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.Whereas through-hole components will stay in place (by gravity) once in placeand can be mechanically secured prior to soldering by bending out two leads onthe solder size of the board, SMDs are easily moved out of place by a touch ofa soldering iron. Without expert skill, when manually soldering or de-solderinga component, it is easy to reflow the solder of a nearby component andunintentionally move it by accident, something that is very difficult to dowith through-hole components.Wiki3.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 onto each of the padsto be soldered with great precision.
This is achieved by using a stencilto screen-print the solder paste through. The same procedure can also beapplied by jet printing. It is this procedure that it usually accountablefor the most defects within assembly, however if controlled properly there canbe very few mistakesThe 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 are usually made from metal but can also be made frompolyurethane.During the print cycle it is important to apply sufficientpressure across the entire length of the squeegee blade to ensure a clean wipeof the stencil. Not enough pressure can cause the paste to smear on thestencil, poor deposition, and the incomplete transfer to the PCB.
Toomuch pressure can scoop too much paste from the board causing excess wear onthe stencil and squeegees, and may cause the paste to seep out between the PCBand the stencil. A typical setting for the squeegee pressure is 0.5Kg ofpressure per 25mm of the blade on the squeegie.In order for the process to run smoothly stencil maintenanceand cleaning must happen regularly during use. Many of the automaticprinting machines have a system that can be set to clean the stencil after afixed number of prints using lint-free material applied with a cleaningchemical such as IPA (IsopropylAlcohol).
The system performs two functions, the first being thecleaning of the underside of the stencil to stop smudging, and the second isthe cleaning of the apertures using vacuum to stop blockages.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.http://www.surfacemountprocess.com/solder-paste-printing-process.html4.
pick-and-place machines or Pare robotic machines which are used to place surface mountdevices (SMDs) onto a printed circuit board (PCB). They are usedfor high speed, high precision placing of broad range of electronic components,like capacitors, resistors, integrated circuits onto thePCBs which are in turn used in computers, consumer electronics as well asindustrial, medical, automotive, military and telecommunications equipment.5.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 board6.7.
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.