The EMPF manufacturing facility has recently added a flying probe tester to its extensive range of manufacturing equipment. The tester is intended for optimization of production yield for printed circuit boards (PCBs) after assembly. During incoming and outgoing inspection, the Seica model S40 Pilot (or “Wing,” outside the U.S.) Flying Probe Tester allows for the detection of misplaced, missing, and incorrect items and incorrect value parts on a board (Figures 2-1 and 2-2).

It takes approximately two days of training to learn how to program and operate the Pilot flying probe tester. The user supplies the CAD file data, from which part reference designations (“Ref Des”) and the position (x,y coordinates) of each board pad or part lead are extracted. Providing a known good or “gold” master board, not only helps to streamline the program debug process but also allows the user to implement vectorless test techniques to test ICs (isolinear chips) and BGA components where access is limited. Although the flying probe tester only requires the CAD file, debugging against a master board will provide adjusted file information which can prevent false failure reports.

The Seica Automatic Program Generator (APG) utility generates the shorts test list, creates the test program, and automatically assigns the appropriate test macro (i.e., #RES macro to test a resistor, #ZEN to test a zener diode, etc.). The user can then run a set of tools to adjust for parallel components, insert guarding test points to achieve maximum test coverage, and optimization routines to minimize probe movements and thus shorten test time (increasing throughput).

Test program debug time varies depending on board size and complexity; however, through a set of tools including an autodebugger and on-line schematic viewer, the average size/complexity board can be completed within a day.

Seica’s Flying Probe Tester model S40 accepts CAD data from over 35 PCB design packages. The control of the equipment is via personal computer (PC) at the user’s console. Commands are provided to the motion controller, which moves the rails, the four probes, and camera, and positions the board during reference setups.

The Seica software algorithms are stored in libraries and are automatically assigned during the APG program generation. Both “User” and “System” libraries are included in the system software. The “System” libraries are fixed, but the “User” libraries are exposed to the programmer, allowing development of specific customer libraries to be stored for future applications without any re-development time. Other user-definable options, such as refinement of probing technique via several probe tip styles, speed/pressure of the probe, and minimization of “fly height” (probe elevation during transit) allow optimization and reductions in test time.

The automated assembly of PCBs can lead to yield issues due to parts dropping off during wave soldering or shifting while inside solder reflow ovens. Parts may also be assembled incorrectly with respect to orientation – an issue with both automated and hand assembly. An incorrect reel of parts may be placed on the pick and place machine for automated insertion. The potential for such mistakes increases as package size decreases and visual identification of part markings becomes impractical. There is a considerable potential for cost savings as a result of inspection at the assembled board level, before the individual board proceeds further into the end-unit assembly process.

The flying probe tester is primarily intended for use with assembled boards rather than bare boards or reverse engineering (Note: Some level of bare board testing can be performed on the Pilot Flying Probe by enabling continuity testing from the APG program generation; however, access to all traces may not be available. Seica’s S24 BBT offers the full suite of bare board testing capabilities). Values of inserted parts are measured in order to determine whether a correct and undamaged part has been installed correctly and properly soldered.

The EMPF has used the flying probe tester to assist a customer who requested assistance with and recommendations for performing quality testing of bare ceramic hybrid PCBs. The customer wanted to verify the integrity of ceramic substrates for shorts, opens, and node continuity before populating with costly components. The customer considered a “bed of nails” type test fixture, but the quantity of boards for each particular design was limited; therefore, modifying the fixture for each new prototype ceramic board layout would be inefficient. The EMPF used the flying probe tester to evaluate the boards. The programmability and versatility of the flying probe machine was a logical solution for the low volume and frequent product changes and revisions encountered. Being fixture-less and grid-less, no constraints were placed on the layout of the design, component leads, connector spacing, or test nodes.

The flying probe system can be used to test fine and ultra fine pitch boards. The accuracy of the linear motor positioning system enables the flying probe to test points as small as 4 mils (0.004 inches) with pad pitch densities of 12 mils (0.012 inches). The flying probe system is suitable for both high volume production and prototype quantities.

The flying probe tester allows for verification of three levels of manufacturing inspection: 1) copper trace errors that occur at board fabrication; 2) solder-paste bridging and solderability/cleanliness issues at pre-assembly; and 3) incorrect part, incorrect part orientation, and part failure at assembly.

For more information regarding how to implement flying probe testing into your assembly process, contact the EMPF Helpline at (610) 362-1320.