Prodrive, a European company based in the Netherlands, has traditionally used a test strategy that combines in-line automated inspection and functional test. However, for sometime there has been some concerns about the functional test approach, due to its inherent complexity, cost and poor maintainability.
These concerns came to a head with a single board robotics controller that challenged the functional test approach. The board is large, greater than A3 in size, and combines digital processing with a full four-axis motion controller. The digital processing section incorporates high-speed processors, FPGAs and DSP. The motion controller section incorporates high power motor control circuits. With 14 separate edge connectors to connect, just fixturing to the board under test during functional test was challenging. Functional test times were excessive and even with Built-in-Test that included Boundary Scan facilities the fault coverage was less than required.
The company decided to investigate In-Circuit Test as an alternative electrical test platform. The problem was in providing sufficient access to the board.
The provision of test pads within a PCB layout imposes an overhead. To ensure reliable connection the test pads have to be at least 0.89mm in diameter, which equates to an area of around 0.62 sq mm. In itself this is a small area, however, test pads can only be located in the free space of the board and there also has to be a keepout area to ensure that the test probes contact with the test pad and not to adjacent components. This means that the area available for test pads is much less than the total area of the board.
Test pads can also have a detrimental effect on the performance of the board. This is particularly true of high-speed circuits. The addition of a test pad on a high-speed trace can impact the performance of the circuit dramatically to the point where they can simply not be included. This is particularly unfortunate since it is in these areas of the board where additional access is often required for good fault coverage.
Agilent Technologies has pioneered a new probing approach called Bead Probe that inverts the probing paradigm. Rather than use a large test pad on the PCB and a sharp edged probe, the approach is to use a small test ‘probe’ on the PCB and a flat head target in the fixture. The bead probe is a solder deposit on a PCB trace (Figs 1 and 2) that is the same width as the trace but with a given length (typically 3 to 5 times the width of the trace) and height (typically 4 mils). This approach consumes no extra surface area on the board and does not degrade the signal path.
A major advantage of the bead probe approach to Prodrive was that it could be applied retrospectively to the existing layout. This avoided a complete re-design of the PCB, which would have required a reiteration of the approval and sign-off procedure with the client. In its implementation Prodrive had to consider three aspects – the solder process, the CAD implementation and the test fixture.
