In Circuit Testers

- notes and details about in-circuit testers or in-circuit test systems detailing key operational points and specifications.

ATE, Automatic Test Equipment tutorial includes:

    •  ICT in-circuit test
    •  In-circuit testers / systems
    •  ICT fixtures & fixturing
    •  ICT programme generation
    •  Design for ICT, DFT

There are many elements to an in circuit tester. They utilise a number of elements to enable them to run the test programme and access the required points on the circuit.

In addition to this in-circuit testers utilise a number of techniques to enable them to be able to access components and measure their values in the presence of other paths within the circuit.

Driver-sensors for ICT

Driver-sensors are the active circuits that are used for making the measurements. Normally drivers and sensors are always present in pairs in an in-circuit test system. As the name suggests the drivers supply a voltage or current to enable a node in the circuit to be driven to a particular state. They normally have a reasonably high capability to enable the node to be driven to the required state despite the condition of the surrounding circuitry. Typically they may need to force the output of a digital IC to a given state despite the natural output state of the device. To achieve this the output impedance of the driver must be very low.

Sensors are used to make the measurements. Like most other measuring devices these need to have a high impedance so that they do not disturb the circuit being measured.

Guarding

The key to the success of in-circuit testing is a technique known as guarding. It is very easy to measure the value of a component when it is not in a circuit. For example a resistor value can be measured by simply placing an ohmmeter across it. However when the component is in a circuit, the situation is somewhat different. Here it is most likely that there are other paths around the component that will alter the value that is measured.

To overcome this problem and gain a far more accurate indication of the value of the component a technique known as guarding is used. Here the nodes around the component under test are earthed and in this way any leakage paths are removed and more accurate measurements made.

Multiplexing

Today's printed circuit boards can be very complicated. On larger boards the node count can easily rise over a thousand and may reach several thousand on some. To have dedicated pins on the tester for each node can be very costly as each one requires its own driver sensor. To reduce this manufacturers introduce a system known as multiplexing. Here a particular node may be placed through a switching matrix so that it can address more than one node. The number of nodes that are addressed by each tester primary node is known as the multiplex ratio.

Whilst it may appear to be an excellent idea to reduce costs, it reduces the flexibility of the tester. Only one of the multiplexed nodes can be accessed at any time. This can cause restrictions in the programming and also in the fixture itself. Considerable thought has to be given to the fixture construction to ensure that two pins on the same multiplex are not required at the same time. It may also cause problems if the pins are allocated automatically by software that generates the test programme and fixture wiring diagram.

When buying a machine it is worth checking whether multiplexing is used and what the ratio is. With this information a judgement can be made of the cost saving against the reduction in flexibility.

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