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What Is PCB Functional Testing?

pcb functional testingPCB testing is a necessary expense that, when done correctly, prevents much larger and more embarrassing damage control when your product goes to market.

Functional PCB (printed circuit board) testing is performed at the end of the manufacturing process to ensure that the manufactured part will not fail immediately or have a shortened service life.

Let’s distinguish between PCB functional testing and other PCB testing, then get into the specifics and advantages of each.

PCB Functional Testing

PCB functional tests include:

  • X-ray inspection
  • Micro-sectioning analysis
  • PCB contamination test
  • PCB solderability test
  • Time-domain reflectometer (TDR)
  • Peel test
  • Solder float test

Why functional testing, specifically, vs. another type of PCB test? Functional PCB testing has some clear benefits, such as:

  • Saves money by removing the need to provide testing equipment to your electronics contract manufacturer (ECM)
  • Works effectively in tandem with in-circuit testing and flying probe testing (see more below)
  • Can test virtually every PCB being shipped (vs. a select few) so the product can be debugged before it reaches the customer

How Functional Testing Works

PCB functional testing examines the whole assembly, not just its components. This kind of circuit board testing simulates the environment in which the actual assembly will be required to perform. With functional testing, it’s possible to test up to 100% of the products before an ECM ships them to an original equipment manufacturer (OEM, AKA you).

Here are some popular PCB functional tests and how they do their job.

X-ray Inspection

Also known as AXI, X-ray inspection involves 2D or 3D digital images that show every layer of the PCB. So, the inspection is completed in one move. The procedure allows a look at the workings of the entire board. A schematic provides a reference for comparison.

Among the advantages of AXI are:

  • An inside view. The hidden elements are no longer hidden, and they can be checked for functionality.
  • Complex PCBs can be inspected, even those with RF shielding.

AXI does require operators with training and experience, but that’s a nonissue if you’re working with a contract electronics manufacturer you trust. The test can result in significant time and cost savings in spite of the additional personnel expenditures.

Micro-Sectioning Analysis

Sometimes called cross-sectioning or metallographic preparation, micro-sectioning analysis takes a 2D sample that reveals inner workings. The test helps identify:

  • Opens
  • Shorts
  • Solder reflow
  • Defective components
  • Thermo-mechanical problems

Micro-sectioning is destructive, but with it, a component that isn’t performing properly can be removed. The component is then put into a curing epoxy, exposed by means of abrasion, and compared with a functioning component.

With a closer look, a technician can see:

  • Thickness of the plate
  • Failure of solder joints
  • Intermittent thickness of a metallic layer

Contamination Testing

PCBs can be contaminated in more ways than we care to admit, including:

  • Flux residue
  • Reaction products
  • Human byproducts
  • Handling

The results of contamination can include, among others:

  • Corrosion
  • Degradation
  • Metallization

PCB contamination happens frequently. So testing for it is essential to proper component performance.

Known also as resistivity of solvent extract, or ROSE, testing, contamination testing is designed to find the bulk ionics and contaminate boards. The process involves a zero-ion or other ionic testing unit drawing ions on the board into a solvent. The ionic contamination is then measured and plotted on a curve to compare it to industry standards.

Solderability Testing

Testing component and PCB pad solderability can help:

  • Reduce assembly problems, such as misapplications of the solder mask
  • Ensure surface sturdiness
  • Increase the probability of reliable solder joints
  • Verify that no adverse effects on solderability will occur during storage

This test is great for a variety of production factors:

  • Solder evaluation
  • PCB coating evaluation
  • Flux evaluation
  • Quality control
  • Benchmarking

Time-domain reflectometer (TDR)

Although designed to identify faults and discontinuities in metallic cable connectors and other electrical paths, it also works for PCB testing.

Specialized TDRs can be used to locate failures in high-frequency boards, particularly those that act like transmission lines. The instrument spots reflections revealing unsoldered pins of a ball grid array, short-circuited pins and more.

Peel Test

As a general rule, peel tests intended for flexible adhesives measure resistance to highly localized stresses. Specific to PCBs, this test measures the strength needed to peel laminate from the board.

Solder Float Test

The float test determines the thermal stress a board’s holes can withstand.

… Or Should Your Project Go in a Different Direction?

Work with your electronics contract manufacturer on a case-by-case basis to determine whether you need functional testing or a more intensive (and expensive) test -- or both.

There are many other tests that may be a better fit than PCB functional tests. These include:

  • In-circuit testing (ICT): Also called bed-of-nails testing, this examination uses needles and access points designed into the PCB to connect with a circuit and determine values. In-circuit PCB testing helps your contractor determine whether any electronic components are defective. With ICT, it’s possible to perform the procedures without introducing power. This protects the component from damage.

  • Flying probe testing: This offers examination without power for a variety of factors, such as resistance, inductance, and shorts.

  • Automated optical inspection: This uses photos of the circuit board to compare it with a schematic.

  • Burn-in testing: This provides information about the PCB’s ability to perform at high temperatures. The trade-off is a smaller volume of yield and potentially reduced product lifespan. Still, the data collected can help engineers understand what caused the defects and modify the design to improve reliability before it hits the burn-in stage.

Your ECM’s PCB Testing Capabilities

To figure out which combination of in-circuit and functional tests will help ensure the highest quality for your PCBs, talk to your electronics manufacturing services provider early and often. Doing so in the design phase will ensure the design is not only manufacturable, but also is compatible with the tests ideal for your project. Bonus points for your ECM if it has both testing and design assistance capabilities!

Ultimate-Guide-to-PCB-Testing-Download-Here

 

Author: Matric