection of the LCD screen
• Perform battery level check on the RTC (real-time clock) battery
• Check the performance of the LCD screen backlight
• Check the quality of the speaker – this included audibility check, distortion check, and power consumption check.
• Measure the resistance of the speaker
• Check the quality of the magnet that is used for backlight switching based on the position of the cell phone flap

The customer clearly wanted the system to be flexible to add any test required at a later stage and also to easily adapt the test station to a newer model of LCD display if required at a later stage, thus protecting their investments. The flexible and reliable PXI platform was chosen to provide the flexibilities the customer was seeking. Off-the-shelf hardware and powerful software tools from National Instruments, such as LabVIEW and NI TestStand, helped us to build the functional tester for the client in less than two weeks.

System Description

The PXI-based test system consists of an NI PXI-1002 chassis housing an NI PCI-8174 controller, NI PCI-1411 image acquisition card, NI PCI-6040 multifunction DAQ card, and an NI PCI-6508 DIO card.

LCD Screen Test

We used the digital I/O card to communicate with the LCD screen driver and generate different test patterns on the LCD screen. After each test pattern is generated, the image of the screen is captured using a Pulnix analog camera interfaced through the PCI-1411 image acquisition card. The pattern generation, capturing of the LCD screen image, and the synchronization between them is taken care by modules written in LabVIEW and NI TestStand. We designed the NI Display Test for testing flat display units; hence the display test is a perfect solution for testing the LCD screen. The image captured is processed using NI Display Test for all the possible defects in the pattern generated. It included pixel-by-pixel verification and complex icons present in the display. NI Display Test made the LCD screen test easy and helped us to put up the entire test system less than two weeks.

Other Tests

All the other tests given below are achieved by using a single card (NI PCI-6040 multifunction DAQ card) connected to the SCC series portable, modular signal conditioning modules. 

1. RTC Battery: We tested the RTC battery by measuring the voltage across the battery terminals, which is achieved by connecting the terminals of the battery to one of the analog inputs of the card. We can then check the battery for the limits.
2. Speaker Performance: To test the speaker, we placed a microphone at an appropriate position near the speaker to catch the response of the speaker. The input from the speaker is passed through SCC-ICP01, a SCC signal-conditioning module that filters the microphone signal and provides the necessary signal conditioning for the microphone. LabVIEW generates a multitone frequency signal from the analog output of the DAQ card, which is fed to the speaker, and the input from the microphone is acquired in one of the analog input channels to check the response of the speaker at different frequencies. Also, a pure sine wave of 1 KHz is generated and is fed to the speaker and its response is studied for the distortion that it introduces. A total harmonic distortion calculation is done in LabVIEW from the acquired waveform. This test using a simple analog output, an analog input and SCC signal-conditioning module replaced the need for an expensive dynamic signal analyzer with a sound meter and source.
3. Speaker Resistance: A constant current input is given to the speaker using SCC-RTD01, a SCC signal-conditioning module, and the voltage is measured to measure the resistance of the speaker. The SCC relay module, SCC-RLY, was used to achieve the switching of the connections between the response test of the speaker and the resistance test of the speaker programmatically.
4. Magnet: The magnet in the assembly is used to turn on the backlighting automatically when the flap of the cell phone is opened. This magnet is tested using a hall-effect sensor and one of the analog inputs is used for this test.
5. Backlight: The backlight is measured by the amount of current drawn by the LCD screen during the LCD screen tests and this tests the failure of backlight.

LCD Test System

We developed the LCD Test system in LabVIEW. NI TestStand performs and synchronizes all the above tests. Wherever possible, we conduct the tests in parallel to reduce the total time taken for the test.

Test Jig

The test jig requirement was complex, as it had to connect to the small ribbon connector without damaging the cell phone flap, which is already assembled with the LCD screen. The client engineers came up with a good design of the test jig onto which the camera, lighting, and mirror arrangement was incorporated to produce a good image as the test system is as good as the image acquired. The test jig was such that, the operator places the cell phone assembly facing down into a closed box, where the LCD screen is reflected by a mirror on to the camera inside the box that is lit with the appropriate lighting.

The operator places the cell phone in the test jig and toggles a clamp, then starts the test by interacting with the user interface. The NI TestStand test sequence and LabVIEW application performs all the various tests on the cell phone and a popup message gives the test result. If the test fails an error code specific to the module is generated for further tracking of the module after rework. If a particular test failed the module would be rejected for that particular reason before other tests are done to save time.

Software Module

A main test module conducts all the tests required by the system as given above, interacts with NI TestStand and NI Display Test for LCD screen test, and performs and synchronizes all the other tests. The tests are conducted in parallel wherever possible to reduce the total time taken for the test. A report is also generated for the user according to each shift and date, having details about the modules that failed the reason and value at which it failed for further usage.

The test system also prepares shift wise reports; keeps track all the faulty assemblies and gives the utilization factor of the test system. All the reports generated are stored in the hard disk of the PXI system. Report module is also designed for the user to check and print historical reports of any shift or any desired period of time. We also provided a histogram of the failed units and other statistical parameters as required by the customer.

Features of Automated Cell Phone Test System

• Fully automated system
• Fully integrated with all the functional tests integrated in one system
• Highly reliable, flexible, and open for further modification or incorporation of new tests
• Continuous production (three, eight-hour shifts, six days a week)
• Reduced test time (about 12 s per unit, reduction by more than 300 percent)

Conclusion       

We built the functional tester based on the concept of virtual instrumentation. By choosing off-the-shelf hardware modules from National Instruments and building an application using LabVIEW and NI TestStand, we built the functional test system in 12 days time and exceeded the expectation of our customer.

For more information, contact:

Senthil Raj Desappan

Applications Engineer

Apna Technologies & Solutions Pvt. Ltd.

#1, Coconut Garden Crs, Outer Ring Rd,

Kalyan Nagar PO

Bangalore - 560043

Tel: +91-80-25426463

E-mail: senthil@apnagroup.com