PREVENTION OF COLD SOLDER
The condition for a proper connection is the cleanliness of the surfaces to be connected, the presence of flux and a sufficiently high temperature to dissolve the copper layer in the tin. Meanwhile, various aggressive chemicals, including those contained in the air, may react with the copper and the alloy of which the tips are made, and in addition, there may be foreign substances on the surface of the elements to be soldered, such as grease, dust, dirt.
The flux contained in the soldering wire will allow you to remove a thin layer of oxides, but cannot cope with grease, dust, dirt or a thicker layer of patina. Therefore, the basic key to success is cleanliness, cleanliness and once again cleanliness. If necessary, the combined elements should be degreased with e.g. spirit or detergent solution (washing liquid). Heavily patined ends should be cleaned mechanically with a knife, fine paper, sponge, abrasive powder or other means. Only well cleaned surfaces guarantee the correct connection. Some people have the habit of covering in solder both the tips of the components and the soldering points before installation.
It shouldn’t hurt anything. Generally speaking, however, you it is not necessary to do this with PCBs of own production. After etching it is worthwhile to clean the paths „to live copper“ with an abrasive sponge, scrubbing powder, and only in the end with very fine sandpaper. The surface of the board cleaned in this way should be varnished with a solution of rosin in denatured – it will ensure perfect soldering without the need to pre-cover the soldering points. And pre-covering of the tips of the elements to be soldered is only recommended if the elements are stored for a long time and have a lot of patina.
It’s also very important not to be overly economical and not to use old, burnt-out, grey recycled solder. Such recovered solder certainly does not contain flux and will not provide good connections. In addition, there is a lot of tin and lead oxides on the surface of such burnt tin, which definitely do not promote a good connection. Stinginess does not pay here, the cost of fresh tin is not a fortune, and you can be sure that it contains flux.
The recovered solder can be used for covering the ends of wires with the use of an additional flux – colophony. It is also necessary to keep the tip of the soldering iron clean – you have to periodically remove the burnt grey tin from the tip. From time to time you should carefully wipe the tip with a special sponge (moist), which can be bought in every electronic shop for two or three zlotys. Such a sponge or so called chip cleaner““ is absolutely necessary euipment of the soldering station.
COLD SOLDERING
Even a skilful and conscious electronics engineer has to deal with cold solder from time to time, resulting from too low a soldering temperature and poorly prepared surfaces of the elements being joined. Often more experienced technicians say that such connections have not been soldered, but branded“. It is hard to be surprised by such a strong term, because it is not only about aesthetics, but also about the durability of the connection and the subsequent serious problems during the system operation.
It often happens to get a device to be repaired, showing strange symptoms: periodic breaks in operation, crackles, noises and breaks appearing and disappearing when knocking, shaking or slightly bending the PCB. This indicates the presence of cold solder.
Usually it is not worth looking for a particular cold solder causing the malfunction. If one cold solder comes out, it is almost certain that more will come out over time. That is why it is worth to solder all the points once again with a well-heated soldering iron. You should carefully examine the board and find out which drops of tin on the soldering points are convex and grey – these are the best candidates for cold solder. Such points should be well warmed up with a small amount of colophony, so that the tin can cover the elements. Sometimes it is worth to remove old tin with a suction gun and solder the leads again with a well-heated solder with fresh tin. It is difficult to give a prescription here and you always have to assess the specific situation: sometimes it is enough to just warm up the soldering points well, sometimes you have to use colophony in addition, and in yet another case you should unsolder the elements, clean them, cover their tips and solder them back.
ACCESSORIES
A special sponge for cleaning the tips, which can now be bought in every electronic shop, is an obligatory accessory, besides the soldering iron, even in the most modest workstation. It is used to wipe the tip during operation and must be slightly damp (not wet). A very important and useful tool is a vacuum extractor – a simple pump for removing molten tin. When buying a vacuum extractor it is worth to spend a few zlotys more and buy equipment that will perform its role for a longer time – photo 28. It is worth to lubricate the devices periodically inside with either oil or, better still, talc. To remove unnecessary tin from soldering points there is also a special copper braid soaked in flux. After touching the soldering point and heating up, this braided copper wire pulls the liquid tin well between its thin fibres.
