**Why do I need test points on the PCB?**
Some may wonder: “In PCB circuit board design, why are test points necessary?” This question might still be unclear to some. I recall when I first started as a process engineer at a PCBA (Printed Circuit Board Assembly) facility, I asked many colleagues about the purpose of test points to better understand their function. Essentially, the goal of adding test points is to verify that the components on the PCB meet the required specifications and solderability. For example, if you need to check if a resistor on a circuit board is functioning properly, the simplest way is to measure it with a multimeter. You can easily check by measuring across both ends of the resistor. The details are as follows:
**Why are test points included in PCB circuit board design?**
However, in large-scale production environments, it’s not feasible to manually measure each resistor, capacitor, inductor, or even the IC circuits on every board with a multimeter. This is where automated test equipment, such as ICT (In-Circuit Test) machines, comes into play. These machines use multiple probes—often referred to as “Bed-Of-Nails” fixtures—that simultaneously contact the designated measurement points on the board. The characteristics of the electronic components are then sequentially measured under program control, in a synchronized manner. Typically, it takes about 1 to 2 minutes to test all components on a typical board, depending on its complexity and the number of parts. Naturally, the more components on the board, the longer the test duration.
However, if these probes directly touch the components or their solder pads on the board, there’s a risk of damaging sensitive parts. To prevent this, engineers developed the concept of “test points”—small, exposed circular pads placed at the ends of components. These pads, which do not have a solder mask, provide a safe location for the test probes to make contact, thus protecting the components from potential damage during the testing process.
1. In the early days when traditional through-hole components (DIP) were used on circuit boards, the component leads often served as test points. The leads of these traditional components were robust enough to withstand probe contact without damage, but frequent issues arose due to poor contact, which led to misreadings during tests. This problem was mainly due to the residual solder paste flux left on the leads after wave soldering or SMT soldering. The flux film formed on the surface had a high resistance, which could cause intermittent contact with the test probes. As a result, test operators on the production line were often seen blowing air with spray guns or cleaning these areas with alcohol to improve test reliability.
2. In fact, even after wave soldering, test points still suffered from poor probe contact. However, with the widespread adoption of SMT, the accuracy of testing improved significantly. The role of test points became even more crucial because SMT components are typically very fragile and cannot tolerate the direct pressure of test probes. By using designated test points, it was possible to avoid direct contact with the components and their leads, thus protecting the parts from damage. This approach also indirectly improved test reliability by reducing the chances of misdiagnoses.
3. However, as technology progressed, circuit boards became smaller, and packing more components onto a board became increasingly difficult. As a result, test points, which occupy valuable space on the PCB, often led to tensions between design and manufacturing teams. This issue is a topic worth exploring in more detail at another time. Test points are generally round in shape, as this matches the probe design, making them easier to manufacture. Additionally, using round test points allows probes to be positioned closer together, increasing the density of probes in the bed of nails.
4. The use of a bed of nails for circuit testing has inherent mechanical limitations. For instance, there is a minimum size for the probe diameter, and probes with too small a diameter are prone to breakage. The spacing between the probes also has constraints, as each probe must be inserted through a hole, with the back end soldered to a flat cable. If the hole spacing is too tight, it can lead to short circuits between probes or interference from the flat cables, which becomes a significant issue.
5. Probes also cannot be placed near tall components. If a probe is positioned too close to a high-profile part, there is a risk of collision, which could damage both the probe and the component. Additionally, to avoid these high parts, the test fixture needle bed may need to be customized with holes, further limiting the placement of test points. This becomes a problem as it makes it increasingly difficult to fit test points for all components, particularly as the size of the circuit board shrinks.
6. As circuit boards continue to get smaller, the debate around the number of test points has intensified. Several methods have been developed to reduce the number of test points, such as Net Test, Test Jet, Boundary Scan, and JTAG. Alternative testing techniques, like AOI (Automated Optical Inspection) and X-ray inspection, are also being explored as potential replacements for traditional ICT testing. However, it seems that no single method can completely replace ICT testing.
7. Regarding the capability of ICT needle implantation, it is important to consult with the fixture manufacturer to understand the minimum diameter of the test point and the required spacing between adjacent test points. Manufacturers typically provide both a desired minimum value and the achievable minimum value based on their capabilities. However, large-scale manufacturers may impose further constraints on the test point spacing, as overly tight spacing could lead to damage to the test fixture.
8. In conclusion, the role of test points in PCB layout is essential for ensuring reliable testing. I hope this explanation helps provide a clearer understanding of their importance and challenges.
