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