Printed Circuit Board Production
a) Transport Negatives of the board layout are required (if double sided boards are required, one for each side). We need them to be:
a) life size,b) Fiber glass board, precoated with light-sensitive resist, is cut to size. Maximum dimensions are 10'' by 12''. Small boards required in large numbers make more efficient use of the board if they are dimensioned as sub-multiples of 12'' (that is, 3'' by 4'' is a better size than 2 1/2'' by 5'')
b) black for tracks, transparent for no copper,
c) have Registration marks for alignment if double sided.
c) Each board and its negative are placed in the Ultra-Violet aligned, using the registration marks, and taped together.
d) The boards are exposed, both sides at once, for about 1 minute. The warm-up time for the unit is about one hour.
e) The exposed board is developed in the spray developer tank, taking about 2 minutes. After washing, the tracks become partly visible as resist colored, against the bare copper background, but are easily scratched. Warm-up time for the developer tank is about 1/2 hour.
f) The board is placed in the etch tank, and spray etched with hot Ferric Chloride solution. All areas of bare copper, corresponding to transparnet areas on the original negative, are etched away in about 2 minutes. Warm-up time for the etch tank is about 1/2 hour.
g) The etched board is washed and dried. If the board is required with Flux/Resist coating, it is now complete; if a roller tinned surface is required, the board is returned to the exposure unit and again exposed for about 60 seconds.
h) The re-exposed board is again placed in the developer tank and spray developed for a further 3 or 4 minutes, to remove all the resist that was covering the tracks.
i) The board is washed and dried and moved to the Roller Tinning machine. This machine takes about 1 hour to heat up to 250 degrees. (and about 2 hours to cool down). The metal roller is rotated and partially immersed in theh liquid solder bath, causing it to become evenly coated with the molten solder. The roller is continuously kept clean and free from dross and burned resist.
j) The board is coated with a strong liquid flux, and allowed to activate for about 1 minutes.
k) The board is run through the Roller Tinning machine, coating all copper tracks on one side with solder. For double sided boards, steps i) and j) are repeated.
l) The boards are soaked under water for about 10 minutes to dissolve
off the flux residue. The finished board is then dried.
Basic Rules For Making PCB Negatives
2) Always view the board from the COMPONENT side (from "above"), even when considering the copper tracks which are actually on the underside.
3) Organize the components with most pins (like DIPs) into rows or columns, for the first attempt.
4) Draw the power rails in a regular array, or organized pattern.
5) Start drawing in the other components, once size is known. Where conflicts exist, with track crossing track, attempt to either:
a) jump across using a component, or
b) keep tracks dominating one axis, and components and links at right angles to the tracks.
6) If still no good solution results, remember some components can be stood on end, or made longer with more lead left on.
7) If still no good solution, return to 3) and select another pattern.
8) If still no good solution, go to double sided board, reWORD 5b) to read: - keep tracks on back dominating one axis, and tracks and components on the front at right angles to them. Also keep top predominantly for ground plane, power rails, etc.,
9) Assume this results in a fairly acceptable solution. Check track widths and clearances and adjust as necessary for current capacity, high voltage isolation, etc.
10) Go over it again to see if some re-routing will give a simpler arrangement.
11) Reposition for smallest area, or best use of the area, consistent with safety and easy access for servicing.
12) Remember you will need to fit the board into a box . . . . do you need space for screws?, etc.
13) Make a rub-down negative by placing the layout underneath an overhead-transparency sheet (thick opaque sheets will give poor contrast, resulting in un-usable boards), add correct size black pads and tracks. Make sure there are no gaps at corners or joins. If you are using Computer Aided Design program, still check that pad and track sizes are appropriate.
14) Identify the negative with your initials, or a description, like "power supply." Turn negative over to put writing on, so that it reads correctly from the copper side. For double sided, top needs to read correctly from above, so it should not be turned over.
15) Also add any details, like "input," "danger high voltage."
16) Make Xerox copy of negative -- turn it face down onto the glass to get a view "from above." Mark in physical outlines of components, see that they fit; check tracks and make corrections.
17) Keep negative and Xerox in safe, clean place!
NOTE: FAILURE TO FOLLOW THESE RULES WASTES OUR MONEY AND OUR TIME, SO WE MAY REFUSE TO WORK ON INFERIOR NEGATIVES.
Additional Points To Remember
a) Boards:
we use 1/16'' thick, fiber glass covered with a 14 thousands of an inch thick layer of copper. (14/1000'' thickness corresponds to 1 ounce per square foot.) The copper is attached to the board with glue that melts well below soldering-iron temperatures. We can make single or double sided boards, but do not have the capability to make plated-thru-holes.
b) Track widths:
Tracks should at all times be greater than .030'', as narrower tracks may cause failures or unreliable operation due to:
c) Pad Sizes:1) poor quality negatives, dust or hair-line scratches on negatives during exposure,Use the largest track widths possible: for 1 oz. copper, avoid going below .045'' whenever reasonable, and increase size when:2) irregular etching in some areas reducing track width to zero.
3) pads separating from tracks during roller tinning or soldering,
4) pads separating from tracks during subsequent desoldering to service/repair boards,
5) atmospheric corrosion making tracks high resistance,
6) normal current load melts the tracks.
1) current is around 100mA (aim for .060'' min),2) current is around 1Amp (aim for .100''),
3) current is around 5 Amps (use .150'' min),
4) when high frequencies are expected, increase track widths by a factor 2 or more and consider using strip-line techniques.
Pads should always be larger than the tracks; typically diameter should be about twice the track width.d) Clearance:
Track to track clearance must take into account that pads will encroach on that clearance, and wherever possible pads on adjacent tracks should be staggered to maintain clearance and avoid difficulties with solder bridging during soldering. Minimum clearance is .025'' for low voltages. Increase clearance:
e) Copper Surface:1) for voltages over 100Vdc or 100V peak to peak, to .100'';Tracking can occur between tracks more readily if they are:2) for voltages up to a few KiloVolts, to .250'' per KV.
1) too close,2) have sharp corners,
3) have solder poorly positioned on the pads,
4) are taking high frequencies,
5) have board surfaces contaminated with flux, dirt, pencil marks, blood, sweat or tears.
Boards can be provided with the pads and tracks already covered with a layer of solder ("tinned"). This is better than bare copper as it does not corrode with contact with air, and is easier to solder to. Alternatively, the copper can be provided covered with a protective flux. This also stops the copper from being attacked by the air, and the additional flux aids with soldering. It does not have to be removed after soldering.