ElectronicConstruction Skills Report This report will cover the various steps that are taken inthe assembly of Printed Circuit Boards (PCB) using both through hole andsurface mount technology. Through Hole Mounting(THM) is the process by which components are kept in place by wires that areplaced through drilled holes on a PCB and soldered on the other side of theboard. It was the more common methoduntil the rise of Surface Mount Technology (SMT) in the 1980s. SMT is the method in which components areplaced directly on to board and soldered on the same side. SMT is much more popular now than PCB as SMTcomponents are smaller thus able to fit more components on to a Printed CircuitBoard.
SMT is also less time consumingare holes do not need to be drilled on to the board. Difficulties in manual assembly and solderingof surface mount technologyManual assembly is more time consuming andexpensive as human’s wouldn’t be able to go as fast as machines. Machines also don’t need breaks, don’t gettired and don’t make mistakes from lack of concentration. Operator skills in manual soldering is alsovery important. It would be much harderto produce the same boards over and over again if there are different peoplemaking them. Hand soldering operations vary from operator-to-operator,therefore hand soldering is less favourable than an automated assembly method. Describethe transport of boards within assembly facility In Assembly facilities PCBs are transportedusually over conveyor belts through machinery designed for different parts ofthe manufacturing process.
Once off ofmachines PCBs can be stored on racks/trolleys with wheels so they can be easilymanoeuvred around the facility safely and in many numbers. DescribeSolder paste depositionSolder paste is the material used to connectcomponents to pads on the Printed Circuit Board. The paste initially keepscomponents in place by being sticky, then it is heated (along with the rest ofthe board) melting the paste and forming a mechanical bond as well as anelectrical connection. When solder paste is used in mass PCB assembly as well as prototype PCBassembly there are a number of stages that are undertaken.
Solder paste isfirst applied to the PCB. Then the paste applied only to where solder isrequired. A solder paste stencil that allows only the solder paste through incertain areas is used to solder where required. Solder paste has an expiry date and must beused within that timeframe. Pickand Place operation Pick and place machines are used widelyin the manufacture of today’s surface mount technology SMT electronics circuitboards. Using these pick and place systems or pick and place machines, it ispossible to accurately place large numbers of small, or large componentsquickly and accurately onto circuit boards.Withsome electronic circuit boards using a lot of surface mount technology (SMT)components, many of which are very small, most of the components thereforerequire very accurate placement, it is not feasible to place them manually. Pickand place machines are used that can place all the components accurately and ina repeatable fashion.
Pick and place machines are used in the manufacture if electroniccircuit boards for the placement of Surface Mount Technology (SMT) componentsonto boards. They pick the components, typically from specially prepared reelsor other packaged forms of components, and place them onto printed circuitboards. The pick and place machines are pre-programmed with the informationabout component positions so that they know where to place the components.
Thisprogramme is normally developed directly from the printed circuit board designinformation. Reflow SolderingReflow soldering is the most widely used methodof attaching surface mount components to printed circuit boards (PCBs) becauseit enable mass production at a very fast rate. The aim of the process isto form acceptable solder joints by first pre-heating the components/PCB/solderpaste and then melting the solder without causing damage by overheating. Cleaningand FluxingFluxing is needed for good soldering practices in an electronicassembly, preparing a circuit board with flux iscritical to the soldering process. Solder flux prepares the copper solderingpads of PCBs and the leads of components so the applied solder will bondproperly.
When melted, solder attaches very well to bare copper,but attaches poorly or not at all to the various oxides of copper. The troubleis that exposed to air, copper tarnishes, or forms oxides on its surface. At normaltemperatures, this is a slow process.
At high temperatures though, the speed oftarnishing increases. During the soldering process fluxing has a couple functions.The first is that it dissolves the oxides on the metal surface, which allowsthe molten solder to flow over the metal. The second is that it blocks oxygen from combining with the metal whilethe metal is hot for soldering. When we talk about cleaning a PCB we are usually talking about unwanteddebris – or matter, residues and corrosion of any kind on the circuit board. Suchthings not only make the assembly look untidy, but could be damaging thefunctionality or long term reliability of the product.
So most of the time,when we’re talking about cleanliness it’s about cleaning off the flux residues.To get the boards clean again we load batchesinto a high powered washing machine. This high temperature, high pressure, allstainless steel dishwasher uses deionized water to remove any residue from themanufacturing process. Deionisedwater is very pure water that lacks any ions. Deionised water doesn’tdamage the circuit as it’s the ions that allow electricity to short variousparts of the device. If a washeris too grand or if there is just a few boards that need cleaning a tooth brushwith the bristles dipped in deionised water would work aswell.Component leadforming Lead Forming machines come in manual and motorised forms.
These machines are used to cut and bendcomponents such as diodes and resistors to the length desired. This saves a lot of time as the machines areable to cut work at a faster rate than a human. It also has the advantage of accuracy as allthe components will now be the same length and bent at the same angle. Wave soldering Wavesoldering is a process by which large printed circuit boards can be solderedquickly and reliably during PCB assembly. The wave soldering process gains its name from the fact that the processpasses the printed circuit boards to be soldered over a wave of solder.In this waya complete board can be soldered in a matter of seconds producing joints that arereliably, both mechanically and electrically.
Apart from being much faster thanmanual soldering, wave soldering is also produces joints with a much higherdegree of reliability.Wavesoldering can be used in PCB assembly for both conventional through holemounted components as well as surface mount components. A wave soldering machine consists of a fluxapplicator, a pre-heat tunnel and solder pot with a pump. A wave soldering machine eliminates handsoldering leading to a faster production and higher quality. Boards are placed on to a pallet and ranthrough the machine. Flux is added tothe boards, and the boards travel through the heat tunnel to pre-heat theboards and activate the flux. Afterexiting the heat tunnel the boards travel over the wave pump and as it does so,the wave solders the connections as it passes. The boards exit the machine and cool down as they await to be moved on.
Wave soldering is not as widely used for PCB assembly as it was at onetime. It is not suited to the very fine pitches required by many of the boardsin manufacture today. However it is ideal for the many boards stillmanufactured with conventional leaded components and some surface mount boardsthat use larger components. In Conclusion I have looked at the many steps involved in the process ofautomated manufacturing of Printed Circuit Boards including the difficulties ofmanual soldering compared with automated soldering, the machinery used in developingthe circuit boards and the processes of the machines being used on theboards. It’s clear to see thatautomation is a much more cost/time effective and accurate way to build circuitboards, so it’s no wonder that it has become the most popular form ofproduction.
