| Printed Circuit Construction and Repair |
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| Written by Bryce Ringwood | ||||||||||||
| Monday, 03 November 2014 14:55 | ||||||||||||
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Printed circuit construction uses a thin sheet of material on which electronic components are mounted. The connections between the components are made from copper tracks on the board material, rather than the older method of wiring between each part. If you take almost any modern appliance apart, you will see the circuit board, often a small brownish coloured board with a few components sitting on it. The pins on the components go right through the board and are soldered to tracks on the other side. If you look at a computer printed circuit board, you will see very tiny components soldered to tracks on top of the board. This is a "Surface Mount Board", as opposed to "Pin-in-hole".
Printed circuit boards (PCBs) have been in use for a very long time – since 1946, and there are certainly many valve radios and TV sets built on them.
I got my first “Printed Circuit Kit” in the mid 1950s. It consisted of a (very) small piece of printed circuit board, some orange paint, a fine brush, some ferric chloride crystals and a small bottle of “Vim”. To make a printed circuit, you polished the board with the “Vim” (Pan scouring powder) and washed and dried it. Next, you painted the pattern of the copper traces with the orange paint. Finally, the board was etched with the ferric chloride. A fretwork drill to drill the holes was supplied. Being about 12 years old at the time, I really made a big mess of the whole thing but I think anyone would be hard pressed to produce a reasonably decent printed circuit board using a brush and orange paint.
Before going any further, there are plenty of suppliers who will take your schematic and turn it into a board directly. If you don't want the experience of making your own boards – skip to the sections you are interested in. Professionally produced boards can have a silk screened top layer for component placement and a green solder mask, not to mention through hole plating on double sided board.
Since my first effort I have designed, produced, repaired and modified more PCBs than I can remember, however, this does not imply I am a professional circuit board designer. This article is therefore aimed at hobbyists who want to put their projects onto a PCB making only one or two boards, or to collectors who might want to repair equipment containing PCBs. The article describes the following processes:
In principle boards are made by printing the artwork onto the copper printed circuit laminate with a material that resists etching, and then removing unwanted copper using an etch solution.
Printed Circuit Laminate
This is made from thin copper sheet bonded to a fibreglass-epoxy (FR-4) or phenolic-paper (FR-2) board. There are other materials, such as Teflon. The boards are available in several thicknesses and the copper may be on both sides. In this article, the discussion will be confined to no-more-than-two layer boards. Although the production and use of multi-layer boards is outside the scope of the average constructor/restorer, there are firms that will offer a service to produce these for you.
Boards may be made on a flexible substrate if you need to replace a flexible board for some reason. Printed circuits made on fibreglass board are tough but they are also tough on drills. They are the best – but if I am honest, phenolic paper is probably perfectly adequate for home construction. In most consumer circuits, phenolic paper is used, usually single-sided with lots of jumpers.
Producing the Artwork
You can still draw the artwork directly onto the board, if you want to. Instead of a fine artists brush and orange paint, you can use a “Decon” marker. If you are just making one fairly simple board, this is a good option. Even so, its best to use a "computer aided design" or "CAD" package to work out how you want the components to be placed. If you really don't want to use a CAD package, then 1 inch squares transparent graph paper and a soft pencil with an eraser can be used to work out where everything on the finished board must go. Why 1 inch? -
If you still don't want to use a CAD package, then you can produce the artwork using transfers (if you can find someone who still supplies them). As before, its best to work with squared paper underneath the clear film on which you will produce the artwork. Nowadays, you can use a CAD package to make a transfer on a laser printer – and, bar the drilling – job done. (In a related process, you can scan PC layouts from magazines and produce transparencies. RS components supply special transparency paper for use with an ink-jet printer. Using it results in dense sharp prints for use with the photo printing process described later.
Be very careful indeed about printing the board the correct way round - its very easy to print the entire project back-to-front. Clearly mark the top right reading, and the bottom in reverse print! Many's the time myself or my board supplier has got it wrong.
When considering CAD software, I consider the following:
Here are some tips when producing artwork:
Remember, its a hobby, not a military project, and provided the connections are correct, your project will work reliably. The communication receiver I made in 1972 still works today with all the rules about “no right angles in traces” etc. all broken and bent.Pay attention to layout from a signal point of view – allow for additional decoupling caps, anode stoppers etc.Always do the silk screen layout, even if you don't intend doing a silk-screen on the component side.Put right-reading text on each side of the board – then there's no excuse for printing the wrong way round
Now – check the artwork by printing a true to scale .pdf file (see note on Kicad) and placing the components on the printout according to the silk screen layout you did. Check the connections are correct. Check the sizes are right and that there is enough space round each part. When finished, shake everything off the paper and do it again to make extra sure. Check the scale is 100% correct – there is nothing worse than trying to squeeze a 40 pin DIP into a PC board that's almost correct.
