Testing PCBs throughout the production process: materials and methods

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What you will learn:

• What methods can be applied to verify the functionality and safety of PCBs? Which are suitable for mass production and prototypes?
• What parameters are checked in each test?
• How to check a PCB without physical testing.

Testing is a key part of printed circuit board (PCB) design. If PCB testing is used at all stages of the production process, manufacturers can save resources and increase productivity by ensuring the quality of their products.

PCB designers pay attention to board checks because they want to be sure that the functionality indicators of the PCB are within allowable values ​​and that the PCB is manufactured with care. Many quality checks are necessary to ensure that the designed product can be produced on a large scale.

These PCB tests can be applied to prototypes and small batches. They check boards for open shorts and bad solder joints, as well as test their performance (Fig.1). The same objectives are pursued by mass production trials.

Manufacturers use different methods for examining PCBs and PCB assemblies (PCBAs). The choice of testing approach is associated with production scale, product characteristics, and novelty.

Let’s take a look at the most used and well-known procedures.

Manual visual inspection

Manual Visual Inspection (MVI), the oldest PCB inspection method, does not require the use of expensive equipment (Fig.2). Examination by a specialist can identify visible defects such as poor welding, lack of details or their bad position, cracked joints and interrupted traces.

MVI is not a very efficient method for mass production due to human error and the increasing complexity of hidden joint boards. However, this approach works well in PCB design and small batch production.

Automated optical inspection

Automated Optical Inspection (AOI) is more suitable for large scale production. An AOI machine compares the photo of the circuit board under test taken by a single 2D camera or two 3D cameras to the given example. This equipment can be placed at the end of the production line for rapid fault detection.

The AOI system can be effective as a first step in PCBA testing. This method is more accurate than MVI, but the installation and programming of AOI equipment requires a lot of time for each PCB design change. That’s why it’s not a good option for prototyping.

AOI can be used with other PCB test methods, such as flying probes, in-circuit testing (ICT), or functional testing, for a more accurate and comprehensive examination.

Automated X-ray Inspection

Automated X-ray Inspection (AXI) is a unique testing tool because it uses X-rays (Fig.3). AXI creates 2D or 3D images of hidden solder joints and can detect solder voids and open connections.

Being a rather expensive and time-consuming technology, it is more suitable for mass production of PCBs and verification of complex boards. It can also be quite useful for prototype design and small batch testing, as it is able to inspect chip elements with pads that are out of sight.

Circuit tests

In-Circuit Testing (ICT), or a “bed of nails”, is a very precise type of electronic control (Fig.4). It is a fully automated and expensive device, so ICT is preferable as the final stage of large-scale product testing with a design that does not need to be changed.

ICT powers each component on the board and provides their automatic one-by-one verification for open shorts and incorrect orientation, and checks resistance and capacitance, providing over 90% fault coverage.

ICT equipment uses fixed probes (“nails”), which are placed in correspondence with the PCB access points. The nails examine the integrity of the solder joints and verify the connection in good condition. ICT test jigs, which can be mechanical or pneumatic, are very effective for mass production testing.

Another advantage of ICT is that it can examine Ball-Grid-Array (BGA) assemblies, FPGAs, and power-up, as well as test LEDs.

Flying probe test

Electronic Flying Probe Test (FPT) is a more economical alternative to ICT – it does not need a PCB tester and a long programming time. However, testing PCBs with FPT is 15 times slower than with ICT.

The FPT device inspects a circuit board using probes; some of them are static (they are under a tested circuit board), others are mobile (they “fly” above the board) (Fig.5). The probes (“needles”) are programmed to check all components of the PCB.

This method is more suitable for prototypes and not for large batch production. It doesn’t require a lot of space and time to install and has a high percentage of coverage, but it’s a bit slow to test.

Resistance tests

Stress tests may be required to assess the ability of printed circuit boards to withstand maximum loads and parameter changes. The operating conditions examined may relate to temperature, current, operating frequency or other significant parameters. These tests are specific and can therefore be used for certain types of PCBs.

Burn-in testing is a way to examine models and PCBs designed for mass manufacturing (Fig.6). Burn-in tests power the circuit board, usually at its maximum specified capacity, for two to seven days. It is intended to disclose any problems or defects with the board using it under extreme or prolonged working conditions.

Stress testing belongs to the group of methods HALT/HASS (Highly Accelerated Life Testing/Highly Accelerated Stress Screening). Their purpose is to determine and eliminate defects as well as to guarantee the quality of the product in the development phase (HALT) or in the production phase (HASS). They are most often made for printed boards used in specific applications: medical and industrial equipment, aerospace and motor vehicles. Stress tests can be carried out for newly launched products to ensure that they are structurally and functionally reliable.

Functional test

The PCB Functional Test (FCT) examines the accuracy of electronics performance (Fig.7). These tests are usually performed at the end of the manufacturing process to verify the functionality of the PCB and its conformance to specifications.

Depending on the complexity of the PCB design and the needs of the consumer, testing may involve simply checking whether the board can be powered on or not, or a complete full study.

Laboratory testing and certification of PCBs

Specially equipped laboratories can examine circuit boards in depth (Fig.8)using devices such as X-ray inner layer registration tester, plating thickness tester, impedance tester, copper adhesion tester, UV spectrophotometer and many more.

Laboratories can confirm that the PCB meets all required requirements and specifications (Fig.9).

PCB Simulations

It is often too expensive to test certain design elements. Simulation tools allow designers to obtain the necessary data without going through physical PCB testing. By using simulation, engineers can achieve cost-effective goals and obtain appropriate results if test equipment and other aspects of investigation are too expensive.

All elements of the PCB can be studied. PCB simulation software uses mathematical models to predict PCB behavior (Fig.10).

Now let’s take a closer look at some simulation tools.

Simulation of spices

The simulation program with emphasis on integrated circuits (Spice) is a widely used simulation tool (Fig.11). Different Spice software packages may have various functions, but almost all of them can perform some basic types of PCB analysis.

DC operating point analysis works with static DC characteristics. It can calculate all DC performance in the datasheet.

Transient analysis works with time response. It calculates voltage and current for each period.

AC analysis works with frequency characteristics. It calculates the small signal response of a printed circuit board and gives simulation results such as gain and phase versus frequency characteristics.

Other SPICE tools, such as temperature scan analysis, Fourier analysis, noise analysis, and worst-case analysis, can also be used for a comprehensive investigation of PCB behavior.

IBIS simulation

The Input/Output Buffer Information Specification (IBIS), which simulates the input/output buffers of an integrated circuit, is a kind of alternative to the Spice simulation (Fig.12). It can be up to 100 times faster than Spice, but is sometimes less accurate. IBIS calculates board behavior and can be used in PCB design at the schematic and layout stages.

The key element of the IBIS model includes a table with current, voltage and timing characteristics. The IBIS model does not contain transistor-level data on the PCB, so vendors are assured of the security of their intellectual property. Most IBIS models are easy to use and can be carried around without constraint.

Conclusion

Testing is an essential step in PCB design and production. Designers and manufacturers can choose and use the appropriate investigation methods themselves or entrust them to specialized laboratories. The tests should ensure the safety and proper functioning of the PCB.

On this front, Built-in sources works with reliable and reputable manufacturers who perform all required PCB testing. Nevertheless, our engineers pay great attention to verifying all the components of the projects, running hardware and software tests.