PCB Testing 101: Important Methods and Metrics

The PCB fabrication process involves a range of quality control measures and testing to ensure a design can be produced reliably at scale. Many of these quality checks happen without the designer's direct involvement, but critical tests like board bring-up and functional testing during prototyping fall on the designer. These tests later become integrated into the manufacturing process for larger production runs, ensuring the design meets the necessary quality standards from prototype to final product.

No matter what level of testing and inspection you need to perform, it’s important to determine the basic test requirements your design must satisfy and communicate these to your manufacturer. If it’s your first time transitioning from prototyping to high-volume production, read our list of PCB testing requirements so that you’ll know what to expect.

PCB Testing During Manufacturing

There are several PCB testing procedures performed during fabrication and assembly. These aim to assess bare PCB quality and yield and to ensure a design has passed through assembly without defects. In addition, electrical testing will be performed during manufacturing/assembly and compared with the design netlist.

For prototype designs, testing doesn’t end with manufacturing. Once the boards are received, the design team should put everything through board bring-up testing and functional testing before finalizing the design. Once you scale to thousands or millions of boards, some of these measurements may need to be automated to ensure high throughput and quality.

Mechanical PCB Testing and Inspection

There is a minimum set of mechanical tests and inspections that are performed during manufacturing to verify the bare board fabrication process and to ensure the board will be reliably assembled:

These tests can be used to determine whether there is some quality problem inherent in the manufacturing process, what rework steps might be required, or whether there is some aspect of the design that leads to a failed test. 

Electrical PCB Testing During Manufacturing

Electrical testing is also performed during manufacturing to check for any faults, impedance deviations, or conductive residues from soldering:

• Continuity test: This measurement checks for opens and shorts with DC current in a bare board.
• Hi-pot test: A high potential test involves bringing the board up to a high potential in order to check that the board has sufficient isolation between different nets. This test is recommended for high-voltage PCBs and on PCBs with thin dielectric layers.
• In-circuit testing: This test also measures the presence of opens and shorts, as well as specific voltage/current values on test points. Sometimes a test fixture is used to measure a specific waveform. In addition, power-on or power-off electrical tests may be used with specific components or test points to check for component faults.
• Resistivity of solvent extract (ROSE) test: This conductivity measurement is used to check for any residue that may be leftover from solder flux.
• Time domain reflectometry (TDR): This test is used to measure impedance in single-ended and differential traces. This may be performed on a test coupon or on a test board with an attached fixture. Some subsequent de-embedding and analysis is needed to fully evaluate signal integrity.

Controlled impedance tests are one area where you should rely on your manufacturer's data and experience before creating your design. If you request a controlled impedance service as part of your manufacturing order, they will be able to verify that your will hit your impedance specifications for their material set. Just make sure this is clearly specified for your manufacturer, such as in your fabrication notes.

PCB Stress Testing

The above list includes the fundamental tests needed to ensure successful fabrication and to spot defects. In addition to the basic tests listed above, your board may need to pass through more stringent tests that are designed to stress a PCB to its limits. Once a PCB has passed through the assembly, it might be put through a battery of stress tests to ensure it will meet minimum performance and reliability requirements. PCB stress tests aim to assess long-term and short-term reliability against idealized environmental conditions. Not all boards will need to have this set of tests performed by a manufacturer. For short prototype runs, these tests aren't generally performed, including by a manufacturer. Instead, the bare board and the finished assembly can be evaluated against reliability standards through inspection procedures.

• Vibration tests, e.g., under mil-aero standards
• NEMA/NFPA/FAA fire safety tests
• Thermal shock tests
• HALT/HASS tests (burn-in testing)
• Environmental exposure tests
• Electrical stress tests
• UL safety tests
• Any other product/industry-specific tests (IEC, ISO, etc.)

