In the world of modern electronic products, PCB (printed circuit board) is an important part of electronic products. It is hard to imagine that there is no PCB in an electronic device, so how the quality of PCB will affect whether electronic products can be normal and reliable for a long time Work has a huge impact. Improving the quality of PCB is an important topic that manufacturers of electronic products should pay enough attention to.
If during the PCB assembly process, excessive solder paste is applied to the pad, or insufficient solder paste is added, or even no solder paste is placed at all, then after the subsequent reflow soldering, once the solder joint is formed, It will cause defects in the electronic connection between the component and the circuit board. In fact, most of the defects can be found with the help of solder paste application conditions to find the relevant quality traces.
At present, many manufacturers of circuit boards have adopted some built-in circuit tests (in-circuit test referred to as ICT) or X-ray technology to detect the quality of solder joints. They will help to eliminate defects due to printing process operations, but these methods cannot monitor the printing process operation itself. A misprinted circuit board may undergo subsequent additional process steps, each of which will increase production costs to varying degrees, so that such a defective circuit board ends up in the placement stage of production. In the end, the manufacturer needs to discard the defective board, or needs to accept expensive and time-consuming rework work. At this moment, there may not be a very clear answer to explain the root cause of the defect.
Poor implementation of solder paste printing process can cause connection problems in electronic circuits. In order to effectively solve this problem, many screen printing equipment manufacturers have adopted online machine vision inspection technology, which will be briefly introduced below.
Online Comprehensive Visual Inspection
In order to help manufacturers of circuit boards to detect defects in the early stages of production process implementation, more and more screen printing equipment manufacturers are integrating online machine vision in their screen printing equipment. technology. The built-in vision system achieves three main goals:
First, they can detect existing defect conditions directly after the printing operation is carried out, allowing operators to deal with related problems in a timely manner before major manufacturing costs are added to the circuit board. This step generally includes when the circuit board is removed from the printing device, after it has been cleaned in the cleaning agent, and when it is returned to the production line after rework.
Second, defective boards can be prevented from reaching the back end of the production line because the relevant defects are discovered at this stage. Therefore, the phenomenon of rework or the phenomenon of abandonment formed in some occasions is prevented.
Finally, perhaps the most important: It can give the operator timely feedback to make it clear whether the printing process in operation is operating well, and thus can effectively prevent defects.
In order to provide effective control during the process operation at this level, the online vision system can be configured to detect the condition of the solder pads on the PCB after the solder paste is coated, and whether there is blockage or tailing in the gaps of the corresponding printing templates. . In most cases, fine-pitch components are inspected to optimize inspection time and focus on the most problematic areas. For this reason, the time spent on testing is worth it when possible problems are eliminated.
Camera localization and detection
In the general conventional online visual inspection application, the camera is placed above the circuit board to obtain the image of the printing position, and can send the relevant image to the processing system of the visual inspection equipment. There, image analysis software compares the acquired image to a reference image stored at the same location in the device's memory.
In this way, the system can confirm whether more or less solder paste has been applied. The same system can also reveal whether the solder paste position on the pad is aligned. It can find whether there is excess solder paste forming a bridge-like connection between two pads? This problem is also known as "bridging" by many PCB manufacturers.
The task of detecting gaps in the printing stencil is in a similar fashion. When excess solder paste is deposited on the surface of the printed stencil, the vision system can be used to detect whether the gap is blocked by solder paste, or whether there is tailing.
After a defect is found, the equipment can immediately automatically request a series of cleaning operations for the screen below, or warn the operator that there is a problem and needs to be repaired. Inspection of printed stencils can also provide users with very useful data on print quality and consistency.
A key capability of state-of-the-art in-line vision systems is the ability to inspect highly reflective PCB board and pad surfaces, as well as inspections in uneven light environments or where dry solder paste structures can make a difference . For example, HASL boards typically exhibit non-uniform flatness, variable surface contours, and reflectivity. Proper lighting also plays a very important role in obtaining the highest quality images.
The light must be able to "target" the board's fiducials and pads, which in turn transform otherwise invisible features into clearly identifiable shapes. In this way, vision software algorithms can be used in the next step to fully exploit their potential capabilities.
In some specific occasions, the vision system can be used to detect the height or volume of the solder paste on the pad, and sometimes only the offline inspection system can be used to do these things. Adopting this procedure means forming a corresponding build-up in a given printed stencil to confirm whether the volume of solder paste is missing on the same pad.
Inspection of Solder Paste
Specifically, it can be divided into two categories: the detection of solder paste on the PCB and the detection of solder paste on the printed template:
a. Detection of PCB
It mainly detects printing area, printing offset and bridging phenomenon. The detection of printed area refers to the area of solder paste on each pad. Excessive solder paste may cause bridging, and too little solder paste may also cause weak solder joints. The detection of printing offset is aimed at whether the amount of solder paste on the pad is different from the specified position. The detection of the bridging phenomenon is aimed at whether the solder paste applied between two adjacent pads exceeds the specified amount. This excess solder paste may cause electrical shorts.
b. Detection of printed templates
The detection of printing templates is mainly for the detection of blocking and tailing phenomena. Detection of clogging refers to detecting whether solder paste has accumulated in the holes on the printing stencil. If the holes are blocked, the next printed dot may have too little paste applied. The detection of smearing refers to whether there is excessive accumulation of solder paste on the surface of the printing stencil. This excess solder paste can be applied to places on the board where it should not conduct, causing electrical connection problems.
In-line machine vision systems can benefit PCB manufacturers in different ways. In addition to ensuring high solder joint integrity, it prevents manufacturers from wasting money due to board defects and resulting rework. Perhaps most importantly, it can provide continuous process feedback, which can not only help manufacturers optimize the screen printing process, but also can increase people's confidence in the process operation.
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