The advantages of needle bed testing in PCB manufacturing
Fast testing speed: The needle bed test adopts a relatively parallel testing method, and the testing speed is very fast. For example, at the threshold of 100V and 10MΩ, the maximum number of test points can reach 10,000 per second. At a threshold of 250V and 100MΩ, 3,000 test points can be measured per second. For large-scale production, it can significantly improve production efficiency.
High stability: Suitable for PCB testing with large batch sizes and dense test points, such as mobile phone motherboards, etc. Its testing process is stable and can ensure the consistency of testing quality in large-scale production.
Accurate fault location: It can immediately identify and determine defects. The tested faults are directly located on specific components, device pins, and network points, ensuring accurate fault location. The repair of faults does not require much professional knowledge, which can significantly improve production efficiency and reduce maintenance costs.
Comprehensive functional testing: It can conduct functional tests on analog devices and logical functions of digital devices, with a high fault coverage rate. It is capable of quantitatively measuring devices such as resistors, capacitors, inductors, and crystal oscillators, conducting functional tests on diodes, transistors, optocouplers, transformers, relays, operational amplifiers, power modules, etc., and performing functional tests on medium and small-scale integrated circuits, such as all 74 series, Memory class, common driver class, switching class and other ics.
Specific test items: It is one of the important testing methods for quality assurance in modern large-scale production. By directly testing the electrical performance of online devices, it can detect manufacturing process defects and component defects. For component types, it can check for component values that are out of tolerance, failed or damaged; for Memory types, it can detect program errors, etc. For process types, it can detect issues such as solder short circuits, incorrect or reverse insertion of components, and missed installation. Faults such as lifted pins, false soldering, PCB short circuits and broken wires.
The limitations of needle bed testing in PCB manufacturing
Mechanical accuracy limitation:
Fixture drilling accuracy: When a well-maintained needle bed drills thick fixture boards, it is difficult to control the accuracy below 25um. For some high-precision PCB testing fixtures, the number of layers can be as high as 8, further increasing the difficulty of drilling accuracy control.
Deviation between PCB hole positions and outer layer patterns: In the manufacturing of multi-layer PCBS, to prevent inner layer breakage and increase the pass rate, it is often the case that after lamination, positioning holes are drilled based on the relative positions of the patterns on each layer. The higher the number of layers, the greater the difference in position between the holes and the outer layer patterns. The positions of the upper and lower surfaces of the PCB may also differ by ±0.15mm, which will affect the accurate contact between the test probe and the test point.
Alignment accuracy issue: When testing PCBS, the alignment accuracy requirements between the PCB and the fixture as well as between the fixture and the equipment are relatively high. To facilitate the placement and removal of the fixture on the needle bed, if pin positioning is adopted, the diameter difference between the pin and the pin hole should be 10-20um. Even the slightest deviation may affect the test results.
Test probe movement issue: In multi-layer fixtures, if there is a slight deviation, causing the probe to rub or get stuck, it will result in an open circuit false alarm. Excessively high density not only leads to a decrease in the strength of each layer of the fixture and the occurrence of bending and other phenomena, but also causes positional deviation of the probe. PCB warping can cause changes in the position of the test target. In severe cases, the probe cannot reach the tested surface, resulting in false alarms.
Dimensional stability influence: PCB dimensional stability and dimensional consistency errors between the fixture and the PCB. For the same type of PCB, due to differences in manufacturing conditions (batch manufacturing), environmental temperature and humidity, the size of the negative and the substrate will change, resulting in minor differences in the graphic size of the same type of PCB. If the board surface is large and the density is high, it will directly affect the test accuracy. Similarly, the size of the fixture may also show microscopic differences due to changes in the environment, which has a significant impact on the test accuracy.
Test point limit: For dedicated needle bed testing machines, there is also a limit on the number of test points, which shall not exceed 15,000 at most. For high-density PCBS, the test area is only within 200mm×200mm. Many PCB manufacturers have no choice but to turn to mobile probe testing. It is relatively economical when the batch size is small, but in cases of large batches and tight schedules, testing will become a bottleneck in production.
Test cost issue:
Equipment cost: The needle bed tester is expensive and difficult to maintain. The needles are selected with probes of different arrangements according to their specific applications, which increases the equipment cost.
Fixture cost: The dedicated needle bed testing machine requires the production of dedicated testing fixtures. The testing fixtures are relatively complex, with high manufacturing costs and long manufacturing cycles. The one-time investment in a general-purpose grid testing machine is relatively high, especially when the grid size decreases or the density of PCB test points doubles (such as from 2.54mm to 1.27mm, or even smaller), the investment will be even higher.
