Production Testing Using Flying Probe Systems
Since the mid 1990’s, moving probe in-circuit test systems have been widely used for electronic structural testing of prototypes and new product introduction of Printed Circuit Board Assemblies (PCBAs). For higher volume and faster production testing, In-Circuit Test (ICT) or Manufacturing Defect Analyzers (MDA) systems, using a Bed-Of-Nails (BON) test fixture, have been the choice for production testing of circuit boards.
The ensuing twenty years have brought many changes to the electronics industry, primarily in the form of smaller components and higher component density. During the same time, innovations with some flying probe test systems now enable electronics manufacturers a new choice of testing volume production of PCBAs with the latest generation of Flying Scorpion™ technologies.
Leading technology moving probe systems, such as the Flying Scorpion, provide the ability to test high density PCBAs, and to do so in production volume applications. The advances in technology are due to:
1. An increase in number of individually programmable flying probes from 4 to 22; 2. An increase in the speed of the mechanical movements of the flying probes and shuttles; 3. The addition of Fixed Channels for continuous access to high usage nets; 4. Incorporation of Boundary Scan as a complementary test method, resulting in reduced test time and increased test coverage; 5. Elimination of redundant tests when multiple test tools are available; 6. Event-driven test software for verification of process changes; 7. Incorporation of a Flying BON, or Mini BON; and 8. Inclusion of application specific probe tips.
The result is higher test coverage, shorter test times, and higher yield at the next stage in the manufacturing process, such as full functional test.2. ICT Background
ICT or MDA systems use a BON test fixture to contact each individual net on a PCBA, typically one test probe, or nail, per net.
A relay matrix is used to switch the ICT electronics to the combination of nets needed to test each component or connection. ICT test systems can keep pace with the highest volume production lines, limited primarily by the physical size of the contact point and adjacent center-to-center distance that a test probes can hit on the PCBA. For each unique PCBA type to be tested, a different BON fixture is needed, adding cost and time delays to the ICT approach. Additionally there is an ever growing number of PCBAs that are untestable with BON testers because of little or no test probe access on the nets, or because possible test points are too close to each other.
Timing is also critical with the ICT approach, Investment in a BON fixture too early could result in having to make expensive and time consuming modifications to the BON if engineering changes to the board design renders the original fixture useless. Investment in a BON fixture too late may mean the fixture is hardly used before production ends. Typical cost for ICT fixtures are $5K to $20K per PCBA part number. And, it is not unusual for a test fixture to cost $60K+ for a large PCBA.3. Flying Scorpion Test Systems Technology Advancements
Increased Number of Moving Probes – The most basic improvement in the Flying Scorpion has been the increase in the number of flying probes from 4 on a single sided system, or 8 on a double sided system, to as many as 22 probes. Double-sided moving probe systems have provided the single most dramatic improvement in moving probe evolution. An increased number of moving probes on both sides of the PCBA reduces the test time and increases the test (fault) coverage. A single test program can now test all of the PCBA components in a single pass. Test time on flying probe systems is also dependent on the time required to move the probes from one position to the next – the more probes, the shorter the test time.
High Speed Probe Mechanics – To decrease total test time even further, the motors that control the up and down movement of the probes have been updated on the Flying Scorpion with the latest technologies, resulting in a 50% decrease in the mechanical movement time.
Probe Placement Accuracy – A dual goal of the new mechanics was improved accuracy and repeatable of probe placement. The Flying Scorpion can reliably hit 4 mil pads 99.99% of the time as a result of the integrated design of the fiducial cameras, shuttles, and probe system.
Fixed Channels – Analysis of any moving probe test program will show that a few nets, such as power and ground, are contacted hundreds, if not thousands, of times during the execution of a single PCBA test program. This means that a considerable amount of time is lost contacting these few nets in a repetitive manner. The addition of Fixed Channels (non-flying) contacts to access these nets can significantly reduce the total test time.
Boundary Scan – This proven technology has been around since the 1980s, providing test coverage as a standalone, or ICT, test solution. The Flying Scorpion incorporates a boundary scan controller in conjunction with its flying probes to increase fault coverage and dramatically reduce test time.
Depending on the number of components on the PCBA that can be tested with Boundary Scan, the PCBA test time may be reduced by 50%, or more. When the Flying Scorpion/Boundary Scan solution is used, the removal of redundant tests is automated.
Event-Driven Testing – The Flying Scorpion can be incorporated directly into a production line, with a beat rate in the tens of seconds, by the use of “Event-Driven” software. This test mode provides a complete test of all components on the PCBA to begin. After a sufficient number of PCBAs have been tested, the test mode is switched to test only the devices whose setup has been changed in the manufacturing line. For example, if a feeder has been changed for a resistor, then testing of that resistor on all PCBAs would be enabled to verify the feeder change is correct. “Event-Driven” testing then allows the ICT test for the subject resistor to be turned off. This assumes that components that have been verified and components without any changes will continue to pass after the initial testing is performed — again, reducing test times to near ICT speeds.
Mini-Flying Bed-of Nails – Another complimentary Flying Scorpion approach is the patented Mini-Flying BON fixture. With up to 128 unmuxed test cannels, this miniature version of an ICT test fixture can be placed anywhere on the PCBA. The MiniFlying fixture allows for very large reductions in test time and, therefore, higher speed production volume testing. This kind of increase in productivity can otherwise only be achieved by ICT, but at a fraction of the cost and implementation time of traditional ICT. For some smaller products that are built in panelized format, the Flying BON has provided 100% of the required test coverage.
Programmable Probe Angles – All probes angles on the Flying Scorpion are not fixed, but programmable from 0º to ±6º, in all directions, dramatically increasing test access and, therefore, fault coverage.
Application Specific Probe Tips – Most flying probers can only use a sharp needle-like probe tip when contacting a PCBA under test. The needle small tip size enables probing of very small test points that are physically located close together. For production testing, different probe tips that compensate for different products and manufacturing methods may be a more robust choice. Flying Scorpion’s probe capability of up to 22 flying probes enables the use of application specific probe tips such as chisel, rounded or even flat that can be mixed in any combination for a custom solution. The use of these different probe tips provides for testing similar to an ICT system.In its new role as a production test solution, the Flying Scorpion test system is an essential part of any PCBA test strategy, serving both prototype and production volume test requirements, providing the maximum flexibility for electronics manufacturers. The Acculogic Flying Scorpion test system has quickly become a tool that manufacturers of all types cannot live without.