Double-sided Flying Probe Solves PCB Test Challenges

Estimated reading time: 5 minutes

By Ray Rattey, Texmac Inc.

Flying probe technology has been in use for over 20 years in the PCB fabrication and assembly industries. Initially used by PCB fabricators to verify bare boards by performing simple shorts and opens testing, flying probe technology eventually spread to PCB assembly houses as an alternative to in-circuit test (ICT). The additional costs associated with ICT fixturing make flying probe test an attractive alternative when used in low-volume/high-mix manufacturing environments.

The first flying probe test systems used two moving probes and provided access to only one side of the PCB. For modern test, flying probe systems evolved into highly sophisticated testers providing much of the same test capably as in-circuit testers, with moving probes capable of simultaneously contacting both sides of the PCB. This allows for increased test throughput as well as greater test capability.

A Challenging PCB DesignPCBs with unconventional designs can pose major testing challenges. Existing single-sided probers may not be capable of addressing some of these test issues. New generation double-sided flying probers can provide solutions to these challenges.

A typical multilayer PCB comprises top- and bottom-side copper, power and ground layers, and internal interconnect layers (Figure 1). Plated via holes provide connections between layers. When developing a test program for in-circuit or flying probe test, software tools evaluate the CAD circuit design files to determine the interconnections between devices. The software evaluates the top and bottom layers to determine the optimal probe contact points (Figure 1). In the case of ICT, these locations are often predetermined by the PCB designer and designated as test points. When test points have not been included in the design, the flying prober has the ability to electrically access other features such as vias or component pads.

In one example, the PCB design consisted of a top copper layer, internal power and ground layers, and internal signal layers. No bottom copper layer was present in the design. What would ultimately become the bottom side of the final assembly was actually contained in the top-side CAD layer. As shown in Figure 2, the final bottom side of the assembly is laid out on the left side of the top copper layer, and the final top side of the assembly is laid out on the right side of the top copper layer. During the fabrication process, the PCB is cut in half and the two halves are laminated onto an insulating center core, producing a final PCB assembly. A flex circuit provides the interconnections between the top and bottom sides of the assembly.

Developing the Flying Probe Test ProgramA typical test development process for flying probe test starts with importing design files (CAD and BOM). The design is analyzed to determine optimal probe contact locations. An automatic test generator (ATG) creates a test sequence that ensures maximum test coverage. The user then debugs the test program.

In this application, test development followed the above steps. The resultant test program, which contains the test sequence as well as the coordinate (X/Y) data that instructs the moving probes where to contact the PCB, was then loaded into the flying prober.*

Typically, at this point in the process, the test engineer would proceed to debug the test program. Due to the unconventional design of this PCB assembly, the process was modified to deal with a final assembly that did not match the CAD data. The CAD data indicated that all components and probing would be from the top side of the assembly; in reality, the final assembly contained both top and bottom sides and would have to be probed from both sides.

From Single-sided CAD to Double-sided TestThe double-sided prober addresses these test issues with two test programs, one for the top and one for the bottom side.

To test the top side of the assembly, the test program contained the entire test sequence for testing the PCB as single top-side assembly, just as described in the CAD data. The flying prober was then instructed to avoid contact to all the test points that were associated with the bottom side of the assembly (Figure 1, PCB left side). Once these test points were defined as "no-contact," any test associated with those test points was set to not be executed in the test program. This new program was then saved as a top-side program.

We then repeated this process for the bottom side of the assembly, defining all test points associated with the top side (Figure 1, PCB right side) as non-contact points. Any tests associated with the these test points were not executed. This program was then saved as the bottom-side program.

The final step was to take the two programs and merge them into a single dual-sided test program. Once the two programs were merged, the tester performed additional circuit analysis on the CAD design to ensure maximum test coverage. Any test that was once untestable could now be included into the final program. Of particular concern was the ability to verify connections between the top and bottom sides of the assembly, through the flex circuit. Because of the dual-sided test capability, those additional tests were easily added to the program.

ConclusionBy making use of the flying prober's dual-sided capability, we were able to create a unique test solution that would have not previously been possible with single-sided flying probers. A similar solution could have been possible using conventional in-circuit testers, but the cost of creating a bed-of-nails fixture for this application would have exceeded the budget for this project. Using previous-generation single-sided flying probers typically meant that the PCB would have to undergo two independent test sequences (top and bottom sides) to completely pass testing. This increases handling and overall test times.

By contrast, the new-generation double-sided flying probe system can now test a complete assembly in a single test program, decreasing overall test times, increasing throughput, and improving overall test coverage. Because of the ability to access both the top and bottom side of the PCB simultaneously, double-sided systems provide cost-effective testing, as well as the ability to provide unique test solutions not possible using other methods.

*The test prober used in this application is Takaya APT-9600.

Ray Rattey, senior applications engineer, Texmac Inc., may be contacted at Texmac Inc., 20 Main Street, Acton, MA 01720; (978) 929-2937; rrattey@texmacusa.com.