Such a colophony-soaked braided wire can be bought, but it can be also be made from a piece of copper-screen cable. After cutting the insulation from a piece of such a cable you have to immediately soak the braided sleeving in flux, e.g. by immersing it in a alcohol colophony solution to prevent copper oxidation. An ordinary cable from typical electric cables is not suitable here because the strands are too thick, whereas speaker cables, which usually consist of many thin strands, are fairly suitable. The best is the shielding of the cable with the thinnest possible strands. Not only older electronic engineers need a strong, preferably backlit magnifying glass, or a magnifying glass and a good lamp. This will allow you to look at the board carefully and find possible cracks and short circuits. Young people usually think that they have a falcon’s eye and they don’t need a magnifying glass, but it’s really worth having.
Special attachments are used to assist in the soldering of integrated circuits (THT and SMD), allowing to evenly heat up all leads. However, these are quite expensive elements, offered for an additional charge as a supplement to professional soldering machines. Despite the high price, the attachments are very useful because they allow to heat up all the chip leads at the same time, which in the case of double-sided PCBs is the only effective way to solder the integrated circuits. Even more expensive, but very useful are hot air (gas) soldering machines. They are usually equipped as standard with a set of attachments for various THT and SMD chips. Photo 30 shows the DIC DEN-ON SC7000Z air desoldering gun from Renex.
The desoldering gun enables the disassembly of electronic THT elements from boards up to 12 layers with metallized holes and the disassembly of SMT electronic elements. Standard, THT elements are disassembled using the „foot by foot“ method with the use of suction nozzles. For the disassembly of SMD elements, not suction, but blowing is used. When disassembling PQFP, SOP and PLCC elements, a steel wire or tape is used, separating the component leads from the board paths after heating the tin with a hot air stream. Recommended heads can also be used for SMD elements. Interestingly, the device is powered directly from 220 V, and the motor and compressor (blow-suction) are built into the device, so there are no separate stations that are typical for this type of tools.
THREATS
Practice shows that you can’t learn to solder without burning your fingers several times. All beginners experience this before they develop the necessary reflexes and habits. Burnt fingers are said to be the entry fee which everyone has to pay. Burned spots hurt a lot, but usually wounds and blisters are not dangerous and heal quite quickly. The risk of being burned with a soldering iron near 400°C only seems serious. Most electronics forget that there is a much worse problem with soldering: the soldering alloy contains large amounts of lead.
And lead is a strong poison. When touching the soldering wire, part of the lead can get on the skin of the hand. Therefore, touch the tin““ as little as possible and wash your hands after soldering. This issue really should not be underestimated, because lead poisoning can become apparent after many years.
It is worth recalling that once, when the connections were made by hand, soldering workers worked in thin gloves. Another noteworthy issue is the fumes of colophony, other fluxes and metals. In professional soldering stations, fume removal systems are usually installed so that the operator does not have to inhale them. This problem must be addressed by people with a tendency to asthma. Additional air extraction, in practice opening a window, is in many cases an absolute necessity.
PRACTICAL TIPS
Colophony should be a permanent piece of equipment in the soldering workstation, but it should not be used unnecessarily. Only some old-fashioned electronic engineers who used flux-free soldering wire a long time ago think that it is essential for normal installation. Today, colophony is practically only used for covering the ends of wires and components. It is worth knowing that an aspirin tablet is suitable for the role of an effective, aggressive flux, but when heated up it emits strong biting smoke. When it comes to substitutes, it is usually not worth to use „tin“ obtained from tinsmiths, roofers, car mechanics and other non-electronic engineers. The problem is not only the lack of a suitable flux in such tin. Often the chemical composition of the alloy differs significantly from the „electronic“ alloy, which significantly reduces the quality of connections.
