Electronic devices used to be large and heavy, like cathode ray televisions, requiring connection to high antennas to capture signals and display vivid images. Flat screen TVs have now become a trend with multiple functions, such as web browsing, connected voice recognition, and even data storage. Miniaturization reduces the size of small tools and improves their functionality. With the development of different technologies in electronic manufacturing, the trend towards small devices has become possible. The internal components are installed inside these electronic devices through printed circuit boards (FPCs).
The older and traditional method of connecting components to FPC is through through-hole technology (THT). This method requires inserting the leads of the components into the holes on the FPC. As mentioned earlier, the continuous reduction in size of electronic products has paved the way for more advanced methods that do not require larger packaging sizes. This can be achieved by mounting components on FPC using a technique called surface mount technology (SMT). We will learn more about SMT assembly, its definition, advantages, and challenges.

The full name of SMT assembly is surface mount technology assembly, which is an assembly technique that uses conductive solder paste to directly mount components on a printed circuit board. The transition from THT to FPC SMT assembly has brought about smaller, higher density, and faster packaging. Unlike wave soldering (THT soldering method), SMT board assembly uses reflow soldering to solidify the adhesive material.

Compared to SMT board assembly, through-hole technology requires more space to connect to FPC. SMT circuit board assembly can achieve more compact and lightweight designs, which is beneficial for various applications such as wearable and portable devices.

Due to the low lead inductance and capacitance in high-frequency applications, reducing signal noise is a major advantage of SMT assembly. Different component types and packaging are suitable for SMT assembly technology.

With SMT assembly, printed circuit boards can be produced at an extremely fast speed using high-speed machines. Even FPC manufacturing has become simpler as there is no need to drill holes to insert lead packages.

The goal is to achieve zero defects in SMT circuit board assembly. With the continuous pursuit of miniaturization, FPC is also prone to potential defects, which need to be considered and addressed to achieve higher quality manufacturing. The following are the main defects encountered during the FPC SMT assembly process.

Insufficient solder coverage can result in weak and disconnected connections between components and FPC substrate pads. This is usually caused by poor wettability, unoptimized template aperture size, insufficient solder paste amount, and template printing misalignment. Insufficient reflux peak temperature can also lead to this defect.

The phenomenon of erecting a monument is a welding defect in which one side of the component is soldered onto the FPC, appearing to be in an upright or vertical position, hence it is called the "monument phenomenon". This defect is caused by incorrect pad design and solder mask application.

Solder bridging refers to two components that should not be electrically connected coming into contact with conductive solder paste, resulting in a short circuit. Excessive use of solder paste and suboptimal heat distribution can both lead to solder bridging.

Another common defect encountered during the assembly process of SMT printed circuit boards is solder balls. Solder balls are small spherical solder materials that separate from the joint. As solder balls are conductive materials, they can have adverse effects during electrical testing. Solder balls may be caused by moisture in the solder paste, inaccurate printing, and ineffective solder flux.

SMT assembly involves processes that require control and standardization to ensure the production of high-quality circuit boards on the assembly line. The following are the main steps of SMT FPC assembly, which must be optimized to achieve a robust process.

Use templates and scrapers to distribute solder paste onto the printed circuit board at a specified pressure and speed. The template contains openings called "holes" used as patterns for solder paste. The scraper carries the solder paste through the wire mesh to wet the FPC pads. It is very important to control the amount of solder paste to achieve good solder joints.

A highly automated pick and place machine with multiple gantry frames, robotic arms, component feeders, and cameras. A gantry is a device for horizontally and vertically moving robotic arms. Each arm has a nozzle for picking up and placing components onto the circuit board. The workbench is used for fixing and securing FPC. Placement accuracy is a key quality attribute for achieving a good picking and placing process. If there is misalignment when placing components, welding bridges may occur.

Printed circuit boards with wet solder paste components need to undergo reflow soldering process. SMT printed circuit board assembly lines typically use conveyor belts, where each process is arranged according to the manufacturing process. Engineering evaluation must be conducted to verify the uniformity and consistency of the temperature curve inside the reflow soldering furnace. The supplier's recommendations for the thermal curve can serve as a benchmark, including preheating, soaking, reflow, and cooling. This step is to activate the flux, completely melt the solder paste, and achieve a fully cured joint. There is inert nitrogen gas inside the reflow soldering furnace to maintain a pollution-free process.

In SMT process, inspection is very important because it can provide early feedback on quality issues caused by assembly process. Automatic Optical Inspection (AOI) is a commonly used method for detecting FPC at extremely high speeds using high-resolution cameras and image processing techniques. AOI machines can be programmed according to the design of FPC to achieve flexible conversion. Calibration and maintenance are necessary to avoid false alarms during AOI inspections.

Another non-destructive testing technique is through X-ray inspection. In this type of inspection, X-rays can pass through materials to generate grayscale images that display internal structures, especially solder joint interfaces. X-ray detection can use 2D or 3D systems, where 2D can provide planar images and 3D can provide volume information of FPC.

Electrical testing in SMT assembly can be conducted through online testing (ICT) or flying pin testing, each of which has specific advantages. ICT settings include fixtures or nail beds for obtaining electrical measurement values from FPC. It is most suitable for large-scale testing. Flying needle testing involves using a testing probe with a robotic arm that can flexibly move around the FPC to test products. Functional testing can also be performed to test the FPC, while simulating the working conditions that the FPC will be subjected to.

If pursuing cost-effective FPC manufacturing services, choosing a Chinese manufacturer is the best choice. As a manufacturing powerhouse, China has abundant resources, cheap labor, and a large number of technical personnel. We recommend Haibo, a Chinese FPC assembly service provider with over 10 years of experience, who has performed outstandingly in various aspects

Efficient and reasonable quotation:There is an efficient quotation process to assist you in making decisions, providing reasonable prices and highly competitive quotes in China.
High quality and reliable products:Produce high standard printed circuit boards, manufacture and test according to specifications, international standards, and internal controls, and conduct pre production inspections to ensure robust processes.
Quick and on-time delivery:The assembly equipment is in good condition, with high productivity, minimal downtime, short delivery time, and also provides urgent services.
Timely and effective response:Always respond to customer needs, provide accurate information, answer questions through multiple channels, and offer one-stop services.
Strong technical strength:China's FPC manufacturing technology is leading, and Haibo has a complete SMT solution that covers assembly, inspection, and other capabilities. It also provides prototype production and customization services.