1, Preface.
In the world of the modern electronic products, PCB (Printed Circuit Board) plays important role in assembling electronic products, it’s hard to imagine that one electronic equipment does not employ PCB, so the quality of blank PCB will bring very big influence to electronic products if can work reliably as normal. It’s important task that electronic products manufacturer must pay enough attention to improve PCB quality.

During PCB assembly, put excess soldering paste on pads, or soldering paste not enough, even never put soldering paste, then carry out reflow soldering, after that once soldering point take shape, there will have defect on connect between components and blank PCB. In fact, most of defect PCBs can be found out relevant quality’s good and bad trace with the help of soldering paste’s situation of application.

Currently, a lot of PCB manufacturer already used in-circuit test (ICT), or say that check quality condition for welding spot by X-ray technology. They will be conducive to eliminate defect that generated by art of printing, but these ways can not supervise art of printing. Misprinted PCB maybe need to accept following increased processing steps, every step will lead different increasing produce cost, finally the defected PCB will be gone to SMT soldering. We have to discard the defective PCB or accept expensive cost and take more time to rework, at same time, may be cannot get specific answer for this defect.

smt soldering

Poor solder paste printing process implementation can cause electronic circuit connection problems. In order to effectively solve this problem, many screen printing equipment manufacturers use online machine vision detection technology, the following brief introduction.

2, integrated online visual inspection
In order to help circuit board manufacturers detect the resulting defects in the early stages of production process, more and more screen printing equipment manufacturers integrated online machine vision technology in their manufacture of screen printing equipment. The built-in vision system can achieve three main goals:
First of all, they can directly detect defects in the printing operation after the implementation of the situation, in the main manufacturing costs are added to the circuit board before, allowing the operator to promptly deal with the problem. This step is typically included when the circuit board is removed from the printing unit, after the cleaning agent has been cleaned, and when it is returned to the production line.

Secondly, since the relevant defects are found at this stage, it is possible to prevent the defective circuit board from reaching the rear end of the production line. Thereby preventing the repair phenomenon or in some cases formed by the phenomenon of waste.
Finally, and perhaps most importantly, it is important to provide the operator with timely feedback on whether the printing process in operation is working well and thus effectively preventing the occurrence of defects.

In order to be able to provide effective control during this level of process operation, the on-line vision system is able to detect the solder pad on the PCB after the solder paste has been applied, and whether the corresponding stencil gaps are clogged or smeared . In most cases, fine-pitch components are tested to optimize the detection time and focus on the areas most prone to problems. To this end, when eliminating the possible problems, when the detection of the cost of this time is worth it.

3, camera positioning and detection
In a conventional, conventional visual inspection application, a camera is positioned above the circuit board to obtain an image of the printing position and to transmit the image to the processing system of the visual inspection device. There, the image analysis software will compare the captured image with the reference image stored in the same location in the device memory. In this way, the system can confirm that the applied solder paste is more or less. The same system can also reveal whether the solder paste on the pad location is aligned. It can be found in the two pad between the excess solder paste whether the formation of bridge-like connection phenomenon? The problem is that many manufacturers of printed circuit boards commonly known as the “bridge” phenomenon.

The work of detecting a print template slot is in one and the same form. When excess solder paste is deposited on the surface of the stencil, the vision system can be used to detect if the slit is clogged with solder paste or if tailing is occurring. After a defect is detected, the device can immediately request a series of cleaning operations on the screen pksaywv. or alerting the operator that a problem exists that requires repair. The inspection 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 PCB substrates and pads with highly reflective surfaces and detect them in heterogeneous light conditions or in the presence of a dry solder paste structure. For example, HASL PCB boards typically exhibit a non-uniform flat surface profile that is variable and reflective. For the highest quality images, appropriate lighting also plays a very important role. The light must be able to “aim” against the board’s reference and pads, turning other imperceptible features into a clearly identifiable shape. So that the next step can use the vision software rules system (vision software algorithms), in order to give full play to their potential.
In some specific cases, the vision system can be used to detect solder paste on the solder paste the height or size of the size, and sometimes may only use off-line detection system to do these things. The use of this procedure means that a corresponding stacking degree is formed in a given printing template to confirm whether the volume of the solder paste is missing on the same pad.

4, the detection of solder paste
Specific can be divided into PCB solder paste on the detection and printing of the template on the solder paste detection of two categories:

A. The detection of PCB
The main detection printing area, printing offset and bridge phenomenon. The detection of the printed area refers to the area of solder paste on each pad. Excessive solder paste may cause bridging phenomena, while too small solder paste can also cause the phenomenon of welding point is not strong. The detection of the printing offset is based on whether the number of solder paste on the pad is different from the specified position. The detection of the bridge phenomenon is whether the solder paste applied between the adjacent two pads exceeds a prescribed amount. These excess solder paste may cause electrical shorts.

B. The detection of the printing template
The detection of the printing template is mainly for the detection of blocking and smearing phenomenon. Detection of blocking refers to detecting the presence of solder paste in the holes in the printing template. If the hole is clogged, the solder paste may appear to be too small at the next printing spot. The detection of tailing means 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 the circuit board should not be turned on the location of the above, causing electrical connection problems.

5, concluding remarks
Online machine vision systems can benefit PCB manufacturers in a variety of ways. In addition to ensuring a high degree of solder joint integrity, it can prevent manufacturers because of the defects of the circuit bo