Whenever soldering has been carried out carelessly and without sufficient flux, there is a risk that the tin will short circuit the adjacent points. Usually it is enough to solder such points with a bit of flux (colophony). The tin in the presence of the flux will shrink in a slightly. If the colophony does not help, remove the excess tin with a vacuum nozzle or braided wire, and possibly solder the points again with a small amount of fresh tin. When replacing components or other manipulations, sometimes the soldering points or paths are damaged (torn off). Particular problems occur when dismantling integrated circuits from a double-sided board with metallized holes. The only effective way is to use an attachment allowing to heat all tips at the same time (photo 19). Even the best suction unit or braided wire usually do not allow to completely remove the tin from the metalized holes. An attempt to tear out such a partially „sucked out“ element often ends up destroying the metallization in the holes and tearing off the soldering points on one side of the board.
Then, when soldering a new element, remember about the metallized holes, thread a thin wire through the hole next to the llead and solder it from both sides of the board to the previously isolated and covered paths. As mentioned earlier, there is no need to remove flux residues (colophony) from the PCB after soldering. On the contrary, some people after soldering cover the PCB on both sides (also elements) with a solution of colophony in alcohol. After drying, the colophony layer additionally protects and serves as an insulating varnish. Years ago it was a common custom, nowadays less so, because various varnishes or more commonly available silicone inlays can be used to protect against moisture.
TIP REGENERATION
Even good-quality tips get dirty over time and resist taking tin. Usually, it is enough to wipe the hot tip regularly with a damp sponge (cleaning cloth) and immediately deep it with fresh tin or colophony. If this doesn’t help much, you have to clean such a tip on a damp sponge, turn it off until it cools down, wind a flux solder wire on the tip as shown in Figure 14 and turn on the soldering iron. When it warms up and melts the tin, the tip should become white. If the effect is not satisfactory, the procedure should be repeated. Some people risk cleaning the „iron“ tip with fine sandpaper, but this method is risky to say the least. Alternatively, after hot cleaning on a damp sponge and cooling down, the blade should be gently cleaned with scrubbing powder, rinsed thoroughly and covered in solder as shown in Figure 14.
TRENDS
Due to the progressive miniaturization of the components, the manual soldering irons have reached the practical end of their capabilities. Nowadays, the whole process of production of electronic devices takes place with the use of various automatic machines. Individual companies offer either complete, fully automatic production lines or individual stations for such lines. However, the need for manual assembly and disassembly of electronic components still exists. Although it is becoming more and more difficult, the first prototypes usually still have to be assembled and soldered by hand. Only for larger volumes it is worthwhile to program the production line for automatic dispensing of soldering paste, automatic component placement and reflow soldering. Although, in view of the low costs of serial production, more and more often searching for defects and repairing them turns out to be unprofitable, service centres still operate. An important problem here is the unsoldering and soldering of the most modern sub-miniature chips, especially with ball leads placed on the bottom (BGA-type housings).
Special stations for manual assembly and disassembly are produced for prototyping and service. An example is the DIC DEN-ON SD-3000 station shown in Photo 31. It is a microprocessor-controlled semi-automatic station using hot air and vacuum tweezers, enabling the assembly and disassembly of SMT (PLCC, PQFP), SMD, and even BGA elements, as well as the disassembly of THT elements. Not only in such extensive workstations, soldering machines and hot air and heaters using infrared are increasingly used. In addition, there are glue and SMD paste dispensers, vacuum tweezers, suction units, vapour traps and others. Please see the articles for a more detailed explanation: SMD in EdW 12/1998, 1/1999 and My first SMD in EdW 8/1999. A traditional electric soldering iron takes a subordinate role there, if used at all. If you are interested in details, you can look for further information on the Internet, starting with the websites of soldering equipment manufacturers and distributors, such as Renex (www.renex.com.pl), which offers such equipment from several foreign companies.