PCB Assembly

SMT Assembly Explained: An Introductory Guide for Beginners

SMT assembly

We are surrounded by electronic devices that have been the result of endless innovations and relentless research and development. From the mobile phones that we use the moment we wake up, to the electric car that sends us to a thousand miles from our current location. Our world has extremely changed from decades ago when human activities had to be done manually such as communicating and traveling. How these electronic devices are made must be intriguing. Within them are printed circuit boards that have been fabricated, assembled, and packaged into the final product that we come to know. In this blog article, we will expound on the PCB technology and PCB SMT assembly processes.

What is SMT assembly?

Before diving into what is SMT PCB assembly, we need to know what is a printed circuit board first. A Printed Circuit Board or PCB is a substrate or laminate populated with electronic components to serve several functions in a device. The most common substrate is FR4 (stands for Flame Retardant) but there are also various substrate types like flexible substrate which consist of polyimide. A PCB can come in various forms and layers: single-sided, double-sided and multi-layered. The PCB can also be categorized based on the substrate type that is being used such as flexible, rigid or a combination of flexible and rigid. 

Surface Mount Technology or PCB SMT assembly, in the most simplistic definition, is the method by which electronic components are assembled into the boards. Surface mount assembly has slowly substituted the more conventional method of assembly called Through Hole Technology ((THT). Instead of “mounting”, THT is a method of “inserting” the component leads into the PCB to make a functional circuit. Surface mount PCB assembly has a better advantage than THT for intricate PCB designs where there is limited spacing and pitching. 

All PCB types can be assembled using SMT PCB assembly technology.  PCB SMT assembly proved to be a reliable method due to other advantages such as high efficiency, better reliability, and better space maximization.

What are the Different Processes Involved in SMT Assembly?

Surface mount assembly

Surface mount PCB assembly processes are easy to visualize and understand. A surface mount assembly line is typically composed of highly automated machines that are programmable and optimized to generate high throughput. Let us go through each process.  

Stencil Printing

A solder paste is applied across the printed circuit boards through a squeegee. The squeegee evenly distributes the paste into the stencil apertures. The objective is to have consistent print volume at the right positions throughout the process. There are variables that need to be controlled during stencil printing such as solder paste viscosity, printing speed, stencil separation speed and squeegee pressure. The quality of stencil printing directly influences the first pass yield of PCB assembly. In fact, it is estimated that around 60% of surface mount assembly defects are brought about by solder printing issues. Immediately after stencil printing, solder paste is inspected for any issues on the board pattern.

Placement of Components

The next SMT PCB assembly step in the surface mount assembly process is the mounting of parts on the printed solder paste. The machines are named “Pick-and-Place”, “Component Placement” or “Chip Shooters” machines. The machine has high precision in positioning the components using a vision system. A vision system is an effective technology consisting of cameras, image processing software, lenses, lighting, and a worktable. 

The key objective in this SMT PCB assembly process is to improve speed and precision with low noise and no jamming issues. First, the PCB is loaded on the worktable, then the nozzle and feeder are selected based on the type of component. The light automatically adjusts to visually locate and align the right placement positions, after which the components are mounted. The PCB consists of fiducials which serve as visual markers for the machine cameras. 

Reflow Soldering

In reflow soldering, the PCBs are passed through an oven with several heating zones to melt the soldering alloy. The temperature profile should match closely with the recommended profile of the solder paste manufacturer. Since different PCB designs will have varying component densities and materials, characterization must be performed to determine the right time and temperature. 

The standard sequence in the reflow process is ramp-up, pre-heat/soak, reflow at peak temperature and cooling. This reflow sequence is followed to achieve a good solder joint. To prevent oxidation of metals, the oven has an inert environment, usually with Nitrogen (N2) gas. Reflow quality is based on the success of solder paste wetting on the contact pads. There should be sufficient solder paste coverage with enough filet formation and zero voids. 

Quality Inspection

There are two visual methods to verify the quality of assembled printed circuit boards. The first one is Automated Optical Inspection (AOI) which has highly advanced vision technology to rapidly inspect issues on the PCBs. The AOI machine gets images on the PCB and compares them with a known good image to be able to determine acceptability. False calls are the primary challenge in AOI inspection as they may lead to the wrong rejection of good parts or the wrong shipment of defective parts. 

X-ray inspection is another quality verification step to identify solder quality problems that could have arisen from the reflow process. In x-ray inspection, the machine can call out solder voiding and incomplete solder coverage. Visual inspection has limited line-of-sight, therefore x-ray inspection is needed to check internal defects on the printed circuit boards.

PCB SMT assembly

What are the common defects encountered during SMT assembly?

Surface mount assembly processes require set-up and process controls to eliminate occurrences of defects. Below are the common defects during assembly:

Solder Bridging

Solder bridging occurs when the solder pastes bridges with the adjacent component which can lead to electrical shorting. Excessive paste can cause this phenomenon; thus, volume must be carefully controlled during stencil printing.

Tombstoning

This defect occurs when there is unbalanced wetting of solder paste causing the component to stand on one end of the lead. Uneven temperature, paste distribution and unbalanced surface tension can lead to this defect. 

Missing Components

Due to increasing numbers of components and reduction in chip sizes, missing components can potentially occur due to mis-pick or machine set-up issues. 

Misaligned Components

Some shifts in the position of the components can occur during pick-and-place or reflow processes.  Misalignment can also lead to electrical failures especially if it touches nearby components.

Solder Voiding

Unoptimized solder reflow processes and contaminated PCB contact pads may develop solder voids or air gaps during the reflow process due to various factors such as expired or contaminated solder paste. 

 

Conclusion

In the future, there may be further developments to further speed up and improve the SMT assembly service. One thing is for sure though, surface mount assembly has the benefit of mounting various component types in a high-density board which makes it more likely to last as a reliable PCB assembly service technology. 

Familiarization with the various surface mount PCB assembly steps helps designers, fabricators and engineers understand the interrelated factors contributing to the overall quality and functionality of printed circuit boards.

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