Before leaving manufacturing houses, bare PCBs (Printed Circuit Boards) and PCB assemblies (PCBA) have to pass electrical tests so as to guarantee that boards go for high performance and high reliability in end products. Electrical tests are implemented to find out electrical and circuitry issues such as shorts, opens, resistance, capacitance etc. all of which indicate whether bare boards or assembled board are correctly fabricated.
Flying probe test originally worked only for bare board inspection while it has been effectively applied on on-line test simulation for both bare boards and assembled boards. The advent of flying probe test has modified the test methods for low volume and quick-turn assembly products. With boards tested through flying probe, product design cycle will be dramatically decreased and time to market will be thereafter cut down.
What is Flying Probe Test?
Actually, flying probe test can be regarded as the upgrading of bed of nails fixture test in that flying probe tester takes advantage of probes to replace bed of nails. Four headers are equipped on flying probe tester along X-Y axis and can move at a high speed.
As a flying probe tester is working, unit under test (UUT) will be first transported to the internal tester through conveyor belt or other UUTs. Then, probes will have contact with test pads and vias so that defects of UUTs can be found out. Probes are connected with drivers (such as signal generators, power supply etc.) through multiplexing system and sensors (such as digital multimeter, frequency counters etc.), through which components belonging to UUTs can be tested. When a component is being tested, other components on the same UUT will be shielded from test to stop reading from being disturbed.
Flying probe tester is capable of testing shorts, opens and component values. Additionally, a camera is equipped on the flying probe tester to help to find out missing components and to inspect component polarity. As positioning accuracy and repeatability of probes reach a scope between 5μm and 15μm, flying probe tester can accurately test the fabrication situation of UUT.
Comparison between Bed of Nails Test and Flying Probe Test
Compared with bed of nails tester, flying probe tester performs better and more effectively in PCB assembly test. First, test development cycle will be reduced so that end products will be able to enter market at a higher speed. Second, cost will be cut down with the application of flying probe tester that no longer calls for fixture that has to be used in bed of nails test. Third, flying probe test is capable of implementing low-volume test at a low cost. Finally, flying probe tester can rapidly test assembly prototype.
How does Flying Probe Tester Work?
Flying probe tester carries out programming more easily and more quickly than traditional ICT (in-circuit test) systems, flying probe test for example.
To implement flying probe test programming, testing staff should first transform CAD (computer aided design) data provided by engineers into applicable file. Then, the newly-generated file will be run through a test program with new files with corresponding formats generated. Last, all the files will be created catering to UUT test demands and requirement.
As soon as test programming is finished, the authentic flying probe test is around the corner. A testing item should be determined first, shorts for example. Then, data of reference points conforming to UUT should be picked up from CAD data. As soon as UUT is fixed on the platform, the programming will be carried out to inspect the fabrication or assembly issues.
It’s necessary to point out that debugging should be done prior to official testing. What’s more, debugging of flying probe test can be completed within shorter time compared with conventional ICT test.
Advantages of Flying Probe Test
Based on the definition and working principles mentioned above, flying probe test features the following advantages:
• Short test development cycle;
• Relatively low test cost;
• High conversion flexibility;
• Rapid feedback provided to PCB design engineers at prototyping period.
Therefore, compared with conventional ICT, flying probe test calls for shorter overall test time. When it comes to PCB assembly, manufacturing can be started just a couple of hours after CAD file arrives. Thus, prototyping PCB assemblies can be tested a couple of hours after being assembled, which differs a lot from conventional ICT that usually spends even a couple of months on test alone. Besides, owing to low difficulty on setting, programming and test, ordinary technicians are able to operate.
Disadvantages of Flying Probe Test
Each coin has two sides. Apart from obvious advantages, flying probe test simultaneously features some disadvantages.
Since flying probes have direct physical contact with vias and test pads and small pits are easily caused on board surface, some OEMs will regard them as fabrication defects. Nowadays, however, as science and technology make constant progress, this will be overcome through the advent of upgrading flying probe testers.
Sometimes, when flying probe tester is working on components without test pad, it's possible that probes have contact with component leads so that loose leads or leads with badly-performed soldering may be missed.
In spite of the disadvantages mentioned, flying probe test is still regarded as a significant test method as far as PCB fabrication and PCB assembly are concerned and will always play a crucial role in leading electronic products to excellent performance and high reliability.