Why Do PCBs Have Those Small Circles? What purpose of Test Points on PCBs?

Why Do PCBs Have Those Small Circles? What purpose of Test Points on PCBs?

Have you ever noticed those tiny circles on PCBs and wondered why they’re there? They don’t seem to have any parts soldered onto them, and some don’t even have solder paste. Well, those small circles are called test points, and they play a crucial role in ensuring that the electronics on the PCB are working correctly during the assembly process.

For those who know about electronics, having test points on a PCB is common knowledge. However, for those with a background in mechanics or other fields, the term “test point” might be a bit confusing.

Simply put, test points are like checkpoints that help us test whether the components on a circuit board meet certain standards and if they’re soldered properly. For instance, if we want to check if a resistor on the circuit board is functioning as it should, the easiest way is to use a tool called a multimeter to measure the electrical connections at both ends of the resistor. The same idea applies to other components like capacitors, inductors, and even complex IC circuits.

Now, in large-scale manufacturing, it’s not practical to manually test every single resistor, capacitor, inductor, or IC circuit on each board. That’s where automated testing machines, such as ICT (In-Circuit-Test) or MDA (Manufacturing-Defect-Analyzer), come in. These machines use special probes (often called a “Bed-Of-Nails” fixture) to quickly test all the necessary circuits on the PCB. They follow a programmed sequence to measure the electrical properties of these components. The time it takes for such testing varies depending on the number of components on the board but typically ranges from 30 to 120 seconds.

However, if the probes were to touch the electronic components or their solder joints directly, it could potentially damage these parts. That’s why clever engineers came up with the idea of “test points.” These are pairs of small circles added to the extra ends of components, with no protective covering. These points allow the testing equipment’s probes to make direct contact without risking damage to the electronic components that need to be measured.

Test points on the PCBIn the early days of circuit boards assembly, people often used the sturdy solder joints of through-hole devices (THDs) as test points. However, this method had drawbacks, as the solder surfaces developed a thin film after wave soldering or surface mount assembly (SMA). This film caused poor probe contact due to its high impedance. To address this, operators would use air guns or wipe the areas with alcohol to improve probe contact.

Even if these test points on PCBs only undergo wave soldering, there can still be issues with poor probe contact because they might get a bit messy with solder flux residue. Until the Surface Mount Technology (SMT) became popular, situations where testing was misled due to flux residue saw significant improvement. The application of test points became crucial as SMT components themselves are typically fragile and cannot endure the direct pressure from testing probes. Using test points allows probes to avoid direct contact with the components and their solder joint. This not only safeguards the components from damage but also significantly enhances the reliability of testing by reducing instances of misjudgment.

As technology advances and PCBs get smaller, fitting numerous electronic components onto a small PCB becomes challenging. The issue of test points occupying PCB space becomes an argument between design and manufacturing. We may explore this topic more in the future.

Test points typically have a circular shape, matching the cylindrical probes. This design is practical for production and allows closer placement of adjacent probes, increasing the probing density of the nail bed.

Conducting tests on PCBs using a bed-of-nails fixture comes with some built-in limitations in terms of mechanics. Here are the key points made simpler:

  • Probe Size: There’s a certain limit to the minimum diameter of the probes. If the diameter is too small, the probes are prone to breaking during operation.

  • Probe Spacing: The distance between probes also has limitations. Each probe needs to come out of a hole in the fixture, and each probe’s base needs to be connected to a wire. If adjacent holes are too small, it not only leads to potential contact short between probes but also poses interference issues with the wires. For large or intricate fixtures, one alternative is creating additional PCB to replace the wires at the base of the probes.

  • Tall Components: Some components with greater height are challenging to probe. If the probes get too close to taller components, there’s a risk of collision and damage. Additionally, due to the increased height, openings must be made in the test fixture bed to accommodate these components, indirectly causing difficulties in probing.

  • Limited Test Points:The increasing difficulty of accommodating all test points on the PCB. As boards get smaller, the discussion around the quantity and viability of test points has become more prevalent. Methods to reduce test points have emerged, such as Net test, Test Jet, Boundary Scan, JTAG, among others. There are alternative testing methods seeking to replace traditional bed-of-nails testing, like AOI and X-Ray, but so far, none seem capable of fully replacing ICT (In-Circuit Test). (Related article: Is removing the In-Circuit Test (ICT) really more cost-effective?)

Regarding the probing capability of ICT, it’s advisable to consult with the testing fixture house. This includes inquiring about the minimum diameter of test points and the minimum distance between adjacent test points. Typically, there’s a defined minimum value and an optimal capability that can be achieved. However, larger companies often impose restrictions on the maximum number of test points and the minimum distance between them to prevent fixture damage.

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