Artwork generation for single-sided boards is a straightforward exercise, whereas preparing artwork for double-sided printed circuit boards, leads to a problem in securing accurate registration between the two sides.
Correct and accurate registration is of great importance in multi-layer boards also where shorting between layers may occur because of improper alignment when the board assembly is drilled for plated-through holes.
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The various methods used for artwork preparation are discussed below.
5.2.1 Ink Drawing on White Card Board Sheets
This is the simplest and earliest method used for PCB artwork design. The materials required are a white cardboard paper, good quality Indian ink and an ink pen. A paper with blue lines grid is usually preferred as these lines do not get reproduced in the photographic process for film master production. Suitable polyester foil, which is dimensionally more stable than cardboard paper, can also be used for drawing the ink pattern. However, the one side of the foil should have a rough or mat surface so that ink will stick to it.
In this technique, circumferences of the solder pads and the centre holes are drawn with a drawing compass and the spaces in between are filled with ink. Conductors are drawn with an ink pen either by directly providing the desired width or as double lines, which are thereafter filled with ink.
Templates in the scale of 1:1, 2:1 and 4:1 are available, which serve as useful aids for drawing of the solder pads. Since this is a totally manual method, it suffers from drawing inaccuracies. For example, conductor widths may vary at least by 0.1-0.2 mm and solder pad locations and conductors can easily by displaced by 0.3-0.5 mm. Also the dimensional instability of the cardboard, 0.01 % °C with respect to temperature and 0.005 %RH (relative humidity) with respect to changes in relative humidity, could be a source of error in high accuracy boards. The method is now rarely used because of the poor stability and precision results.
5.2.2 Black Taping on Transparent Base Foil
Manual artwork generation has become very convenient since the advent of self-adhesive or transfer type pads and precision tapes, and no longer relies on the high drawing skills and patience of the draftsman.
Self-Adhesive Tapes
They are available in a wide range of widths, which can be selected depending upon the application.
They are supplied in rolls with standard widths of 0.5 mm, 1 mm, 2 mm, 4 mm and 8 mm; etc.
(Figure 5.1a). The width tolerance can be 0.05-0.1 mm depending upon the tape width and the manufacturer.
0.5 mm 1 mm
2 mm 4 mm 8 mm Tapes
Fig. 5.1(a) Sizes of tapes
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Self-Adhesive Pads
Pads are available in standard shapes such as donut, square, hex, oval, and tear drop and triangular.
These are shown in Figure 5.1b. The two different types of pads which are commonly used are:
Self-adhesive type and the Transfer Type.
Donut pads Donut pads (solid)
Square pads Square pads (solid)
Hexagonal pads Teardrop pads (double end)
Oval pads (one hole offset)
Teardrop pads (with fillet radius)
Oval pads (two holes)
Teardrop pads (with straight side fillets)
Fig. 5.1 (b) Standard pad shapes, all pads are 1.27 mm
Transfer Pads
They are printed on a thin adhesive film of typically 10 mm thickness, which is mounted on the top side against a transparent carrier strip. The pad can be transferred from the carrier strip onto the artwork base (polyester base sheet) by just rubbing with a wooden stick end or pencil. The carrier strip can then be lifted from the artwork base leaving behind the pad.
In order to minimize the artwork preparation time and to achieve accuracy in positioning, pads in the form of solder pads, pre-arranged patterns like integrated circuit footprints, contiguous patters for connectors and special patterns for TO casing components can be used. These pre-arranged patterns are usually for the multi-lead components and are generally referred to as footprints: for
example, the 16 pin IC pattern is called DIP 16 footprint and so on. Some typical footprints are shown in Figure 5.2.
A) SolderPads
3.8 mm 4.8 mm 6.35 mm 7.9mm 12.7mm
B) IntegratedCircuitPads
DIP- 8 DIP- 8
DIP-14 DIP-14
DIP- 16 TO - 5/8 TO - 5/10 TO - 5/12
C)ConnectorPads
7.924 mm 5.08 mm
D) OtherPads
Transistors Dimensioning Registration
Fig. 5.2 Special pads commonly used in 2:1 artwork generation
Polyester Films
They are usually employed as the artwork base foil as they provide an excellent dimensional stability.
The typical values are 17 ppm§°C with respect to temperature changes and 11 ppm§% RH with respect to changes in relative humidity (ppm = parts per million). Polyester films are available in
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many thicknesses. However, the minimum thickness should be 100 mm to facilitate sufficient mechanical stability against wrinkles. Polyester films are available with pre-printed grids.
The artwork for double-sided PCBs may be generated by using either the ‘Two Layer Artwork’
or the ‘Three Layer Artwork’.
Two Layer Artwork
In this method (Figure 5.3), separate artworks are prepared for the component side and the solder side of the PCB. The method is tedious as it is quite difficult to achieve accurate registration between the two layers.
Tape and pads for the component side traces
Use 5 to7mil thick polyester
Tape is on farside
Tape and pads for the non-component side traces
Pads Tape
Polyester film
Tape Pads Polyester film
(Registration is very important from pads on the first sheet to pads on the second sheet)
Fig. 5.3 Two-layer Method for artwork generation
Three Layer Artwork
This method solves the registration problem for the common pads by preparing a separate artwork for the: (i) component side conductors, (ii) solder side conductors (solder pads, pads or ICs, via holes, etc), and (iii) the pads, which are common to both sides and form the third layer. Special registration marks need to be provided for each of the layers of the artwork.These are required to align the final master films at the time of fabrication.
A major disadvantage of the three-layer method is that for any artwork modification, a simultaneous rectification of two or even three layers is required which is sometimes a very cumbersome procedure.
5.2.3 Red and Blue Tape on Transparent Polyester Base Foil
By using special tapes, it is possible to draw the two sides of the artwork on the two sides of the same transparent film. The component pads, which have to appear on both sides of the PCB are generated by using black self-adhesive pads. The conductor pattern is, however, done by using red transparent tape for one conductor side and with blue transparent tape for the other conductor side.
While producing the two film masters, colour separation is achieved by using special filters. For example, a red filter used with the camera will filter off the red-taped conductors (normally solder side) while only the blue tapes and black pattern will appear. Similarly, a blue filter will filter out the blue taped conductors (normally component side) leaving only the red tapes and black pattern as being visible. The end result is absolute registration for double-sided PCBs and is the most convenient method for their preparation.