Vision system makes PCB defects nowhere to escape


In the world of modern electronic products, PCB (printed circuit board) is an important part of electronic products. It is difficult to imagine that there is no PCB in an electronic device, so how the quality of PCB will be long-term and reliable for electronic products. Work has a very big impact. Improving the quality of PCBs is an important issue that electronics manufacturers should pay attention to.

If during the assembly of the PCB, excessive solder paste is applied on the pad, or the solder paste is insufficiently added, or even no solder paste is disposed at all, once the solder joint is formed, the solder joint is formed after the subsequent reflow soldering. This can lead to defects in the electrical connection between the component and the board. In fact, most of the defects can be found by the application of solder paste.

Many manufacturers of circuit boards have used some in-circuit test (ICT) or X-ray technology to detect the quality of solder joints. They will help eliminate defects due to printing process operations, but these methods cannot monitor the printing process operation itself. A printed circuit board may accept subsequent incremental process steps, and each process step will increase production costs to varying degrees, resulting in such a defective circuit board eventually reaching the placement stage of production. Finally, the manufacturer needs to discard this defective board, or it needs to accept the costly and time-consuming waste of rework. There may not be a clear answer at this point to explain the root cause of the defect.

Poor solder paste printing process implementation can cause problems with the connection of 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 board manufacturers find defects in the early stages of production process implementation, more and more screen printing equipment manufacturers are synthesizing online machine vision in their screen printing equipment. technology. The built-in vision system enables three main goals:

First, they are able to directly detect defects that exist after the printing operation is implemented, allowing the operator to deal with the problem in time before the main manufacturing cost is added to the board. This step typically involves when the board is removed from the printing unit, after cleaning in the cleaning agent, and when returning to the production line.

Secondly, because the relevant defects are found at this stage, it is possible to prevent the defective circuit board from being delivered to the rear end of the production line. Thus, the phenomenon of rework or the abandonment formed in some cases is prevented.

Finally, perhaps the most important: to give the operator timely feedback to make it clear that the printing process in operation is good, and thus can effectively prevent defects.

In order to provide effective control during this level of process operation, the online vision system can be configured to detect the presence of pads on the PCB after the solder paste is applied, and whether the corresponding print template gap is clogged or smeared. . In most cases, fine-pitch components are tested to optimize inspection time and focus on the areas that are most prone to problems. For this reason, when the problems that may arise are eliminated, the time spent on the detection is still worthwhile.

Camera positioning and detection

In a typical conventional online visual inspection application, a camera is placed over the circuit board to capture an image of the print location and to transmit the relevant image to the processing system of the visual inspection device. There, the image analysis software will compare the acquired image with a reference image stored in the same location in the device memory.

In this way, the system can confirm whether the applied solder paste is more or less. The same system can also reveal whether the position of the solder paste on the pad is aligned. It can be found that there is excess solder paste between the two pads to form a bridge-like connection phenomenon, which is commonly referred to as the "bridge" phenomenon of many printed circuit board manufacturers.

The work of detecting the print template slits is in the same form. When excess solder paste is deposited on the surface of the print stencil, the vision system can be used to detect if the gap is clogged with solder paste or if a smearing has occurred.

After the defect is discovered, the equipment can automatically request the cleaning operation of the screen below, or warn the operator that there is a problem and need to be repaired. The detection of printed templates can also provide users with very useful data on print quality and consistency.

A key feature of state-of-the-art in-line vision systems is the ability to inspect highly reflective PCB boards and pad surfaces, as well as in uneven light conditions or in the presence of dry solder paste structures. . For example, HASL boards typically exhibit non-uniform flatness, surface contour variability, and reflective capabilities. For the highest quality images, proper lighting also plays a very important role.

Light must be able to "target" the board's fiducials and pads, turning other, imperceptible features into clearly identifiable shapes. In this way, the vision software algorithms can be used in the next step to fully exploit their potential capabilities.

In some specific cases, the vision system can be used to detect the height or volume of the solder paste on the pad, and sometimes it is possible to do this only with an offline detection system. The use of this procedure means that a corresponding degree of accumulation is formed in a given print stencil to confirm that the volume of solder paste is missing on the same pad.

Solder paste inspection

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. Inspection of the PCB

Mainly detects the printing area, printing offset and bridging phenomenon. The detection of the printed area refers to the area of ​​solder paste on each of the pads. Excessive solder paste may cause bridging, and too small solder paste may cause solder joints to be weak. The detection of the print offset is based on whether the amount of solder paste on the pad is different from the specified position. The detection of the bridging phenomenon is for whether the solder paste applied between two adjacent pads exceeds a prescribed amount. These extra solder pastes can cause electrical shorts.

b. Detection of printed templates

The detection of printed templates is mainly for the detection of blocking and smearing. The detection of the clogging means detecting whether or not solder paste has accumulated in the holes in the printing stencil. If the hole is blocked, then the solder paste that may be applied at the next printing point will appear too small. The detection of the tailing refers to whether or not an excessive amount of solder paste is deposited on the surface of the printing template. These excess solder paste may be applied to locations on the board that should not be turned on, 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 prevents manufacturers from wasting costs due to board defects and resulting rework. Perhaps most importantly, it provides continuous process feedback that not only helps manufacturers optimize the screen printing process, but also adds more confidence to the process.
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