Printed circuit boards: guidelines

Here's a step-by-step procedure you can follow to make use of the Printed Circuit Board (PCB) fabrication facility offered by the Technical Services Unit (TSU) within this Faculty.

  1. Decide whether a PCB is your best choice of construction method. See the comparative guide to the full range of these.
  2. Check that the limitations of an TSU PCB are acceptable to you. We cannot produce boards with:
  3. Design your layout. You may use whatever software package you prefer. How seriously you regard their different capabilities and limitations will depend upon your individual circumstances. Staff in the Teaching Labs or TSU can advise. Whichever package you choose, make sure that it is able to output in a format useable by our process. If you choose the Gerber format then make an independent check on your artwork with a third party viewer, such as gerbv.
  4. Include with your design some element which enables us to identify the chirality of your artwork. Errors on your part here are liable to prove expensive.

    Include also in your artwork a rectangular border or crop marks to show where we should guillotine the board. We cut to the inside edges of these. These dimensions also set the board costing.

  5. Choose your board material, and surface finish.
  6. Check that the cost is acceptable. A charge is made to recover our consumables and equipment expenses and, if you are from outside this Faculty, for labour as well. Obtain an estimate on-line and when you are happy with the details follow the link to reproduce them as printable job request form. If you are a project student, then you do not pay this charge directly, but must still obtain a figure so that you and your supervisor can verify that it is reasonable.
  7. For undergraduate work you must get the form above signed by a technician in the Teaching Laboratories or your project supervisor. It might be useful to have a print of your artwork to hand when you do this - obvious mistakes can be spotted early.

    Postgraduate and other designers must supply a valid University project cost code or Interdepartmental Transfer form.

  8. Take the completed form to the TSU room 04 block AB Floor 4. We understand that it is sometimes difficult for students to get their forms signed immediately. We are prepared to start work on jobs costing less than £10 with an unsigned copy of the form. You must still present the signed form when you collect your board.
  9. PCB Job turn-round times
    Submit your job
    between 09:00 and
    16:30 on this day
    for collection between
    09:00 and 16:30
    on this day
    Monday Wednesday
    Tuesday Thursday
    Wednesday Tuesday
    Thursday Tuesday
    Friday Tuesday
    Saturday No service
    Sunday No service
    As a guide, the current lead times on most boards are as set out at right. However, these times should be extended if there are intervening bank holidays or University holidays (see calendar). The limits on delivery times depend upon staff availability and loading on the system. If you let us know your requirements are urgent then we may be able to process your job at a higher priority.

    We will notify you by e-mail when your board is ready, or if a problem arises. When you collect your board you can also get the artwork film (in case you later need copies of the board).

  10. After you collect your board make sure that the hole sizes are adequate. It is a lot easier to widen out holes before any components or wires have been soldered onto it.

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Tracks wider than 0.2 millimetres can usually be fabricated without problems. You may submit artwork with tracks which are of any width. However, be aware that as the track width shrinks the limitations of our production process means that the width as fabricated may be be significantly different to your artwork specification. This uncertainty amounts to about 0.15 mm for artwork of reasonable quality. Likewise, insulation gaps less than 0.18 millimetres may result in shorted tracks. For stretches of track over small distances with good artwork we can try to go lower. Thicker tracks and wider gaps give us fewer problems.

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Chiral Text

Each year a dozen boards we produce are useless to the people who asked for their manufacture. The reason is that the tracks are etched in a pattern which is a 'mirror image' to the one that the designer actually required. Look at the two boards below and decide which is correct and which has 'mirrored' tracks:

Correct and mirrored boards
It's none too clear, is it? This is our dilemma! We must have some way to tell whether the artwork you supply us is a view looking directly at the tracks or looking 'through' the board. In fact Board B is probably the correct one because there is a square pad identifying pin number one of an 8-pin DIL package IC.

It's easy to avoid this ambiguity: add your e-mail address as text on your artwork so that on the finished board it may be read without using a mirror.

Correct and mirrored boards
It's now clearly a doddle to identify the correct way round. It has the added advantage that, on busy days when we have many customers, both your artwork and finished board are less likely to be muddled with those for someone else.

