Surface Mount Devices(SMDs) are attached to the same side of the printed circuit board (PCB) as the solder pads, and these pads can be on both sides of the PCB.

Pin-Through-Hole devices (PTH) are placed on the top side of the PCB. They have their leads inserted through holes in the PCB, and are soldered from the bottom, usually through a wave soldering process.

PCB circuit assemblies using both types of components, SMD and PTH. They require special handling because they each use different soldering techniques.

Reflow soldering is used to attach SMDS to PCBs, or they can be wave soldered.

In the reflow process, solder paste is applied to the solder pads of the PCB using a stencil, and the SMDs are placed with their leads on top of the solder paste. Both the SMDs and the PCB are heated, reflowing (melting) the solder, connecting the leads of the components to the solder pads on the PCB.

To wave solder, when connecting the SMDs to the PCB, instead of reflowing the solder paste, a wave of solder can be used. However, after the components are placed on the board, it is imperative to turn over the board for proper functioning. To keep the components from falling off the PCB as it turns over for wave soldering, and having the wave of solder wash off any remaining SMDs, the SMDs first have to be glued in place onto the PCB.

Pin-Through-Hole devices, or Plated Through-hole Devices (PTH), must be wave soldered

After the PTH components are inserted into the holes of the PCB, and without turning the board over, the PCB is passed over the wave of solder, connecting the component leads to the printed circuit board.

When both SMT and pin-through-the-hole devices are used on the same assembly, several steps are needed for soldering:

1. On the “component side” of the PCB, the SMDs are placed and reflow soldered.
2. The board is turned over and on the “solder side,” without soldering them; the SMDs are then glued in place.
3. The board is turned over again, and passing their leads through the holes in the PCB from the “component side”. PTH components are machine-inserted.
4. At this time, special-handling components are inserted into the PCB by hand.
5. The “solder side” of the PCB is wave solder, soldering the SMDs glue to the “solder side” of the PCB, the leads of the PTH components, and the leads of the special handling components, all in one step
6. Test the board.

Before submitting the PCB for manufacturing, some design issues are of special concern and need attention:

• To allow maneuvering of the insertion machines, PTH devices require extra room on the PCB.
• When making decisions about assembly using lead-free techniques, the thermal profile issues affecting the soldering process should be addressed.

Our mixed PCB assembly services are widely used across:

• Consumer electronics
• Automotive systems
• Medical devices
• Industrial control systems
• Telecommunications
• Aerospace and defense

We tailor our processes to meet the strict standards and requirements of each industry.

Rush PCB has both experience and expertise in all three types of assembly: SMD, PTH, and a mixture of both SMD and PTH.

Let our engineers at RUSH PCB work with you on the engineering and design of your mixed component assemblies.

So, for more information, contact us at sales@rushpcb.uk or call us at 0203 750 0201.