PCBA Testing

Functional test (FCT) is used as a final manufacturing step. It provides a pass/fail determination on finished PCBs before they are shipped. An FCT’s purpose in manufacturing is to validate that product hardware is free of defects that could, otherwise, adversely affect the product’s correct functioning in a system application. In short, FCT verifies a PCB’s functionality and its behavior. It is important to emphasize that the requirements of a functional test, its development, and procedures vary widely from PCB to PCB and system to system.
Functional testers typically interface to the PCB under test via its edge connector or a test-probe point. This testing simulates the final electrical environment in which the PCB will be used.
The most common form of functional test simply verifies that the PCB is functioning properly. More sophisticated functional tests involve cycling the PCB through an exhaustive range of operational tests.

Customer Advantages of Functional Test:

• Functional test simulates the operating environment for the product under test thereby minimizing the expensive cost for the customer to provide the actual testing equipment
• It eliminates the need for expensive system tests in some cases, which saves the OEM lots of time and financial resources.
• It can check the functionality of the product anywhere from 50% to 100% of the product being shipped thereby minimizing the time and effort on the OEM to check and debug it.
• Prudent testing engineers can extract the most productivity out of functional test thereby making it the most effective tool short of system test.
• Functional test enhances the other types of tests such as ICT and flying probe test, making the product more robust and error free.

A functional test emulates or simulates a product’s operational environment to check its correct functionality. The environment consists of any device that communicates with the device under test (DUT), for example, the DUT’s power supply or program loads necessary to make the DUT work properly.
The PCB is subjected to a sequence of signals and power supplies. Responses are monitored at specific points to ensure functionality is correct. The test is usually performed according to the OEM test engineer, who defines the specifications and test procedures. This test is best at detecting wrong component values, functional failures and parametric failures.
Test software, sometimes called firmware, allows production line operators to perform functional test in an automatic way through a computer. To do this, the software communicates with external programmable instruments as a digital multi-meter, I/O boards, communication ports. The software combined with the fixture interfacing the instruments with the DUT make it possible to perform a FCT.

Circuit testing (ICT) is traditionally used on mature products, especially in subcontract manufacturing. It uses a bed-of-nails test fixture to access multiple test points on the PCB’s bottom side. With sufficient access points, ICT can transmit test signals into and out of PCBs at high speed to perform evaluation of components and circuits.
A bed of nails tester is a traditional electronic test fixture. It has numerous pins inserted into holes, which are aligned using tooling pins to make contact with test points on a printed circuit board and are also connected to a measuring unit by wires. These devices contain an array of small, spring-loaded pogo pins making contact with one node in the circuitry of the device under test (DUT).
By pressing the DUT down against the bed of nails, a reliable contact can be made quickly with hundreds and in some cases thousands of individual test points within the DUT’s circuitry. Devices that have been tested on a bed of nails tester may show a small mark or a dimple which comes from the sharp tips of pogo pins used in the fixture.
It takes a few weeks to create the ICT fixture and do its programming. A fixture can either be vacuum or press-down. Vacuum fixtures give better signal reading versus the press-down type. On the other hand, vacuum fixtures are expensive because of their high manufacturing complexity. The bed of nails or in-circuit tester is the most common and popular in the contract manufacturing environment.

ICT provides OEM customer such benefits as:

• Although a costly fixture is required, ICT covers 100% testing so that all power and ground shorts are detected.
• ICT testing does power up testing and eliminates customer debug needs to almost ZERO.
• ICT does not take a very long time to perform, for example if flying probe takes 20 minutes or so, ICT for the same time might take a minute or so.
• Checks and detects shorts, opens, missing components, wrong value components, wrong polarities, defective components and current leakages in the circuitry.
• Highly reliable and comprehensive test catching all manufacturing defects, design faults, and flaws.
• Testing platform is available in Windows as well as UNIX, thus making it slightly universal for most testing needs.
• Test development interface and operating environment is based on standards for an open system with fast integration into an OEM customer’s existing processes.
• ICT is the most tedious, cumbersome, and expensive type of testing. However, ICT is ideal for mature products requiring volume production. It runs the power signal to check voltage levels and resistance measurements at different nodes of the board. ICT is excellent at detecting parametric failures, design related faults and component failures.