Outcome 3 report
In this report I am going to discuss the assembly methods of both surface mount technology and through hole soldering and to help me with this I have broken it down into 8 main points.
There are many difficulties in each of manual assembly and surface mount technology within soldering. For manual assembly some of these problems include disturbed joints which is when the solder moves as it solidifying. Another problem that commonly occurs is a cold joint where the solder has been removed from the board too early and not been allowed to melt properly. Also a frequent problem is an overheated joint where the solder has not yet flowed and the residue of the burnt flux makes this joint hard to fix. Insufficient wetting is when the solder has wet the leads but then has not went on to form a good bond with the pad. There are also many problems with surface mount technology these include, solder balls which is mainly caused by poor process conditions which could include someone moving the board while the solder is settling. Another problem is solder beads which could occur when the reflow ramp up is too fast or too slow. A further problem is bridging which can occur both due to hot slumping and cold slumping. Also known as a common problem is tombstoning which is most commonly caused by an unequal components placement on the board before reflow results in unbalanced solder forces.
Circuit boards are made up of extremely sensitive components, and handling them in a careless manner will almost certainly cause the board to be damaged, especially putting pressure on the small components, also the smallest amount of static electricity can completely destroy the full board so the best way to prevent damage is to keep the generation of static electricity near the board to a minimum, discharging any static charge that has built up within the circuit and handle the circuit with care. While transporting boards you are recommended to put on rubber soled shoes to ground yourself before handling a board. Make sure the humidity within your workplace is high as static electricity builds up much more in dry air. Also you are recommended to put on gloves to handle a circuit board.
Solder paste is a material used in the manufacture of printed circuit boards to connect surface mount components to pads on the board, it is also possible to solder through hole pin in paste components by printing solder paste over the holes. There are three kinds of solder paste deposition these are Step stencil which have the possibility of soldering all chip component types into cavities, also has a low cycle time and more space for different depths, the other kind is dispensing which has the possibility of soldering all chip component types into cavities, the cycle time of this depends on the needle which is used to create dispensed dot sizes and the third and final type is jetting, a jetting machine is capable for processes and for different volumes, a jetting machine is designed to feed a and place machine and can sometimes mount 40,000 components an hour. The most appropriate and cost effective method to use will depend on the final product confirmation since each method each have its own limitations in terms of capability to support design features or cycle time. Transferring solder paste in high density cavities is a process that can be done with high quality and capable of achieving IPC-610 class.
A pick and place machine helps with the assembly of circuit boards by automating the placement of small surfaces mount components on to boards, removing the process of manually planning and soldering small components while using pick and place preparation is key throughout soldering, solder paste must be applied to the solder pads of the board before operating the machine. Companies tend to use a laser cut vinyl strider as a mask with holds cut over the solder pads. Then with the mask on the board, solder paste can be forced onto the solder pads. The pick and place machine does exactly what the name states, it picks up the components and places them in a specified location. Solder paste is applied to the board before they are placed into the machine. Once the machine has placed the components the solder paste holds the components in place while the band is transferred to an oven. When the solder paste is melted, reflowed and then cooled, permanent solder joints are created at each component, completing the process. Also the pick and place machine does not require an air compressor as it has a vacuum and pump within the machine.
Reflow soldering is a process in which a solder paste is used to temporarily attach one or more electrical components to their contact pads after which the entire assembly is dependant on controlled heat, which then melts the solder, which leads to the joint connecting. Heating may be achieved by putting the assembly through a reflow oven or by soldering individual joints with a hot air pencil. Reflow soldering is the most common method of attaching surface mount components to a circuit board. The aim of the reflow process is to melt the solder and heat the touching surfaces without overheating and damaging the electrical components. There are 4 sections of reflow soldering these are ; preheat zone, thermal scale zone, reflow zone and cooling zone. Preheat zone is the first stage of the reflow process, during this reflow phase, the entire board assembly climbs towards a target soak or dwell temperature. The main goal of the preheat phase is to get the entire assembly safely and consistently to a soak or pre-reflow temperature. The second stage is thermal soak zone which usually takes around 1-2 minutes to remove the solder paste and the flux components start the oxide reduction process on the component leads and pads. The third stage is the reflow stage where the temperature reaches its highest point. The final zone is a cooling zone which slowly cools the board and finally solidifies the joints on the solder.
According to website ‘copperplumbing.org.uk’ they say you should clean both outside and inside of the surface of the fitting. They recommend using fine sand paper or a brush but go against the use of steel wool. They also recommend Abrasive impregnated nylon scouring pads for potable services in order to prevent particles of steel entering the system. According to website ‘http://www.youtube.com/watch?v=s4uK21E1xXk’ using a new proprietary technology, magic cleaner 3D manages to remove particulates attached to printed circuit boards, this made it the most powerful cleaner in the world when the video was released in 2011. As the years go on the size of the components used are becoming much smaller whilst the density of components on the PCB is increasing. There are 2 types of cleaners they are the brush type cleaner and adhesive roller type cleaners. Flux is necessary to reduce oxide when there are hot metals in contact in the air, many pieces of solder normally have flux in the center of them. Which is more helpful as it manages to reduce oxide of the hot metals as when the solder melts it can reduce the oxide almost instantly.
Lead forming machines and lead forming tools performs both lead forming and lead cutting tasks. Manual and motorized component lead farmers accommodate a wider range of axial and radical through hole components. Five things that affect lead forming are the length of the component from lead tip to lead tip, the distance from the top of the board to the bottom of the component, the length of the lead foot, the thickness of the leads and the length of the lead from the component body to the first bend in the lead. Lead thickness is the most important of all of the specifications.
Wave soldering is a massive soldering process which is used in the manufacture of printed circuit boards. The circuit board is passed over a pair of molten solder in which a pump produces an amount of solder that looks like a standing wave. As the circuit board makes contact with the wave, the components become soldered to the board. Wave soldering is used for both through hole printed circuit assemblies and surface mount. In the latter stage, the components are glued onto the surface of a printed circuit board by placement equipment, before being run through the molten solder wave. As through hole components have been largely replaced by surface mount components, wave soldering has been supplemented by reflow soldering method in many large scale electronic applications. However, there is still significant wave soldering where surface mount technology is not suitable.
I have successfully described the assembly of both solder mount technology and through hole soldering in this report by going through 8 main points.