PCB Technical - PCBA inspection process/inspection technology∕process overview

PCBA inspection process/inspection technology∕process overview
The overall process of PCBA inspection process!
Note: The detection equipment and installation layout corresponding to various detection methods are generally divided into online (in series in the pipeline) and offline (independent of the pipeline). Under the following conditions, the online inspection process layout should be used first to improve the inspection efficiency and the efficiency of assembly line operations:
2. Inspection technology/process overview
The inspection technology applicable to PCBA products can be mainly divided into: solder paste coating inspection SPI, automatic optical inspection AOI, automatic X-ray inspection AXI, online inspection ICT, flying probe inspection FP, and functional inspection FT.
1. Automatic optical inspection
Detection principle: When the AOI detector automatically detects the PCB, the machine automatically scans the PCB through the camera, collects images, compares the detected solder joints with the qualified parameters in the database, after image processing, detects the defects on the PCB, and uses the display or automatic marking Display/mark the defects for repair by maintenance personnel
Functions and characteristics of detection:

1) Automatic optical inspection (AOI) uses high-speed and high-precision visual processing technology to automatically detect various mounting errors and soldering defects on PCB boards. PCB boards can range from fine-pitch high-density boards to low-density large-size boards, and can provide online inspection solutions to improve production efficiency and welding quality;
2) By using AOI as a tool to reduce defects, find and eliminate errors in the early stage of the assembly process to achieve good process control. Early detection of defects will avoid sending substandard products to the subsequent assembly stage. AOI will reduce repair costs and avoid scrapping unrepairable PCBs.
2. AOI inspection content:
1) Check the top surface reflow soldering components;
2) Check the through-hole components before wave soldering;
3) Check the through holes and SMD/SMC after wave soldering;
4) Check the connector pins before press-fitting
5) Check the connector pins after press-fitting.
3. Check the setting of monitoring points.
AOI can be applied to multiple inspection points on the production line, but three inspection points are the main ones, namely after solder paste printing, before reflow soldering, and after reflow soldering
1) After solder paste printing. If the solder paste printing process meets the requirements, soldering defects caused by printing defects will be greatly reduced. Typical printing defects include the following:
Insufficient solder paste on the pad; too much solder paste on the pad; poor overlap of the solder paste on the pad; solder bridges between the pads.
The inspection of this inspection point most directly supports process tracking. The quantitative process control data at this stage includes printing offset and solder volume information and qualitative information about printed solder paste
2) Before reflow soldering. The inspection of this inspection point is completed after the component placement is completed and before the PCB is sent to the reflow oven. This is a typical inspection point where most defects from solder paste printing and machine placement can be found. The quantitative process control information generated at this location provides information on the calibration of high-speed chip machines and fine-pitch component placement equipment.
This information can be used to modify component placement data or indicate that the placement machine needs calibration. The inspection of this inspection point meets the goal of process tracking.
3) After reflow soldering. This inspection point is checked at the last step of the SMT process. It is the most important inspection point of AOI, and all assembly errors can be found. Post-reflow soldering inspection provides a high degree of security and can identify errors caused by solder paste printing, component placement, and reflow processes.
Although each inspection point can detect defects with different characteristics, the AOI inspection equipment should be placed in a detection position that can identify and correct the most defects as soon as possible.
3. Online Testing (ICT)
1. Detection principle.
ICT detection is mainly to detect the open circuit, short circuit, and welding of all components of the PCBA circuit by contacting the detection point of the PCB assembly with the detection probe. And can accurately identify the fault location of the PCBA (has a high recognition ability for the welding detection of the components).
2. Testing functions and characteristics:
1) Within a few seconds, all the components on the assembled circuit board can be detected: resistors, capacitors, inductors, transistors, FETs (field effect tubes), LEDs (light emitting diodes), ordinary diodes, Zener diodes, Optical lotus root, IC, etc., whether they meet the design requirements;
2) Be able to find out the defects of the process in advance, such as short circuit, open circuit, missing parts, reverse, wrong parts, empty welding, etc., and feedback to the improvement of the process;
3) The above detected fault or error information can be printed out through the printer. This information mainly includes the fault location, the standard value of the part, and the detected value for the reference of the maintenance personnel. It can effectively reduce the personnel's dependence on product technology, without the need to understand the product line, and also have the ability to maintain;
4) The defect information can be detected and statistically output, and the production management personnel can analyze it to find out the causes of various defects, including human factors, so that they can be solved, improved, and corrected one by one, thereby improving PCBA manufacturing capabilities.
3. Flying probe detection (FP)
1. Detection principle:
1) The open circuit detection principle of flying probe detection is the same as that of ICT. Two probes are connected to the end of the network at the same time to energize, and the obtained resistance is compared with the set open circuit resistance to judge whether the circuit is open or not. But the principle of short-circuit detection is different from that of ICT
2) Due to the limited detection probes (usually 40032 probes) and the number of points contacting the board surface at the same time is very small (corresponding to 40032 points), if the resistance measurement method is used to measure the resistance between all networks, then there are N networks For the PCB, it is necessary to perform N2/2 inspections, and the probe moving speed is limited, generally 10 points/sec to 50 points/sec, so the efficiency of flying probe detection is relatively low.