In-circuit test
The In-Circuit-Test ( ICT ) is a test procedure in electronics production to prove the correct function of electronic assemblies. In ICT, the focus is on testing the components and electrical connections on an assembled circuit board . Each individual circuit board is checked for faults in the conductor path (such as short circuits or interruptions), soldering faults and component faults with a special test adapter. Entire circuit blocks ( clusters ) can also be tested.
The ICT test system can measure analog parameters of components ( resistance , capacitance , inductance , etc.) using various measurement methods (such as two-wire, four-wire measurement, etc.) and thus detect incorrectly assembled or defective components. To test digital components, defined test signals can be fed in and their effects compared with previously defined samples.
If an ICT test system is limited to the measurement of analog components, it is also referred to as an MDA ( manufacturing defect analysis ).
Test strategy
The test of circuit boards is usually carried out directly after the manufacture of the same or immediately before the circuit board is assembled. This is usually a Go / NoGo test in which the faulty circuit boards are sorted out.
The IC test of assembled assemblies can be carried out directly after the last assembly and soldering step, even before the assembly is subjected to a functional test for the first time or the assembly is connected to the operating voltage. A Go-NoGo test is also carried out during the IC test of the assembled modules, whereby the errors can be displayed in the case of non-functional modules. In the case of the assembly, these errors, e.g. B. component faults, missing soldering point or solder bridge between two neighboring networks, ... can be repaired. The assembly is then checked again to prove that the repair was successful.
The situation is different with the functional test of an assembly, in which the overall or partial function of the circuit is in the foreground and less the measurement of individual component values. If a function test is integrated in the ICT, the focus is often only on a certain partial function of the overall circuit.
Sometimes the components of the assembly are also programmed or Boundary Scan is used as part of the ICT test.
Adaptation of the assembly
Feeding or handling of the assemblies
The feeding of the circuit boards or assemblies to the test system can be done in different ways:
- manual, is particularly useful for long test times or smaller series
- Feeding from the magazine, for stand-alone machines
- Feeding from trays, for stand-alone machines
- Inline system, especially for larger series with linked process steps
- Feeding into workpiece carriers, which can be delivered from magazines or from an inline system.
Magazine and tray handling can either be integrated into the machine or implemented with an attached handling system.
Contacting the electrical networks
The electrical networks are contacted by an adapter. Special spring-loaded test pins (also known as test needles) with various head shapes are used for contacting. These meet certain solder mask-free areas on the board, the so-called test points. The contacting unit can be constructed with a wide variety of adapters and types of contact. The contact is often made with the support of a vacuum or compressed air. The vacuum adapter z. B. the assembly is pressed against the needle bed by the negative pressure. Purely mechanical clamping of the circuit boards or assemblies is also possible.
In principle, contact can be made in two different ways with an assembled module. In the first case, the soldering points of the assembled components or the components are contacted by the needle. In the second case, additional test points are included on the circuit board of the assembly. These are rectangular, square or round copper surfaces without solder mask that can be used for contacting. Since components or their soldering points can be damaged in the first process, the process with additional test points is usually used.
When testing an unassembled circuit board, the contact points of the components to be soldered can be contacted directly as test contacts.
Adapter types
A general distinction can be made between two types of test adapters: spring pin adapters and rigid needle adapters.
Spring pin adapter
This adapter is used in most ICT test systems. The test points and component pins are contacted and measured directly with spring contact pins. In practice, contact distances of 0.8 mm can be achieved with this contact system. Due to the wobble play of the spring pins, however, the test surfaces should have a diameter of at least 0.6 mm. Special additional processes and design features also enable smaller contact distances and test areas, whereby the contacting force and the service life of the spring contact pins are reduced. In mass production, test point diameters of 1.0 mm or larger are used in order to minimize measurement problems due to incorrect contacting and thus necessary rework.
The spring pin adapters can be divided into vacuum adapters , compressed air adapters or mechanically contacting adapters.
Rigid needle adapter
The rigid needle adapter is mainly used when contact is to be made on very small structures (test points> 0.2 mm, contact spacing> 0.25 mm) or when an adapter with a very long service life is required. Due to their complex structure, they are more expensive than a spring pin adapter, but this additional investment pays off very quickly, as there is much less service work and the associated system downtimes. The full advantages of these adapters can only be exploited if the position of the substrate is optically detected by the ICT test system and a position correction is carried out in X, Y and θ.
