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I don't know if the question is straightforward. I am not interested in the manufacturing process, soldering, testing, etc. When I am working on a school project, first of all I try my design on a breadboard and then if it's a request, I design the PCB.

I have always thought how a motherboard/video projector main board/etc that has many components (SMD) is tested in the beginning ?

Is the functionality split and tested separately ?

How about SMD technology ? How do you test SMD circuits without a breadboard ?

How big designs begin ? Starts with a breadboard ? I hope I am not silly.

EDIT: Another question: If I want to design a power supply (just an example) in SMD technology, where I can start ? To test first with THT components ?

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    \$\begingroup\$ Big designs are usually first prototyped on some larger boards designed for test. Like in less layers, many testpoints, many configurable jumpers. Sometimes it takes more than one iteration to get the production-grade board. \$\endgroup\$ – Eugene Sh. Dec 12 '18 at 16:48
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    \$\begingroup\$ Larger designs generally do not rely on bread-boarding. Bread-boarding gives you very little control on how the circuitry is designed, as you cannot precisely control component placement, trace width/routing constraints, impedances, or thermal management. Furthermore, key components for something like a motherboard (like DDR or CPU) simply aren't available in breadboard packages. \$\endgroup\$ – Chris Fernandez Dec 12 '18 at 16:51
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    \$\begingroup\$ As commented previously, many designs are never breadboarded at all. The components themselves may not be easily breadboardable, and the layout can be critical for the whole thing to work. So they go to PCB immediately. Sometimes early PCB designs (spins) have zero-ohm resistors (or similar), added to allow breaking up the circuitry. \$\endgroup\$ – mike65535 Dec 12 '18 at 17:34
  • \$\begingroup\$ We don't do breadboards. For designs with many new features we put time in the schedule to do multiple design revisions. Chip manufacturers often have evaluation kits which can be used for proto-typing. For example, you can modify a power supply evaluation kit to make sure your planned inductor, output capacitor, compensation network will work correctly. \$\endgroup\$ – mkeith Dec 12 '18 at 19:14
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Typically everything that can be simulated is simulated. Specially power supplies and everything that is analog. The rest is designed by reading the datasheets and application notes very carefully and following them.

Using a bread board is quite rare actually. Building test circuits on veroboard is slightly more common. You can use SMD components with those. They can be used to test some specific circuit, which can't be simulated for some reason.

Big part of the design work is just reading the datasheets, and relying on them. With experience you learn to know how things work. Projects typically have an experienced lead designer who knows what to do. Junior designers of the project "obey" the lead designer and learn from him/her.

Peer reviewing is also important. When a design is readyish, it is passed to colleagues who check what you have done and try to find errors or something that can be improved.

Then there are things like EMC design, RF design, etc. The project might include specialists for these areas, that take care of them. Big projects are very much team work.

Here is one more detailed description https://www.radio-electronics.com/info/electronics-design/development-process/tutorial-basics-summary.php

The details of the design process vary a lot. The design houses have different processes, and it also depends on the type of the project.

Then answers to your specific questions:

Is the functionality split and tested separately?

As much as possible, yes. You can't split everything, e.g. you can't test a digital accelerometer without connecting it to a CPU and actually running code that uses the accelerometer. Most of the analog stuff can be tested in smaller parts, but you still have to also test it all together to know it works

How about SMD technology ? How do you test SMD circuits without a breadboard ?

As much as possible using simulations. With veroboards it is also easy to solder SMD components. Mostly everything is tested with a prototype that is built on an actual PCB

How big designs begin ? Starts with a breadboard ? I hope I am not silly.

The very first thing is specifying requirements for the design. Then simulations, etc.

If I want to design a power supply (just an example) in SMD technology, where I can start ? To test first with THT components ?

With simulation.

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