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Some of readers may find these advices are obvious or very simple. That's better, this means they are already learned some from their own experience. This section is intended for these who won't repeat other's errors.


Design


Standard PCB dimensions

Standard dimensions simples housing design and inner connections a lot. I used 25 mm step for years, and 50x50 and 50x100 PCB module sizes were most frequently applied.


Standard PCB terminals placement

Inputs are placed at left or bottom, outs - at right or top side. Pins and terminals numbers are going from left to right, this make connections clearer.


PCB layout

Circuit layout can be drawn on notebook sheet. Prefer sheets with cells grid mathing on both sides. As more modern method CAD software can be used. There are variety of this type of software, some of these programs are proprietary and some are free. CAD can be targeted at non-PCB design, such as CorelDRAW or it's free analog, InkScape.


Power bus

Integrated circuits are often difficult for layout, requiring power connections that are crossing signal lines. To prevent jumpers, you can use power bus made of tin-plate or glass fiber PCB material. Power bus making layout less complex.


Temperature conditions and ventilation

During operation, electronic parts are heating. If no ventilation provided, parts overheating can happen, causing them to failure.

Natural air convection is sufficient cooling in most cases, but high power blocks require active ventilation. Computer fans and heat sinks can be used for cooling. Fans are quite productive and less noisy.


Mismatching connector types

Different connectors should be used for different signals, preventing their misconnection. Extra attention should be paid to power supply connectors. There should be no way to supply reversal, inputs connection instead of outputs or high-current circuits connection to low-current parts. Sockets and connectors must differ in sizes and colors.


PCB production


Primary cleaning

Copper clad on sheets is oxidising. PCB should be cleaned and degreased before etching. This can be done with finishing paper, acetone, abrasive cleaners. There should be no spots on copper clad and even no fingerprints. Copper is oxidising quicly, so PCB should be etched immediately after cleaning. Cleaned copper clad sheets storage is useless because of their oxidising.


Etching container

Photo film solution containers can be used, or any jar or square container. For faster etching, solution can be shaked. This ca be done with mistrimmed computer fan - put the weight to one of blades, or remove some of them and attach fan to container.

Another container is vertical cuvette with air pump. Air from pump is sprayed at bottom of container. If air bubbles does not go to some areas of container, you can move PCB sometimes. Distance between cuvette sides should be minimal (1 cm is enough) to use less solution amount.


Etching solution temperature

Ferric chloride etches better at 50-60°C (120-140°F). Fresh solution is self-heating, but becomes cold after 20 minutes. Ferric chloride powder is put in water (do not pour water in FC) with spoon. Do not use aluminium spoon.

Also, etching can be fastened via solution heating. Use aquarium heater, hot water bath or power resistor for it. Heater should be coated with anti-corrosive material. Aquarium heater is best since it made of glass. Cold solution is working slower.


Etching solution aging

During etching, solution becomes less active. If air bubbles from pump cause foam to appear, solution color is dirty-brown or bright green (and muddy), it should be replaced. Solution can be reactivated for little, if you put some ferric objects in it (nails or something like this). This reqires HCl acid to be added to solution. Acid concentration should be more than 3%.


Cleaning out the protection layer

After etching, PCB is washed in water an then protective layer is removed.


Examining PCB at light source

It's most simple PCB quiality control method. Lit from other side, the PCB shows unetched areas or other possible defects.

More informative method is testing with multimeter. Each track is tested for solidity, and then resistance between separate tracks is tested. There should be no short circuits between different tracks. If some are found, it can be cut out with knife.


Drilling

All holes are drilled from Ø0.8 mm (0.03") and then drilled to Ø1 mm (0.04") for powerful parts.


Holes countersinking

Holes are countersinked with 2.5 mm drill (0.1") sharpened to 90° at other side of PCB. If there two copper layers are used, no countersinking of these holes is required.

Drilling machine should be used carefully, avoid lowering drill a lot. If there are few holes, they can be countersinked without machine. 0.8-1 mm holes are countersinked with 2.5 mm drill, and 2.5-3.3 mm holes are countersinked with 5.5-6.5 mm (0.22-0.25") drill.


