PCB News - The visual system makes PCB defects impossible to escape

In the world of modern electronic products,printed circuit board is an important part of electronic products. It is hard to imagine that any electronic equipment does not use PCB, so the quality of PCB will have a great impact on the long-term normal and reliable operation of electronic products. Improving the quality of PCB is an important issue which should be paid enough attention to by electronic product manufacturers.

If excess or insufficient solder paste is applied to the pad during PCB assembly, or if no solder paste is placed at all, the electronic connection between the component and the circuit board will be defective once solder spots are formed following subsequent reflow soldering. In fact, the majority of defects can be found by the application of solder paste related quality traces.

At present, many circuit board manufacturers have adopted some in-circuittest (ICT) or X-ray technology to detect the quality of solder joints. They will help eliminate defects arising from the printing process operation, but they cannot monitor the printing process operation itself. A misprinted circuit board may be subjected to additional process steps, each of which increases the cost of production to some extent, leading to the final assembly stage of the defective circuit board. The manufacturer may have to discard the defective circuit board or undergo costly and time-consuming repair work. At the moment, there may not be a clear answer to the root cause of the defect.

Poor solder paste printing process implementation can cause problems in 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 integrated visual inspection

An increasing number of screen printing equipment manufacturers are incorporating on-line machine vision technology into their screen printing equipment to help circuit board manufacturers detect defects in the early stages of their process implementation. The built-in vision system accomplishes three main goals:

First, they are able to detect defects directly after a printing operation has been carried out, allowing the operator to deal with problems before major manufacturing costs are added to the board. This is usually done when the board is removed from the printing unit, after it has been cleaned in a cleaning agent, and after it has been repaired and returned to the production line.

Second, because defects are found at this stage, defective boards can be prevented from reaching the back end of the line. So the prevention of repair phenomenon or in some occasions formed by the abandoned phenomenon.

It may be important to be able to give the operator timely feedback as to whether the printing process being operated is performing well and thus effectively preventing defects.

In order to provide effective control at this level of process operation, an online vision system can be configured to detect the condition of the pad on the PCB after the paste has been applied, and whether the gap between the corresponding printing templates is blocked or dragged. In the vast majority of cases, fine-spaced components are tested to optimize test time and focus on problematic areas. For this reason, the time spent in testing is well worth it when the possible problems are eliminated.

Camera positioning and detection

In conventional online visual inspection applications, the camera is positioned above the circuit board to obtain the image of the printing position, and the related image can be sent to the processing system of the visual inspection equipment. There, image analysis software compares the captured image with a reference image stored in the same location in the device's memory.

In this way, the system can confirm whether too much or too little solder paste has been applied. The system also reveals whether the solder paste is aligned on the pad. It can find whether there is excess paste between the two pads to form a bridge like connection phenomenon? This problem is also known as the "bridge" phenomenon by many PCB manufacturers.

The work of detecting gaps in the printing template is done in an identical form. When excess solder paste is accumulated on the surface of the printing plate, the visual system can be used to detect whether gaps are blocked by solder paste or if there is a trailing phenomenon.

After the defect is found, the equipment can immediately automatically request the following screen cleaning series of operations, or alert the operator to the existence of problems need to be repaired. The inspection of printing templates can also provide users with very useful data on printing quality and consistency.

A key feature of the on-line vision system is the ability to detect PCB and pad surfaces with high reflectivity, in uneven light conditions or when dry solder paste structure makes a difference. HASL boards, for example, tend to be uneven and flat, with variable surface profiles and reflective properties. Proper lighting also plays a very important role in obtaining quality images.

The light must be able to "target" the reference and pad of the board, in turn turning other obscure features into clearly identifiable shapes. The next step is to use visionsoftwarealgorithms to reach their full potential.

In certain situations, the visual system can be used to detect the height or volume of solder paste on the pad, and sometimes only an off-line inspection system can be used to do this. Using this procedure means that a corresponding degree of accumulation is formed in a given printing template to confirm whether the paste volume is missing on the same pad.

Testing of solder paste

Specifically, it can be divided into two categories: detection of solder paste on PCB and detection of solder paste on printing template:

A. PCB detection

Mainly detect printing area, printing offset and bridging phenomenon. The inspection of the printing area refers to the area of solder paste on each pad. Excessive solder paste may lead to the occurrence of bridging phenomenon, and too small solder paste will also lead to the phenomenon of unstable welding points. The detection of printing offset is to see if the amount of paste located on the pad is different from the specified position. The test for bridging is to see if more than the specified amount of paste is applied between adjacent pads. The excess solder paste may cause an electrical short circuit.

B. Inspection of printed templates

The detection of printed templates is mainly for blocking and trailing. The detection of blockage refers to the detection of solder paste accumulation in the holes on the printing plate. If the hole is blocked, too little solder paste may be applied at the next printing point. The detection of trailing refers to the accumulation of excessive solder paste on the surface of the printed template. This excess solder paste can be applied to areas of the 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 a high degree of solder joint integrity, it prevents manufacturers from wasting money on board defects and resulting rework. Perhaps importantly, it provides continuous process feedback, which not only helps manufacturers optimize their screen printing process, but also gives people more confidence in the process.