Transcribing the Artwork on to the Copper
At this point, you have the artwork in computer readable form, or as a physical lines and traces on a piece of paper or film, produced by pen and ink, or other direct means. If you are getting the board produced for you, then your supplier will tell you what he wants, or what he is prepared to accept. Most in my area will want Gerber files, but some will accept .pdf files containing the artwork. If using Gerber files, try using Viewmate to ensure that the board looks the way you think it should look.
Assuming you want to take your artwork and print it yourself, then the first step is to produce a transparency. This is easily done on a laser printer using transparent film – be sure to set the copy as dark as possible, because you want the greatest possible contrast. Hold the transparency up to the light and check for any “less dense” areas.
Mix the developer according to the instructions on the spray can (7g per litre for Positiv 20). Usually, this involves a small amount of caustic soda dissolved in a large quantity of water. Developer goes off after a time – don't expect to make a good board with an old can of developer. RS Components sell a commercial developer that is much better than caustic soda and not as finnicky to mix.
Now prepare the board – you do not need a darkroom, but working in a blaze of light might be a bad idea..
Inspect the result in a dimly lit room.
This process needs to finish up as perfect as possible, and if you are not completely satisfied – do it again. You can buy pre-sensitized boards, if this becomes a problem.
Now expose the board.
At the end of this process you should see that the exposed lacquer has a slightly different colour. Put some developer into a plastic container and put the board into the developer. After a few minutes, the tracks will become clear and stand out against the copper. Remove from the developer. Be sure to wash your hands if you didn't use plastic tweezers. Don't develop the board in bright sunlight - or your track will magically disappear!
Dry the board thoroughly and inspect the result for breaks in the tracks, underexposed areas etc. If the board is underexposed – sorry, but you'll probably have to do it again. You can put it back in the developer and see if it works out, but it usually doesn't.
Breaks in tracks can be fixed with a Decon pen. Shorts across tracks can be scraped away with a modelling knife.
Is it good enough? - If so – time to etch.
Etching the Board
Most articles advise the use of Ferric Chloride as a chemical etching solution. I don't use it because it stains clothes horribly and seems to get everywhere. Just one small crystal escaping from the bottle is bound to find its way onto something and spoil it. It's up to you, but I prefer this mix of swimming pool chemicals: ! BE CAREFUL - WEAR PROTECTIVE EYEWEAR AND GLOVES. Add theacid to the water first.
300ml Hydrogen Peroxide (200 Vols) Pool conditioning chemical
300ml Concentrated Hydrochloric Pool Acid
300ml Water (More if you want a slower, more controlled etch)
Don't mix it in a whisky glass like I did - you might drink it and then your teeth will feel rough for several days.
My etching and developing baths are used ice-cream containers.
This produces a very clean etched product. The blue coloured fluid (copper chloride ?) that remains contains copper and can be put in the swimming pool afterward – its an algaecide. (Everyones a winner with this mix!).
Dry and inspect the board before drilling. Clean the resist off using lacquer thinners. Finally trim the board to finished size. (I use a band saw.)
Drilling
The finished holes should be free from burrs and ridges, so have a few 1mm drill bits at the ready. If you are using fibreglass laminate, these will
become blunt after only a few holes (You might get about 100 holes.) If you have to force the
Don't be tempted to try tungsten carbide drills – they are incredibly brittle, even if they do make fantastic holes for their brief, expensive life.
If you are not going to assemble the board immediately, spray it with a plastic coat (eg Plastik 70. Clear lacquer will do.) to protect the copper. Otherwise – assemble the board.
Assembly – Soldering and Cleaning
This is very straightforward. Always use a temperature controlled soldering iron with a suitable bit – say 1.2mm and a 25 Watt iron. Keep the tip clean and solder quickly making sure the solder flows around the component and the pad. It should have a slightly concave appearance between the wire and the copper all the way round. If you have to use lead-free solder for some reason – take extra care because it has a higher soldering temperature than leaded. The solder joint will not be quite as concave and often won't be as shiny. Beware of too much heat, as this will cause the copper to become unglued from the laminate. ("Lift"). My favourite solder is "Savbit Alloy".
Traces of flux must now be removed using flux cleaner (lacquer thinners again) and a stiff artist's brush. Flux looks horrible, is sticky, corrosive and attracts dust. You can buy a "clean solder"that leaves little residue - but it lead-free. You have to use it if you are using components thatobject to solvents.
