BTC and SMT Rework Challenge [electronics manufacturing expo News]

10 Jul 2019

With the rapid development of electronic manufacturing technology, more and more customers are increasingly demanding PCB rework, and new solutions and technologies are needed to solve these new demands.

Most of the customers need to effectively carry out the repair of electronics manufacturing expo BTC (Bottom Terminated Component) and SMT PCB. Now, in the next few years, the following topics will be more and more widely discussed and discussed:

  • BTC devices and their characteristics: Handling of bubble problems
  • Smaller devices: miniaturized (return capability of 01005)
  • Large-size PCB processing: Dynamic warm-up for large board rework
  • Reworkability of the rework electronics manufacturing expo process: flux and solder paste application (dip technology), removal of residual tin (automatic tin removal), material supply, handling of multiple devices, and traceability of rework.
  • Operational support: a higher degree of automation, software-guided operation (friendly human-machine interface)
  • Cost-effective: Rework systems that meet different budgets, ROI assessment

However, the above electronics manufacturing expo topics do not express their relevance in practice. There has been a lot of discussion about the 010005 rework ability in the industry, but so far, none of the technologies claiming this rework ability has been proven to provide real success for this tiny device in the daily actual rework. Rework process.

In sophisticated flow lines, many parameters need to be observed and controlled, including:

  • When soldering and extracting the device, it cannot affect other electronics manufacturing expo devices in the immediate vicinity
  • Add new solder paste to small solder joints
  • Device pick-up, calibration, placement
  • PCB coating
  • PCB cleaning and other

However, due to the presence of electronics manufacturing expo 01005 devices, the challenge of rework has inevitably emerged. On the one hand, the device size is getting smaller and smaller, the assembly density is getting higher and higher, and on the other hand, the size of the PCB is getting larger and larger. Thanks to the development of communication products and network data transmission technologies (cloud computing, Internet of Things), the computing power of computer centers has been growing rapidly. At the same time, the motherboard size of computing systems has also been growing.

This has the challenge of uniformly and completely preheating a 24" x 48" (610 x 1220mm) multilayer PCB during the rework process.

In addition, in the growing field of electronics manufacturing, rework processes have been recognized as part of electronic assembly, and the recording and tracking of individual circuit boards for rework has become an essential requirement. Among the topics mentioned above, the blueprint for rework ability by 2021 is mainly described, and three of them will be introduced next. Other electronics manufacturing expo issues are also very important in future rework, many of which are viable through practice certification, so they only need to be added or improved in existing rework equipment.

Bubble treatment during rework

In assemblies containing bottom terminal elements (BTC), the presence of air bubbles has been a very serious problem for many applications. For the definition of bubbles, the following is a description of the welding defects:

The tin melt quickly fills the appropriate vacancies and captures some flux in the solder joints. The bubbles trapped by the electronics manufacturing expo flux are voids; these voids prevent the filling of the tin. In such solder joints, the solder cannot fill the entire solder joint because the flux has been sealed inside.

In the SMT field, bubbles can only produce the following effects:

Since there is very little solder that can be applied to each solder joint, the reliability of solder joints is of much concern. The presence of electronics manufacturing expo bubbles has been a common drawback associated with solder joints, especially during reflow soldering of SMT. The presence of bubbles can exacerbate the strength of the solder joints and ultimately lead to solder joint failure.

The impact on the quality of the formation of bubbles in solder joints has been reported many times in numerous discussions:

  • Reduces heat transfer from component to PCB, increasing the risk of excessive component body temperature
  • Reduced mechanical strength of solder joints
  • Gas instinct escape from the solder electronics manufacturing expo joint – can cause solder splatter
  • Affect the current carrying capacity of the solder joint (ampere capacity) – the junction temperature is too high due to the increased resistance of the solder joint
  • Affects signal transmission – in high frequency applications, bubbles attenuate the signal

Especially in power electronics, the generation of bubbles on thermal pads (such as QFN package components) has become an increasing electronics manufacturing expo problem at this stage. Heat needs to be transferred from the component to the PCB for heat dissipation. This important process cannot be achieved, which seriously shortens the life of the component.

Remove some conventional ways to reduce bubbles:

For example, using low-bubble solder paste, optimizing the reflow soldering temperature profile, and optimizing the opening of the electronics manufacturing expo stencil to obtain the best amount of solder paste, there is also a solution for bubble treatment when the solder paste is in the liquid phase, and also throughout the electron. A new option during the assembly process.

So the question is, how do you apply the bubble treatment process to an open environment like rework equipment? The vacuum technology used for reflow soldering is clearly not applicable. A technique based on sinusoidal excitation of a PCB substrate is more suitable for rework (Figure 1). The first is that the PCB is excited by a longitudinal wave with a amplitude of less than 10 μm. This sine wave excites the electronics manufacturing expo PCB at a set frequency. In this region, both the PCB body and the solder joints on the PCB are self-resonant by this stress. When the PCB is scanned by energy, the components remain in place and the bubbles are trapped in the edge regions of the liquid solder and have the ability to escape from the solder joints.

