The Whys and Wherefores of Manufacturing Testing
“Quality is never an accident; it is always the result of intelligent effort.” – John Ruskin
Turning ideas into electronic devices that are functional, reliable and cost effective is no small feat. Some of the least understood, but critically important steps in manufacturing are related to testing – bare Printed Circuit Boards (PCBs), assembled PCBs, and fully assembled devices. Choices that are made for manufacturing testing will directly and indirectly impact the cost and quality of the final manufactured products. Manufacturing test plays a critical role in identifying and correcting potential issues before a product is shipped.
Three Primary Points for Testing
Bare PCBs, Assembled PCBs, and Assembled Devices
Before a new electronic device can be assembled, Device Solutions’ engineers create plans for all of the significant components, their interconnections, and how they and the final product will be tested. It is important to know what the test methods will be because many of those tests require that special features (such as test points, user interfaces, and communication ports) be included in the design and layout of a PCB.
Once the schematic has been created, connections between the components (resistors, capacitors, inductors, integrated circuits and displays) are laid out using a PCB design tool. These designs are then used to create the physical PCBs, a process known as fabrication. “Bare-board” tests can be done to ensure that the PCBs have been fabricated properly before components are placed on the PCBs.
Once components have been placed and soldered to the PCB, called “populating the board,” and before the populated PCB is enclosed in a protective outer case (such as the plastic case of a mobile phone) their functionality can be tested. Depending on the equipment needed, the number of tests, and the time required to complete the tests, human testing or more automated types of testing may be used. In some cases, additional PCBs, modules, and other peripherals may be connected together and tested before placing into the enclosure.
Another approach, referred to as “In Circuit Test” or ICT, is an automated test performed by having “probes” make contact with pre-defined test points on a board. These tests measure the electrical responses between contact points and compare them to expected values. Including test points for purposes of running ICT on a board can complicate the PCB layout and can take up additional space on the PCB. However, ICT makes rapid, automated testing possible. Two common approaches to ICT include “Bed of Nails” and “Flying Probe” tests.
“Bed of Nails” refers to a custom built fixture that tests a specific PCB layout. It consists of anywhere from tens to hundreds of spring-loaded probes that simultaneously make contact with the test points on a PCB (bottom and/or top of the board).
“Flying Probes,” on the other hand, are computer controlled test probes that make contact with test points on a PCB to take electrical readings. The advantage of this type of testing is that the fixture can be configured and programmed to test many different kinds of PCBs. However, testing times are longer and programming the probes adds additional costs.
The final step in manufacturing testing is referred to as “Final Assembly Testing.” These tests are performed on fully assembled products and can be done in a couple of ways. The first way is a manual test by a human operator interacting with the device. The second way is a fully automated or “self” test. Since the device will be testing itself, significant expense is required to develop the software needed to program the device to run its own diagnostics. For very high volume products, however, this can be the most cost effective testing option.
Not All Testing is Created Equal
Although manufacturing testing is a necessary part of product manufacturing, determining acceptable quality levels is also important. The cost of ensuring perfection, especially when perfection is not strictly necessary, can be prohibitive. On the other hand, if perfection is the goal (as is the case with a life-saving medical device), then additional testing costs are warranted. Device Solutions works with each individual client to evaluate all the data and parameters before deciding on the appropriate level of manufacturing testing. To quote Michael Pail, Device Solutions’ Hardware Engineering Manager, “Deciding on the appropriate level and method of manufacturing test can be reduced to a business case decision like most, if not all, other aspects of product design. The goal is to deliver a product with a known, acceptable quality level at the lowest possible overall cost.”
– Jena Ball