Surface mount technology, SMT with its associated surface mount devices, SMDs enable PCB assembly of electronic equipment to be far more efficient than had the old leaded technology been used.
When it was introduced, SMT revolutionised PCB assembly, making it very many times faster, and the finished results more reliable.However to fit in with the PCB assembly methods for soldering that enable volume PCB assembly and manufacturing need to be employed.
The soldering processes required for SMDs during the PCB assembly needs to ensure the components are held in place during the soldering, the components are not damaged, and the final soldered quality is exceedingly high.
One of the main causes of equipment failure in the past has been the quality of the soldering, and by ensuring the soldering quality is very high, the PCB assembly process can be optimised and the overall equipment reliability and quality is able to meet the highest standards.
Rationale for specialised SMT soldering techniques
Although in the very first days of using surface mount technology, SMT, soldering was sometimes achieved manually, this is not feasible in the vast majority of cases today for two reasons:
- The minute size of the components and tracks is too small for manual operations and traditional soldering.
- The quantities of circuits normally produced could not be achieved using manual methods.
Obviously some manual soldering is required for activities like repair, modification and rework.
SMT soldering process
There are several stages required to solder SMDs to boards. However there are two basic methods of soldering that are used. These two processes require the board to be laid out with slightly different PCB design rules, and they also require the SMT soldering process to be different. The two main methods for SMT soldering are:
- Wave soldering: This technique for soldering components was one of the first ones introduced. It entails having a small bath of molten solder which is flowing out causing a small wave. The boards with their components are passed over the wave and the solder wave provides the solder to solder the components. For this process, components need to be held on place, often by a small dot of glue so that they do not move during the solder process.
- Reflow soldering: This is by far the preferred method these days. Within the PCB assembling, the board has solder applied through a solder screen. Components are then placed onto the board and held in place by the solder paste. Even before soldering it is sufficient to hold the components in place provided the board is not jolted or knocked. The board is then passed trough an infra-red heater and the solder is melted to provide good joint for electrical conductivity and mechanical strength.
The soldering process is an integral element of the overall PCB assembling process. Typically board assembling quality is monitored at each stage and the results fed back to maintain and optimise the process for the highest quality output.
Accordingly the soldering techniques required for electronics assembly are honed to meet the needs of SMDs and the processes used.