If a PCB has a combination of leaded-components and SMD-components, The SMDs can be mounted on the topside and or the bottom-side of the PCB. If the SMDs are mounted on the topside of the PCB, the assembly process starts after mounting the components in the solderpaste with reflow soldering of the SMDs.
Next the leaded-components are mounted and these components are often soldered in a wave soldering process. During wave soldering the whole PCB is exposed to the thermal demands needed for soldering the leaded components. One has however to keep in mind that there are extra thermal restrictions in this case, due to the already soldered SMDs on the topside of the PCB.
If these restrictions hamper the soldering of the leaded-components, one should use another soldering technique for the leaded-components, such as selective soldering.
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