1.Manual assembly and manualcomponent repair is more difficult and requires highly skilled operators and moreexpensive tools, this is due to the small component sizes and lead spacing’s ofmany SMDs. Handling of small surface mount technology components canbe difficult, requiring tweezers, unlike nearly all through-hole components. through-hole components will stay in place (bygravity) once in place and can be mechanically secured prior to soldering bybending out two leads on the solder size of the board, SMDs are easily movedout of place by a touch of a soldering iron. Without expert skill, whenmanually soldering or de-soldering a component, it is easy to reflow the solderof a nearby component and unintentionally move it by accident, something thatis very difficult to do with through-hole components.2.
Within the PCB assembly facilities board are often worked onin an assembly line. This process allows them to be moves from one section toanother whilst something is added at each stage. The boards can be moved onconveyor belts passing the boards from one state to the next. This process allowsthe board to be worked on quickly as at each section there is something beingworked on and there is never an idle stage. The disadvantages of this are thatit can often take a while to notice something is wrong and by the time that ithas been noticed a considerable number of units may also be defected.
3.One of the most crucial parts in the process of surface mountassembly is applying the solder paste to the printed circuit board (PCB). The aim of this process is to deposit the correct amount of solder paste on toeach pad in order to be soldered with great accuracy. This is done by usinga stencil to print the paste through.
The same procedure can also be applied byjet printing. It is this procedure that it usually accountable for themost defects within assembly, however if controlled properly there can be veryfew mistakes.The most frequently used way of applying solder paste to aPCB using a stencil is a special squeegee blade. The squeegees are thetools used to apply the solder paste across the stencil and on to thePCB. They can sometimes be made from polyurethane however they are usuallymade from metal.
During the print cycle it is important to apply the rightamount of pressure across the entire length of the blade to ensure a clean wipeof the stencil. Too little pressure can cause the paste to smear on thestencil and can cause an incomplete transfer of the paste to the PCB. Toomuch pressure can scrape up too much paste from the board causing excess wearon the stencil and squeegees, and may cause the paste to seep out between thePCB and the stencil. To verify the process, automatic inspection can be used toaccurately check solder paste deposits. There are two types of solderpaste inspection available which are 2D inspection which checks the area of thepaste deposit and 3D inspection which checks the volume of the paste deposit. 4.
Pick-and-place machines are robotic machineswhich are used to place surface mount devices onto a printed circuit board.They are used for high speed and precision placing of a wide variety ofelectronic components, like resistors s, capacitors, integrated circuits ontothe PCBs which in turn are used in computers, consumer electronics as wellas medical, industrial, automotive, military and telecommunications equipment. Themachines are programmed with the information of where to place certaincomponents and can work in groups pacing multiple components together or in an assemblyline fashion where one piece is placed and is then moved along to the next machine5.Reflow soldering is a process in which multiple componentsare temporarily secured to a PCB before a controlled temperature is applied andfixes the components in place on the board. There are 5 basic stages to thisprocess. Stage 1: preheating. This should get rid of any solvents and activate theflux.
Stage 2: bring all components tothe same temperature. The soak zonebrings the temperature of all components and board areas to an equal level. Componentsdo not heat up at the same speed. This is especially the case with infra-redheating, due to uneven absorption of infra-red energy by components.
Stage 3: Heating. In the reflow zone the temperature is increasedto above the melting points of the solder tis causes it to fuse with the copperand wets the pads and component pins better when it is hotter, this creates better joints. Stage 4: Cooling. The components need to cool fastbut also at a rate which does not cause thermal stress.
Room temperature coolingis usually fine6.Solder needs flux to help with the fusing between metals athigh temperatures. For example tin and copper fuse well together but at hightemperatures, like those that occur during soldering the oxides of copper donot fuse well with tin.
The flux helps to reduce the oxides and help bond thetwo metals. In order for the boards to be produced at a high standardthe stencils and equipment has to be cleaned regularly. Boards are oftencleaned with a cleaning chemical such as IPA (Isopropyl Alcohol). This rubbing alcohol is used to remove the flux fromthe stencil to allow for it to be clean ready for it to be used again.7.8.
The basic equipment used during the process is a conveyorthat moves the PCB through the different zones, a pan of solder used in thesoldering process, a pump that produces the actual wave, the sprayer for theflux and the preheating pad. The solder is usually a mixture of metals. Atypical leaded solder has the chemical makeup of 50% tin, 49.5% lead, and 0.5%antimony.