Finally, spray the board with plastic film (e.g. Plastik 70) to protect it.
Now plug it in and switch it on and enjoy, or in my case (and probably yours) switch on and figure out whyit doesn't work in spite of every effort to be perfect.
Repairs to Boards
Before you begin, its worthwhile closely inspecting the board for the following:
Breaks in the track near a pad are often caused by “lift”. The pad was overheated during soldering and the copper has come unstuck from the laminate. Remove the solder and the pad. Trim the track near the break and scrape off any insulation with a modelling knife to expose clean copper. If possible – bend the component lead on to the clean copper and solder. If not, loop a piece of wire wrapping wire round the component lead, solder and solder to the track. Some components might have to be supported with epoxy.
Burned and missing tracks can be replaced with a link of fine insulated wire. Secure the wire to the board with superglue. The components at each end of the burned out section will most likely have to be replaced in any case. (How bad is it ? Can it realistically be repaired ?)
Fractured boards and tracks are usually phenolic. The board can be butt-jointed with epoxy. Next, the fractured tracks need to bridged across the fracture with a small piece of wire soldered at each end. For some reason, its impossible to simply put a blob of solder on the break – the solder just flows away either side.
Components are often secured to the board by a combination tool, which cuts and bends the leads over the pad prior to soldering. This makes a really good soldered joint, but can lead to trouble if the lead touches or almost touches an adjacent track. It can be hard to spot.
IC Sockets are not my favourite – all manner of things stop working until I press the ICs back into their sockets. Then the cycle repeats. The only time I have used them is in the case of a repair to a double sided board, where the via has decided to detach itself and remain round the component leg. When designing your own boards, you have to use them for microprocessors which need to be removed and re-programmed.
Mistakes will be made from time to time. Traces that don't go where they ought to can be cut and a small section removed to ensure the ends don't join again. The correct trace can be made with a piece of fine insulated wire. Don't forget to correct the artwork.
SMD Devices
I used Kester solder paste and a dispenser. This really does take some getting used to. The needle clogs and has to be freed quite often. The paste doesn't hold the parts in place very well, so I used a toothpick to hold them in place while I swirled a hot-air pencil over the part until the solder flowed. Most parts "blow away" from the stream of hot air, even if you direct the flow vertically. Maybe I just need practice. After testing the board, you might find the odd part hasn't taken and has to be resoldered. On one of the boards, the chip only half worked. I simply held it in the tweezers that came with the kit and blew hot air over it to remove it. I then painted flux over the pads and placed the new part on the board and resoldered it using the hot air pencil. No further solder was needed. For some reason that I don't understand, the board survives all this heat without degradation.
I do the assembly on a sheet of fire resisting material, to prevent burning the house down.
Repairing SMD boards is something I find very difficult. The boards are complex and the parts can't be identified with certainty. The circuit diagram or schematic is often only known to the suppliers.
Unsoldering
Solder can be removed from the board using a solder sucker while heating the joint. It is often more easily removed if coated with a liquid flux beforehand. This job must be done quickly to avoid overheating the board and “lift”.
Double sided boards are difficult to unsolder, even if you have a vacuum de-soldering tool. The solder must be entirely sucked free from the joint using the tool. Paradoxically, you sometimes have to re-solder the joint with fresh solder before it can be sucked clean. Next, the component wire may have to be freed from the hole by moving the wire from the side of the hole with a toothpick. Take care not to pull the tube of copper forming the via out of the hole along with the component wire, but if you do, remember to solder the component back on both sides of the board.
If a surface mount component is faulty, remember that the top priority is to save the board, not the component. I cut the leads with a modelling knife, if it is an IC, discard the body, then carefully remove each lead at a time, leaving the board as it was before assembly. A good solder wick can help here. Next, the board is coated with liquid flux and the component replaced. You won't need any additional solder.
There are hot-air and rework stations available. They are also useful for assembling your own SMD boards. They are supplied with a kit cotaining fine tweezers, desoldering braid and spring wire for unsticking the pins on those tiny complicated chips. When working on SMD boards, I use a head magnifying-glass. Beware manufacturers like Tivoli Audio, who also glue the components down. You have to get everything hot enough to melt the glue.
Don't be afraid of surface-mount components and designing boards to use them. You can purchase “practice boards” for SMD – but why bother? There are a mountain of junk and scrapped boards to practice on.
