Journey from Pre-Reflow AOI to Post-Reflow AOI
Surface Mount Technology (SMT)
Look inside any piece of commercially made electronic equipment these days and it is filled with minute devices. Rather than using traditional components with wire leads like those that may be used for home construction and kits, these components are mounted onto the surface of the boards and many are minute in size.
This technology is known as Surface Mount Technology (SMT). The components in this technology are directly mounted on the surface of the Printed Circuit Board (PCB) without any holes/drilling. There are special components used for this technology that are called Surface Mount Devices (SMD). The components are placed over the PCB that already has conduction paths, and finally, the components are soldered in a reflow oven. They are directly attached to the board and provide better performance than its predecessor: Through-hole Technology (THT).
Printed Circuit Board Assembly (PCBA)
A PCB with components mounted on is called an assembled PCB and the manufacturing process is called PCB assembly or PCBA for short. There are copper lines on bare board, called traces that electrically link connectors and components to each other. They run signals between these features, allowing the circuit board to function in a specifically designed way. These functions range from the simple to the complex, and yet the size of PCBs can be smaller than a thumbnail.
So, how exactly are these devices made? The PCB assembly process consists of several automated and manual steps with each step of the process, a board manufacturer has both manual and automated options from which to choose. PCBA process encompasses the following production steps:
- Solder Paste Printing
The first step of PCB assembly is applying a solder paste to the board. This process is like screen-printing a shirt, except instead of a mask, a thin, stainless-steel stencil is placed over the PCB. This allows assemblers to apply solder paste only to certain parts of the would-be PCB. These parts are where components will sit in the finished PCB.
2. Pick and Place (PnP)
After applying the solder paste to the PCB board, the PCBA process moves on to the chip placement machine where a robotic device places surface mount components, or SMDs, on a prepared PCB. SMDs account for most non-connector components on PCBs today. These SMDs are then soldered on to the surface of the board in the next step of PCBA process.
3. Reflow Soldering
After the pick and place process concludes, the PCB board is transferred to a conveyor belt. This conveyor belt moves through a large reflow oven, which is somewhat like a commercial pizza oven. This oven consists of a series of heaters which gradually heat the board to temperatures around 250 degrees Celsius. This is hot enough to melt the solder in the solder paste.
4. Inspection and Quality Control
Once the surface mount components are soldered in place after the reflow process, which doesn’t stand for completion of PCBA and the assembled board needs to be tested for functionality. Often, movement during the reflow process will result in poor connection quality or a complete lack of a connection. Shorts are also a common side effect of this movement, as misplaced components can sometimes connect portions of the circuit that should not connect.
Checking for these errors and misalignments can involve one of several different inspection methods. The most common inspection methods include:
Automatic Optical Inspection: Automatic optical inspection is a more appropriate inspection method for larger batches of PCBAs. An automatic optical inspection machine, also known as an AOI machine, uses a series of high-powered cameras to “see” PCBs. These cameras are arranged at different angles to view solder connections. Different quality solder connections reflect light in different ways, allowing the AOI to recognize a lower-quality solder. The AOI does this at a very high speed, allowing it to process a high quantity of PCBs in a relatively short time.
Why is Inspection and Quality Control Important?
Surface mount technology is an electronics assembly production process in which the electronic components such as resistors, transistors, ICs etc. are mounted (by soldering) directly onto the surface of a printed circuit board (PCB). This method gives a variety of benefits like:
- smaller size and weight of final product,
- electrical noise reduction
- speed up of the electronics production process
- decrease of potential reliability defects
In consequence, the SMT reduces manufacturing costs and uses PCB surface area more efficiently than its predecessor: through-hole technology (THT).
Nevertheless, SMT is not a zero-defect process and due to components miniaturization and high density of them on the PCB area, repair of potential errors becomes much more difficult, and this becomes valid especially when the defect is detected at the final stage of the assembly process. In production, this means putting a lot of focus on detecting and correcting any manufacturing defects or Manufacturing Defects Analysis (MDA).
In many aspects, PCB is the ‘brain’ of the product and, as such, it is a critical component with its own precise and rigorous design requirements. And just as PCBs are getting smaller, they’re also becoming more complex. Even a relatively simple board can be made up of literally thousands of soldered joints, which means it is simply not practical, or viable, to attempt to inspect manually.
Therefore, the sooner in the production process that faults are spotted, the easier and more cost efficient it is to rectify them to avoid replicating the same problem on a larger scale, making AOI a “must-have” on any PCB assembly manufacturer’s checklist.
