Printed Circuit Board (PCB) Assembly
By Akber Roy – Chief Technical Officer at RUSH PCB
Included in the assembly process is automated optical, electrical, and X-ray testing, and sometimes the application of a conformal coating.
PCB, PWB, PCBA
A PCB (Printed Circuit Board) is a term commonly used for:
- A Printed Wiring Board (PWB) which is an unpopulated PCB – no parts on it yet; it isn’t compete
- A Printed Circuit Board Assembly (PCBA) which is a populated PCB – all components and connectors are soldered onto it; it is complete
Most people use PCB as a global term, and whether the term PCBA or PCB is used, it’s still the same populated printed wiring board (PWB).
Fabricated Printed Wiring Board
A printed wiring board is an automated method of wiring; it makes sure the wiring between components is always the same with every finished PCB Assembly. The boards can be large or small, the wiring can be simple or intricate, the signals carried can be digital or analog.
At one time, PWBs were insulating boards with the conducting wiring paths actually printed on the boards. Now, the insulating boards are laminated with copper sheets or are electroplated with copper. Then the excess copper is etched off (chemically dissolved), leaving copper conducting paths on the Printed Wiring Board.
Assembling the Components on the PWB
Most components are either surface mount (where the components are attached only to the surface of the PCB) or pin-through-hole (where component leads go through the board from surface to surface of the PCB)
Typical component types:
- Surface Mount Devices
- Pin-Through-Hole Components
- BGA and CGA Packages
- Hand Soldered Components
Using solder, components are electrically and mechanically attached to the board. For further protection against mechanical shock or for thermal conduction (getting rid of excess heat), large components may have extra means of attaching them to the PWB.
Because advanced assembly procedures are used, the joining of components to the Printed Wiring Board is usually done by machine, but in special cases, assembly is done by hand.
Method Used for Soldering PCBAs
To solder the components to the PWB, two automated methods are used: wave soldering and reflow soldering.
Wave soldering is passing the bottom side of the PWB with its components across a wave of solder welling up from a large solder pot. The solder attaches all the surface mount devices on the bottom and all the pin-through-hole devices to the PWB.
To make sure solder follows the leads through the holes from surface to surface of the PCB, as the PCB is soldered, pin-through-the-hole components have to be wave soldered. Making sure surface mount components don’t come loose In the wave of solder, as they’re mounted by machines, they’re glued to the PWB.
Using infrared heat or hot air, reflow soldering is attaching surface mount components to the PWB by melting solder paste (a mixture of solder and flux).
Similar to silk screening, stencils are used to apply solder paste to the solder pads of the PWB. Then, before heating the solder, surface mount components are set on the solder paste.
A few components, that can’t be machine applied, are hand placed on the circuit board and then hand soldered.
Testing of the PCBA
The finished PCBA is tested and inspected automatically with cameras for visual errors (Automatic Optical Inspection or AOI), electrically for wiring errors, and if needed to make sure hidden connections are complete, X-rayed.
Because of the complexities of the circuits, functional testing is accomplished by the design company.
Some completed Printed Circuit Board Assemblies have a conformal coating.
Similar to a thick paint, a conformal coating is a clear or opaque protective covering that seals the components and circuits of a PCBA. It prevents dust, chemicals, or moisture from attacking the finished PCBA.
Because the components become sealed, conformal coating makes rework or repair difficult. Conformal coating is only applied after all testing and rework is finished.
Contact RUSH PCB
For further information on Printed Circuit Board Assembly, contact the engineers at RUSH PCB .