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Why do we need test a product at all I can hear you saying, surely all we need to do is manufacture the product correctly in the first place. This is certainly a valid viewpoint; however there are a number of factors at work which simply prohibit this strategy in many cases.

For example

  • Legal and Statutory requirements need to be observed e.g. EMC
  • Customers expect their products to be functionally tested
  • Interference between component tolerances can cause products to work at sub-optimal levels or not at all
  • Alternative components fitted in certain locations can cause products not to work
  • Complexity of products makes it sensible to test 'sub-assemblies' prior to final assembly
  • To catch early life fails e.g. accelerated aging tests such as HASS

The objective of test in an electronics manufacturing environment it to ensure by optical, electrical or physical measurement of a characteristic that components or circuits meet agreed design specifications.

In order to maximise test coverage of products and reduce costs many companies employ a 'combinational approach' to test where more than one strategy is used.

This guide takes a look at some of the most common methods and critiques the usefulness of each.

Bare Board Test

PCB fabricators will bare board test the PCB to ensure there are no internal faults or damaged tracks. We would recommend that when you buy PCBs you always ask for 100% bare board tested product as faults in PCBs sometimes will not show up until the final unit is under test and when all of the value had been added.

Automated Optical Inspection (including Comparator)

This method uses the computer aided design data (CAD) for each PCB assembly to generate a programme based upon optical comparison. A range of AOI machines are available which will check for component presence, polarity and form right through to the more expensive machines which will also inspect for solder (bridges, joint integrity etc) related defects as well as component legends.

In circuit (ICT)

Also known as 'bed of nails' testing. This is usually used for products which are manufactured in reasonable volumes as it involves capital expenditure on test fixturing and programming per product.

Flying Probe

Used where volumes are relatively low and the cost of ICT is prohibitive. Also know as fixtureless testing, this test solution uses a range of mobile test probes to physically 'probe' the board under test.

Functional Test

This test is designed to ensure that the unit under test performs the functions for which it was designed. Whilst it is possible to buy functional test hardware and software many companies design their own 'black box' functional testers which use emulation to ensure their products are fit for purpose.

Environmental Stress Screening (ESS)

Also known as thermal cycling, this test is usually carried out when the PCB assembly is fully populated. A powered up unit is placed into a thermal chamber capable of both elevating (up to approx. +185degC) and reducing temperatures (down to approx. -55degC). The purpose of this test is to catch early life failures by mechanically stressing the solder joints. This can be a controversial test because some schools of opinion believe that it also prematurely ages components causing them to fail in the field at an earlier stage than they otherwise would had they not been through an ESS process.


Highly-Accelerated Life Testing (HALT) and Highly Accelerated Stress Screening (HASS) are two techniques used to identify the weakest point in the design and manufacture of products, thus contributing to improving the quality.

HALT exposes products to step by step cycling in shock, temperature and vibration and its objective is cause the product to fail at its weakest point.

Whereas HASS is an abbreviated form of HALT testing. The objective here is not to causes failures within the product under test but rather to monitor that products continue to operate properly under test conditions which mimic the real world.


Electromagnetic Compatibility Testing which is commonly referred to as "EMC testing" has become an important part of the product design process. EMC is a term for the behaviour of equipment in terms of the electro magnetic interference it generates and the immunity to other electro magnetic fields around its enclosure and cables.

Typically a product design team must:

  • ensure that any electric or electronic device will create no more than a limited amount of radio frequency interference so that other products are not functionally affected
  • ensure that an electric or electronic device will withstand a certain amount of electro magnetic fields while operating as intended within specifications

EMC testing thus should give the consumer a level of confidence that equipment will operate in a reliable manner when it is used or installed in its intended environment. EMC Testing also allows a harmonised standard to be applied across both geographies and manufacturers.

Test Topics

Flying Probe

In Circuit Test

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Guide Written by Paul Wilson

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