In this way, it is possible to reduce the bubble ratio to 2% in the welding of new components (Fig. 2). Even with this technique, significant bubble removal can be performed on the target solder joints on the assembled PCB during the secondary reflow process. In this re-bubble rework process, only the selected electronics manufacturing expo area on the PCB is heated to the reflow state, and only this area is sinusoidally excited, so there is no negative impact on the entire product.

The scanning wave is longitudinally advanced along the PCB substrate and excited by a linear scanning wave produced by the piezoelectric actuator

Figure 1. Air bubbles in PCBA are processed by a piezo driver.

Figure 2. Turning on the excitation function during reflow can significantly reduce the proportion of bubbles in the MLF (before and after application).

Residual solder removal - tin removal

Unlike the production process, removing solder remaining on the pads is a very important step when reworking on an electronic board. This step is essential for electronics manufacturing expo reinstalling a new component to a designated location and for obtaining repeatable, highly reliable solder joints.

The common manual operation today depends on the skill of the operator and hides the risk. This step is essential when a new component is reinstalled to a designated location and a reproducible, highly reliable solder joint is obtained.

Figure 3. The tin removal system to remove excess solder during SMT rework and placement.

Manually operated, but non-contact removal of residual tin modules has the following advantages:

  • High repeatability and non-contact extraction of solder
  • No risk of soldering pad breakage and peeling
  • No risk of damage to the insulating electronics manufacturing expo paint
  • Eliminate contamination of solder or solder on PCB

The method is to preheat the bottom of the PCB with a gentle hot air until the solder remaining on the PCB melts and then pump away from the surface of the PCB using a vacuum nozzle with a fast liquid solder (Figure 3).

In addition to this known technique of melting solder by hot air and then removing residual tin by vacuum, it is also important to control the height of the electronics manufacturing expo nozzle in order to ensure that the board is not damaged. In the first operation, the height of the nozzle often needs to be adjusted manually, while in advanced systems, the device can automatically adjust the height by pressure sensing or triangular laser measurement.

For high-quality daily operations, solder separation and flux filtration and convenient maintenance are important product features.

This module does not exist separately from the rework station equipment and needs to be integrated into the device. During the tin removal electronics manufacturing expo process, the temperature of the board needs to be sensed, and the preheating function of the bottom of the PCB must be proactive and controllable.

Operational guidance - software assisted operation

The market clearly requires that the rework station has flexible adaptability, simple operation, and higher automation, and the equipment does not depend on the operator. To aggregate the above requirements, a set of interactive interfaces with clear structures for operators or device managers is required.

A clear expectation from the customer electronics manufacturing expo regarding the rework process is to achieve an independent operator and achieve high quality return results. This is why more and more automated processes are being applied to rework equipment, and improvements in HCI (Interpersonal Interface) have also been proposed. In the process of rework, the system with operational guidance and assistance can significantly reduce the operator's mistakes. A software interface with a clear architecture provides all the information necessary for each decomposition step, from temperature profile selection, component removal, to final placement soldering, and the need to add solder paste or solder paste (Figure 4).

All process steps are assisted and the operator is guided throughout the process and directed to the next process operation. When the system is not automatically executed, the electronics manufacturing expo pictogram will tell the user what to do next (Figure 5).

Figure 4. Curve selection interface with clear architecture during the rework process.

Figure 5. The pictogram guide guides the user through the placement process step by step.

Computer-Aided Mounting (CAP) is another means of obtaining the best results by processing images and assisting the operator. The picture called “Comparator Mode” in this example shows the colored pads and component solder fillets, as well as the overlapping virtual electronics manufacturing expo colors (Figure 6). The solder fillet of the component is shown in red, and the solder pads on the PCB are shown in green. Once the image of the solder fillet and the solder pad overlap, it will be blue, indicating that the best placement conditions have been reached.

Unlike images from real-time cameras, the computer-assisted placement feature provides the best color contrast image through image processing. Just like an air traffic control chart (Figure 7), the system clearly displays meaningful data to the user. Therefore, the operator can concentrate on the task of position correction, and the fatigue of the person is also minimized.

Another element of computer-aided placement is the implementation of a digital split optic. It provides multiple images (such as larger QFP components) at high magnification for the most accurate positioning (Figure 8). It replaces the optical electronics manufacturing expo magnification technology that requires an optional lens. The high-resolution camera image is divided into four areas, each showing a corner of the component. The positioning work is performed in high-magnification images, and the process of matching the solder fillet and the solder pads will also become easier.

Summary and future prospects

In the electronics industry, even if the quality of the product continues to improve, rework will still be a challenge for the next few decades. The integration of electronic assembly is getting higher and higher, and the products are becoming more and more complex. The task of the supplier of the rework system is to follow the pace of the market and provide a solution that can successfully repair the circuit board. At the same time, in the part of the rework electronics manufacturing expo equipment, the degree of automation will continue to increase, and the operation assistance function will become more important. In addition to the technical challenges, integrating the most versatile high-end rework systems, you need to find their way to derive entry-level products that meet market needs. Business and environmental protection will also benefit from less scrap and electronic waste.

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