PCB-Prototyping
There are various ways of making your own
pcb's, and some that we've tried will be described.
Drawing directly onto the copper with one of those etch resist pens that seem to smudge everywhere. This is only any good if you want a big prototype, with leaded components.
Using that iron on stuff (after its gone through the Laser printer). It seemed impossible to get the temperature right (of the iron : due to very primative controller), to avoid creating a diabolical mess all over the pcb.
Printing out on a Laser printer, the artwork, on normal paper, then glueing the paper to the pcb, then marking the outlines of the tracks with a sharp scalpel, then washing the paper off, then filling in the tracks with a preferably newish etch resist pen. Whew ! . Although this can take hours, it's more reliable than the first method.
Ironing the artwork printout from standard Laser paper onto the pcb (use hottest setting of iron). Peel off while still hot. Then using the result as a guide for a standard etch resist pen. Time consuming, but neat.
Get a UV exposure box and do it properly and use UV sensitive pcb's. The artworks are printed out on standard Laser transparency sheets (Some touching up of the artworks with an indelible pen is usually required). You can, if you don't want to spend the money on this box, just buy a simple small flourescent light fitting, replace the tube with a UV type, put this in a box with a window on top and expose through that. Its best to walk out of the room whilst this is going on to save being blinded, if you don't have a convenient lid. Double sided pcbs are made by sellotaping the two artworks together (You will probably need to put diagonally opposed registration marks on the artworks), and then taping the pcb inside this sandwich, before exposing each side separately. We've had quite good results with this, where there have been a number of SMD flatpack packages on the board. This also works (but not quite as well) with inkjet transparencies.
Below are two samples, the first being done in a fairly primative setup. (Health & Safety (COSHH) regulations may apply, depending on where this is done, and also what kind of chemicals are used).
The next was designed using Protel99SE and consists of 4 copper layers. This is a PCI interface POST card, and had to be painfully hand tracked out, as there were no PCI connectors available in the schematic/pcb Libraries. Due to following the design practice of having the artwork checked by another Engineer, zero mistakes resulted in the manufactured pcb. Prototyping in this case was impossible.The IC's were programmed in-situ via a JTAG scan chain, the connector for which, is at the top left.
PCB Layout Tools
Having tried a number of packages for Windows,
we prefer to use Protels (Altium)old DOS Autotrax CAD package, for a number
of reasons.
- Free Download.
- Can Generate Gerber files; These can be checked with a Gerber viewer, before manufacture.
- Can define Library parts. Important.
- Hardware demands relatively benign.Works on an old 486 Laptop.
- Don't need to create a whole Database (As with Protel 99SE), when you just need to design a small pcb.
One problem with the package, which was carried over into at least its
Protel99SE suite was that it seemed to make hole sizes for some standard
components too small.
The Autotrax layout package doesn't have any surface mount
components. Some of these have been added to the Library, as and when they are
needed, and are inside the following download. Most are also incomplete, in that
the resist layer has not been added. Its assumed that they will be used in a
hobby environment, where only prototypes will need to be made.
PCB Design Checklist
When you get to the stage of needing professional board
manufacture, then the following list should prove useful. Print out and tape to
your monitor, to make sure that when the gerber data is sent to the
manufacturer, time is not wasted on trivialities. Some of these rules were
obtained from Hallmark Electronics Ltd (Chesterton in Staffordshire),
when having a number of pcb's made.
PCB DESIGN CHECKLIST
- Are holes big enough ?. Check Library parts too.
- Make sure there is a keep-out layer present.
- Solder resist, where it should be.
- Connected copper check carried out.
- Aperture file to be included with Gerber data.
- Does Gerber Data import back ?.
- Is ground striping required ?.
- Do End-Tracks need terminating ?.
- Your Company name to be on top copper.
- Board name, design reference and issue in top copper.
- Legend preferably in copper layer.
- No tiny writing.
- Minimum hole size = 0.3mm.
- Pad sizes. Minimum annulus is 5mil. 10mil for ease of soldering.
- Trace width and spacing. x/y rules (x=min trace width, y=min trace spacing). Typical rules are 8/10. Apply to any metal on the pcb. Hallmark , x=5mil.