SPI or solder paste inspection is a technical procedure to analyze or inspect solder paste deposits in a printed circuit board or PCB. This analysis takes place during the manufacturing process. Solder paste inspection examines PCB defects like solder percentage, the quantity of solder, missing solder, and the like. 

Hence, SPI is an essential process that confirms the right deposition of solder in the PCB. Moreover, half of the PCB panel solder joints are detected to be damaged by the inappropriate solder paste joining. Thereby we can pledge that SPI is one of the most significant processes as you can skill-fully cut down the extra amounts afterward connected to soldering. 

Keep reading the article to know more about the importance of solder paste inspection in detail.

What is solder paste?

Solder paste, a blend of minute metal solder grains and sticky flux, comes in a paste consistency, as the name suggests. The special category of flux and the size of the metal particles decide the outcome of the paste. The particle size can be anywhere between 150 to 2 mm, and the flux can make different types of solder paste-like no-clean solder paste, rosin-based solder paste, and more. 

This paste has a unique usage in the printed circuit board (PCB), where it attaches the mount particles to pads on the board. Later the whole thing goes through an infra-red reflow device, and the melted paste permanently gives a solid soldered joint. Below let us evaluate the significance of SPI and discover the working of the whole process.

Printed Circuit Board (PCB) Solder Paste Inspection techniques:

Printed circuit boards SPI inspection is followed by mainly two techniques – Automated Optical Inspection or AOI and Automated X-Ray Inspection or AXI. Both of these techniques are discussed briefly below.

• Automated Optical Inspection(AOI):

In this technique, the PCB is inspected for having significant flaws like surface defects, component defects, dimensional defects, etc. This machine has advanced technology to detect any error in the very first level and adequately inspect it. AOI machines can examine the PCB by capturing images and finds out defects if any components are missing? Are they placed in an appropriate position or not? Are there any defects or not?

AOI can inspect various sizes of components with both 2D and 3D models. 3D AIO machines can sort out the defects like- component skew, area defects, wrong part, solder bridging, high defects, volume defects, lifted leads, tombstoning, and more.

• Automated X-Ray Inspection (AXI):

AXI is another Solder paste inspection device established on X-ray radiation rather than ordinary visual light. AXI gives greyscale images, not a full-color image like the AOI. In this process, the bottom of the PCB reveals, and the X-ray light goes through there. It helps determine some of the PCB defects like thermal defects, QFN part defects, BGA poor wetting, BGA missing balls, solder gaps in pad areas, bridging under a BGA, etc. 

Both of the AOI and AXI techniques are used in Solder paste inspection to identify the errors and mistakes in PCB to prevent this from further defects and extra charges.

How does solder paste inspection work?

SPI goes through several methods and several inspection procedures. Fundamentally, the printed circuit board is captured as a still picture or image and then compared with the board’s goal model. It includes several processes like- template, pattern, and statistical comparison. 

In template matching, the image of the circuit board is contradicted with the expected one. Similarly, the other two follow the comparison of pattern and statistical appearances. 

On the other hand, a particular type of camera is used to stream a video recording as a part of SPI inspection. It is not highly popular, but people like it because of the high speed of this system.

In addition to that, good lighting is essential as well as good pictures. Usually, in this method, fluorescent lighting, LED lighting and infra-red or ultra violate lighting is used to identify the defects efficiently. Once these essential steps are done, the lighting source’s right positioning is required to highlight every single detail of the board. Otherwise, the lighting sometimes hides the components, and that is hazardous. 

If everything is in the right place, it will be a wise choice to start the solder paste inspection without any further delay.

Importance of Solder Paste Inspection Process:

If you’re reading this article until now, then the importance of solder paste inspection is evident. Let’s have a look to understand this more clearly.

• After a quick and detailed solder paste inspection, the PCB becomes error-free and long-lasting. The inspection process includes minute observation, and the 3D pictures help to detect the faults easily. Additionally, it helps to improve the whole infrastructure of the print and provides a better execution. 
• The usage of high-quality appliances always provides better supervision and legibility to the PCBs. In this process, the use of high-quality angled cameras allows clicking three-dimensional images. Well, a 3D picture is way better and reliable than the ordinary one. In this way, the solder paste inspectors can go to the components’ very tiny and delicate details. 
• Solder paste inspection helps identify the problems and errors, which aid the manufacturer, look into this matter efficiently. Sometimes, the quantity of the paste can be a bit less or more, which is not a problem when you look with naked eyes, but it can damage the printed circuit board in the long run. Inspecting this thoroughly will rescue the PCB from early damage, and it will cost no additional money naturally. 

Conclusion:

If you’re looking for a wholly error-free and high-quality printed circuit board or PCB, don’t ever think to remove the solder paste inspection process or SPI from your list. Not only will it make your PCB defect-free, but it will also help you save other charges related to future damages. If you are a manufacturer, a proper SPI will reduce your delivery time for sure.