When you order printed circuit boards (PCBs), you know that there is a high cost associated with failures. The financial loss you want to avoid is your PCB suddenly failing or having a shortened lifespan due to design or quality assurance issues.

　　PCB assembly testing methods are an integral part of the manufacturing process. Reputable Electronics Contract Manufacturers (ECMs) offer a variety of PCB testing methods, but there are seven main types:

　　In-circuit testing;

　　Flying probe testing;

　　Automated Optical Inspection (AOI);

　　Burn-in testing;

　　X-ray inspection;

　　Functional testing;

　　Other functional tests (solderability, contamination, etc.);

　　Here's a primer on the more important PCB testing types. This should better prepare you for discussions with your ECM:

　 　7 PCB Testing Methods

　 　1. In-Circuit Testing

　　In-circuit testing (ICT) is currently the most reliable type of PCB testing. The high price reflects this—tens of thousands of dollars, although the cost depends on factors such as board and fixture size.

　　Also known as bed-of-nails testing, ICT can power up and activate individual circuits on the board. In most cases, the test aims for 100% coverage, but the actual coverage will be closer to 85-90%. The advantage of ICT is that the 85-90% coverage you get is completely free from human error.

　　This test involves using fixed probes whose layout matches the PCB design. The probes check the integrity of the solder joints. The bed-of-nails tester simply needs the board pushed onto the bed of probes to begin testing. Access points are pre-designed on the board, allowing the ICT test probes to make contact with the circuits. They apply a certain amount of pressure to the connections to ensure they remain intact.

　　ICT is typically performed on larger connections and Ball Grid Arrays (BGAs).

　　This test is suitable for "mature" products that are expected to have few revisions. If your goal is not Design for Manufacturing and there are no suitable pads on the board, in-circuit testing may not be possible. Unfortunately, you can't change your mind mid-production and switch to an ICT strategy.

　　2. Flying Probe Testing

　　Flying probe testing is a tried and tested method that is cheaper than in-circuit testing. This is a non-powered test that checks for:

　　Opens

　　Shorts

　　Resistance

　　Capacitance

　　Inductance

　　Diode issues

　　A flying probe tester works by using needles connected to probes on an xy grid obtained from the basic CAD. Your ECM program will coordinate to match the board, then run the program connected to these test points.

　　We've talked about the common comparison of flying probe versus ICT. They each have their pros and cons.

　　In some cases, ICT makes flying probe testing unnecessary, but the PCB must be designed to fit the test fixture—meaning higher initial costs. ICT is faster and less prone to error than flying probe testing, so you may find the extra cost worthwhile. While flying probe testing may be cheaper initially, for large orders, it may actually be less cost-effective.

　　A final note: PCB flying probe testing does not power the board.

　 　3. Automated Optical Inspection (AOI)

　　AOI takes pictures of the PCB using a single 2D camera or two 3D cameras. The program then compares the picture of the board to the detailed schematic. If there is a mismatch between the board and the schematic, the board is flagged for a technician to inspect.

　　AOI can be used to catch problems early to ensure production stops as soon as possible. However, it does not power up the board and may not have 100% coverage of all part types.

　　Never rely solely on automated optical inspection. AOI should be used in conjunction with other tests. Our favorite combinations include:

　　AOI and Flying Probe

　　AOI and In-Circuit Testing (ICT)

　　AOI and Functional Testing

　 　4. Burn-in Testing

　　As the name suggests, burn-in testing is a more intense type of PCB testing. It is designed to detect early failures and determine load capacity. Due to its intensity, burn-in testing may destroy the components being tested.

　　Burn-in testing runs electricity through your electronics, usually at a higher-than-rated capacity. Electricity is run continuously through the board for 48 to 168 hours. If the board fails, it is called an early failure. Boards with a high early failure rate are obviously not ideal for military or medical applications.

　　Burn-in testing is not suitable for all projects, but it makes perfect sense in some cases. It can prevent embarrassing or dangerous situations before the product reaches the customer.

　　Keep in mind that burn-in testing may shorten the lifespan of the product, especially if the test puts more stress on the board than it is rated for. If few or no defects are found, the test limits can be lowered after a shorter period to avoid putting too much stress on the PCB.

　 　5. X-Ray Inspection

　　This "test," also known as AXI, is really more of an inspection tool, at least for most ECMs.

　　During this test, an X-ray technician can spot defects early in the manufacturing process by looking at:

　　Solder joints

　　Internal traces

　　Voids

　　There are 2D and 3D AXI tests, with 3D offering faster test cycles.

　　X-ray inspection can check components that are usually hidden from view, such as connections with solder under chip packages and ball grid array packages. While this inspection is very useful, it does require a trained and experienced operator.

　　Also, please note that your ECM may not be able to use an X-ray machine to inspect every layer of the circuit board. We can indeed detect internal defects through the circuit board, but this is a very time-consuming and expensive process (for both the ECM and the customer).

　　 6. Functional Test

　　Some customers do prefer a good old-fashioned functional test. Your ECM will use this test to verify that the product is able to start up.

　　This test does require a few things:

　　External equipment

　　Fixtures

　　Requirements of UL, MSHA, and other standards

　　The functional test and its parameters are usually provided by the customer. Some ECMs can help develop and design such tests.

　　This does take time. If you want a quick product launch, this may not be your best option. But from a quality and longevity perspective, functional testing can save face and money.

　　 7. Other Functional Tests

　　Depending on the specific situation, there are other types of functional tests that can be used to inspect your PCB.

　　PCB functional testing verifies the behavior of the PCB in the final use environment of the product. The requirements, development, and procedures for functional testing can vary greatly depending on the PCB and the final product.

　　Other PCB assembly test types include:

　　Solderability test: Ensures surface robustness and increases the chances of forming reliable solder joints

　　PCB contamination test: Detects high levels of ions that may contaminate the board, leading to corrosion and other problems

　　Microsection analysis: Investigates defects, open circuits, short circuits, and other failures

　　Time Domain Reflectometry (TDR): Locates faults in high-frequency boards,

　　Peel test: Determines the strength required to peel the laminate from the board

　　Floating solder test: Determines the level of thermal stress that the PCB holes can withstand

　　Advantages of functional PCB testing include:

　　Simulates operating environment, minimizing customer costs

　　Can eliminate the need for expensive system testing

　　Can check product functionality—you need to check and debug 50% to 100% of the products you ship

　　Works perfectly with other tests such as ICT and flying probe testing

　　Ideal for detecting incorrect component values, functional failures, and parametric failures

　 　Consider your situation

　　Figuring out which PCB test is right for you can be a challenge; there are certainly many ways! Your ECM will know which tests are right for your specific needs, so consult them often.

　　Don't forget about PCB prototyping. This fundamental element of product launch is a test in itself, allowing you to see a real product before it hits the market.