PCB Selective soldering


Through-hole technology is becoming less and less frequent on PCB assemblies these days. There are however a number of components that are widely used in their through-hole packages and they still need to be soldered. Wave soldering is a bulk soldering process that is used mainly when dealing with through-hole components, but it can also be utilized with SMT components if an adhesive material is used to secure the components to the board. The wave soldering process, as the name suggests, requires that the planted board pass through a wave of molten solder. While pins can withstand the thermal shock, most SMT components are more sensitive and can be easily damaged.

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The preferred method of soldering SMT components is via the reflow method. In this process, solder paste is applied to the pads using a stencil. Components are placed in position and due to the viscous nature of the solder paste they are kept in place. In some cases, special adhesives are used to ensure that the position of the components does not change during the process. The next step is the actual reflow process. The planted board is placed in a reflow oven and a specific temperature profile is used to ensure that the components are not damaged while still allowing for the solder to melt properly.

Some boards contain both SMT and through-hole packages and since reflow soldering with through-hole parts is problematic at best, manufacturers are left with two options: hand soldering, a process that is quite slow and inaccurate, not to mention costly, and using a selective soldering machine.

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Selective soldering applies heat to only a specific portion of the board. This can cause flux residue to spread to other sections of the PCB. Flux residue contains ionic contaminants that are not easily detected. The main problem here is that these kinds of contaminants can lead to dendritic growth that can ultimately reduce the field life of the board. One way to overcome this problem is preheating the PCB before the selective soldering process begins. This way, the flux in the solder paste can better activate. Contamination tests can also help identify flux migration.

There are several ways of automating selective soldering.  The simplest is to use a controlled soldering iron and sequentially apply the solder and iron to the leads of the components. Alternative methods are available for high volume applications: pin transfer, mini-wave or stencil-dip.

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