Various Printed Circuit Board Assembly Processes

Printed Circuit Board Assembly involves several processes, depending on the type of mounted PCBs that will be used. It should be noted that PCB fabrication is different from PCB assembly. When a PCB is fabricated, electronic components have to be soldered onto it so that it can be used by any electronic equipment. This assembly of electronic components depends on the type of circuit board, the type of electronic components, and the purpose of the circuit board.

Generally, there are several common things that are needed in the assembly process and these are:

Printed circuit board, electronic components, soldering materials, soldering flux, and soldering equipment. Soldering materials include the solder paste, the wire, and the solder bar. Soldering equipment include wave soldering machine and SMT equipment. There are different processes for single-side surface mounting and double-sided surface mounting, as well as for single-sided mix assembly and double sided mix assembly.

For single-side surface mounting, there are only three steps to the process:

It starts with solder printing, followed by adding SMD parts; the final step is all about doing reflow soldering. Solder printing uses solder paste, a sticky substance and a stencil to transfer solder paste into a bare circuit board. When the stencil is correctly aligned onto the board, solder paste is then applied over the openings using a metal squeegee. Once the steel foil is separated from the board, the solder paste will remain, which is now ready for placement of the SMD. Foil thickness and opening size control the volume of paste.

Printed circuit boardToo much solder paste cause bridging, tomb-stoning and solder balling. Too little solder paste applied creates insufficient solder joints. These all affect the electrical functionality. The last process in this PCB assembly process is reflow soldering. This is when a solder paste is used to attach electrical components to the contact pads. Then, the entire board is subjected to controlled heat, melting the solder. Afterwards, it is heated with a reflow oven, through which the PCB assembly is passed. Reflow soldering, on the other hand, is the conventional means of mounting components onto the board.

For single-sided plug-in mounting, there is also a three-step process:

forming, THT parts and wave soldering. Wave soldering is a process where the PCB is passed over a pan of heated, molten solder. There is a pump that produces an upwelling of solder that is akin to a standing wave, hence the name ‘wave soldering.’ When the circuit board comes in contact with this wave, the components become soldered unto the board. Forming causes inefficiency in the process of wave soldering. It may cause electrostatic damage and the probability for mistakes is also increased. The wave soldering process includes the insertion of electronic components, flux application, preheating, cleaning and testing.

The PCB assembly process for double-sided surface mounting involves both sides of the board. The steps for both sides are: solder paste printing, adding all the necessary SMD parts, reflow soldering and flipping. Flipping is done so that it would be possible to work on the other side. To fully maximize PCB space as well as to improve efficiency, the A side is usually laid out with IC components and the B side consists of chip components.

The single-sided mixed assembly has five steps: solder-paste printing, surface-mounting parts, reflow soldering, adding THT components, and lastly wave soldering. This type of assembly allows for a second heating, which results in higher efficiency. For double-sided mixed assembly, the PCB assembly is heated three times, but it exhibits low efficiency.

All processed boards, no matter what type, are carefully tested and inspected, particularly for solder shorts or tombstones. Then, all necessary repairs are made.