PCB News - Does the vision system help PCB technology?

In the world of modern electronic products, PCB (printed circuit board) is an important part of electronic products. It is difficult to imagine that an electronic device does not use PCB, so how the quality of PCB will affect whether electronic products can be normal and reliable for a long time Work has a very big impact. Improving the quality of PCB is an important issue that electronic product manufacturers should pay enough attention to.

If excessive solder paste is applied to the pad during the PCB assembly process, or the solder paste is insufficiently added, or even no solder paste is placed at all, then after the subsequent reflow soldering, once the solder joints are formed, It will cause defects in the electronic connection between the components and the circuit board. In fact, most of the defects can be found with the help of the solder paste application to find the relevant quality marks.

At present, many circuit board manufacturers have adopted some built-in circuit test (in-circuittest, referred to as ICT) or X-ray technology to detect the quality of solder joints. They will help eliminate defects due to the printing process operation, but these methods cannot monitor the printing process operation itself. The circuit board with the wrong printing may accept the additional process steps, and each process step will increase the production cost to varying degrees, so that such a defective circuit board will eventually reach the placement stage of production. Finally, the manufacturer needs to discard the defective circuit board, or need to accept the costly and wasteful repair 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 is briefly introduced below.

Online comprehensive visual inspection

In order to help circuit board manufacturers to find defects in the early stages of the implementation of the production process, more and more screen printing equipment manufacturers have integrated online machine vision in their screen printing equipment. technology. The built-in vision system can achieve three main goals:

First of all, they can directly discover the existing defects after the printing operation is implemented, and before the main manufacturing cost is added to the circuit board, they can allow the operator to deal with the related problems in time. This step generally includes when the circuit board is removed from the printing device, after it is cleaned in the cleaning agent, and when it is returned to the production line after being repaired.

Secondly, because related defects are found at this stage, it is possible to prevent defective circuit boards from reaching the back end of the production line. Therefore, the repair phenomenon or the abandonment phenomenon formed in some occasions is prevented.

Finally, and perhaps the most important: to be able to give timely feedback to the operator to make it clear whether the printing process in operation is operating well, and thus can effectively prevent the occurrence of defects.

In order to provide effective control in the process of this level of process operation, the online vision system is configured to detect the condition of the solder paste on the PCB after the solder paste is applied, and whether the corresponding printing template gap is blocked or tailed . In most cases, the inspection of fine-pitch components is to optimize the inspection time and focus on the most problem-prone areas. For this reason, when the possible problems are eliminated, the time spent on detection is still worthwhile.

Camera positioning and detection

In general and conventional online visual inspection applications, the camera is placed above the circuit board to obtain images of the printing position, and can send the relevant images to the processing system of the visual inspection equipment. There, the image analysis software compares the acquired image with a reference image stored in the same location in the device's memory.

In this way, the system can confirm whether more or less solder paste is applied. The same system can also reveal whether the solder paste position on the pad is aligned. It can be found whether there is excess solder paste forming a bridge-like connection between the two pads? This problem is also known as the "bridging" phenomenon commonly known by many printed circuit board manufacturers.

The work of detecting gaps in the printing stencil is in the same form. When the excess solder paste is accumulated on the surface of the printing template, the vision system can be used to detect whether the gap is blocked by the solder paste, or whether there is a tailing phenomenon.

After the defect is found, the equipment can immediately automatically request a series of cleaning operations on the screen below, or warn the operator that there is a problem that needs to be repaired. The detection of printing templates can also provide users with very useful data about printing quality and consistency.

A key function of the most advanced online vision system is the ability to inspect PCB circuit boards and pad surfaces with high reflectivity, and inspect under uneven light conditions or under conditions where the structure of dry solder paste causes differences. . For example, HASL circuit boards generally exhibit uneven flatness, variable surface contours, and reflectivity. To obtain the highest quality images, proper lighting also plays a very important role.

The light must be able to "aim" at the fiducials and pads of the circuit board, turning other undetectable features into clearly recognizable shapes. In this way, the vision software rules system (visionsoftwarealgorithms) can be used in the next step to give full play to 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 an offline inspection system may be used to do these things. Adopting this procedure means forming a corresponding degree of accumulation in a given printing template to confirm whether the volume of solder paste on the same pad is missing.

Solder paste inspection

Specifically, it can be divided into two categories: inspection of solder paste on PCB and inspection of solder paste on printed templates:

a. Inspection of PCB

Mainly detect printing area, printing offset and bridging phenomenon. The inspection of the printed area refers to the area of solder paste on each pad. Excessive solder paste may cause bridging, and too small solder paste may also cause weak solder joints. The detection of printing offset is based on whether the amount of solder paste on the pad is different from the specified position. The detection of bridging is for whether the solder paste applied between two adjacent pads exceeds the prescribed amount. This excess solder paste may cause electrical shorts.

b. Detection of printing templates

The detection of printing templates is mainly for the detection of blocking and trailing phenomena. Detection of clogging refers to detecting whether solder paste has accumulated in the holes on the printing template. If the hole is blocked, then the solder paste applied at the next printing point may appear to be too little. The detection of smearing refers to whether there is excessive solder paste accumulated on the surface of the printing stencil. These excessive amounts of solder paste may be applied to areas on the circuit board that should not be conductive, causing electrical connection problems.

Online machine vision systems can benefit PCB manufacturers in different ways. In addition to ensuring the high integrity of the solder joints, it can prevent manufacturers from wasting costs due to circuit board defects and the resulting rework. Perhaps most importantly, it can provide continuous feedback on the process, which will not only help manufacturers optimize the screen printing process, but also increase people's confidence in the process.