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The term bed of nails is a rather graphic description of what many fixtures look like, having a large number of test points or probes proud of a board that holds them in place.
Although the concept of the in-circuit test fixture or bed of nails is broadly the same whatever manufacturer is used, there are a number of variations on the basic theme.
In circuit test fixture basics
The In-circuit test fixture is required to interface the main tester with the particular board under test. It will have a main connector that interfaces to the tester and wires that are taken from the connector to individual pins / probes / or "nails" that make contact with the required nodes on the board under test.
The probes are held in place by what may be termed a base-board. This is precision drilled to ensure that the probes are held in exactly the right place for the fixture to make contact with the required nodes on the board.
The board is held in place accurately by the fixture and pulled onto spring loaded pins that make contact with connections on the board. The board may either be pulled down under the action of a vacuum or it may be achieved mechanically.
At one time when board component densities were much lower it was often possible to place special ATE pads onto the board to enable good connection to be made. Nowadays with very much more compact boards this is not possible. Instead connections are made onto the component pads. This is obviously more difficult because of the solder and the component connection itself, but can still be achieved to a high degree of reliability. Typically each spring exerts a force of between 100 and 200g to ensure that good contact is made. This obviously means that the total force required for all the pins on a board can be very significant. Sometimes supports for the board are required to ensure that it does not flex too much as this may result in cracking some delicate surface mount components.
Typically pins are placed on a 0.1 inch matrix. Many new surface mount IC packages require a much finer pitch, and to achieve this an adapter is often used.
There are about three main methods for pulling the board onto the probes:
- Vacuum: This form of fixture uses a vacuum to pull the board down onto the pins. It has the advantage that as the vacuum exists over the whole area of the board, the board is evenly pulled down onto the pins, but it does require any holes in the board to be sealed before the in-circuit test stage of the production process.
- Pneumatic: This form of fixture uses a compressed air source, present in most manufacturing areas to be used.
- Mechanical: This uses a simple lever or other mechanical arrangement to pull the board down onto the pins.
Wireless In-circuit test fixtures
Another form of ICT fixture is known as a wireless fixture. This does not mean that it uses wireless / radio communications, but instead the fixture does not use traditional wires but it uses a printed circuit board. This provides a number of advantages:
- Reduces complexity of fixture: Most ICT fixtures require many wires to run between the individual probes and the fixture connector that interfaces to the main ICT system connector. There can often be several hundred wires, making the fixture very complicated and difficult to work upon.
- Reduces spurious resistance The wires within an ICT fixture need to be long enough to allow the fixture to be opened to enable proper access. The length of the wires can introduce a significant amount of resistance that can reduce the overall measurement accuracy of the system. Using a wireless ICT fixture enables the track lengths to be shortened and resistance level decreased.
- Improves reliability: the large number of wires in an ICT fixture introduces a means of failure. Wires can easily break and become disconnected. The use of a wireless fixture, using PCB technology significantly improves the level of reliability.
- Reduces fixture cost: Using modern software, it is possible to reduce the cost of the fixture production by using a PCB. Using automatic routing of the tracks in the PCB layout software means that PCB design is automated to a large degree. This means that the complex wiring is removed from the fixture production process.
Test pins / probes for in-circuit test fixtures
There is a great variety of different types of pin or test probe that can be used for in-circuit test fixtures.
The in -circuit test probes or test pins or probes are spring loaded and comprise a barrel with its internal spring, and the plunger. The test probes fit into a socket that enables them to be replaced when they become worn of damaged.
The major design changes are within the head or tip that contacts the board under test. Each type of head has a particular application for which it is best suited.
- Concave tips: These in-circuit test probes are often used for connecting on to terminal posts.
- Spherical radius convex tips: These may be used when mating with an edge connection on a printed circuit board.
- Cone tips: This format for an in circuit test probe is often used for mating with a PCB via hole, or directly onto a PCB track..
- Single pointed tips: These are often used for mating with solder joints as the tip is able to penetrate the oxide film on the solder to make good contact.
- Multi-pointed tips : These may be used when a connection is required through a larger area of solder - the multiple serrations mean that several points of contact can be made through the oxide layer on the solder. They may also be used to mate with the connection to a conventional component, i.e. not surface mount. The probe serrations will connect to the solder and wire that protrudes through the board.
The wiring in the fixtures is generally not neatly loomed together. Whilst this may not be as aesthetically pleasing, it reduces the levels of cross-talk and spurious capacitance. It also reduces the wire lengths within the fixture as the shortest route between two points can be taken within reason.