Flying Probe and Boundary Scan Testers Unite

Using the combined ATE strategy on a particular PCB unit-under-test (UUT) (see Figure 1), the Test Centre achieved complete diagnosis and fault identification on 98% of the volume of test throughput. In addition, the UUT includes some programmable devices, so the on-board programming facility of the boundary-scan tester is an added advantage.

As a further bonus, Test Centre engineers also raise fault coverage by using the boundary-scan tester’s programming feature to simulate hardware failures. They achieve this by programming temporary faults into special data registers implanted in the board’s ASIC/FPGA hardware. Not only does this technique extend general fault coverage, it also helps to identify even more faults to component or board level.

Board Includes Non-Scan ICs
The UUT in this application is a control unit PC plug-in card for data exchange between two interfaces. The board includes three boundary-scan ICs (see Figure 2). The microprocessor IC includes boundary scan as a standard device, but the dual port controller IC and interface IC are FPGAs and have boundary scan specially designed-in. The local bus controller IC and several bus driver ICs are non-boundary-scan ICs.

Figure 2. Combined flying probe and boundary scan ATEs communicate via RS-232C.

Although boundary-scan implementation seems fairly high at first sight, in fact the resulting testability using boundary scan alone is quite low. The problem arises in this design because the location of the non-boundary-scan ICs highly isolates the boundary-scan ICs. Table 1 shows a detailed distribution of the full-, partial, and non-boundary-scan devices. In total, the boundary-scan chain includes 2278 scan cells.

  ATEs Link Via RS-232C
The combined ATEs in this application consists of a Takay APT 8400CE flying probe ATE and Göpel Electronic boundary-scan ATE. The combination results from an earlier co-operation between the two companies (Ref. 1). Due to the closed software environment of this flying probe ATE, the boundary scan tester uses a separate PC. This PC connects to the host of the flying probe ATE via an RS-232C line. The two-part solution allows parallel programming via the boundary-scan ATE, and testing of other UUT parts by the flying probe ATE.

Developing the Test Strategy
Siemens Test Centre engineers developed their test strategy based on a statistical analysis of failure data gathered over six months of production using several forms of test on a variety of PCBs in a range of conditions. Applying these failure probabilities to the UUT in this application predicts that 18% of board throughput will have at least one fault. Now using the combined ATEs, the overall aim was to precisely identify the majority of these faults and limit remaining undiagnosed failures to 2% of boards. (The statistics indicate that 2% of faulty throughput will contain dynamic or temperature-related faults, which are beyond the scope of these testers.)

AOI & BIST Play a Part
Further study of the statistical failures indicates that the optimal test strategy for this UUT is a mixture of optical inspection, boundary scan, flying probe ATE and built-in self test (BIST). Table 2 shows the individual test steps. In arriving at this test sequence, you need to consider several points.

For example, boundary scan can detect and diagnose pin-level faults only if a direct connection exists between at least one scannable driver and a scannable sensor. Also, minimising repeat checks, especially avoiding double checks at pin level, allows you to reduce the number of flying probe test steps. In this way, you can restrict the flying probe ATE to parametric tests and coverage of non-scannable pins.

To test isolated boundary scan pins you can use the flying probes as virtual boundary scan pins. In this way, the test temporarily transforms isolated pins into fully testable boundary scan pins. Standard in-circuit testers with parallel pin electronics and bed-of-nails contacts already employ this principle. The advantage here is higher boundary-scan test speed and higher fault coverage compared to normal flying probe capability.

Because the UUT in this application includes a microprocessor IC and programmable devices, the strategy can include BIST. The system loads the self-test into the programmable parts via boundary scan. Boundary scan can also read out test results at the completion of the overall test.

Table 3 shows the final sequence of individual test steps and the time taken at each step. For comparison, the table includes a similar test sequence using only a flying probe ATE (with an opens check option). You can see that the combination ATEs execute more tests in less time and add in-circuit programming and BIST.

Software Has Flying Probe Tool
Although the combined ATE system controls only four probes in this application, the CASCON-GALAXY software can control up to 20 probes simultaneously. Data links via DDE or DLL on the host, or standard interfaces of two interconnected PCs allow coupling to other flying probe ATE models. To incorporate flying probe control the standard ATPG tool suite includes a flying probe tool and a corresponding fault diagnosis processor.

The software allows free batch run definition, which means each user can define individual test sequences. The software also provides testability analyses for different filter functions on nets that only the flying probe ATE can test.

In practice, test times reduce roughly linearly in proportion to the depth of boundary scan implementation. Around 30% of boundary scan devices can achieve a test time reduction of 50% in some applications, although much depends on an individual application.

The individual test steps, except for the built-in self-test, were generated almost entirely automatically by using CASCON-GALAXY boundary-scan software or Fabmaster computer-aided-manufacturing software.

Looking Ahead
Siemens engineers are currently looking at ways to improve the system by using just one PC and coupling the individual ATEs via a software link based on a multi-tasking operating system. For example, one method is to couple the APT8400 flying probe ATE via a control bus, (such as IEEE-488 or VXIbus), with the control unit. Using the same UUT, you can connect external devices, such as DMMs, scopes, and signal generators directly to the probes or via a connector. In this way, you can increase the value of the flying probe tester and make testing more efficient and economic. This coupling is also an effective and reasonable solution for users planning to shift flying prober tester usage from prototype test to manufacturing test.

Tamás Marosvölgyi works with Siemens Competence Centre Test Engineering, Electronics Technical Services, Munich. Herbert Tietze is product manager for flying probe testers at Itochu SysTech in Düsseldorf. Thomas Wenzel is a founder of Göpel Electronic and has been manager for Boundary Scan Test Systems since 1991.

References
1. Itochu SysTech, “Product Information Flying Prober with Boundary Scan”, 1998.

2. Siemens Test Engineering Competence Centre, “Mit Test zum Erfolg aus elektro AUTOMATION”, 8/98.