For those studying electronics with FR4 PCBs, it is common to place test points on circuit boards. However, for those studying mechanics, the concept of a test point may not be as familiar.
The primary purpose of setting test points is to verify whether the components on the circuit board meet the required specifications and solderability. For example, if you need to check for faulty resistance on a circuit board, the simplest method is to measure the two ends with a multimeter. However, in a mass production facility, it is not practical to manually measure the correctness of each resistance, capacitance, inductance, and IC circuit on every board. This is where the In-Circuit-Test (ICT) automatic test machine comes in, utilizing multiple probes (commonly known as “Bed-Of-Nails” fixture) to simultaneously make contact with all the parts and circuits on the board that need to be measured. The characteristics of these electronic parts are then sequentially measured under program control, with sequence as the main part and parallel as the auxiliary part. Generally, it takes only 1 to 2 minutes to test all the parts of a standard board, with the testing time increasing based on the number of parts on the circuit board.
However, directly allowing these probes to make contact with the electronic parts on the board or their soldering points could potentially damage some electronic components, which would be counterproductive. To address this issue, engineers have implemented the use of test points. Test points lead out a pair of circular small points at both ends of the component, without a mask on the upper surface, allowing the test probe to make contact with these small points instead of directly with the measured electronic parts. This method serves to protect the electronic components while enabling efficient testing.
In the early days of circuit board technology, traditional plug-in (DIP) was commonly used. During this time, the solder leg of the electronic part served as the test point due to its ability to withstand needling. However, poor contact often occurred due to a residual film of solder paste flux forming on the solder surface after wave soldering or SMT eating tin. This film has a high impedance, leading to poor probe contact. As a result, test operators often resorted to blowing with an air gun or wiping with alcohol.
Even after wave soldering, poor probe contact at the test point was still a concern. The prevalence of SMT improved the situation, as it made test misjudgments less common. The use of test points became crucial as SMT parts are fragile and cannot bear direct probe pressure. Test points ensure that the probe does not directly touch the parts or their welding feet, protecting them from damage and improving test reliability.
As technology advances and circuit boards shrink in size, the issue of test points occupying space becomes a battle between design and manufacturing. This issue will be further discussed later. Test points are usually round to accommodate round probes and increase needle planting density of the needle bed.
There are limitations to using a needle bed for circuit testing. The minimum probe diameter has a limit, and too small probes are prone to breakage. The distance between pins is also restricted, as each pin must come out of a hole and be welded with a flat cable at the back end. Needles cannot be planted beside high parts due to the risk of collision and damage. The shrinking size of circuit boards makes it increasingly difficult to accommodate test points for all parts.
As board size decreases, there has been ongoing discussion on reducing test points through methods such as Net test, Test Jet, Boundary Scan, and JTAG. There are also alternative test methods like AOI and X-Ray, but none seem to fully replace ICT 100%. Manufacturers should be consulted on the needle planting ability of ICT, including the minimum test point diameter and distance between adjacent test points. Manufacturers with scale may impose additional requirements to prevent fixture damage.
The primary purpose of setting test points is to verify whether the components on the circuit board meet the required specifications and solderability. For example, if you need to check for faulty resistance on a circuit board, the simplest method is to measure the two ends with a multimeter. However, in a mass production facility, it is not practical to manually measure the correctness of each resistance, capacitance, inductance, and IC circuit on every board. This is where the In-Circuit-Test (ICT) automatic test machine comes in, utilizing multiple probes (commonly known as “Bed-Of-Nails” fixture) to simultaneously make contact with all the parts and circuits on the board that need to be measured. The characteristics of these electronic parts are then sequentially measured under program control, with sequence as the main part and parallel as the auxiliary part. Generally, it takes only 1 to 2 minutes to test all the parts of a standard board, with the testing time increasing based on the number of parts on the circuit board.
However, directly allowing these probes to make contact with the electronic parts on the board or their soldering points could potentially damage some electronic components, which would be counterproductive. To address this issue, engineers have implemented the use of test points. Test points lead out a pair of circular small points at both ends of the component, without a mask on the upper surface, allowing the test probe to make contact with these small points instead of directly with the measured electronic parts. This method serves to protect the electronic components while enabling efficient testing.
In the early days of circuit board technology, traditional plug-in (DIP) was commonly used. During this time, the solder leg of the electronic part served as the test point due to its ability to withstand needling. However, poor contact often occurred due to a residual film of solder paste flux forming on the solder surface after wave soldering or SMT eating tin. This film has a high impedance, leading to poor probe contact. As a result, test operators often resorted to blowing with an air gun or wiping with alcohol.
Even after wave soldering, poor probe contact at the test point was still a concern. The prevalence of SMT improved the situation, as it made test misjudgments less common. The use of test points became crucial as SMT parts are fragile and cannot bear direct probe pressure. Test points ensure that the probe does not directly touch the parts or their welding feet, protecting them from damage and improving test reliability.
As technology advances and circuit boards shrink in size, the issue of test points occupying space becomes a battle between design and manufacturing. This issue will be further discussed later. Test points are usually round to accommodate round probes and increase needle planting density of the needle bed.
There are limitations to using a needle bed for circuit testing. The minimum probe diameter has a limit, and too small probes are prone to breakage. The distance between pins is also restricted, as each pin must come out of a hole and be welded with a flat cable at the back end. Needles cannot be planted beside high parts due to the risk of collision and damage. The shrinking size of circuit boards makes it increasingly difficult to accommodate test points for all parts.
As board size decreases, there has been ongoing discussion on reducing test points through methods such as Net test, Test Jet, Boundary Scan, and JTAG. There are also alternative test methods like AOI and X-Ray, but none seem to fully replace ICT 100%. Manufacturers should be consulted on the needle planting ability of ICT, including the minimum test point diameter and distance between adjacent test points. Manufacturers with scale may impose additional requirements to prevent fixture damage.
