What is the difference between AOI and SPI20240904

Understanding SPI Inspection : A Key to Reliable Electronics Manufacturing

In the realm of electronics manufacturing, precision and reliability are paramount. Surface Mount Technology (SMT) has revolutionized the industry, enabling the production of compact and highly efficient electronic devices. However, with the increasing complexity of circuits and components, ensuring the quality and functionality of these electronic assemblies has become more challenging. This is where Solder Paste Inspection (SPI) comes into play. SPI inspection is a critical quality control process in SMT that helps maintain high standards in electronics manufacturing. In this article, we will delve into the details of SPI inspection, its importance, methodologies, and its impact on the overall quality of electronic assemblies.

What is SPI inspection? 

Solder Paste Inspection (SPI) refers to the process of evaluating the application of solder paste on a printed circuit board (PCB) before the placement of electronic components. Solder paste is a mixture of solder flux and solder powder that is used to create solder joints between electronic components and the PCB. The correct application of solder paste is crucial because it affects the reliability and performance of the final product. SPI ensures that the solder paste is applied accurately, in the right quantity, and in the correct locations, reducing the likelihood of defects in the final assembly.

Why use SPI inspection in electronics manufacturing?

1.Prevention of Defects: SPI plays a vital role in preventing common defects such as solder bridges, insufficient solder, and misalignment of components. By detecting these issues early in the manufacturing process, SPI helps to avoid costly rework and repair.

2.Improved Reliability: Proper solder paste application is essential for creating reliable solder joints that can withstand mechanical stress and thermal cycles. SPI ensures that the solder paste is applied uniformly and accurately, leading to improved reliability and longevity of the electronic device.

3.Cost Efficiency: Detecting and addressing solder paste application issues at the early stages of production is more cost-effective than dealing with problems after components are placed or soldered. SPI helps in minimizing production downtime and reducing material waste.

4.Compliance with Standards: Many industries require adherence to specific quality standards and regulations. SPI helps manufacturers meet these standards by ensuring that solder paste application meets the necessary specifications.

What are the methods of SPI inspection?

SPI can be conducted using various methodologies, each with its own set of advantages and applications. The primary methodologies include:

1.Automated Optical Inspection (AOI): This is the most common SPI method, which uses high-resolution cameras and image processing algorithms to inspect solder paste application. AOI systems can detect anomalies such as excessive or insufficient solder paste, misalignment, and bridging. These systems are highly efficient and can process a large volume of PCBs quickly.

2.X-ray Inspection: X-ray inspection is used to detect issues that are not visible to the naked eye or through standard optical methods. It is particularly useful for inspecting the internal structures of multi-layer PCBs and detecting issues like hidden solder bridges or voids.

3.Manual Inspection: Although less common in high-volume manufacturing, manual inspection can be used in smaller-scale production or as a supplementary method. Trained inspectors visually examine the solder paste application and use tools such as magnifying glasses to identify defects.

4.Laser Inspection: Laser-based SPI systems use lasers to measure the height and volume of solder paste deposits. This method provides precise measurements and is effective in detecting issues related to paste volume and uniformity.

What are the key parameters in SPI Inspection?

Several critical parameters are evaluated during SPI inspection to ensure proper solder paste application. These parameters include:

1.Solder Paste Volume: The amount of solder paste deposited on each pad must be within specified limits. Too much or too little paste can lead to defects in the solder joints.

2.Paste Thickness: The thickness of the solder paste layer must be consistent to ensure proper wetting and adhesion of the components. Variations in paste thickness can affect solder joint quality.

3.Alignment: The solder paste must be accurately aligned with the PCB pads. Misalignment can result in poor solder joint formation and potential component placement issues.

4.Paste Distribution: Uniform distribution of solder paste across the PCB is essential for consistent soldering. SPI systems assess the evenness of paste distribution to prevent issues such as solder voids or bridging.

How to Guarantee Solder Paste Printing Quality?

● Squeegee Speed: The speed of travel of the squeeze determines how much time is available for the solder paste to “roll” into the apertures of the stencil and onto the pads of the PCB. Typically, a setting of 25mm per second is used but this is variable depending on the size of the apertures within the stencil and the solder paste used.

