Question

Can somebody explain me about the different types of circuit boards? Please explain broadly.... Help me...

Answer 1

Well there been different types over time, in my day, 70's, we would actually use a tape to form mask for board layout before etching. Then they came out with mylar photo mask, there's computer program you could buy for layout, this was for individuals board. Mass produced board use similar procedure but have some sort of contacts. In the 70's I worked on military boards that had 20 contacts (10 on each side) to be inserted into a slot. My understanding it's not much different today except they have smaller components and use a flow soldering technique which help remove stray resistance, capacitance, and inductance and make fabrication easier. There's company out there that will fabricate a PC board for you if given a layout. Of course you should breadboard your design and ensure it works before sending it to them.

Answer 2

PC board fabrication is a separate EE specialty that I never got into. I designed some boards in 1978 and gave general layout. Others actually made calculation and created the mylar for photo etching, and this was done in the machine shop where I worked. I actually did the etching.

Answer 3

Today's circuit boards are almost all made out of fiberglas. The primary characteristics that are involved are:
1) Number of layers (from 2 to 20+), and
2) Whether the board uses Surface Mount (SMD aka SMT) components or "Through hole" components, and
3) The smallest trace width that the process can reliably make (5 mils is common, 2.5 mils is here now and not too expensive)

The process has several high-level steps:
1) Choose a CAD system - the most common industrial EE CAD system in use in the US is probably AutoCAD. I use Eagle from CadSoft - which offer a free version that is pretty much full featured but is limited to small boards (something like 3" x 4")
2) Create the schematic in the CAD system using the CAD system. This process is called "schematic capture", and I recommend you always create the schematic from the beginning using the CAD system you are going to use to create the board layout. You will create or load (from a huge database of existing parts) a part in the CAD system for each component you use on the PCB. A "part" in the CAD system includes a schematic symbol with pin numbers and a physical board layout "footprint" for each part, ie pads for each pin. Then you will draw wires on the schematic using the system's drawing capability.
3) After the schematic is finished, you will switch to board layout mode. The system will automatically include all your parts and will draw "airwires" making all connections. But the parts are sort of in a pile and the airwires are not physical representations. You pick a board size and draw its outline.
4) Placement: Then you "place" each component on the board in approximately the best location based on the connections that have to be made, proximity to connectors etc. This is almost always done manually, but some high end CAD systems offer some automatic placement tools. However, even in those cases, you will almost always move the parts around a bit manually.
4) Routing: This is the process of drawing physical traces on the board making all the necessary connections. Digital boards of modest speed are frequently routed automatically by the software. The software frequently will only be able to complete a portion of the routing, leaving some for you to do manually. High speed digital boards and analog boards are usually routed manually. You, the designer, carefully place and connect all parts taking into account distributed capacitance and inductance and trace width and length and, therefore, resistance. The quality of the work you do in this step determines speed, noise, EMI susceptibility, and radiation, etc.
5) Output: The output of the CAD based design is a set of files that can be used by the PCB fabrication house to make the PCB. The standard format for this files is "Gerber", so named because we used to print out the design on a Gerber large format graphic printer and then use the hard printed sheets to define the PCB using a photolithographic process. Nowadays, the files run machines that use lasers to expose photoresist on bare PCB material, and thereby define the layout.
6) Fabrication: the PCB is fabricated by a vendor who takes your Gerber file as input and sends you finished circuit boards, which includes all traces, vias, holes, contacts, solder mask, and silkscreen.
7) Assembly: There are other companies who specialize in assembling your PCB. You send them your Gerber files, the already fabricated PCBs, a BOM (Bill of Materials), and all components. They return a finished PCB. Some houses will also test the boards for you if you provide the necessary information and/or test fixtures.
8) Low volume PCBs: we face a special problem when we want a small number of boards. Most PCB fab houses do not want to work with you or charge a great deal for small quantities. But there are specialty houses that will make 1 - 1000 boards for you at remarkably low prices. You can get five 3"x5" 2 layer boards fabricated for something like $200 for the batch. You can then get those boards assembled for something like $200 (plus the cost of all the components). Check out PCBFabExpress.
9) High volume PCBs: once you have larger volumes (at least 1000 units per order) there are vendors who will take your CAD files, BOM, and test vectors and they simply supply finished boards. They buy the components and inventory them and test the finished board. Almost everyone who has volumes of 100,000 units/year or more use this approach.
10) Use of inner layers. The most common multilayer boards have 4 layers. The top and bottom are used almost exclusively for interconnections while one inner layer is a ground plane and the other is used to supply Vcc, or Vdd power - whether one voltage or more than one voltage. Really complex, dense boards may use many more layers - maybe as many as 20. Many of these boards are for very specialized military applications where cost is not a constraint but weight and density are.

Does this help?

Answer 4

I don't know if it helped him but it was very informative to me!! I've been out of the loop for 27 years and I see where PCB fabrication is definitely a EE specialty. Working generally on my own now I stick to analysis of discrete components and new specific ways of using them to minimize error and source drain by circuit. I really don't like circuit simulations that much, it seem to take away some understandings. It may be useful to use after a through design, to check where you may have gone wrong or for stray errors. Well anyways I don't know how to give Best Response or Metals but you deserve them both. Thank You

Answer 5

Thanks for the kind words Ken! I hope it was a helpful comment.

You make a good point about simulation. I probably should have mentioned it.

The high end CAD systems can do simulation of the whole system. IC designers use simulation extensively, as do those working at the module or board level with very high speed and RF circuits. But such simulators are very expensive and many small and medium sized companies don't perform system level simulations - it is often cheaper to go through a couple of prototype spins. However, circuit level simulation is widely used for analog circuits. Linear Tech's LTspice is one such simulator that is very good. At the other end of the spectrum you have 5spice, which is free (but limited). I agree with Ken that over use of simulators can lead to trial and error iterative solutions that do little to develop your understand, and I agree that one should do one's best circuit analysis and design before checking the result with a simulator. 5spice allowed me to find a mistake in a power control circuit I designed recently. I was watching TV and decided on a whim to put the circuit in 5spice and was very surprised when it didn't work. I was able to find the mistake and modify the design using the simulator - the first prototype worked perfectly. So 5spice saved me a prototype spin.

Answer 6

1) Cordwood construction can save significant space and was often used with wire-ended components in applications where space was at a premium (such as missile guidance and telemetry systems) and in high-speed computers, where short traces were important. In "cordwood" construction, axial-leaded components were mounted between two parallel planes. The components were either soldered together with jumper wire, or they were connected to other components by thin nickel ribbon welded at right angles onto the component leads. To avoid shorting together different interconnection layers, thin insulating cards were placed between them. Perforations or holes in the cards allowed component leads to project through to the next interconnection layer.

2) Multiwire is a patented technique of interconnection which uses machine-routed insulated wires embedded in a non-conducting matrix (often plastic resin). It was used during the 1980s and 1990s. Multiwire is still available in 2010 through Hitachi. There are other competitive discrete wiring technologies that have been developed.

3) Surface-mount technology emerged in the 1960s, gained momentum in the early 1980s and became widely used by the mid 1990s. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly on to the PCB surface. Components became much smaller and component placement on both sides of the board became more common than with through-hole mounting, allowing much higher circuit densities. Surface mounting lends itself well to a high degree of automation, reducing labour costs and greatly increasing production and quality rates. Carrier Tapes provide a stable and protective environment for Surface mount devices (SMDs) which can be one-quarter to one-tenth of the size and weight, and passive components can be one-half to one-quarter of the cost of corresponding through-hole parts. However, integrated circuits are often priced the same regardless of the package type, because the chip itself is the most expensive part.