Why Manual Soldering is Still Important in SMT Assembly Manufacturing Process

As the electronics technology evolves, many of today’s modern electronics are becoming smaller, lighter, and even increasing performance. This, in turn, led to improvements in the SMT assembly manufacturing process whereby tasks are further optimized to increase the rate of production of throughputs while maintaining quality and reliability.

With this new and improved manufacturing process, manufacturers are now able to produce boards at a much faster rate, while driving the cost of each board down. Additionally, with the use of automation in the SMT assembly manufacturing process, there will also be fewer chances for errors.

Now, this begs the question, why choose manual soldering in SMT processes despite it being less efficient and effective when compared to automated assembly?

Automated Assembly vs Manual Soldering SMDs

The most notable difference between the two processes is the equipment. In an automated assembly, many pieces of machinery are involved to perform an efficient process, whereas, manual soldering requires almost none, as it mainly involves manually operated devices.
The automated assembly process for SMT components starts with the dispensing machine applying soldering paste to the PCB with a stencil as a guide. Next, pick-and-place machines are used to mount SMDs on the solder paste which was applied on the PCB. Finally, the PCB with SMDs is passed along a conveyor belt into a reflow oven, where the solder paste is then melted using intense heat to securely join the components to the PCB, creating a PCB assembly.
In contrast, manual soldering requires a trained operator to manually pick the appropriate SMD using tweezers/ suction pick and place it in the correct position on the PCB. Following that, a hot soldering iron and solder wire is used to apply molten solder to join the PCB with its component terminal. This process is then repeated for all the terminals of each component.
In manual soldering, the components can move freely on the board during the soldering process, which can be challenging for human operators to produce consistent results when soldering components are not secured in place.

The Importance of Manual Soldering

It’s undeniable that automated assembly of SMT is more efficient and can produce better and consistent results when compared to manual soldering. However, there are still some things that can only be achieved with manual soldering, these includes:
Touch Ups
Even in automated assembly, there are cases where smaller components are overshadowed by larger components on the PCB. This results in incomplete melting of solder paste during the reflow process, which can lead to defects in the product. To remedy this problem, manual soldering can be used for a minor touchup to correct this error without wasting the entire lot.
Other scenarios where a manual touch-up may be necessary are mishandled components with a bent pin, a tombstoned component, and components that require rework or repair.
Rework and Repairs
This is needed when an SMD is damaged or misplaced and requires a correct one. In this process, the operator will have to remove the damaged/misplaced component from the board, and replace it with a new component or place it back in the appropriate position.
For this process, operators will typically utilize hot-air gun for heating the solder joints of components. Hot-air guns usually come with different nozzles for adjusting the size and temperature that are ideal for certain types of components.
The process begins by applying hot air to melt the solder joints. Once the solder joints are melted, the operator then lifts the component away from the board. To replace the component, excess solder must first be removed from the pads, and then reapply the solder paste manually onto the pads. Next, the operator will use the hot air gun again to melt the freshly applied solder paste to form the joints. This technique is commonly used for both discrete SMDs and SMD ICs such as QFPs.
With that being said, manual rework is not suitable for BGAs and gull-winged components as the pins and solder balls are kept hidden and difficult to access between the body of the IC and the PCB.
Special Components
These components are more fragile against temperature changes and have difficulty withstanding prolonged heat exposure. This can be a problem when they need to be inside the reflow oven during the PCB assembly process. To solve this problem, operators must solder these ‘special’ components by hand within a short period, to prevent damages from the heat of the soldering operation.
On PCBs, you will typically find a mix of large and small components. During the reflow process, the smaller components tend to heat up at a much quicker rate when compared to the larger components. This can lead to problems such as overheating of the smaller components which prevents them from being soldered properly. With manual soldering, operators can hand-solder the larger components after completing the reflow without damaging the smaller components.

What Do You Need for Manual Soldering of SMDs?

To effectively perform manual soldering SMDs, there are several things you will need. These include:
  • Soldering iron
  • Special heating tips for soldering irons
  • Solder flux
  • Solder wick
  • Hot-air gun
  • Nozzles for hot-air gun
  • Vacuum pump
  • Tools – Shear cutters, wire cutters, tweezers, safety goggles, and others 
  • Experienced operators
To properly perform manual soldering of SMDs, it is a must to have experienced operators to facilitate the process. The operators should be able to manually handle boards with multiple electronic components, as well as implementing ESD regulations to prevent inadvertent damages to the PCB assembly.
The handling of smaller components requires high attention to detail, especially when being heated. To ensure this process goes smoothly, soldering irons with varying tip temperature settings are necessary to allow for quick temperature changes for quick soldering and desoldering, without overheating the components.
When removing excess molten solder is required, a hand-operated vacuum can be a great tool to make the process easier. However, another alternative is to use hot-air guns which have both blowing and sucking features. The tip of a hot-air gun is hollow, which allows for the pumping and sucking of air and molten solder. In most hot-air guns, there will also be special tips provided that allow for the heating of pins on all four sides of quad ICs for soldering or de-soldering them.
When working with SMDs, it is important to have lead-free solder available, either as wires or the pest, as well as matching solder flux. Solder wicks are handy tools to have to remove excess solder.
In addition, many other tools can help improve the manual soldering process. When handling smaller SMDs, a tweezer is a great tool to move parts around. Also, a reliable vice is crucial to keeping the board in place when soldering. As for inspection after the soldering process, an illuminated magnifying glass can help identify defects on the board. Apart from the one just mentioned, there are many more tools that are available to enhance the manual soldering process.

Conclusion

From what was discussed in this article, manual soldering still plays a pivotal role in the SMT assembly process. Although automation allows for many advantages such as increased efficiency, higher quality, and lower costs. There are still some things that can only be achieved with manual soldering.

If you are looking for a company that can help you manufacture your product which include the mix of automated and manual soldering, we have the solution for you! At ECI Technology, we have professionals with extensive experience in soldering processes to provide high-quality and reliable products to meet the needs of your business.

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