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In Circuit Test

When electronic products are manufactured using either manual or automatic means, there is a potential opportunity for assembly error arising from incorrect manufacturing processes.

If undetected, errors can cause damage to the circuit when the assembly is powered up or the assembly may suffer from either reduced performance or possibly even catastrophic failure at functional test and beyond.

As a result, a test method was evolved which allowed the electronics manufacturer to test each PCB assembly for solder shorts (also known as solder bridges), opens, and a limited amount of component values. This type of test machine became known as a Manufacturing Defect Analyser or MDA.

However, test engineers needed more capability and in particular needed the ability to check for criteria such as resistance, capacitance and inductance as well as the correct operation of various analogue/digital devices. This lead to the development of In-circuit Testers (also known a 'bed of nails' testers).

In circuit test equipment consists of the test machine itself (inside which are driver/receiver cards plugged into a large backplane) and a bespoke test fixture. The complexity of the unit under test will determine how many driver/receiver cards are needed to be present in the machine.

In circuit testers use a 'specific to type' test fixture. The pcb assembly is fitted to the test fixture and located in exactly the correct position by tooling pins. The platen of the test fixture is sucked down by a vacuum pump on to metal test pins which are brought into contact with the underside of the PCB assembly through designated holes in the platen. Test pins of various shapes and sizes, each designed for a specific role (e.g. crown, chisel points etc) are used to make contact with test pads, vias or solder joints allowing the tester to test various parts of the circuitry.

Test fixtures are generally wired in what would appear to be a rather untidy manner, however this method reduces wire lengths and minimizes the potential for rogue readings caused by the resistance/capacitance of the materials used.

Test fixtures can be changed over by means of a simple mechanical lever and a software change.

In circuit test machines operate by testing each component in turn, checking for presence, value and in some cases orientation. Whilst In circuit test is not a functional test a high level of confidence can be derived from an in circuit 'pass' as it indicates all of the components have been correctly assembled and there are no solder bridges or opens present.

The driver/receiver cards drive parts of the circuit under test to a particular state by supplying a voltage or current and sensors then measure the resulting outputs. Measuring the value of a component when it is not in a circuit is relatively easy, however it is more problematic when a component is connected to other components and as a result a technique called guarding is often employed by testers.

Guarding earths the components around the component under test allowing for a much more accurate test measurement to be taken.

It is possible to test high complexity products with high node counts using a multiplexed test machine and a technique called cluster testing.

Advantages of ICT

  • Relatively easy to Programme
  • High fault coverage for manufacturing defects
  • Test results relatively easy to interpret
  • Not limited to use by test technicians, can easily be used by Production Operators
  • Short test time (Usually 1-2 minutes per assembly)
  • Very efficient for medium to high volume through hole conventional assemblies
  • Relatively low maintenance costs
  • Wide choice of ICT platforms and manufacturers
  • Machine can be configured in terms of node count to your exact requirements i.e. no need to pay for redundant features

Disadvantages of ICT

  • High capital outlay
  • Each product needs its own test fixture
  • Not suitable for small batch products where test fixture/programme cost may prove prohibitive
  • PCB densities sometime make probing difficult if not impossible
  • Test contact failures can arise when test pins do not make proper contact with the appropriate test pads
  • Test pins need to be regularly cleaned and replaced
  • Some components may be shielded by other components so no test access can be achieved
  • This is not a functional test although some of the larger machines can power the assembly up and simulate functional conditions
  • Struggles with high density, small package size SMT components
  • Large node count assemblies may need you to purchase additional driver/receiver cards - which are not cheap
  • Equipment manufacturers offer relatively expensive support contracts

Test Topics

Flying Probe

In Circuit Test

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Guide Written by Paul Wilson

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