Tinning the PCB

Etched PCB copper tracks are oxidising and should be coated with tin as soon as possible.

Whole tracks are covered with flux and then tinned with soldering iron and cable shielding (for unnecessary tin removal). Shielding provides additional tracks cleaning. If there is too much tin on shielding, it can be cut with nippers.


Pistons placement

If needed, wire pistons can be placed. These are made of tinned AWG wire spirals. It should be beaded with screwdriver after placement, and then soldered with some amount of tin. Outer spiral diameter is 2.5 mm (0.1"). Pistons are used for easy wire connections between blocks, minimizing risk of tracks peeling from PCB.

If there is power bus, it is placed after pistons and before parts.


Preparing to parts mounting


Parts control

Preliminary parts examining (especially, used parts) should be a rule. Before placing, electronic part requires seconds for testing, and are very difficult to locate when connected to circuit while faulty. Sometimes, new parts also requires examination, since some part defects are still possible.

Oxide capacitors are losing their capacitance. This is due to their electrolyte vapourising. On the contrary, old resistors and ceramic capacitors become more thermal stable.


Preliminary terminals cleaning

Part teminals are oxidising through time. To prevent malfunctions in circuit, clean terminals of old parts before soldering. It can be done with thin finishing paper, knife or aspirine tablets. Aspirine is used for tinning instead of rosin flux.

Do not overheat parts, especially semi-conductive parts such as transistors, diodes, triacs, etc. Hold terminals with tweezers and do not tin second terminal right after first. Take another part and tin one of terminals on it, and after 5-10 sec you can return to prevoius and tin next terminal. This ensures sufficient cooling of parts while tinning or soldering.

Bend terminals of resistors, diodes in way not preventing marking reading. This is not necessary for resistors and inductors with color code.


Mounting and soldering


Soldering tip sharpening

Soldering iron tip should be sharpened (with rasp, knife, finishing paper) and forged with hammer. Forging slows down copper dissolution in the tin. Burn-proof tips are more convenient in work since their material does not corrode in tin. These tips are made of massive copper core, coated with aluminium shield. Working edge of tip is tinned.


High-quality soldering

There is only one quality soldering secret - loads of rosin. It prevents tin oxidising (happening at high temperatures faster) during soldering. Oxidising prevents termilals wetting, causing unreliable contact. These defects are nasty since they does not appear immediately, but through time. Unreliable contact is difficult to find out.

Also, prevent soldering iron overheating. Too hot soldering tip burns flux too quicly leaving no time for it's action.


Too cold soldering tip

Tin is melted slowly and becomes mattes after cooling, with noticeable crystallization. These contacts fail after some time, destroyed by vibration or corrosion. Increase soldering temperature or use more fusible solder (more tin, bismuth or cadmium additives).


Too hot soldering tip

Flux burns in seconds, before soldering. Black rosin spots on PCB indicates too high temperature. Decrease tip temperature or use soldering iron with less power. Also, power grid voltage is unstable sometimes, so low voltage soldering iron is preferred.


Parts overheating

Solder partd quicly and precisely. 1-3 seconds is enough for quality soldering. If soldering was unsuccesfu, do not attempt to redo it immediately, wait some seconds to part is cooled down.

Keep soldering iron by side of parts, preventing placing it over parts and get heater too close to part. Remember that heat is conducted to parts not by soldering tip only, there is also infrared radiation of heater.


ESD protection

Some parts are ESD sensitive. These ones comes in packages with short-circuited terminals, foil or such. Keep their terminals short-circuited until they are mounted on PCB. Also, prefer earthed soldering iron and wear natural cloth. If you have no soldering iron with ESD protection, unplug it from power grid and solder while it's hot.


Mounting device to housing


Marking mounting holes

Most simple way for housing holes markings is pencil drawing on notebook sheet. Edges or centers of holes are punched with core and then drilled, milled or cut. Insudtrial oil can be used as sheet adhesive. It can be cleaned with rag, with alcohol or acetone if needed.


