Basic settings of the visual inspection machine

Basic optimization

Each PCB can be inspected using optical or X-ray technology and using appropriate algorithms. The basic principle of image-based inspection is that every image with obvious contrast can be inspected. The main problem in AOI is that when some inspection objects are invisible, or there is some interference on the PCB, the image becomes blurred or hidden. However, actual experience and systematic testing have shown that these effects can be prevented or even reduced through PCB design. In order to promote this optimization design, some seemingly ancient additional methods can be used (these methods are still being respected in many fields), and its advantages include:

Reduce programming time

Minimize false alarms? Improve failure detection.

Formulating design guidelines can effectively simplify inspections and significantly reduce production costs. ViscomAG and KIRRONGmbH&CoKG jointly developed a special test program, the purpose is to fundamentally study and prove the effect of these designs in the inspection. The PCB layout based on the IPC-7350 standard is recommended as the benchmark for these tests. In order to explore the inspection effect of each layout, it is recommended to use this benchmark on a large number of PCB layouts; after that, the wrong layout of the PCB is intentionally used to make it produce some process defects, such as tombstones and pin suspensions.

Layout suggestions

PCB overall layout for AOI inspection

There should be at least a 3mm (0.12”) process edge from the device to the PCB edge. Chip devices must be prioritized over cylindrical devices. It is recommended to consider sensor technology in the layout, because sometimes inspection can only pass through a vertical (orthogonal) angle, while others At that time, an auxiliary angle is needed to proceed.

Components

For a stable process, an important factor is the components, which is not only related to the direct device layout on the PCB, but also more or less related to the "process design". The purchasing trend of components is to be as cheap as possible, regardless of their differences in color, size and other parameters. Unfortunately, the impact of these choices on the AOI or AXI inspection process in the future is often overlooked. Always using the same materials and products can significantly reduce inspection time and false alarms, and these problems are mainly caused by sudden changes in components and PCBs.

Component size

The IPC-7350 standard describes the size of the device and suggests the size of certain pads. According to the IPC standard, the length of the device and the width of the pin can have a larger range, on the contrary, the size of the pad is relatively fixed. In addition, the impact of PCB manufacturing tolerances relative to the changes in these devices is also very small.

PCB color and solder mask

Usually, the equipment can check the colors of all different veneers, although some details in the inspection are not dependent on the color. For example, a white and a green PCB have different contrast ratios, so the device needs some specific compensation. In one extreme case, the bridge appears black against a bright background, while in the other extreme case, the bridge appears bright against a black background. Here we recommend using a matt solder mask. In our practice, the area between the pads (even the fine-pitch pins) should also be covered with a solder mask. This suggestion has also been responded to by the solder supplier.

Printing pattern

All PCBs printed with patterns can also be inspected, for example, when the borders of components or components

When the letter on the body of the component appears alone in a certain area of the component and interferes with the inspection of other parts, the inspection procedure can be manually adjusted. Nevertheless, there is still a larger choice for the printing range of patterns within the scope allowed by the production. Therefore, the reduction of non-reflective logo printing (black or dark yellow) is worthy of consideration. Another possible situation is the need to selectively print logos: for example, when certain specific devices (such as Hall sensors) face down, they must be printed in white; and the other situation is printed with polarity logos. Tantalum capacitors with oblique angles; this can make the logo and the background form a sharp contrast, making the inspected image clearer.

Reference point

The equipment can check all types of reference points, and any component of the visual inspection machine can be defined as a reference point.

Although three reference points can compensate for the deformation of a single board, usually only two reference points need to be determined. Each fiducial point is at least 5mm (0.2”) away from the edge of the veneer. Cross, diamond, star, etc. are more suitable, and a uniform black background is recommended. In addition, the fiducial of the cross is particularly advantageous because they are under the detection light The image is very stable and can be determined quickly and easily.

Confirm bad board

The equipment has the ability to check all known types of bad board identification. Any component on the board can be defined as a bad board identifier. It is recommended to use a method similar to the above-mentioned reference point determination, that is, to confirm by checking the entire board or the single bad board identification on the single board that has been assembled, if possible. Each individual bad board mark on the board will only be checked one by one after the bad board mark check of the whole board fails; the bad board mark of the whole board should be located on the edge of the PCB.

Avoid solder joint reflection

The shape and contact angle of the solder joints are the source of solder joint reflections. The formation of solder joints depends on the size of the pad, the height of the device, the quantity of solder and the reflow process parameters. In order to prevent welding reflections, symmetrical arrangement of devices should be avoided.

