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Author: Jon Hawkins

Date: 2018-10-15

A Basic Overview of the Process of Design to Manufacture for Electronics

When an engineer first starts the development of an electronic circuit, the terminology, data and general knowledge of manufacturing processes can be overwhelming. That said, the basic structure and the processes that must be followed are quite straightforward really, it’s just a question of having them explained simply: this is the purpose of this post.

If your design is straightforward the advice in this post is probably adequate – you don’t need to follow the follow-on post links I’ve placed throughout the document.

I’ve assumed that an approved specification of requirements has already been compiled, and I will not delve too deeply into testing other than the engineer’s basic functional testing.

Get it wrong quickly, fail fast

It is entirely understandable that those at the beginning of their careers are straining every sinew to please, but trying to perfect a theory without trying something is an error many engineers learn at a cost. Just have a go, and if it’s wrong you’ll learn.The sin is if you don’t learn from such experiences; the cost of a few boards is as nothing compared to weeks of lost time, which is certainly what will happen if the design of a board has been agonised over without manufacturing it and trying it.

So, what resources are required to kick things off? The short answer is: PCB design and layout CAD [for example, Altium (professional), Easy PC (intermediate), or Design Spark (free download)]. In addition to this electronic components will need to be purchased, once you’ve decided which you need (inevitably researching across the web), there are some big distributors that it is always sensible to use for prototypes, e.g. Farnell, Digikey, Mouser and RS Components. If you want to be manufacturer friendly pick non-critical passives in accordance with my post Design for Manufacture

Design your circuit schematic using the CAD system, and that involves generating a schematic (circuit diagram), BoM (Bill of Materials) and footprints. Doing the schematic is just a question of putting symbols of the components you intend to use on the circuit and connecting them with each other. The diagram below illustrates this:

 

Your CAD package will allow you to do this.

As you place the components it will be necessary to give each component properties, which will then allow the construction of a BoM. A BoM lists all the components, the suppliers and part numbers, designators (R1, C10, etc) that show what position the component will be fitted in on the PCB, when it’s made.

Next come the footprints.

Footprints are the copper areas, e.g. pads, designed to line up with the pins of a given component. Your CAD software will have many of the standard ones in its libraries, but if this is not the case you will have to design them using the CAD and the dimensions given in the datasheet for the component in question.

Every device mounted on the board will need a footprint, and every symbol on your schematic will need a footprint. If one is missing – there will be nowhere to fit one of the components on the board. Below I’ve tried to illustrate this by showing: a component being matched with its footprint on a board; and a view of footprints without components. The datasheet is the place to go to get the details necessary to design a footprint.

Next, comes PCB layout and design.

You have now selected all the components needed, defined how they are to be connected, and designed the pads for them so that it is possible to sit and connect them on the PCB.

Next, you need to position the components on the PCB and define the PCB size/border using Altium/Design Spark, or whatever. These tasks seem easy – just draw the border of the PCB to size and drag the components into it (which is known as placement), but there are some important considerations:

  • Partitioning: I’ve done another post on this (coming soon), but as I am sure you can imagine, digital, power and analogue signals should not all be randomly mixed up – it is asking for trouble with noise performance.
  • Fitting: don’t forget you’ve got to connect all the pins to each other, and that means positioning the chips so this can most easily be achieved.
  • Mechanical issues: connectors, switches and the like may need to be in very specific positions, the whole product may need to fit in a box (post coming soon), etc.

Next comes tracking

A topic in its own right, and covered in more depth in my post (linkcoming soon). If you imagine for a moment that the design is a basic low-frequency digital design not particularly subject to noise issues, then the tracking is simply joining the pins and terminals of all the components up so that a PCB circuit is created that matches the schematic circuit mentioned above.

As I am sure you can imagine, it is very easy to get this wrong, so don’t neglect to use the DRC (design rule check) feature that’s sure to be an integral part of the CAD package you are using. (Also note that many CAD packages have auto-routers, but these are usually less than perfect [a straightforward, low-frequency digital design may be ok, but otherwise use auto-routers with caution]). Don’t proceed to the next stage until the DRC reports no errors (ignore this advice and the omission is sure to come back and bite you later on).

I’ve glossed over the use of planes, but these are covered in my post (coming soon).

When you’ve finished, and your DRC is clear of errors, you need to produce manufacturing files for the PCB. The exact instructions for this will depend upon the CAD package that you’ve used, but what you’re looking for is ODB++, or plotting and printing, which will produce gerber files and NC drilling files (you should also make sure there is XY placement data in the pack that you provide for your manufacturer). Do either ODB++ or gerbers, but not both.

The PCB board specification is something which should be very straightforward if the design processes above has been done thoroughly.  There are all sorts of arcane variations, such as stack design, which is the detail of the laminating in a PCB, that really should not require specification for an uncomplicated design. If you have a good manufacturer, like Newbury Electronics Ltd, the following will be more than adequate (NE will take care of the rest).

  • The thickness of the PCB – 1.6mm is a common standard
  • The material the board is to be made from – FR4 is the most common material
  • The colour of the board – use green unless there is a good reason not to – most common
  • The copper weight (unless you are intending to carry high currents on the board the standard 1oz Cu will almost certainly be adequate (note 1oz Cu means 1 sqft of copper at the thickness specified will weigh one ounce). 2 and 4oz Cu are readily available should it be desired.
  • if Cu pads are left bare they will oxidise over time, which makes soldering problematic or ineffective. To avoid this there is usually a surface treatment applied to the pads, and it is most commonly ‘silver immersion’.
  • You may state the number of layers the board has and the dimensions, but this information is included in the manufacturing files above, so you shouldn’t need to.

Design Manufacturing

So, now you’ve reached a key stage – you should be ready to get your design manufactured. The files you’ve generated that your manufacturer will need are:

  • BoM
  • The Board Specification (thickness, PCB material, colour, copper weight and immersion type)
  • The PCB manufacturing files (ODB++ or gerbers, NC drilling files and XY data)
  • A BoM (bill of materials)
  • If there is firmware to be blown onto the board, the object (hex) file needed to do this (for firmware outside the scope of this post see coming soon.).
  • The quantity you want to order
  • Now just call Newbury Electronics Ltd on 01635 40347, and the Sales team will see you through the process and get your order placed.

That should be it, but Newbury Electronics Ltd has a team of technicians who examine the data – if there are problems they will tell you, and help you rectify whatever might be wrong.

Don’t try and define the panels for production of your PCBs – let the experts do it. If you don’t know what I mean by this – good. A more detailed description of the manufacturing process can be found in my post(coming soon).

Then you have a bit of a wait whilst your PCBAs (assembled PCBs) are made. This may be anything from about a week to a month, depending on the delivery option you’ve chosen.

Testing

Testing of your PCB is a key stage. I am not going to go into a lot of detail on that here, but it is always good to start slowly. Some basics follow:

  • Visually inspect the boards – check out anything that doesn’t seem right
  • Apply power and check that all the power rails are up.
  • If there’s firmware, check that it will program.
  • Get the schematic out, and start checking the major functions on the board.
  • Don’t get too upset if something is wrong. A design engineer who tells you his/her boards always work is a liar.
  • If there are problems decide how best to get over them: cut and strap; remake; whatever.

More detailed coverage of testing can be found in my post (coming soon).

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