PCB Trace fabricates and assembles various types of printed circuit boards. Depending on the board technology under use, we select the most optimal method of soldering technology. The primary aim of soldering is to anchor the electronic components on the board. Anchoring the components to the board by soldering serves two purposes. One, fixing each component in its respective position so that it does not fall off the board. The other, providing each component with an effective solder joint that connects it electrically to its copper pads and tracks, allowing the assembly to function as intended.
The soldering technique assemblers use depends on the type of components the board is using. Typically, there are two types of components in use in the electronic industry:
Through Hole Components
These are components with long connecting leads protruding either axially or radially from the component body. Mounting through hole components requires the printed circuit board to have multiple holes. The operator passes the leads of the component through the hole, and cuts and clinches the lead to prevent it from falling out.
The design of through hole components prevents them from being very small. Moreover, the requirement of holes for mounting them requires a minimum spacing between the pads. This prevents achieving high component density in printed circuit boards that use the through hole component technique. The presence of large number of holes in the board reduces the number of channels available for routing traces in the inner layers, leading to a larger multilayer board size.
Surface Mount Components
These are components without leads and therefore, do not require holes in PCBs to mount them. Instead, they have end caps that require mounting them on pads on the top surface of the board.
The design of surface mount components allows manufacturers to make them in very small sizes. Therefore, it is possible to achieve very high component density. The absence of through-holes in the board increases the number of channels available to the designer for routing traces, resulting in a reduction not only in the board size, but also in the total number of layers necessary.
The two types of components above leads to three types of printed circuit board assemblies:
Through Hole Technology (THT)
This technology involves boards assembled only with through hole components. All holes in the board pass through the entire thickness of the board, with the diameter of the holes matching the lead diameter of the specific component meant for that hole.
Printed circuit boards with through hole technology may be single-, double-, or multi-layered. All holes in double- or multi-layered boards are of the plated through type. This requires each hole to have a copper pad on the two outside surfaces of the board. An electroplating process creates a thin-walled copper barrel on the inner wall of each hole connecting the two pads. This makes an electrical connection between copper traces on the top and the bottom surface of the board. Other copper traces on any inner layer may also connect to the copper barrel in the same way.
Surface Mount Technology (SMT)
This technology does not require through holes in the board. The design of electronic components uses end caps instead of leads. This allows placement of surface mount components on copper pads on the top surface of the board. Vias allow connections to copper traces on inner layers. These are small holes with a depth of one layer only. Similar to through holes, an electroplating process creates a thin-walled copper barrel on the inner wall of each via, creating an electrical connection between copper traces on adjacent layers. Therefore, vias may or may not be visible on the outer layers. Vias visible on the surface, but not passing through the board, are blind vias. Those embedded within inner layers, and not visible on any outer layer, have a special name—buried vias.
Mixed Technology
Printed circuit boards may use both types of technologies—THT and SMT. Here, some may be surface mount type of components, and others through hole types. Surface mount components require mounting them on pads on the top surface of the board, whereas all through hole components will require threading them through holes in the board.
Mounting all the through hole components requires placing them only on the top surface of the board, but it is possible to place surface mount components both on the top and the bottom surfaces. To prevent components mounted on the bottom surface from falling away, the general practice is to glue them in position.
For soldering boards with the above three technologies, three soldering methods are in popular use:
Wave Soldering
This method of soldering is suitable for boards using through hole and mixed technology. The assembled board passes over a tub of molten solder, such that the solder adheres to the copper pads on the underside of the board and the component leads protruding through them.
The tub of molten solder usually has an impeller driving a wall of molten solder towards the underside of the moving board assembly. Operators must preheat the board assembly and apply flux on its underside to facilitate proper soldering. As the board assembly grazes the top of the wall of molten solder, the solder adheres to the board and anchors the component as it cools. The molten solder also adheres to and anchors any surface mount component present on the underside of the board using mixed technology.
Reflow Soldering
This method of soldering is suitable only for boards using surface mount technology. However, this method requires a precondition. The operator uses a stencil to deposit a small amount of solder paste on the copper pads on the top surface of the board, before they can place any component.
After a satisfactory deposition of the solder paste, a pick-and-place machine mounts individual surface mount components in their respective positions on top of the solder paste deposits on the board.
A conveyor system then carries the entire assembly through a heated chamber, the reflow soldering machine. Infra-red heaters in the machine heat the board and the components until the solder paste melts and flows.
Flux contained in the solder paste helps in soldering and anchoring the surface mount components on their respective pads on the board, as it passes to a cooler region.
Selective Soldering
This method of soldering is useful for boards using mixed technology, where the majority of the components are surface mount types, with a few of the through hole kind.
After the board assembly has undergone reflow soldering for soldering and anchoring the surface mount components, the board undergoes selective soldering for anchoring the through hole components.
Although the process is similar to that for wave soldering, there is a major difference. Instead of a wall of molten solder, there is a round spot of welling solder. The operator positions the board such that the spot is just below the joint of the through hole component. Once they solder one joint, they move the board to solder the other joint/joints of the component.
Conclusion
The process of soldering is a vital one for any printed circuit board assembly. According to PCB Trace, the quality of soldering primarily determines the quality of the assembly. We run experiments to decide the soldering profile of any PCB assembly before we begin full-scale production. This allows us to achieve high-quality solder on any PCB assembly.