Wave soldering

After wave soldering, all parameters of the solder joint will change greatly. This is mainly due to the aging of the tin in the soldering furnace, which causes the reflection characteristics of the pad to change from light to dark. Therefore, these changes must be included in the algorithm during inspection. In wave soldering, the typical lack of

The traps are short circuits and weld beads. When a short circuit is detected, if the printed pattern or non-reflective printing is reduced and the solder mask is applied, these false alarms can be eliminated. If the reference point is not covered by the solder mask and passes through wave soldering, it may cause a hemisphere to be tinned on a circular reference point, and its internal reflection characteristics will change; use a cross as a reference point or use solder mask The layer covering the reference point can prevent this from happening.

Chip components, MELF devices and C-leads devices

On chip components and MELF devices, the meniscus-shaped solder joints must be correctly identified; the solder joints on both sides of the device body are difficult to check because the solder does not climb. In addition, the distance Xc from the edge of the pad to the solder end also needs attention. The ratio of Xc (the outer pitch of the pad) to Xi (the inner pitch of the pad) should be >1. The same rules apply to the meniscus type of C-leads devices and the pad design on both sides of the device body. Here, we suggest that the ratio of Xc to Xi is slightly greater than 1.5. It is worth noting that the length change of any component must also be included.

"Gull Wing" Type Pin Device

Generally, the criteria for determining this type of device can be found in the analysis of the capillary effect in the vertical direction. Due to capillary force, the solder crawls from the end of the pad to the pin to form a solder joint. Due to process fluctuations and the blocking effect of the edge of the device, a complete upper half-moon solder joint cannot be completely formed. Although there is no upper half-moon-shaped solder joint, it can be considered to be well soldered. The side climb of the "gull-wing" pin solder cannot always be checked due to device changes or pad design. This is because the solder climb direction must be checked at an angle perpendicular to the pin direction. If the climb is small, it must be inspected from other angles, and only through such auxiliary inspections can it provide rich image information to evaluate the quality of the solder joints.

Oblique angle detection: PLCCs type device

The pads of the pins of PLCCs devices have different designs. If it is a long pad design, the climbing effect of the solder on the PLCC pin can be checked. If the pad remains bright, then the solder has climbed to the pin end, so the device is considered to be soldered. If this design principle is followed, defects can be detected through vertical inspection.

For PLCC solder joints, sometimes there will be less tin. Since the climbing condition of the pins with little tin is the same as when there is no solder, the vertical inspection of the PLCC solder joints cannot be passed, and the little tin defects must be inspected by oblique angle inspection.

Layout suggestions

The overall layout of the PCB For the general AXI test PCB layout, all pads must be solder masked. This is because the solder resist layer and the actual pad are not really in contact, and there is a certain gap between the solder resist layer and the pad. The advantage of this is that the solder can accumulate in the pad after being heated, which also makes it easy to reproduce the solder climb in the XRAY image.

2Dx-ray

When applying 2Dx-ray technology, all components need to be placed on the front side of the PCB. When using 2Dx-ray to detect these devices, a place without devices must be defined as a "forbidden zone." For some BGAs, it is recommended to use

A teardrop-shaped asymmetric pad design is used, which makes the forming properties of the solder erroneously judged by the system as a geometric connection form; in addition, some special QFN inward or outward meniscus pad designs This is also the case.

QFN pad design

The pad size and solder paste printing area of the QFN device are the same size as its pin size, and the pins of the device are staggered at the bottom of the package.

Therefore, the QFN pad design suggestion is: the pad protrudes from the outer end of the device pin, and

It is retracted at the inner end of the device so that a meniscus pad is formed inside and outside the pin of the device. An important point here is that the tolerance range of the device must be considered when designing and calculating.

BGA design

In BGA design, the shape of the solder joints (such as the teardrop type) can be made visible through a special layout; that is, in addition to the strange shape of the teardrop solder joint, its direction is also very random. All in all, the pads on the device surface and the pads on the PCB are exactly the same size as the BGA solder balls. At the University of Erlangen, Germany, scholars have done a lot of research to evaluate the model of a single pad shape; they found that regardless of whether the pad is circular or non-circular, the solder paste printing pattern should remain circular.

in conclusion

As a rule, in production, the test system should be established and optimized according to the requirements of the production batch. It has been proven countless times in actual operations that this alone is far from enough. If you want to test a new batch of PCBs within two weeks of production time, it may happen that the color of ELKO suddenly changes from black to yellow, or the pins of the transistor are shortened or bent; or The color of the resistor changed from yellow to blue, and so on.

The AOI inspection program must and be able to deal with the problems caused by these various changes. However, some of these changes take time to process, because we cannot know in advance whether a new component is used or there is a wrong component layout. At the same time, in the face of quality difficulties, a large number of possible situations that are allowed also need a consistent and feasible explanation.