Mechanical Drawings (Electronic formats.)

In order to ensure that we get the mechanical features of your PCB correct, a mechanical drawings may be submitted, showing the important features.

If you don't have drawings in electronic format, don't worry just post us a paper copy. (Unfortunately the print quality of faxes is usually too poor to be much of use!)

File formats

Any common electronic format is acceptable. But in general Adobe^® Portable Document Format (PDF) are usually most convenient for reference. (These can be easily  created.)

AutoCAD^® DXF format files can usually be imported to our PCB design software, and that helps us ensure that the PCB we design matches your mechanical drawing.

If you can provide mechanical drawings in both formats, this will help.

There are two important caveats regarding DXF format files however. The first is that the import process does not always properly preserve the component scale, so that the imported PCB profile is the wrong size.

We can overcome this if one of the dimensions is shown as 'free text' beside the dimension line proper. (It is also another reason for sending a PDF copy along with the DXF. The second is that the import can fail if the DXF contains a lot of detail or a lot of layers.)

Creating PDF files

You will probably already be familiar with the Adobe^® Reader^® reader for viewing PDF format files, but did you know that it is possible to create PDF format files from almost any program?

If you cannot export directly to PDF, the simplest solution is to obtain a PostScript^® printer driver, which creates a PostScript format printer file. This can then be converted to PDF format using open source software (free!). (PDF and PostScript are closely related.)

You should be able to download a suitable driver from the Adobe website. You will need to download two files, the driver program itself, and a PostScript Printer Definition (PPD) file for the printer you want to use. You don't have to choose a PPD for your own printer, or even have a printer at all, just select the printer port 'FILE' and the the output goes to a file!

Choose a printer that handles a conveniently large paper size, so that you don't lose any detail in the drawing. (Tip: Hewlett Packard made some very big printers!) This can always be scaled to fit your actual printer in the PDF viewer if you need to. (You can find out what paper sizes a particular 'printer' supports by reading the PPD, it's a text format file.)

 

 Download and install the programs GSView, and GhostScript. (You will find links to the download sites on the 'Links' page. If you use Linux^®, you probably already have versions of them, they are included in most distros.) These will allow you to view a variety of file formats, including Postscript and PDF and convert between them. 

(Don't be put off because the programmes are free to dowload and use. They are not cut-down demos, or pirates hacks, or 'mickey mouse' applications in any way. They are fully functional, proffessional software applications,  at least as reliable as anything you can buy,  completely legal, and safe to use. Don't believe anyone who tries to tell you otherwise, they either don't know what they are talking about, or (frankly!) they are lying.) 

The files produced in this way will not have the advanced features that documents produced using Adobe^® Acrobat^® provides, but should be more than sufficient for our needs.

See our Links page, for download sites.

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High-speed considerations, EMC, Ground/Power planes, In-House standards.

High-Speed signals

If your design requires any special consideration with regard to high-speed circuitry, such as matched or paired traces, special supply treatments, filtering or segregation of power planes, please indicate clearly these and any other special treatments.

EMC

In recent years the various regulations and EU directives mean that electromagnetic emissions from electronic equipment have to meet certain standards. These standards are generally compatible with similar regulations applied in countries outside Europe, but some countries have more stringent requirements. This means that some consideration may have to be made with regard to controlling such emissions in your design.

There are a number of myths surrounding the subject of EMC, and we recommend consulting a specialist if you have any doubts with regard to your product.

Usually the simplest method of controlling such emissions is the use of inner layers dedicated as power and ground planes, and the use of surface-mount components. Using planes reduces the resistance and high frequency impedance of the supply paths, leading to reduced transient voltages, and hence, radiated signals. Employing surface-mount components not only reduces or eliminates component lead impedance, but also brings the electrical elements of the components in closer proximity to the ground/power planes, which also reduces emissions.

These two measures alone can reduce emissions by up to 40db!

Despite the likely increase in cost of the PCB and components this is, almost without exception, less expensive than the added cost of mechanical shielding which might otherwise be required. Other techniques can also be employed reduce emissions for general digital circuits, but few will give as large an effect. For designs that entail the risk of high levels of radiated emissions, this technique should be regarded as standard practice.

It is also worth bearing in mind that your design may be susceptible to interference from other equipment, such as mobile phones. As a rule, good emitters are good receivers, so these techniques can also reduce the effect of such sources on your product.

It may be that your design may not require any special treatment with regard to EMC. If you are in any doubt, please feel free to contact us, or check out our links page for specialist EMC websites and services.

In-House Standards

Many companies have their own set of rules for PCB design. If you, or your client, are one of these, please do not forget to provide us with a copy of the relevant rules. You will almost certainly be charged for remedial changes to a designed PCB to meet these standards.

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Prototypes

This section deserves a Latin motto: Amat victoria curam (Victory favours those who take pains.)

New PCB's

It is always best to treat each new PCB design as a prototype. While this probably goes without saying for a new electronic circuit, prototype PCB's are often considered an unnecessary expense when a new board is designed for an existing tested circuit.

This can be a risky approach. Despite taking every precaution to avoid errors, there is always a finite possibility that the PCB design may be incorrect in some detail. Ordering production quantities of an untried PCB could be a very expensive gamble. In many cases PCB corrections can be made for very little, if any, extra cost.

New Electronics

We have already stated that all new electronic circuits should be prototyped. Even relatively simple circuits can be wrong, and most engineers will admit to apparently very obvious mistakes. (The good ones anyway. Those who don't admit such errors are not likely to have achieved much.) Most errors seem obvious once they are uncovered. The difficulty is in discovering the fault.

With new electronics, it is very important that the PCB is produced to the highest possible standards. Some errors on 'production' PCB's are invisible to the naked eye, either because the error is on an internal layer, or may be just too small to see. (Very thin whiskers, or films of copper, can have resistances as low as a few tens of ohms, and still be quite invisible.) Even the best and most meticulous PCB fabricators can occasionally produce rogue boards.

Many expensive engineering hours have been spent trying to trace a circuit design error, only to find that the problem is due to a PCB fabrication fault. It is often not possible to repair the faulty PCB, and more boards must be ordered. It may then not be possible to salvage key components, which at this stage may be difficult to replace, and you still don't know if the your new design works properly!

In the past, testing individual prototype PCB's was considered uneconomic as it required setting-up a so-called 'bed-of-nails' test jig. These jigs take time to create, and this can add considerably to the set-up cost, and introduces further delay. Fortunately the introduction of 'roving probe' testers offers electrical testing of prototypes at a price comparable to the cost of production testing.

A further reduction in prototype board cost can be had from specialist 'multi-board' fabricators. Several boards from different customers are created at once, as a single job, reducing set-up costs. The possible downside to this approach is that, you may have to wait a bit longer for your board, and the comparative cost saving diminishes with the total board area, and soon reaches, and may ultimately exceed the price from a 'traditional' fabricator. You may be lucky and find one who is competitive in price for large quantities, but in any case setting up the newly proved design for production will incur a cost at that stage.

'Soak' Testing

We strongly recommend all power supplies, or any design involving large currents or voltages, is extensively soak tested. Also we would advise that these types of circuit are always tested for continuity and 'partial shorts'. Failure to do so can have catastrophic results. Literally. There is a very real risk of fire or explosion, as quite a lot of power can be delivered to very small regions of the PCB.

I have personally witnessed such a failure. It occurred 49 (!) hours into soak testing.

You should therefore be prepared to soak test the design for several days. The test should be continuosly supervised. Do not assume that the equipment is safe once the power is removed, parts of it may still be hot!

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