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The Printed Circuit Board Primer
Краткое содержание статьи: A fine printed circuit board (PCB) is a mixture of high art, and solid engineering. Here is a short primer on what goes into the making of a PCB, the terminology, and the features that enhance reliability, and lower cost.

The Printed Circuit Board Primer

Редакция THG,  10 августа 2001
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Component Mounting And Soldering

The final step is to mount and solder the components. Both THT and SMT components are placed on the PCB by machines.

THT components are most often soldered in an automated process called 'Wave Soldering'. This enables all components to be soldered simultaneously. Their legs are first cut near the board and slightly bent over to keep the component in place. The PCB is then moved over a wave of liquid flux, such that the bottom side strikes the flux. This removes any oxide from the metal surfaces. After heating the PCB it is similarly moved over a wave of melted solder. The solder attaches to the solder pads and component legs, and the soldering is complete.

A common way of soldering SMT components automatically is 'Over Reflow Soldering.' A solder paste containing both flux and solder is then applied to the solder pads before the components are placed on the PCB. The PCB is then heated in an oven such that the solder in the paste melts. Cooling the PCB completes this type of soldering and the PCB is ready for final testing.

Where The Cost Savings Come In

To make the cost of the PCB as low as possible a lot of factors have to be considered:

  • The size of the board is of course significant. The smaller the board is, the cheaper it is. Some PCB sizes have become standard sizes for manufacturing, and sticking to one of these sizes helps reduce the cost. The website of CustomPCB has info about some standard sizes.
  • Using SMT is cheaper than THT because it makes the PCBs more dense (and therefore smaller).
  • On the other hand, if the board becomes very dense the wires in the conductor pattern must be thinner, and more high tech machines have to be used to manufacture the boards. Higher quality materials must also be used, and the routing of the wires must be done more carefully to avoid any leakage currents that could affect the operation of the circuit. All this could increase the cost of the PCB more than what is gained by reducing its size.
  • The cost increases with the number of layers, but fewer layers will often increase the size of the PCB.
  • It takes time to drill the holes, so as few vias as possible is desirable.
  • Buried vias are more expensive than vias that go through all the layers. This is because buried vias makes it necessary to drill each layer separately before they are laminated together.
  • The size of the holes in the PCB depends on the diameter of the component legs. If components with different types of legs are required on the same board the machine that drills the holes cannot use one single drill to drill all the holes. The more times the drill has to be changed while processing one board, the more expensive the PCB is to manufacture.
  • An electric test with a 'Flying-Probe' is more expensive than an optical test. Often an optical test is sufficient to make sure that the PCB does not have any defects.

It all adds up to a significant amount of work for the manufacturer as devices become more complex. The process of creating a PCB is useful to understand because, it gives us an indication of the abilities of a manufacturer when we are comparing like boards, which may deliver similar performance, but may vary in cost, or reliability.

A good engineer can look at a board and draw a conclusion as to the quality of the design. You may not want to go that far, but the next time you look at a motherboard, or graphics card, you just might have an appreciation of the art of PCB design.


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