Mixed Assembly

Due to the rapid evolution of computerized technology, an increasing number of printed circuit boards are being made with smaller and smaller parts. This means many PCBs are made today with a combination of methods, commonly referred to as mixed technolog

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Mixed Technology

Due to the rapid evolution of computerized technology, an increasing number of printed circuit boards are being made with smaller and smaller parts. This means many PCBs are made today with a combination of methods, commonly referred to as mixed technology. An assembly that involves mixed technology will encompass one of the following approaches:

  • A single-sided mixed assembly, where a printed circuit board undergoes surface mount technology as well as through-hole technology, both on the same side of the board.
  • A split assembly, where one side of the printed circuit board is assembled with surface mount technology and the other side of the board is assembled with through-hole technology. PCBs like these have normal-size components on one side and tinier components on the other side.
  • A double-sided mixed assembly, where both sides of the printed circuit board are assembled with a combination of through-hole technology and surface mount-technology. A board of this type features regular components as well as tiny and delicate ones on both sides.

Before a PCB design goes into production, the boards intended for use must be inspected for quality purposes. On the components, foot oxidation or oily residue could serve as red flags. If stored in a cool environment, the solder paste can only be applied once it has been thawed and stirred. The PCB must be dry before any paste is applied to the surface.

On printed circuit boards with mixed technology, a more complex set of processes are required within the pick-and-place machine. Here, a mixture of different component sizes will need to be accounted for, either on one or both sides of the board.

If the printed circuit board consists of a double-sided assembly, the soldering process will need to be applied to both sides. Basically, everything that happens on one side also happens on the other. The only difference is the specific components and their placements, as one side might contain smaller components than the other. Wave soldering is not possible for two-sided PCBs because resubmitting the board for the second side would ruin the delicate parts on the first side.

Any printed circuit board comprised of mixed technology should be submitted to an automatic optical inspector. This way, technicians can be ensured a foolproof inspection that will detect even the tiniest imperfections, such as minor misplacements of microscopic parts.

To account for the complexities of a two-sided mixed technology board, a more thorough set of tests are required for the initial PCBs produced in such lines. Since there are more components to account for, there are more potential problems if even one part is out of alignment.

How is a PCB with Mixed Lead Components Assembled?

PCBs With Surface-Mount Devices

Printed circuit boards (PCBs) with surface-mount devices (SMDs) can be wave or reflow soldered, while PCBs with pin-through-the-hole devices (PTHs) can only be wave soldered. PCBs with a mixture of SMDs and PTHs require both wave and reflow soldering, and have to be soldered in several steps.

Reflow Soldering

Reflow soldering requires a screen process. In the screen printing process, a paste containing both solder and flux is administered to the PCB solder pads.

In the next step, making sure the surface-mount-devices will have their leads in the paste, a machine places them on the PDB

Reflow Soldering
A paste, containing both solder and flux, is applied to the solder pads of the printed circuit board. The components are then placed on the PDB and the paste holds the components in place.

To solder the components to the board, a stream of hot air or radiant heat is applied to the whole board. This evenly heats up the board, the components, and the solder paste, melting the solder in the paste and connecting the components to the board.

Reflow Heat
When all the components are placed on the PCB, heat in the form of infra-red radiant heat or hot air is used to melt the paste, which reflows to make permanent connections.

Wave Soldering

Instead of heating a paste, a solder wave can connect the Surface Mount Devices to the PCBA. However, so the Surface Mounted Devices don't fall off the PCBA as it's turned over, and the solder wave knocks off any that haven't already fallen off, the components require gluing.

PTH components cannot be reflow soldered, but require wave soldering after their leads are inserted through the holes in the PCB.

Wave Soldering
The wave of solder is produced with a pump, so that a welling up of solder will touch the PCB as it is passed over the solder-pot. All exposed metal on the bottom of the PCB will be coated with the solder.

PCBs With Both PTH and SMD Components

If PTH and SMD components are used on the PCB, more steps are required to attach them:

1. Component side SMDs are reflow soldered

Component side SMDs are reflow soldered
Solder paste is only applied to the pads that receive components, so the other pads will remain clean for the next steps. The Surface-Mount-Devices going on the component side are placed on the PCB and the solder paste and then reflow soldered.

2. Solder side SMDs are glued in place on the solder side of the PCB.

3. Pin-Through-Hole devices are machine inserted into the holes on the PCBA

4. Hand installed devices are installed on the PCBA

Hand installed devices are installed on the PCBA
The Surface-Mount-Devices going on the solder side of the PCB are glued in place. The PDB is turned over. The other machine installed devices are insserted with their leads through the holes in the PCB. Any hand inserted devices are then installed.

5. The glued SMDs, PTHs, and the hand installed devices are soldered in one wave

The glued SMDs PTHs soldered in one wave
The Surface-Mount-Devices going on the solder side of the PCB are glued in place. The PCB is turned over. The other machine installed devices are inserted with their leads through-the-holes of the PCB, from the component side.

6. Other components that have to be hand soldered to the PCB are then installed

7. The assembly is tested