Simply the purpose of setting test point is to test the circuit boards have components conform to the specifications and welding, such as when we want to check the resistance of the circuit board if have any questions, the simplest way is to test its two ends by multimeter.
But In mass production of the factory there is no way to let you use the meter to slowly on the measurement Of each board to each measuring resistor, capacitor, inductor, and even the IC Circuit is correct, so have the so-called ICT (In Circuit Test) - automated Test machine, it USES the dogan probe (commonly called "needle Bed (Bed - Of - Nails)" fixture) and contact the board all parts need to be measured on line, and then through the program control step by measuring the characteristics Of the electronic parts, usually such testing generally all parts Of the board only need 1 ~ 2 minutes time to complete, depending on the Circuit components on the amount, the longer the more parts.
If you allow the probe contact directly to the board on electronic components or above its leg, it is likely to be crushing some electronic components, the opposite, so clever engineers invented the "test point", in the ends of the additional parts to elicit a pair of round dots, there is no welding (mask), can make a test probe access to these small points, without direct contact to the measurement of electronic parts.
Early time due to parts of traditional welding feet strong enough, don't worry about needle, but often have poor contact probes of misjudgment occurs, because the general electronic components after wave soldering, or SMT solder, on the surface of the solder usually form a layer of solder paste flux residual film, the film of the impedance is very high, often cause poor contact probe, so at that time, are a common test operators of production line, regularly take a hard blow air gun, or with alcohol to wipe the need to test.
Later after SMT prevailed, test misjudgment situation had very big improvement, the application of the test point has also been greatly rewarded, because of the SMT parts are usually very weak, cannot bear the direct contact with the pressure of test probe, using the test point can not let the probe in direct contact to the parts and its welding feet, not only to protect parts from harm, also indirectly promote reliability testing, because the misjudgment of less.
The test points is usually round, because the probe is also round and relatively good, and it is easy to make the adjacent probe close to the point so that the needle density of the needle bed can be increased.
As the boards are smaller and smaller, the test points of the lower parts of the board are becoming more and more difficult to hold.Now there are some methods for reducing test points, such as Net test, test Jet, Boundary Scan, JTAG, etc.There are other test methods that want to replace the original needle bed test, such as AOI, X-ray, but at present every test seems to be unable to replace ICT 100%.