PCB Reliability and Repeatability

Defects and faults are virtually the same. One leads to the other. For instance, a digital IC-output pin that doesn’t toggle correctly is a defect that results in a fault. In-circuit (ICT) and functional tests detect faults. But there can be defects that do not show up as faults. These include insufficient or excessive solder, misaligned components, marginal joints, and open power pins.

Thermal profiling details temperatures in a solder reflow oven at different zones located on top and bottom of the oven. A Printed Circuit Board’s complexity and construction including the number of ground planes for multilayer boards dictate what temperature profile works best while the number of components and their “spread density” on a board determines the profile complexity.

Thermal profiling has two primary objectives:

• Determine the process settings for a given printed circuit board assembly and
• verify process consistency for repeatable results. Without verifying consistency, the assembly or PCB can miss reliability targets. And poor thermal profiles risk poor solder joints and damaged components.

Flying probe testing is best for low volume, highly complex assemblies. It is easy to set up, conduct, and check for open/short circuits and wrong values. This test also verifies component placement and identifies missing components. However, it does not perform power-up testing or check for functional failures.

ICT testing is the most tedious of all PCB tests. It is cumbersome and expensive. Creating an ICT fixture costs from $10,000 to $50,000 and takes four to six weeks to build. However, ICT is ideal for mature products requiring high volume production. It runs the power signal to check voltage levels and resistance at different nodes of the board. ICT is excellent at detecting parametric failures, PCB design-related faults, and component failures.

Functional testing verifies board operation and behavior. The PCB is subjected to a sequence of signals and supply voltages. Responses are monitored at specific points to ensure the board operates correctly. An engineer usually specs the test and the OEM defines test procedures. This test is best at detecting wrong component values, and functional and parametric failures.

If a PCB project is limited to a prototype, the California PCB Assembly will probably not want to pay the high prices associated with ICT fixtures. Instead, it will likely rely on a basic flying probe or power-up test. However, if a particular PCB is mature and has a long product life, and has been designed for a highly reliable Design For Testability (DFT) system, the OEM will probably pay for the ICT costs. Each testing stage finds failures, but Printed Circuit Boards that pass the final test are sure to work properly.