Solder Paste Inspection, SPI, is a key technique used in the manufacture and test of electronics printed circuit boards, PCBs. Solder Paste Inspection, SPI enables fast and accurate inspection of the solder paste on PCBs to ensure that the quality of paste on PCB are  printed correctly and without manufacturing faults.

Need for Solder Paste Inspection, SPI systems

Despite the major improvements that have been made, the solder paste on modern circuits are far more complicated than boards were even a few years ago. The introduction of surface mount technology, and the subsequent further reductions in size mean that boards are particularly compact. Even relatively average boards have thousands of soldered joints, and these are where the majority of problems are found.

This increase in the complexity of boards also means that manual inspection is not a viable option these days. Even when it was an accepted approach, it was realized that it was not particularly effective as inspectors soon tired and poor and incorrect construction was easily missed. With the marketplace now requiring high volume, high quality products to be brought to market very quickly very reliable and fast methods are needed to ensure that product quality remains high. SPI Solder Paste Inspection is an essential tool that ensure costs are kept as low as possible by detecting faults in the solder printing stage of production line.

Solder Paste Inspection systems can be placed into the production line just after the solder printing process. In this way they can be used to catch problems early in the production process. This has a number of advantages. With faults costing more to fix the further along the production process they are found, this is obviously the optimum place to find faults. Additionally, process problems in the solder and assembly area can be seen early in the production process and information used to feedback quickly to earlier stages. In this way a rapid response can ensure that problems are recognized quickly and rectified before too many boards are built with the same problem.

Solder Paste Inspection, SPI basics

Solder Paste Inspection, SPI systems use visual methods to monitor printed solder paste for defects. They are able to detect a variety of defects such as nodules, scratches and stains as well as the more familiar dimensional defects such as Volume、Area、Height、Shape、Shift、Bridge、Overflow.  As such they are able to perform all the visual checks performed previously by manual operators, and far more swiftly and accurately.

They achieve this by visually scanning the surface of the board. The board is light by several light sources and one or more high definition cameras are used. In this way the SPI machine is able to build up a picture of solder paste on the board

The Solder Paste Inspection, SPI system uses the captured image which is processed and then compared with the knowledge the machine has of what the board should look like. Using this comparison the SPI system is able to detect and highlight any defects or suspect areas.

SPI uses a number of techniques to provide the analysis of whether a board is satisfactory or has any defects:

  • Template matching:   Using this form of process the SPI, Solder Paste Inspection system compares the image obtained with the image from a “golden board”.
  • Pattern matching:   Using this techniques the SPI system stores information of both good and bad PCB assemblies, matching the obtained image to these.
  • Statistical pattern matching:   This approach is very similar to that above, except that it uses a statistically based method of addressing problems. By storing the results of several boards and several types of failure, it is able to accommodate minor acceptable deviations without flagging errors.

In order to build up the database of what the board should be, both known status boards and PCB design information is used as described later.

As technology has improved it has been able for SPI systems to very accurately predict defects and have a small number of defect not found scenarios. therefore, SPI systems form a very useful element in a sophisticated manufacturing environment of solder print.

SPI image capture and analysis

One of the key elements of an Solder Paste Inspection, SPI system is the image capture system. This captures an image of the printed circuit board, PCB assembly which is then analysed by the processing software within the SPI system. There are many variants of image capture system dependent upon the exact application and the complexity / cost of the SPI system.

Imaging systems have more than one to provide better imaging with 3D capability. The cameras should also be able to move under software control. This will enable them to move to the optimum position for a given PCB assembly.

In addition to this the type of camera has an impact on performance. Speed against accuracy is a balance that has to be struck and will impact on the camera type used:

  • Streaming video:   One type of camera used for Solder Paste Inspection takes streaming video from which complete frames are taken. The captured frame then enables a still image to be generated on which the signal processing is performed. This approach is not as accurate as other still image systems but has the advantage of very high speed.
  • Still image camera system:   This is generally placed relatively close to the target PCB and as a result it requires a good lighting system. It may also be necessary to be able to move the camera under software control.

When analysing an image of a board, the SPI system looks for a variety of specific features: Volume、Area、Height、Shape、Shift、Bridge、Overflow, background colour and reflectivity, etc. As an important element of its task the SPI system also inspects the soldered joints to ensure they indicate that the joints are satisfactory.

When analysing the boards the AOI system must take into account many variations between good boards. Not only do components vary considerably in size between batches, but also the colour and reflectivity. Often there are also differences in the silk screening where ink thickness and color typeface may change slightly.

SPI light source

Lighting is a key element in the SPI system. By choosing the correct lighting source it is possible to highlight different types of defect more easily. With the advances that have been made in lighting technology in recent years, this has enabled lighting to be used to enhance the images available and in turn this enables defects to be highlighted more easily with a resultant reduction in processing required and an increase in speed and accuracy.

Most SPI systems have a defined lighting set. This will depend upon the operation required and the product types to be tested. These have usually been optimised for the anticipated conditions. However sometimes some customization may be required, and an understanding of lighting is always of use.

A variety of types of lighting are available:

  • Fluorescent lighting:   Fluorescent lighting is widely used for Solder Paste Inspection, SPI applications as it provides an effective form of lighting for viewing defects on PCBs. The main problem with fluorescent lighting for SPI applications is that the lamps degrade with time. This means that the automated optical inspection system will be subject to a constantly changing levels and quality of light
  • LED lighting:   The development of LED lighting has meant that Solder Paste Inspection, SPI systems are able to adopt a far more stable form of lighting. Although LED lighting does suffer from a reduction in light output from the LEDs over time, this can be compensated for by increasing the current. Using LED lighting, the level of lighting can also be controlled. LEDs are therefore a far more satisfactory form of lighting than fluorescent or incandescent lights that were used years ago
  • Infra-red or ultra-violet:   On some occasions infra-red or ultra-violet lighting may be required to enhance certain defects, or to enable automated optical inspection to be carried out to reveal certain types of defect.

Apart from the form of lighting, the positioning of the lighting for Solder Paste Inspection, SPI  is equally important. The light sources require positioning to not only to ensure that all areas are well light, which is particularly important when certain components may cast shadows, but also to highlight defects. Careful adjustment may be needed for different assemblies.