In the world of automatic test equipment, there are two competing technologies to achieve board test: a bed of nails (BON) approach and the flying probe. Regardless of the approach, board testing itself is a key requirement for OEMs and a clear competitive advantage for contract manufacturers. For OEMs, board testing at both the component and system level is critical to assure the performance and reliability of the circuit board. If there are problems with a production build, catching and correcting these errors early saves time and money.
Contract manufacturers who possess board testing capability can offer a superior value package to their customers. This added capability helps enormously for prototype development by decreasing the time to market. First to market is often a decisive advantage. Many companies outsource all their manufacturing tasks to contractors, and if these contractors have the ability to both build and test circuit boards, the design and production can be confirmed in days rather than weeks and months.
In essence, the mechanical method of how the circuit board is presented and secured in either of the two types of electrical test machines defines the strengths and weaknesses of the two electrical test approaches.
With a BON test fixture, the circuit board is placed in a vacuum fixture or some similar device. The vacuum is turned on, the circuit board is pulled onto the conductors (or nails), and electrical contact is made simultaneously to all the test points (Figure 3-1). Input signals are sourced to the components through the conductors. Other conductors then read output signals to confirm the placement, identity, connection integrity and performance of the components and the electrical system. Usually, each circuit board requires a unique fixture.
With the flying probe test machine there is no fixture required. Circuit boards are placed and fastened in the flying probe machine. The flying probes are under computer control and move from component to component, checking electrical performance of the chips and the system. The probes are moved with three-axis, high powered linear motors to achieve electrical contact with the component under test (Figure 3-2). Electrical signals are sourced into the board with computer controlled flying probes and the resultant output signals are read with other flying probes. With the probes acceleration at seven “Gs” (224 feet per second squared), the term “flying” probe is no exaggeration. The machine then moves the probes to other test points on the board and the process is repeated. Locating the components on the board is performed with a vision system, and the machine identifies the location of the test points based on the board fiducials or other reliable locating point on the circuit board. Flying probe test machines have four or more moving electrical probes. Probes are built into the machine to access both the top and bottom of the board. The probes move extremely fast and can make up to 15 high speed tests per second. As with the BON, the probes make contact to the board to source signals to the components and read output signals.
The cost of a comprehensive test program centered on BON fixtures revolves around the base machine plus the number of different fixtures. Each unique circuit board requires a separate fixture and each modification of the circuit board usually requires modifying the matching fixture. Delivery time to purchase a BON fixture usually runs about two to four weeks with prices from $5,000 to $10,000.
For prototype work, the delivery time associated with acquiring the fixtures can be a problem. Large size fixtures can be especially expensive and difficult to acquire promptly. Test points are a necessity for a BON vacuum fixture. On high density boards with very small components (e.g., 0402s) bunched close together, circuit boards must have enough room for the pins on the bed of nails fixture to make contact with the pads on the boards.
The BON fixture requires test points or test pads on the circuit boards to assure a good electrical contact. Special attention must be given to board layout to assure access to the components and a comprehensive test program. Also for BON testing, these test pads should be flat and parallel to the circuit boards. This is required since the conductor nails are
stationary and the circuit board is moved vertically to make electrical contact. If the conductors are forced to make contact on an angled surface, the conductor can be deformed and the electrical contact will be intermittent and unreliable.
A key characteristic of the flying probe machines is that they can achieve reliable electrical contact on non-flat surfaces and can target very small areas with good repeatability. The probes themselves are under direct machine control and are constructed on an angle. The probe point has a targeting accuracy of +/-3 mils (.003"), so the probe point can be programmed to land on the leg of a component or any other point on the circuit board.
The cost of the two approaches (flying probe versus fixed bed of nails) can differ widely. If a manufacturer has a single modest complexity board with low to medium volumes of production, the start-up cost for the bed of nails is far less than a flying probe machine. If a company has large production runs and a low mix of boards, the bed of nails would be the most cost effective approach. If board throughput is the decisive issue, the bed of nails has a performance advantage since all the test points make contact immediately.
The BON testing approach is less favorable when there are short runs of individual products but high overall volume of these products. Rapid changeover favors the flying probe because a fixture is not required and all the required component set-up information is provided in the CAD (computer aided design) data.
It is not uncommon for OEMs to outsource both circuit board layout and construction while controlling the design tasks themselves. This avoids the cost of acquiring manufacturing capabilities. The contract assembly house that has the ability to offer testing and documentation of the performance of the customer’s circuit board can provide the necessary assurance of a quality build.
To summarize, where the project requires dedicated testing of large quantities of circuit boards and board throughput is the decisive requirement, a test strategy utilizing a BON may clearly be the most compelling. The initial cost is reasonable, and test coverage can be complete assuming the test points are designed into the board. Also, where low initial cost is critical a BON will be the best choice.
The flying probe machines excel in a test environment where there is a high mix and low quantities of different circuit boards to test and where turnaround time does not allow for building a dedicated fixture. The flying probe is also a superior choice where several revisions of a design are expected and the time between prototype designs must be held to a minimum. The initial cost of a flying probe machine is substantially higher than the initial cost of a base machine using the BON approach. Due to the exceptional flexibility and speed of the flying probe, it may be the more cost effective approach; however, the cost of the machine is higher than a BON based machine.
A state of the art SPEA Flying Probe machine with four test probes and the capability of performing up to 15 tests per second is available for demonstration at the EMPF factory facility. Please contact Ken Friedman at 610.362.1200, extension 279 or via email at kfriedman@aciusa.org.
