Learn about SPI (Solder Paste Inspection) quality control in PCB assembly

SPI Quality Control is a step in PCB assembly that serves to inspect the application of solder paste on PCBs.

What is SPI?

 

The process of mounting a board requires a lot of planning and study. And there are several steps to be completed and one such step is SPI control. SPI inspection means Solder Paste Inspection, which in Portuguese means inspeção da pasta de solda. It is an important tool for the execution of board assembly. The inspection of solder paste, is a process carried out through an equipment in 3D that analyzes if the volume of solder paste is adequate. Thus SPI checks if the amount of paste is insufficient, i.e. if there is excess or shortage of solder paste. This procedure helps to minimize various risks, including the short circuit of the final part for example. The SPI equipment exists so that there is better control and monitoring of production.

The SPI (Solder Paste Inspection) has angular cameras, which produce sharp 3D images, these cameras are able to capture the height of the printed solder folder, thus facilitating accurate measurement of the solder folder volume. With the help of SPI and in conjunction with automated optical inspection, manufacturers easily monitor the gluing and soldering component printing processes.

Weld paste inspection is a process that can help increase product reliability and quality. The use of this tool can reduce between 50% and 70% Welding defects, defects that are often attributed to the processes of applying the weld paste to the printed board. It is also important to point out that the SPI not only locates defects but also optimizes the quality control of the board assembly. 

How is SPI Quality Control performed?

Quality control in PCB assembly is of utmost importance, and the SPI process plays a key role for parts to meet quality standards. The welding moment in the board assembly is crucial for the whole project to be well executed. Therefore the welding moment must be planned and carried out with great precision. The engineering team involved in the project plans thoroughly so that there is no error at this stage. It is of utmost importance that all professionals are trained so that the execution is successful. This from the choice of suppliers to the development of the project.

The precision in the welding of the components, needs a thorough inspection. It is at this point that the raw material chosen in the supplier's approval process, such as welding paste, must perform excellently. The use of SPIs, is very important to detect faults at the moment when the solder paste is applied. The technology of the SPI equipment used is also very important, today there are 3D equipment of excellent quality. The periodic maintenance of these equipments is also very important, so that there are no failures. These equipments achieve this by visually scanning the surface of the board. The board is illuminated by various light sources and one or more high definition cameras are used. In this way, the SPI machine can create a solder folder image on the board. The captured image is processed and then compared with the machine's knowledge 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 various techniques to provide application analysis.

• Model matching:   Using this form of process, the SPI weld paste inspection system compares the image obtained with the image of a standard board.
• Pattern matching:   Using these techniques, the SPI system stores good and bad PCB assembly information, corresponding to the image obtained to these.
• Statistical pattern matching:   This approach is very similar to the one described above, except for the use of a statistical method to solve problems. By storing the results of multiple boards and various types of failure, it can accommodate small acceptable deviations without signaling errors.

After welding, inspections are performed to detect faults at the time the weld paste was applied. This way, if there is any fault, the correction is applied before it follows the production of the board by the assembly line. 

Many defects can be avoided with SPI, e.g. insufficient welding volume, because if the amount of paste used on the board surface is too small, the parts to be welded will not be fixed. Also the excess of solder can be a problem, because when the volume of paste is deposited on the plate is greater than indicated, the pads may result in short. And short ones in turn are errors in the welding position that can result in short circuits. The reasons that normally cause these short circuits can be: Stencil with solder paste residue; Non-conforming print parameters; Excess solder paste in the stencil opening is also an aggravating factor as solder paste may leak. With an effective quality control on the produced parts, no doubt will result in the success of the final product.

How important is SPI (Solder Paste Inspection) in PCB assembly?

The inspection of the solder paste is very important because it minimizes the possible problems that may exist in the other processes of the board assembly. This inspection checks the solder paste deposit in the PCB manufacturing process. The solder paste volume is an important element for the success of the board assembly because there is a precise quantity so that there are no future problems with the PCB. 

With a well applied SPI process to the board design, threats will be minimized, thus generating long-term reliability of the welding joints. And greater credibility to the final product. SPI uses advanced equipment for this inspection, so there is less risk of defects in the parts produced. By using angular cameras to take 3D photos, it is possible to check if the folder volume, is insufficient or if there is excessive welding. These cameras are very useful for measuring the alignment and volume of the soldering folder. This helps to improve performance as well as print quality. In addition, it helps to improve PCB performance.  