Connections between blocks

Blocks and modules are connected with multi-core wire in PE or PTFE isolation. Good idea is using color wires. Blue, dark, black and other "cold" shades are best for low-level or ground wire. Bright, "warm" shades such as yellow and red are best for high-level and power wires. If there are multiple power supplies, AT/ATX color coding is preferred, i. e. red goes to +5 V, yellow to +12 or +24 V, orange to +3.3 V and so forth. If there are negative voltages in supply, their wires should be shaded "cold" - blue, purple, green and such. If there are RG, RGB LED matrices or strips used, their channels wires should represent color, and common wire should represent supply voltage polarity - white for common plus, or black with common minus. If there are AC power grid wires connected to boards, their color and thickness should differ strongly from another wires. Best are heavy wires with marks, e. g. blue wire with thin yellow strip. PTFE isolated wires have only one color, so color heat-shrinkable tube can be used for their marking.

Line-demanding signals, such as audio, high-frequency, require shielded wires. Keep also these wires short. If there are too many wires, cross-board can be used, where complex wires connections made in printed way.


Heat-shrinkable tube

Useful thing for contacts isoletion, color code marking and wire grouping.


Assembled products control

Each module should be tested for a few hours to prevent parts overheating problems and additional diagnostics. Mechanical strength of module is tested on shaking table or by giving shock on falling from 1 m to hard surface, for 3-5 times. Assembled modules should operate 24 hours without failures, and most heated components temperature should not exceed 60°C (140°F).


Tools


Tweezers

Tweezers is main mounting tool. 15 cm medical tweezers is best. Closing force of it can be corrected with pliers.

Some users put rubber tubes on, but this is unnecessary in most cases. Rubber surface is slippery, while servicing device under voltage is very bad practice.


Cutter knife

These knives are used by elecricians, builders. It is very handy to cut wood, rubber, plastic, vinyl film, PVC and such, and also for wire stripping and cutting. Knife have breakable stabped blade of steel, which is held by catch. Best catch is springed metal latch, or screw with nut on wheel. If you want cut hard materials (PVC, PE sheets, thin wood), prefer knife with metal guides. Worst knives have full-plastic body, these ones have slippery surface and easily breaks down under heavy force.


Metallic ruler

Ruler can be 20-40 cm long (0.5-1.5 feet). This ruler is often need because most of working materials are small. For PVC sheets best is 1-1.5 m ruler. Put wood under PVC sheet to prevent floor or table spoils. To improve ruler grip, thin rubber tape can be used.

Plastic and school rulers are inappropriate since their accuracy is poor. If you are using multiple rulers, make sure their scales are matching. Plastic rulers mismatch can be up to 0.2"/ft.


Shears for metal

There are variety of shears, best for PCB cutting is medium. If you can't hold both copper clad sheet and the tool, you can mount one of shears grips on table. Try to leave minimal offset from marked line, PCBs does not require a lot of mechanical working. Keep cutting until line ends, not clamping shears blades completely, or cut steps will appear on cut edges. Thick copper clad sheets can be also cut with plexiglas cutter.


Drilling machine

High RPM drill, such as Dremel. If no drill available, power DC motor with collet can be used. Drills are 0.5-3.3 mm in diameter.

If you drill a lot of boards, you can save time with multiple drillers, for example one for 0.8 mm drills and one for 2.5 mm drills.
Drills

Most used diameters are 0.8 mm (0.03"), 1 mm (0.04"), 1.5 mm (0.06"), 2.5 mm (0.1"), 3.3 mm (0.13-0.15"), 6.5 mm (0.25").

Drills are blunted while working, and should be sharpened. Small drills can be sharpened with diamond needle file. These drills breaks often, so some backup is preferred.


Chemicals and materials


Tin (tin-lead solder)

Soldering alloy - tin and lead (most popular proportion is 40/60). There are low melting temperature alloys, containing cadmium, bismuth and other rare elements, but RoHS standard prohibits most of them.


