Printed Circuit Board Soldering
Nowadays, PCB through hole technology is infrequently used in PCB assembly. However, many components are still widely used in through-hole packages, and they need to be soldered.
Wave soldering is a bulk soldering process that is primarily used to handle through-hole components, but can also be used with SMT components if an adhesive material is used to secure the components to the board. Wave soldering process requires planted boards passing through a wave of molten solder.
Although pins can bear thermal shock, most SMT components are more sensitive and tend to damage. The better way to solder SMT components is to use reflow method. In this process, solder paste is applied to the pads using a stencil.
Components are placed in certain position, and they are held in place due to the stickiness of the solder paste. In some cases, special adhesives are used to ensure that the position of the component does not change during the process. The next step is the actual reflow process. The planted board is placed in a reflow oven and use a specific temperature to ensure that components won’t be damaged while still allowing the solder to melt.
Some boards involve SMT and through-hole packages, and because reflow soldering with through-hole components is always problematic, manufacturers only have two options: manual soldering which is a rather slow, inaccurate and costly process, or use selective soldering machine.
Selective soldering applies heat only to a specific portion of the board. This may cause flux residue to diffuse into other parts of the PCB, but flux residues contain ionic contaminants that are not easily detected. The main problem here is that these contaminants can lead to dendritic growth which can ultimately reduce board life. One way to solve this problem is to preheat PCB board before the selective soldering process begins. In this way, the flux in the solder paste can be better activated.
Automating selective soldering can be used in several ways. The simplest is to use a controlled soldering iron and apply solder and iron to components’ lead one by one.