If you don't have room on your layout for seven letters, or your software can't manage text, or can't get it the right way round, then use the alternative symbol (an arrow with the right hand side of the tip removed) positioned near a corner of your board please. Important: if we see neither your mail address, nor the alternative symbol then we will go by any other text that we do see; such as component numbers - you might then get a mirrored board :-(

A mistake often made by customers is to imagine that the need for chiral text can be avoided by telling us that artwork represents either 'the bottom layer', 'the solder side' or 'the top side'. Unfortunately, those attempts are useless because the chirality remains ambiguous - unless you physically write 'top' or 'bottom' on the artwork the correct way round.

When you apply your text please take time to stop and think about how it is going to appear not just on your screen but on the finished board. In the most common case of a single sided board using through-hole components, being designed on screen as though you were viewing the component side, then your text needs to appear mirrored on screen. Whatever way you are designing a layout it is up to you to get the chiral text correct.

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Output Formats

There are two ways of presenting us with the artwork for your board: as a printout or as a file.

Artwork supplied as a print

Our production process can accept a positive image on film at 1:1 scale. That is, areas representing tracks should be opaque and areas of insulation should be transparent to the near ultra violet region of the spectrum. Note to users of exotic board materials: see the section on user supplied substrates. The tracks should have as high a contrast as possible.

The best such artwork is produced commercially using a photoplotter; but this is impractical for most users. You can instead supply a printout on polyester film (with a specially prepared surface) available to undergraduates from the Teaching Labs or to others from the TSU. The contrast ratio is marginal and only a limited number of laser printers are known to produce adequate results. Most Hewlett Packard LaserJet printers are satisfactory. The film costs about 50p per sheet; so make sure your print looks right on paper first.

Set your software to generate an image printed on the side of the film that will be in contact with the board during exposure. Normally, this will mean that a print on paper of the 'solder side' of your board should appear as it does on screen, whereas a print of tracks on the 'component side' should appear as a mirror image of its appearance on screen.

Artwork supplied as a file

If you are not able to produce a print yourself then we can make one from a computer file you generate in one of the standard image formats, eg

Artwork File Formats
Format name File
Notes *
Autocad DXF 1
Gerber original (RS274D) GBR 1, 2, 4
Gerber Extended (RS274X) GBR 1, 4
Graphics Interchange Format GIF 3
Hewlett Packard Graphics Language HPGL 1, 2
Illustrator AI 1
Macintosh Picture PICT 2, 3
Microsoft Word DOC 1
Microsoft Powerpoint PPT 1
Photoshop PSD 2, 3
Portable Document Format PDF 1
Portable Network Graphic PNG 2, 3
PostScript PS, EPS 1
Scalable Vector Graphic SVG 1
Tagged Image File Format TIF 3
Windows Bit Map BMP 3

* Notes:

  1. These are vector graphics formats and thus yield good results on printers of any resolution. The file sizes are usually far smaller than bitmap formats (3).
  2. Support for these formats is currently weak. Please check with TSU before submitting jobs using them.
  3. These are bitmap graphics formats. Results are usually unsatisfactory at resolutions below 300 pixels per inch. File sizes are larger than with the vector formats (1). To ensure the scaling is handled correctly include the dimensions, in millimetres, of your board outline.
  4. We recommend that you verify Gerber format artwork is correct using an independent viewer, such as that online.

Most of the CAD software native file formats (.PCB etc) are not supported. This is either because we have no licence for the package, no expertise driving it, or because printed results depend upon the 'postprocessor' settings which aren't specified in the file.

Inkjet prints made by Technical Services currently use a gelatine coated film giving superior results to the polyester laser prints. However this is costly (£1.30) and is not at present separately available to customers.

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User supplied substrates

If you supply substrate material yourself then we prefer that it is without a photosensitised coating; we will apply one for you whose exposure time and development method is known to us. This coating has negative exposure characteristics, that is areas of copper track must be represented on original artwork film by transparent areas. If your software cannot generate reversed opacity then supply a file on disk and make clear to us that you require it to be reversed.

We will need a 'border' around the circuit of about 4 millimetres. Please ensure that your material is sized with this in mind.

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Computer controlled mill

For specialist work there is available a high speed milling machine (an LPKF ProtoMat 91s/VS) suited to PCB production. This needs Gerber format files but its use is outside the scope of this document.

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Surface finish

We cannot produce conventional gold or tin plated surface finish, nor can we produce the green epoxy 'silk screen' protective layer. Your options are:

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Last modified: 12th Dec 2017