● Squeegee Pressure: During the print cycle, it is important to apply sufficient pressure across the entire length of the squeeze blade to ensure a clean wipe of the stencil. Too little pressure can cause “smearing” of the paste on the stencil, poor deposition, and the incomplete transfer to the PCB. Too much pressure can cause “scooping” of the paste from larger apertures, excess wear on the stencil and squeegees, and may cause “bleeding” of the paste between the stencil and PCB. A typical setting for the squeegee pressure is 500 grams of pressure per 25mm of squeegee blade.

● Squeeze Angle: The angle of the squeegee is typically set of 60° by the holders they are fixed to. If the angle is increased it can cause “scooping” of the holder paste from the stencil apertures and so less solder paste to be deposited. If the angle is reduced, it can cause a residue of solder paste to be left on the stencil after the squeeze has completed a print.

● Stencil Separation Speed: This is the speed at which the PCB separates from the stencil after printing. A speed setting of up to 3mm per second should be used and is governed by the size of the apertures within the stencil. If this is too fast, it will cause the solder paste to not fully release from the apertures and the formation of high edges around the deposits, also known as “dog-ears”.

● Stencil Cleaning: The stencil must be cleaned regularly during use which can be done either manually or automatically. The automatic printing machine has a system that can be set to clean the stencil after a fixed number of prints using lint-free material applied with a cleaning chemical such as Isopropyl Alcohol (IPA). The system performs two functions, the first being the cleaning of the underside of the stencil to stop smudging, and the second is the cleaning of the apertures using vacuum to stop blockages.

● Stencil and Squeegee Condition: Both stencils and squeegees need to be carefully stored and maintained as any mechanical damage to either can lead to undesired results. Both should be checked before use and thoroughly cleaned after use, ideally using an automated cleaning system so that any solder paste residue is removed. If any damage is noticed to squeegee or stencils, they should be replaced to ensure a reliable and repeatable process.

● Print Stroke: This is the distance the squeegee travels across the stencil and is recommended to be a minimum of 20mm past the furthest aperture. The distance past the furthest aperture is important to allow enough space for the paste to roll on the return stroke as it is rolling of the solder paste bead that generates the downward force that drives the paste into the apertures.

Which Kind PCBs Can Be Printed?

No matter rigid, IMS, rigid-flex or flex PCB (refer to our PCB Manufacturing), if the PCB strength is inadequate to support PCB itself as absolute flat as requirement on the rails of the SMT lines, the PCB assembly manufacturer will ask to customize SMT carrier or carrier (made of Durostone).

This is an important factor to ensure the PCB is held flat against the stencil during the printing process. If the PCB, no matter rigid, IMS, rigid-flex or flex, is not fully supported, it can lead to printing defects, such as a poor paste deposit and smudging. PCB supports are generally supplied with printing machines which are a fixed height and have programmable positions to ensure a consistent process. There are also adaptable PCB and are useful for double-sided assembly.

Printed Solder Paste Inspection (SPI)

The solder paste printing process is one of the most important parts of the surface mount assembly process. The earlier a defect is identified the less it will cost to correct – a useful rule to consider is that a fault identified after reflow will cost 10 times the amount to rework than that identified before reflow – a fault identified after test will cost a further 10 times more to rework. It is understood that the solder paste printing process presents far more opportunities for defects than any of the other individual Surface Mount Technology (SMT) Manufacturing Processes. In addition, transition to lead-free solder paste and use of miniature components, has increased the complexity of the printing process. It has been proven that the lead-free solder pastes do not spread or “wet” as well as tin lead solder pastes. In general, a more accurate printing process is required in a lead-free process. This has pushed the manufacturer to implement some type of post-print inspection. To verify the process, automatic solder paste inspection can be used to accurately check solder paste deposits. At RICHFULLJOY , we can detect some defects of printed solder paste, line insufficient deposits, excessive deposits, shape deformation, missing paste, paste offset, smearing, bridging and more.