If your manufacturer cannot perform these more advanced tests, there are specialty testing companies that will qualify new products with a comprehensive methodology and a series of tests to order. UL tests and electrical stress tests are usually the most important when developing consumer or commercial products as they provide the baseline requirements for reliability. For other products in areas like medical, automotive, or aerospace, there will be much stricter standards both in terms of IPC Classes and other industry standards (SAE, MIL-STD, etc.).

Reliability and PCB Failure Analysis

What goes into reliability analysis and understanding the root causes of failure? Once a board is stressed to the point of failure, or it simply doesn't pass the qualifications listed above, some investigation is needed to determine the root cause of failure. The first place to start is with functional testing (see below) to determine which specific features or capabilities have failed. If you start there, then you can narrow down to the specific point in the design where the failure likely occurred. In addition to electrical testing around the board, microsections are often used to investigate which specific points in the design may have failed and to determine the exact mechanism.

If you have access to simulation applications and plenty of computing power, you can even run stress simulations to quantify things like mean time to failure, exact location and types of thermally-induced mechanical failures, and design exploration procedures to determine how the design or fabrication process should change.

When failure is noticed and it is found to occur outside of normally anticipated operating conditions, you can consider this a success as long as the design is compliant with your design and reliability standards. No design is invincible, so don't be surprised if, eventually, the design fails under extreme stress. The goal is to determine that the design can perform reliably under some reasonable expectation of the conditions encountered during deployment. Reliability standards have been developed to address this specific point, and designing your PCB to be compliant with these standards is the first step in ensuring reliability.

Qualification

Before subjecting your board to a battery of reliability tests, make sure you're designing with reliability and safety standards in mind. Certain aspects of a design that determine reliability are mandated by some of the IPC standards:

• IPC-6011 General Performance Specification for Printed Boards
• IPC-6012D Qualification and Performance Specification for Rigid Printed Boards
• IPC-6013D Qualification and Performance Specification for Flexible/Rigid-Flexible Printed Boards

These standards provide specific dimensional guidelines and tolerances to which a manufactured board must adhere. To be clear, the guidelines do not specify specific pad, trace, hole, or other feature sizes that your board must hit. However, they do specify a set of minimum criteria that must be met in a manufactured board for each of the IPC classes of products. Depending on a manufacturer's fabrication allowances and the class of product, certain dimensional targets with which the fabricated board must comply can be determined. A prototypical example is annular rings for Class III devices under the IPC 6012 standard.

PCB Functional Testing

Functional testing for electronics includes a range of possible tests, many of which are focused on ensuring the product provides the desired user experience and end functions that were intended in the design. This is the responsibility of the design team during the prototyping phase, rather than the responsibility of the manufacturer. Remember, your manufacturer’s job is to give you a PCBA that electrically matches the design data you give them, it is not typically their responsibility to perform functional verification unless you can help automate this testing.

In the event the design doesn’t produce the expected functional test results, it’s up to the designer to troubleshoot and debug the design to determine the problem. The designer or test engineer may need to manually gather some electrical measurements, experiment with firmware, and retrace problems through the design to locate the causes of any defects. Once these are located, they can be addressed in the next design revision and, ideally, can be incorporated as test requirements as the product is moved into higher volume.

If you’re making the transition into higher volume, and your product’s functionality or standards conformance requires passing specific electrical, thermal, or mechanical tests, you should specify these for your manufacturer, develop testing procedures in-house, or contract these services through an external testing firm. Talk to them early to make sure they understand what you need and that they have the capabilities to automate these tests to ensure product quality. It takes time up-front to complete these tasks, but you’ll have some piece of mind knowing every possible fault has been anticipated in the design.

The best PCB design tools in Altium Designer® give you everything needed to define PCB testing requirements in your schematics and as attached documents. When you’re ready to send your design out for manufacturing, you can easily release your design data to your manufacturer with the Altium 365™ platform. Altium 365 and Altium Designer give you everything you need to pass a design review, communicate test requirements, and communicate design changes.

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