Bead probe implementation on a large complex PCB

21 April 2008

Fig 1:  Diagram of bead probe

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. Elfried Keers reports

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.

Fig 2:  Image of bead probes

A bead probe is manufactured using the same steps as all other solder features. The solder mask is opened up over the trace where a bead is required. Then a new aperture in the paste stencil is opened over this solder mask hole. This aperture is deliberately oversized slightly to precisely control the amount of metal that forms a bead. Solder paste for beads is applied at the same time as all other paste features are applied. During reflow, solder flows and is drawn to the copper trace. Surface tension causes the bead to have a curved surface and rise above the solder mask, where it solidifies into a Bead Probe.

The height of the bead is controlled by two factors - the volume of the solder deposited and the solder stencil aperture. The solder mask hole is formed as a rectangle with rounded ends (‘obround’) with its width related to the width of the trace. The length runs in the same direction as the trace. The solder stencil hole is square, rotated 45 degrees to the trace and centred on the bead location. Fig 3 shows the board, solder mask and solder stencil stack-up for a bead probe.

The bead probes have to be added to the layout of the board at the CAD stage. Bead probes are built as library elements to be placed either on the top or bottom of the board. They are similar to surface via’s in their characteristics, however they differ in an important aspect. Via’s are round so one library element can be used for all instances since there is no requirement to rotate them.

The bead probe in comparison is ‘obround’, and has a specified direction consistent with the direction of the trace. Library elements for standard bead probes had already been developed within Prodrive’s CAD system so they could be applied directly to the board.

When incorporating bead probe elements it is also essential to include ‘keepout areas’ that take account of the geometry of the flat head probe to be used. The aim is to ensure that the position of the bead probe does not include component bodies or device leads within the area. This avoids the possibility of the flat head probe hitting these higher profile elements rather than the bead probe. Once these precautions are taken the addition of bead probes to a CAD layout is as straightforward as adding any other library element.

Fig 3:  Board, mask, stencil stack-up

Since the concept of bead probing is no different from conventional bed-of-nails fixturing then the same design and production techniques can be used. There is a complication in that in most cases boards will use a mix of bead probes and conventional test pads. However, ultimately there is no reason why an entire board cannot be implemented with bead probes, which would eliminate this. In Prodrive’s case Agilent constructed the bed of nails fixture using a combination of conventional and large flat head probes.

Prodrive was particularly impressed with the reliability of the fixturing. Debug of the test program and fixture was conducted on a limited number of sample boards. As a result each of these was subjected to many activations of the fixture. The cycle counter recorded around 600 fixture activations in total, which implied 100 to 200 fixtures cycles per board. Throughout this the connectivity between the fixture and the board under test was maintained.

The test strategy for the Prodrive’s robotics board no longer uses end-of-line functional testing. All structural and electrical testing takes place in-line using Agilent automated inspection and In-Circuit Test systems. Environmental and functional testing continues to take place at sub-assembly stage.

The effect on test time has been significant. The functional test of the controller board was taking 15 minutes. The In-Circuit Test takes 90 seconds. As a result of this, over the next 6 to 12 months Prodrive anticipates that a further 60 boards will use its new In-Circuit Test philosophy and utilise Bead Probes for improved access.

Ref 1. K. P. Parker and D. DeMille, “A Bead Probe CAD Strategy for In-Circuit Test”, Proceedings, International Test Conference, Paper 18.2, Santa Clara CA, Oct 2007

Elfried Keers works as the European Market Development Manager at Agilent Technologies focusing on Manufacturing Test and Inspection Systems as well as Functional Test Systems for the Automotive and Wireless Markets.

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