1. of the stencil. Not enough pressure can cause

1.Manual prototype assembly or component-level repair is moredifficult and requires skilled operators and more expensive tools, due to thesmall sizes and lead spacing’s of many SMDs. Handling of small SMTcomponents can be difficult, requiring tweezers, unlike nearly all through-holecomponents. Whereas through-hole components will stay in place (undergravitational force) once inserted and can be mechanically secured prior tosoldering by bending out two leads on the solder size of the board, SMDs areeasily moved out of place by a touch of a soldering iron.

Without expert skill,when manually soldering or de-soldering a component, it is easy to accidentallyreflow the solder of an adjacent SMT component and unintentionally displace it,something that is almost impossible to do with through-hole componentsWiki3.One of the most important parts of the surface mountassembly process is the application of solder paste to the printed circuitboard (PCB).  The aim of this process is to accurately deposit the correctamount onto each of the pads to be soldered.  This is achieved byscreen-printing the solder paste through a stencil or foil but also can beapplied by jet printing.  It is widely believed that this part of theprocess, if not controlled correctly, accounts for the majority of assemblydefects.The most common method of applying solder paste to a PCBusing a stencil printer is squeegee blade.

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  The squeegees are the toolsused to apply the solder paste across the stencil and on to the PCB.  Theyare usually made from metal but can also be made from polyurethane.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 “bleeding” of the paste between thestencil and PCB.

  A typical setting for the squeegee pressure is 0.5Kg ofpressure per 25mm of squeegee blade.The stencil must be cleaned regularly during use.  Manyof the automatic printing machines have a system that can be set to clean thestencil after a fixed number of prints using lint-free material applied with acleaning chemical 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.