If your have any questions about PCB ,please contact me info@wellcircuits.com
Some may wonder: “In PCB circuit board design, why are test points necessary?” This question might still be unclear to some. I recall when I first started as a process engineer at a PCBA (Printed Circuit Board Assembly) facility, I asked many colleagues about the purpose of test points to better understand their function. Essentially, the goal of adding test points is to verify that the components on the PCB meet the required specifications and solderability. For example, if you need to check if a resistor on a circuit board is functioning properly, the simplest way is to measure it with a multimeter. You can easily check by measuring across both ends of the resistor. The details are as follows:
**Why are test points included in PCB circuit board design?**
However, in large-scale production environments, it’s not feasible to manually measure each resistor, capacitor, inductor, or even the IC circuits on every board with a multimeter. This is where automated test equipment, such as ICT (In-Circuit Test) machines, comes into play. These machines use multiple probes—often referred to as “Bed-Of-Nails” fixtures—that simultaneously contact the designated measurement points on the board. The characteristics of the electronic components are then sequentially measured under program control, in a synchronized manner. Typically, it takes about 1 to 2 minutes to test all components on a typical board, depending on its complexity and the number of parts. Naturally, the more components on the board, the longer the test duration.
However, if these probes directly touch the components or their solder pads on the board, there’s a risk of damaging sensitive parts. To prevent this, engineers developed the concept of “test points”—small, exposed circular pads placed at the ends of components. These pads, which do not have a solder mask, provide a safe location for the test probes to make contact, thus protecting the components from potential damage during the testing process.
1. In the early days when traditional through-hole components (DIP) were used on circuit boards, the component leads often served as test points. The leads of these traditional components were robust enough to withstand probe contact without damage, but frequent issues arose due to poor contact, which led to misreadings during tests. This problem was mainly due to the residual solder paste flux left on the leads after wave soldering or SMT soldering. The flux film formed on the surface had a high resistance, which could cause intermittent contact with the test probes. As a result, test operators on the production line were often seen blowing air with spray guns or cleaning these areas with alcohol to improve test reliability.
2. In fact, even after wave soldering, test points still suffered from poor probe contact. However, with the widespread adoption of SMT, the accuracy of testing improved significantly. The role of test points became even more crucial because SMT components are typically very fragile and cannot tolerate the direct pressure of test probes. By using designated test points, it was possible to avoid direct contact with the components and their leads, thus protecting the parts from damage. This approach also indirectly improved test reliability by reducing the chances of misdiagnoses.
3. However, as technology progressed, circuit boards became smaller, and packing more components onto a board became increasingly difficult. As a result, test points, which occupy valuable space on the PCB, often led to tensions between design and manufacturing teams. This issue is a topic worth exploring in more detail at another time. Test points are generally round in shape, as this matches the probe design, making them easier to manufacture. Additionally, using round test points allows probes to be positioned closer together, increasing the density of probes in the bed of nails.
4. The use of a bed of nails for circuit testing has inherent mechanical limitations. For instance, there is a minimum size for the probe diameter, and probes with too small a diameter are prone to breakage. The spacing between the probes also has constraints, as each probe must be inserted through a hole, with the back end soldered to a flat cable. If the hole spacing is too tight, it can lead to short circuits between probes or interference from the flat cables, which becomes a significant issue.
5. Probes also cannot be placed near tall components. If a probe is positioned too close to a high-profile part, there is a risk of collision, which could damage both the probe and the component. Additionally, to avoid these high parts, the test fixture needle bed may need to be customized with holes, further limiting the placement of test points. This becomes a problem as it makes it increasingly difficult to fit test points for all components, particularly as the size of the circuit board shrinks.
6. As circuit boards continue to get smaller, the debate around the number of test points has intensified. Several methods have been developed to reduce the number of test points, such as Net Test, Test Jet, Boundary Scan, and JTAG. Alternative testing techniques, like AOI (Automated Optical Inspection) and X-ray inspection, are also being explored as potential replacements for traditional ICT testing. However, it seems that no single method can completely replace ICT testing.
7. Regarding the capability of ICT needle implantation, it is important to consult with the fixture manufacturer to understand the minimum diameter of the test point and the required spacing between adjacent test points. Manufacturers typically provide both a desired minimum value and the achievable minimum value based on their capabilities. However, large-scale manufacturers may impose further constraints on the test point spacing, as overly tight spacing could lead to damage to the test fixture.
8. In conclusion, the role of test points in PCB layout is essential for ensuring reliable testing. I hope this explanation helps provide a clearer understanding of their importance and challenges.
If your have any questions about PCB ,please contact me info@wellcircuits.com
