1. per 25mm of squeegee blade. The stencil


Manual  assembly and manual
component repair is more difficult and requires highly skilled operators and more
expensive tools, this is due to the small component sizes and lead spacing’s of
many SMDs. Handling of small surface mount technology components can
be difficult, requiring tweezers, unlike nearly all through-hole components.
Whereas through-hole components will stay in place (under gravitational force)
once inserted and can be mechanically secured prior to soldering by bending out
two leads on the solder size of the board, SMDs are easily 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 accidentally reflow the solder of a
nearby  component and unintentionally
displace it, something that is almost impossible to do with through-hole

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One of the most important parts of the surface mount
assembly process is the application of solder paste to the printed circuit
board (PCB).  The aim of this process is to accurately deposit the correct
amount onto each of the pads to be soldered.  This is achieved by
screen-printing the solder paste through a stencil or foil but also can be
applied by jet printing.  It is widely believed that this part of the
process, if not controlled correctly, accounts for the majority of assembly

The most common method of applying solder paste to a PCB
using a stencil printer is squeegee blade.  The squeegees are the tools
used to apply the solder paste across the stencil and on to the PCB.  They
are usually made from metal but can also be made from polyurethane.

During the print cycle it is important to apply sufficient
pressure across the entire length of the squeegee blade to ensure a clean wipe
of the stencil.  Not enough pressure can cause the paste to smear on the
stencil, poor deposition, and the incomplete transfer to the PCB.  Too
much pressure can scoop too much paste from the board causing excess wear on
the stencil and squeegees, and may cause “bleeding” of the paste between the
stencil and PCB.  A typical setting for the squeegee pressure is 0.5Kg of
pressure per 25mm of squeegee blade.

The stencil must be cleaned regularly during use.  Many
of the automatic printing machines have a system that can be set to clean the
stencil after a fixed number of prints using lint-free material applied with a
cleaning chemical such as IPA (Isopropyl
Alcohol).  The system performs two functions, the first being the
cleaning of the underside of the stencil to stop smudging, and the second is
the cleaning of the apertures using vacuum to stop blockages.

To verify the process, automatic inspection can be used to
accurately check solder paste deposits.  There are two types of solder
paste inspection available which are 2D inspection which checks the area of the
paste deposit and 3D inspection which checks the volume of the paste deposit.


4.  pick-and-place machines or P
are robotic machines which are used to place surface mount
devices (SMDs) onto a printed circuit board (PCB). They are used
for high speed, high precision placing of broad range of electronic components,
like capacitors, resistors, integrated circuits onto the
PCBs which are in turn used in computers, consumer electronics as well as
industrial, medical, automotive, military and telecommunications equipment.


Reflow soldering is a process in which multiple components
are temporarily secured to a PCB before a controlled temperature is applied and
fixes the components in place on the board



8. The basic equipment used during the process is a conveyor
that moves the PCB through the different zones, a pan of solder used in the
soldering process, a pump that produces the actual wave, the sprayer for the
flux and the preheating pad. The solder is usually a mixture of metals. A
typical leaded solder has the chemical makeup of 50% tin, 49.5% lead, and 0.5%