The inspection performed throughout the application of solder paste (SPI) besides helping to reduce defects in the joints, helps to save money, because in this way the cost of scrap and the minimum cost of rework is reduced. This way there is less waste of raw material for example, and the optimization of the estimated production time. The earlier a defect is identified, the less it will cost to correct. Some studies say that a fault identified after backflushing will cost 10 times the amount of rework than that identified before backflushing, i.e. a fault identified after the test will cost 10 times more to rework. 

As assemblies become more complex and the components used become smaller, and manual inspection is not a viable option today, it becomes more difficult to control the welding paste printing process without automating the inspection process. Weld paste inspection machines are a significant investment, but the benefits of rigidly controlling the manufacturing process outweigh the high investment.

How important is the solder paste?

Solder paste is one of the most important items used in the manufacture of printed circuit boards. Solder paste is used to connect the mounting components to the surface of the board. For specialized applications, other metals can be added to the composition, making it more efficient.

Solder paste must have some properties that make it highly useful in several sets of circuits, such as viscosity. Viscosity is what determines the strength of a material. As the metal content increases, the viscosity also increases, which in turn depends on the temperature, that is, when the temperature increases the viscosity decreases. 

An important point for the solder paste is rheology, because it determines the flux and deformation of the solder paste, this occurs when the temperature or pressure determines the condition of the paste. This affects the way the paste is deposited on the board. Implicating in the ability to bond the solder paste, making it more efficient.

The "Fall", which is also a property of the pulp, refers to the dispersion of a material after application. This depends mainly on the temperature, height of the pulp deposited and viscosity. In other words, it depends on all the other properties mentioned above. The presence of a fall can create a risk for the assembly line, as the presence of this "fall" can form uneven weld bridges or deposits between the two successive lands, thus causing a short circuit. 

In the SMT line the applications of Solder Paste have some very important characteristics. Weld joints, for example, are a homogeneous mixture of weld alloy. It is capable of forming metallurgical bonds with the welding conditions provided, thus helping to create reliable welding joints. To simplify the design, solder pastes are used with lead components, also improving component density and lowering manufacturing cost, thus increasing the reliability of the project. 

SPI process in Cromatek

Cromatek meets large industrial demands for THT and SMT assembly, the SMT assembly lines are composed of state-of-the-art equipment, and the THT assembly line also has a rigid system of quality and excellence, thus making Cromatek a reference in PCB assembly. 

Cromatek's production has quality control in SPI and AOI Line from start to finish, this way the boards go through automatic optical inspection, with state-of-the-art equipment and a highly prepared team for product analysis. 

For Welding Folder Printing Cromatek does not require the customer to use our own welding folder model, but it is necessary to share the model that will be used to find out if the folder used conforms to Cromatek's qualification levels. SPI (Solder Paste Inspection) is performed on 100% of the parts produced and the result must be recorded and linked to the PCB serial number. 

The SPI process is carried out at Cromatek with great precision and state-of-the-art equipment. All Cromatek professionals are trained to perform the analysis of this procedure, and are also able to perform the proper handling of the equipment of Solder Paste Inspection, as training is held on a regular basis for Cromatek employees.

To ensure quality control, Cromatek aims to be within the standards of certification ISO-9001:2015. It also has an ecologically correct assembly line without the use of lead and other heavy metals in its production; Cromatek has an ESD Clean Room, which has antistatic protection so as not to damage the electronic components and all the boards are submitted to rigorous functional tests that validate their operation.

Perform your project in Cromatek. From the project to the finished product, Cromatek delivers solutions for all industrial segments, acting according to the need and particularity of each client, aiming that the client focuses on its focus. 

Aiming at the service of each client, Cromatek has resources for PTH assembly with axial, radial, jumpers and special components using the 5S system, Kaizen, Lean Manufacturing. Determines work cells and continuous production line according to the particularity of each product and flexibility of demand. Cromatek's SMT line is composed of state-of-the-art machines, capable of assembling boards of different technical requirements, through the rigid quality control and stability of the production process, Cromatek is able to meet various applications, demands and market segments. 

To contract the Cromatek PCB Assembly Service, choose a type of contract, Labor or TurnKey, send the project specifications (BOM, Gerber, Stencil).  Count on our experts to turn your projects into reality.