Rosin (flux)

Pine rosin in amber-yellow pieces. Chinese rosin is producted in rectangular or round cakes, poured in containers. There are also rosin in cracked pieces on market, this can be used too. If there is too much rosin flour from these pieces, it can be dissolved in alcohol or acetone. Melting rosing in hot water gives bad results because of too low temperature. Rosin is passive flux. Active fluxes are aspirin, ortophosphoric acid, water zinc chloride solution. After soldering or tinning with these fluxes, wash soldered surface with acetone and then solder with rosin. It may also be required to repeat washing and soldering with passive flux. Active fluxes causes corrosion and should be throughly removed.


Cable braid shielding

Useful for surplus tin removing from PCB tracks. 1 meter of cable shielding is working for months.


Acetone

Organic solvent with least pharmacokinetic effects, except ethyl alcohol. Used for rosin removal from soldered connections or PCB. For less usage, can be used in 1:1 solution with water. Should be stored in sealed containers. Volatile, flammable.


Ethyl alcohol

Used for liquid flux (rosin solution), and for rosin removal. Unlike acetone, does not solute PVC isolation and can be used for assembled PCBs cleaning. Also, used for sockets cleaning.

Pure alcohol (95% solution) is hygroscopic and should be stored in sealed containers. Flammable. For plastic cleaning best is isopropyl alcohol, but it is poisonous. Sold as plastic cleaner liquid.

Less alcohol concentrations gives worst results of cleaning. Do not use alcohol drinks for cleaning, on the contrary they causing more pollution.


Ferric chloride etchant

Dark powder, stored in sealed jars due to hygroscopicity, known as green etchant. Oxidising by air. Put in water, and do not pour water into powder (causes a lot of heating). Can be replaced with blue etchant - ammonium persulfate.


Copper clad sheets

Fiber glass sheets with copper coating, producted in up to 1x1.5 m dimensions. Coating can be on one or both sides, thickness vary from 0.3 to 3 mm. There are also PTFE sheets for high frequency applications. 100x100 mm is minimal dimensions available for sale.

It is not necessary to etch second side if only one used. Other side is used as shielding in this case.

Wear protective glowes while working with copper clad sheets since thin glass dust causes skin irritation. Copper is oxidising, so sheets should be stored in package or bag, preventing contact with air.


PTFE isolation wires

Single color multi-core mounting wire.


PE/PVC isolation wires

Multi-core mounting wire, variety of colors, with or without silk shielding.


PVC sheets

Sheets are produced up to 2x3 m, 1-20 mm of thickness. There are some colors, most popular is white. PVC is less strong than metal but more flexible than wood, and is easily cut, drilled or milled. Housings with heavy mechanical load can be combined with aluminium profile carcas.

PVC parts are attached to each other with screws bolts, nuts, glues etc.


Cyanacrylate adhesive

Cyanide acrylic adhesives is class of plexiglas derived adhesives based on hydrocyanic acid. These substances require high quality ventilation or working outside. Cyanacrylates are one of best plastic adhesives, making glued seam more durable than PVC fiber. Adhesive is quick-drying, making ware ready for use within hour. Cyancrylates are intended for nonfood application only. Avoid fingers contact with adhesive, it is not harmful but is nasty and takes time to clean out.

Most popular cyanacrylic glue is Loctite 401, 402, 405 produced in South Korea.


Plexiglas sheets

This plastic is hard and transparent. It can be shaded variety od colors, can be matte or glass-transperent and is 1-10 mm of thickness. Degrades under direct sunlight and heavy UV radiation. UV causes plexiglas to shade yellow and make it fragile.

Cut with special cutter or saw blade. Finished with rasp file and finishing paper. Can be bended if heated, like a PVC. Can be glued with dichloroethane, cyanacrylic adhesives or welded with plexiglas bar.


Aluminium profile and sheets

Can be used as chassis, carrier of housings and heat sinks. Easy to drill, mill and other mechanical treatment. Like thin steel, can be cut with carbon dioxide lasers. Does not corrode, but this require special inert fluxes for soldering. Because of this most popular connection of aluminium parts is mechanical - bolts, screws, nuts and such.


Finally, some advices and secrets from author.

 

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