Printed Circuit Board (PCB) Assembly
By Akber Roy – Chief Technical Officer at RUSH PCB
Include in the assembly process is automate optical, electrical, and X-ray testing, and sometimes the application of a conformal coating.
PCB, PWB, PCBA
A PCB (Print Circuit Board) is a term commonly use for:
- A Print Wiring Board (PWB) which is an unpopulate PCB – no parts on it yet; it isn’t compete
- A Print Circuit Board Assembly (PCBA) which is a populate PCB – all components and connectors are solder onto it; it is complete
Most people use PCB as a global term, and whether the term PCBA or PCB is use, it’s still the same populate print wiring board (PWB).
Fabricated Printed Wiring Board
A print wiring board is an automate method of wiring; it makes sure the wiring between components is always the same with every finish PCB Assembly. The boards can be large or small, the wiring can be simple or intricate, the signals carry can be digital or analog.
At one time, PWBs were insulating boards with the conducting wiring paths actually print on the boards. Now, the insulating boards are laminate with copper sheets or are electroplate with copper. Then the excess copper is etch off (chemically dissolve), leaving copper conducting paths on the Print Wiring Board.
Assembling the Components on the PWB
Most components are either surface mount (where the components are attach 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 Solder Components
Using solder, components are electrically and mechanically attach 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 advance assembly procedures are use, the joining of components to the Print 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 automate methods are use: 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 solder, pin-through-the-hole components have to wave solder. Making sure surface mount components don’t come loose In the wave of solder, as they’re mount by machines, they’re glue to the PWB.
Using infrare 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 use 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 apply, are hand place on the circuit board and then hand solder.
Testing of the PCBA
The finish PCBA is test and inspect automatically with cameras for visual errors (Automatic Optical Inspection or AOI), electrically for wiring errors, and if need to make sure hide connections are complete, X-ray.
Because of the complexities of the circuits, functional testing is accomplish by the design company.
Some complete Print 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 finish PCBA.
Because the components become seal, conformal coating makes rework or repair difficult. Conformal coating is only apply after all testing and rework is finish.
Contact RUSH PCB
For further information on Print Circuit Board Assembly, contact the engineers at RUSH PCB .