AOI Role in Inspection and Quality Control
AOI is the automated visual inspection of PCB manufacturing, which uses a camera to scan the assembly for defects that would lead to board failure. Depending on the inspection goals, which the AOI manufacturer wants to achieve, the AOI machine are placed just before (pre-reflow) and just after (post-reflow) the soldering process (reflow). An electronics manufacturing provider that is focused on reliable assembly services, would utilize two AOIs in one SMT line mainly for complex, advanced products that require in-depth inspection These systems have become more important for manufacturers because they provide incomparable reliability to help maintain high-quality boards with thousands of solder joints and complex and intricate components.
The AOI process delivers the real-time inspection required by an EMS as well as providing empirical data that is invaluable for process control and refinement as production volumes increase. An AOI system uses a number of strategies to analyse whether a PCB has been produced to the required standards by pattern matching algorithms which keep a record of both successful and unsuccessful PCB assemblies. By constantly reviewing and analysing fault data, EMS partner can identifying trends, enabling improvements in the manufacturing process, minimizing the risk of future rework and increasing the efficiency of production.
What does AOI inspect?
Automated optical inspection (AOI) uses optics to capture and analyze images of a PCB to verify the following for assembled electronic components:
- Nodules, scratches and stains
- Open circuits, shorts and thinning of the solder
- Incorrect, missing and skewed components
- Insufficient paste area, smearing, and bridging
- Missing or offset chips, skewed chips and chip-orientation defects
- Solder bridges, and lifted leads
- Line width violations
- Spacing violation
- Excess copper, and missing pad
- Trace shorts, cuts, jumps
- Area defects
- Component offsets, component polarity,
- Component presence or absence, component skew from surface mount pads
- Excessive solder joints and insufficient solder joints
- Flipped components
- Paste around leads, solder bridges, and solder paste registration
Types of AOI systems and Capabilities
Both 2D and 3D AOI inspection systems are available on the market. However, 3D inspection has allowed more accurate measurements and a more stable inspection process over 2D inspection.
2D AOI Systems:
2D inspection technology, which is less expensive than 3D AOI, is the most common solution used by contract manufacturers and OEMs. There are several 2D AOI systems available on the market. The most advanced systems among them have multiple high-resolution cameras (8 to 15 MP resolution) and precise lenses. They also use sophisticated inspection algorithms to inspect defects.
2D systems offer a wide range of performance features:
- Flexible inspection capabilities
- Optical character recognition and verification (OCR, OCV)
- Data matrix code reading
- Straightforward recognition of certain polarity marks
- Less susceptible to shadowing issues
- Multiple solutions for additional tasks beyond solder joint inspection, such as measurement tasks or color analysis.
However, 2D technology also has limitations such as it cannot inspect hidden solder joints and provide volumetric measurement data (required to measure coplanarity and solder volume/shape). Furthermore, 2D AOI shows higher level of false defect rates compared to 3D machines.
3D AOI Systems:
The 3D AOI is available on the market since only a few years, and has been adopted by more advanced and experienced electronics manufacturing providers. The main advantage of an 3D inspection over 2D inspection within the PCB assembly process, is that it provides true volumetric height information. It can measure the volume of each and every visible solder joint of every component. Coplanarity on lifted leads and other height-sensitive devices can be detected without difficulty. Since height data can be measured, AOI programmers can specify the precise height tolerance acceptable for a particular component. Thus, in summary 3D AOI systems can:
- measure volumetric inspection data
- perform true coplanarity inspection
- reduce false call rate
Summary
As boards become more complex with increasing component miniaturization, the inspection process and how it can be achieved accurately is increasingly important to electronic components longevity.
In short, although AOI can be used for real-time SMT preliminary quality analysis for immediate feedback of SMT quality, so that SMT process operations can be improved, which can effectively increase the SMT yield rate. Although ICT test machine can also be used to identify PCB problem, there is usually a time difference of more than 24 hours before responding to the SMT to correct it. Therefore, from the perspective of real-time quality control, AOI does have its advantage.
In addition, with the development of 3D technology and the improvement of MCU computing capabilities, many equipment manufacturers have begun to develop stereo AOI technology which purpose is in addition to the existing pattern matching technique, stereo AOI can provide better imaging technology than the original 2D/3D AOI.
References
[1] Russell Poppe, JJS Manufacturing. (2018). Why AOI is a Must-have for PCB Assembly. SMT007.
[2] Przemysław Prolejko. (2020). The benefits of 3D AOI in SMT assembly. ASSEL.
[3] Pamela Lipson. (2006). A Test Strategy for Pre-Reflow AOI. ASSEL.
[4] VCTA Team. (2019). Detection Principle of Automatic Optical Inspection (AOI). Zhen Hua Xing.
[5] Yamaguchi Koji. (2020). High aspect ratio solder printing technology enabling mixed mounting from 0402M components to large components. Omron.
[6] ElectronicsTalk Team. Let Us Explore The Secret Of SMT Machine. ElectronicsTalk.