Prototype Boards - Stripboard and Veroboard
Many years ago, I received an item of lab equipment to repair – made in Switzerland. I thought it had to be a great piece of construction. Imagine my surprise, when I opened it up to find it was all made on Veroboard (Stripboard). My own efforts with this material have not been crowned with particular success and always look messy, but it has its place if you want to try out a circuit and perhaps haven't grasped all the finer points of its operation. They are very good for school and science projects, but because of the projects I tend to undertake, I don't use them except when I'm doing something simple, like the Morse Code Project, or the Battery Eliminator. To be fair, strip board is very handy for power supplies.
Stripboard/Veroboard design software is available on the Internet, most of it is commercial or crippleware, but there is “Stripboard Magic” - which is “abandonware”.
Conclusion
If you have an idea for a project, but have limited time to spend on bringing it to fruition, then printed circuits are the way to go. The key is to prepare the project well – sound design and good artwork. Making repairs to simple single and two – layer circuit boards is also straightforward. On the other hand, better leave multilayer and cell-phone repairs to the professionals.
Talking of professionals, we have cell-phone repair experts at just about every street corner in Johannesburg – each one kitted out with a hot-air rework station. I really need to talk to them, because they know things that I don't. I wouldn't know where to start repairing a mobile phone.
Software I have tried
DOS
Tango PCB - Not tried much
Wintek's Smartwork – quite expensive commercial software.
PCB Turbo - Bought many years ago. It needs a dongle and a real DOS computer.
Windows
FreePCB for which you will also need Viewmate
DesignSpark (RS Components)
Kicad
MS Paint
Linux
Kicad
Stripboard Software
VeeCAD – Commercial and free crippled version
Stripboard Magic – Free and quite fun to use.
LochMaster – Commercial, not tried.
References
These are a bit out of date now, but some hings haven't changed:
Horowitz and Hill “The Art of Electronics” - Cambridge University Press 1980
Radio Communications Handbook – RSGB - 1983
Appendix - KicadI tried a simple project having a microprocessor using MS Paint with predefined components. I finished the board and it certainly looked quite neat., but it had quite a number of mistakes, the most horrible of which was getting two of the CPU power pins the wrong way round. The other mistakes were design errors - forgetting the pull-up resistors. It took a very long time to design the board this way and I would never do it again. This experience prompted me to use learn and use Kicad. Compared to professional design
packages, Kicad may not be the greatest, but then I wouldn't know. Its certainly way ahead of the DOS package I bought in the 1980s. With Kicad, you begin with the schematic. Then associate the parts in the schematic with their footprints on the board. The final step is to design the board by spreading the components out and routing (or autorouting) the tracks. I found that I could not produce a correct check plot in .pdf format with Kicad - I think this must be a "bug". In the end I used the Kicad PostScript plot option and a program called GhostView to check the layouts. Kicad has a number of vacuum tubes in its component library - mostly those valves that are beloved of the audio fraternity. Kicad also has a tool kit to allow you to calculate the correct track widths for a given current. The charge balancer shown carries quite heavy currents, so some of the tracks have to be thickened up with solder. The tiny black square in the photo is an LM2903 dual comparator IC. There are plenty of tutorials for Kicad on the Internet - the one I used was a YouTube tutorial called "Getting to Blinky". The first board I made was a "Charge Balancer". I got a number of boards made locally by BOSCO (Who have no trouble at all reading Kicad's Gerber files), and unlike my unfortunate MSPaint experience, these SMD boards all fitted together and worked correctly first time. (See above illustration). Appendix - Ultraviolet Light Exposure UnitThis is said to be the last stripboard project you'll ever make. It wasn't in my case. The unit consists of an array of UV LEDs on stripboard, arranged as 4 rows of 57 LEDs wired in parallel. Each row is wired as a panel of 3 x 19 LEDs. There are four panels connected in series, so, because the forward voltage of the LEDs is 3.0
volts approximately, a total of 12 volts is required to drive the array. Each LED draws 20mA, so the current required is 57*20 = 1140mA. The array is 19 x 12 = 228 LEDs arranged at 0.2" centres. (3.6 x 2.2 inches - Max size of the board.) The only LED driver I could find that was somewhere near suitable was the Meanwell LDD 1000H, which will supply 1000mA only, so the LEDs will not run at full brilliance. The panel and LED driver are built in to the LID of a plastic enclosure and the sensitised board is placed on a layer of foam, followed by the "positive" and then a sheet of glass. The lid is placed on top. Exposure time is around 3 minutes. You should use UV resistant (yellow) safety glasses when using this. It may give you headaches, as the UV is quite strong.
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| Last Updated on Wednesday, 05 November 2014 12:04 |
