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 use of PCB in an electronic equipment, so how the quality of PCB will have a very big impact on the long-term normal and reliable work of electronic products. To improve the quality of PCB is an important issue that should be paid enough attention by electronic manufacturers.
If too much solder paste is applied to the solder pad during PCB assembly, or if the solder paste is insufficient or not installed at all, then subsequent reflow soldering, once solder joints are formed, will cause defects in the electronic connections between the components and the circuit board. In fact, most defects can be traced to the application of solder paste.
Many circuit board manufacturers now use in-circuit test (ICT) or X-ray technology to check solder joints. They will help to eliminate defects due to the printing process operation, but these methods cannot monitor the printing process operation itself. A misprinted circuit board may accept an additional process step, each of which increases the cost of production to a different degree, so that such a defective circuit board may eventually reach the assembly stage of production. ** manufacturers need to discard the defective circuit board or accept costly and time-consuming rework, and at this point there may not be a very clear answer to the root cause of the defect.
Poor solder paste printing process can cause problems in electronic wiring. 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
In order to help the PCB manufacturers to find the defects in the early stage of production process, more and more screen printing equipment manufacturers have integrated online machine vision technology in their screen printing equipment. The built-in visual system can achieve three main objectives:
First, they are able to detect defects directly after a printing operation is carried out, allowing the operator to deal with problems in a timely manner before major manufacturing costs are added to the board. This step generally involves removing the board from the printer, washing it in the cleaner, and returning it to the production line after repair.
Second, because the defect was found in this stage, the defective circuit board can be prevented from being delivered to the back end of the production line. Therefore, the phenomenon of repair or in some cases the formation of the abandoned phenomenon.
Perhaps most important: the ability to provide timely feedback to the operator on the quality of the process in operation, thus effectively preventing defects.
In order to provide effective control at this level of process operation, an online visual system is configured to detect the solder pads on the PCB after the solder paste has been coated, and whether there is clogging or trailing in the gap of the corresponding printing template. In the vast majority of cases, the detection of micro-spacing components is to optimize the detection time and focus on * problem prone areas. For this reason, the time spent on detection is well worth it when 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 printed position and can send the relevant image to the processing system of the visual inspection equipment. There, the image analysis software compares the captured 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 system can also reveal whether the paste position on the pad is aligned. Can it detect if there is excess solder paste between the two pads forming a bridge-like connection? The problem is what many PCB manufacturers call “bridging”.
The inspection of printing template gaps is done in the same form. When excess solder paste is deposited on the surface of a stencil, the visual system can be used to detect gaps in the stencil that may be blocked by the solder paste or may have a trailing effect.
After the defect is found, the equipment can automatically request the following screen cleaning series, or alert the operator that there is a problem that needs to be fixed. The inspection of printing templates can also provide users with very useful data on printing quality and consistency.
A key function of the online vision system is the ability to detect highly reflective PCB boards and pad surfaces, as well as in uneven light conditions or under conditions where dry paste structures cause differences. HASL circuit boards, for example, tend to have uneven, flat surfaces, variable surface profiles, and reflective characteristics. Proper lighting also plays a very important role in obtaining quality images.
The light must be able to “target” the circuit board’s reference and solder pads, turning other imperceptible features into clearly identifiable shapes. The next step is to use vision software algorithms to realize their potential.
In some specific situations, the visual system can be used to detect the height or volume of the solder paste on the solder pad, sometimes only using an offline inspection system to do this. The application of this procedure means the formation of a corresponding build up in a given stencil to confirm that the paste volume is missing on the same pad.
Solder paste inspection
Specifically, it can be divided into two categories: solder paste detection on PCB and solder paste detection on printing template:
A. PCB testing
It mainly detects printing area, printing offset and bridging phenomenon. The inspection of the printing area refers to the solder paste area on each pad. Excessive solder paste may lead to the occurrence of bridge phenomenon, and too small solder paste will also lead to the phenomenon of weak welding joint. The test for printing offset is to see if the amount of solder paste on the pad is different from the specified position. The bridge phenomenon is detected when the amount of solder paste applied between two adjacent pads exceeds the specified amount. This excess solder paste may cause an electrical short circuit.
B. Inspection of printing templates
The detection of printing template is mainly for blocking and trailing phenomenon. Blockage detection is the detection of the presence of solder paste in the holes in the stencil. If the hole is blocked, the solder paste applied at the next printing point may appear to be too little. A trailing inspection is performed to see if excess solder paste has accumulated on the surface of the stencil. This excess solder paste may be applied to areas of the circuit board where it should not lead, 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 due to circuit board defects and the resulting rework. Perhaps most importantly, it can provide continuous process feedback, which not only helps manufacturers to optimize the screen printing process, but also increases people’s confidence in the process operation.