Further adapter criteria
For the appropriate positioning of a circuit card in relation to the ICT needle bed, there should be two asymmetrically arranged circuit board holes (anti-twist protection) in the test item or in the panel. Two so-called positioning pins are then placed in the needle bed, which bring the circuit board into the optimal test position.
In general, care must be taken that the force applied by the needle bed adapter (test needles, support points, hold-down devices, etc.) does not bend the test item and thus damage it. There are therefore enough support points to support the circuit board, but also as a counterpart to the test needles, a corresponding number of hold-down devices for optimal clamping of the circuit board to the needle bed.
It is also possible to position the test item via the outer contour of the test item or panel, but this results in lower positioning accuracy (e.g. due to unclean separators on the circuit board). The test points on the test item must then be dimensioned correspondingly larger. Furthermore, with contour clamping, care must be taken to ensure that the test item has a certain stability so that it can be clamped at all (especially with assemblies without edge strips).
Assembly for one-sided contact
If the circuit board only has test points on one side and is airtight, it can be sucked in with a vacuum table and a correspondingly adapted negative seal. If the test item has many holes or if you want to save yourself the need to make a special seal, a vacuum adapter with a hood must be provided. Appropriate hold-down devices are to be provided in the hood. The circuit board should be picked up and guided using positioning pins.
Assembly for double-sided contact
If circuit boards have to be contacted from both sides with needles, a vacuum seal cannot be used. In the case of a vacuum adapter, a hood adapter with an integrated second needle bed and hold-down devices or a purely mechanical adapter with a lower and upper needle bed must be built up. The circuit board must be centered on the needle bed using positioning pins and the upper needle bed must also be centered with the lower one so that optimal contact is guaranteed.
Single-stroke system
With the one-stroke system, the contacting process takes place with a stroke, that is, the circuit board is positioned in the contacting system and the adapter moves down onto the circuit board. If the test pins hit the circuit board, the necessary contact force is built up with the remaining stroke.
Two-stroke system
A two-stroke or double-stroke system is used when the test item cannot be contacted with the full needle bed in order to carry out a special ICT measurement. Test needles with different lengths are used for this. Only the longer test needles contact the test item in the first stroke. In the second stroke, short as well as longer test needles contact the test object. A good application example is to carry out the ICT with the needle bed completely in contact and then to carry out a small additional function test or programming of components with reduced needle contacts.
Single-chamber or two-chamber adapter
The various ICT test systems often offer the option of setting up a so-called two-chamber / double-chamber adapter for the assembly. As a result, the processing time for inserting an assembly into one test chamber can be completely eliminated from the test time while the test system is testing the assembly in the other chamber.
Measurands
Measurement voltages and measurement currents
The analog component measurements in ICT testers are typically carried out with low voltages and currents. By default, voltages in the range from 0 V to 1.0 V can be used. The measuring currents are typically in the range from a few microamps to a few milliamperes. Larger measurement voltages are often not permitted for assemblies. There is always the risk that other components can be damaged by the measuring voltage or that diode sections become conductive, so that measurements within the circuit of a module can no longer be sensed. For the same reason, the maximum measuring current is also limited, since in very unfavorable cases the current carrying capacity of the lines on the circuit board or the components limits the maximum measuring current. In ICT test systems, a direct voltage is typically used to measure resistance values, while an alternating voltage is often used in capacitors and coils.
Measurable sizes for components
Most ICT test systems can typically perform the following measurements on analog components.
