Ultimate Guide To PCB Testing - Matric Group

IN ELECTRONICS MANUFACTURING, PRINTED CIRCUIT BOARDS ARE SEPARATED INTO THREE CATEGORIES:

• Class 1 Electronics: General Electronics Products
• Class 2 Electronics: Dedicated Service Electronics Products
• Class 3 Electronics: High-Reliability Electronics Products

The categories reflect the level of quality of each circuit board type, from lowest (Class 1 standards) to highest (Class 3 standards). This classification system was developed and is monitored by IPC under the IPC- standard.

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Perhaps the major difference between each class is the degree of inspection electronics assemblies must undergo and the quality standards to which they're subjected.

Understanding the classes and their requirements can be helpful to OEMs who aren’t sure to which class their product should belong. Factors like customer requirements and cost can weigh heavily in deciding which class to pursue.

CLASS 1 -- GENERAL ELECTRONIC PRODUCTS

The first electronic products class is referred to as the “general electronics” category. This class includes boards with the lowest quality requirements and is mostly found in products with an expected short life cycle.

This is basically the “you get what you pay for” class. These electronics are held to the lowest standard of quality and thus are usually found in inexpensive, high-volume productions.

CLASS 2 -- DEDICATED SERVICE ELECTRONIC PRODUCTS

Class 2 electronic devices encompass all electronics where continued performance and an extended life cycle are required -- to a point. Uninterrupted service is desired, but not critical. Along with what's in the chart above, IPC Class 2 examples include:

• Televisions
• Air conditioners
• Tablets

In other words, these are items where an early life cycle failure would have you red-faced and frustrated, but wouldn’t put your life at risk.

CLASS 3 -- HIGH-RELIABILITY ELECTRONIC PRODUCTS

The third class of circuit boards is subject to strict guidelines due to their importance in the field. Class 3 electronics are typically mission-critical products.

Whether it’s a pacemaker or a military radar, a product that needs to meet IPC Class 3 requirements must use high-reliability electronic components to ensure uninterrupted service.

These electronics are usually the highest of quality, and many OEM products that could pass as Class 2 opt for the IPC Class 3 standard because the benefits of higher-quality electronics outweigh the cost of additional testing and inspection.

In-Circuit Testing

In-circuit testing (ICT) is the most robust type of PCB testing. 

An ICT, also known as a bed-of-nails test, powers up and actuates the individual circuitry on the board. In most cases, the test is designed for 100% coverage, but you’ll get closer to 85-90% coverage (totally free of human error).

ICT is often performed on bigger connections and ball grid arrays (BGAs).

This test is for a “mature” product with very few revisions expected. 

Flying Probe Testing

Flying probe testing is a tried-and-true option that’s less expensive than in-circuit testing. It’s a nonpowered type of test that checks for:

• Opens
• Shorts
• Resistance
• Capacitance
• Inductance
• Diode issues

The test works through the use of needles attached to a probe on an x-y grid obtained from basic CAD. Your ECM program coordinates to match the circuit board and then runs the program.

In some cases, ICT makes it unnecessary to use flying probe testing, but the PCB has to be designed to fit with the test fixture -- which means a higher initial cost. While flying probe testing can be cheaper initially, it may actually be less cost-effective for large orders.

One final word of caution: A PCB flying probe test does not power up the board.

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Automated Optical Inspection (AOI)

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AOI uses either a single 2D camera or two 3D cameras to take photos of the PCB. The program then compares the photos of your board to a detailed schematic. If there is a board that does not match the schematic to a certain degree, the board is flagged for inspection by a technician.

AOI can be useful for detecting issues early to ensure production is shut down ASAP. However, it does not power up the board and may not have 100% coverage for all part types.

Burn-In Testing

As the name suggests, burn-in testing is a more intense type of testing for PCBs. It’s designed to detect early failures and establish load capacity. Because of its intensity, burn-in testing can be destructive to the parts being tested.

This is done by running a power supply through the electronics at an elevated temperature, often its maximum-specified capacity.

Electronics have a higher failure rate at the beginning of their life cycle, leveling out in the middle and rising again as they reach the end of the life cycle. If a PCB with an infant mortality failure like this were to make its way into military or medical equipment, it could result in a potentially serious failure.

Burn-in testing reduces the number of latent defects by triggering them through this testing, resulting in a more reliable batch of electronics for the OEM, the trade-off being a smaller yield and potentially reduced product lifespan.

The data collected through this process can in turn help engineers understand what caused the defects and modify the design to improve product reliability before it even hits burn-in.

X-Ray Inspection

Also referred to as AXI, this type of “testing” is really more of an inspection tool, at least for most ECMs.

There are 2D and 3D AXI tests, with 3D offering a faster testing period.

X-ray testing checks elements that are usually hidden from view, such as connections and ball grid array packages with solder joints underneath the chip package. While this check can be very useful, it does require trained, experienced operators.

Advantages of functional PCB testing include:

• Simulates the operating environment, minimizing customer cost
• May eliminate the need for expensive system tests
• Can check product functionality
• Pairs well with other tests, such as ICT and flying probe
• Great for detecting incorrect component values, functional failures, and parametric failures

PCB SOLDERABILITY TESTS

Evaluating solderability will tell your PCB tester whether or not the attached components, like leads and terminations, can withstand the scorching temperatures that come with soldering.

It also determines whether storing these components has adverse effects on their ability to solder to the PCB when putting them to use. Understanding the solderability of components and the board can minimize PCB failures and improve final product quality.

Three of the most common solderability testing formats are:

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