| Component | Measurands |
|---|---|
| Short circuit test | The short-circuit test between two neighboring electrical networks checks whether an impermissible, very low-resistance electrical connection has occurred during the manufacturing process of the circuit board or the assembly, e.g. B. a solder bridge. |
| Resistances | Resistance value |
| Capacitors | capacity |
| Coils and transformers | Inductance |
| Diodes and bipolar transistors | The forward voltage between the base and the emitter, the blocking behavior between the base and the emitter, in the conductive state the forward behavior between collector and emitter and the forward voltage, in the blocked state in the reverse direction the blocking behavior between collector and emitter. |
| Field effect transistors | The blocking behavior between gate and drain / source, in the conductive state the forward behavior between drain and source and the forward voltage, in the blocked state the blocking behavior between drain and source. |
Tolerances of the measured quantities
The components to be measured in each case have all tolerances. For example, consider the tolerance limits of an electrical resistor with a nominal value of 10 kΩ at room temperature and a tolerance of the resistance value of ± 1% of the nominal value. The actual resistance value can therefore be between the lower limit of 9.9 kΩ and 10.1 kΩ. The ICT test system is also not zero fault tolerant and therefore this tolerance must be added to the actual component tolerance. So does the ICT test system z. B. for a measurement of a resistance in the range 10 kΩ a measurement tolerance of ± 0.8%, then in this case the measurement range for the test of the resistance with a lower limit of 9.82 kΩ and the upper limit to 10.18 kΩ can be set.
Quantities that cannot be measured or cannot be measured correctly for components
Various electrical quantities cannot be recorded or cannot be recorded correctly with ICT test systems.
| Component | Measurands |
|---|---|
| Resistances | For resistance values less than 100 Ω, four-point measurements should be used so that contact resistance can be eliminated. Very small resistance values in the milli- or micro-ohm range can usually not be measured with the required accuracy with a measurement voltage of typically 1 V and a measurement current of a few milliamperes, the same applies to very large resistance values in the mega- and gigaohm range. |
| Capacitors | Very low capacitance values in the picofarad range, since the capacitance of the connecting line or the adapter can already be in this order of magnitude or can even be greater; very large capacitance values, as this is difficult to measure with a low measuring voltage and a low measuring current |
| Voltage dependent resistance
(VDR resistors) |
The VDR resistors are used to limit overvoltages and only conduct when the response voltage is exceeded, which is typically significantly higher than the measuring voltage of the ICT tester |
| Zener diodes | Zener voltage , provided that the Zener voltage is greater than the measuring voltage of the ICT tester |
| Parallel connection of the same components with very different values | Example: If a capacitor of 1 µF is connected in parallel with a capacitor of 100 pF, measurement problems can arise because the necessary tolerance of the overall measurement includes the measured value of the small capacitor in its entirety - so the 1 µF capacitor can be detected in the overall measurement but not the 100 pF capacitor. |
Other measurands
In addition to the purely analog component test, larger ICT test systems can often also supply the assembly to be tested with voltage and carry out further tests. The playing field ranges from simple digital ICT, in which the input pins of a component are stimulated and the expected signals at the output pins of the component are observed, to (extensive) function tests.
Typical test sequence
- Discharge routine, especially for discharging electrolytic capacitors (serves for the safety of the assembly and test system as well as for measurement stability; this step is always carried out as the 1st step)
- Contact test (to check whether the test system is correctly connected to the assembly)
- Short circuit test (test for soldering defects)
- analog component test (test of all analog components for presence and value)
- Comparison test of ICs for the presence and correct soldering
- Test the correct polarity of capacitors
- Supply the module with operating voltage
- Powered analog test (test of analog components that require operating voltage, e.g. relay)
- Powered digital test (test of digital components: stimulation of input pins, monitoring of output pins; comparison with target values)
- Boundary scan tests
- Flash, ISP and other component programming
- Remove the module from the operating voltage
- Discharge routine to transfer the assembly in a voltage-neutral manner (as at the beginning).
While most typical ICT test systems have the appropriate equipment in the system to carry out the tests listed above, additional hardware components are often required for special additional tests:
- Camera system for checking the presence and polarity of otherwise unmeasurable components
- Photodetectors for testing LED color, intensity, homogeneity
- external frequency meters for testing very high frequencies
- Additional equipment for measuring signal analysis such as B. Edge steepness, envelopes (FA08 Aeroflex card)
- external equipment for high-voltage measurements (e.g.> 100 V DC) or AC voltage sources.
Products and standards
- Testjet (Agilent)
- FrameScan (Genrad / Teradyne)
- QTest2-Probe (Aeroflex)
- ElectroScan (SPEA)
Other testing techniques
The following are a few more test techniques that are often used in the manufacturing process of electronics production:
- AXI automatic X-ray inspection
- AOI Automatic optical inspection
- FKT function test / final test
- Boundary Scan technology.