1. is wrong and by the time that it


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.  through-hole components will stay in place (by
gravity) once in place 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 reflow the solder
of a nearby component and unintentionally move it by accident, something that
is very difficult to do with through-hole components.

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Within the PCB assembly facilities board are often worked on
in an assembly line. This process allows them to be moves from one section to
another whilst something is added at each stage. The boards can be moved on
conveyor belts passing the boards from one state to the next. This process allows
the board to be worked on quickly as at each section there is something being
worked on and there is never an idle stage. The disadvantages of this are that
it can often take a while to notice something is wrong and by the time that it
has been noticed a considerable number of units may also be defected.


One of the most crucial parts in the process of surface mount
assembly 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 to
each pad in order to be soldered with great accuracy.  This is done by using
a stencil to print the paste through. The same procedure can also be applied by
jet printing. It is this procedure that it usually accountable for the
most defects within assembly, however if controlled properly there can be very
few mistakes.

The most frequently used way of applying solder paste to a
PCB using a stencil is a special squeegee blade.  The squeegees are the
tools used to apply the solder paste across the stencil and on to the
PCB.  They can sometimes be made from polyurethane however they are usually
made from metal.

During the print cycle it is important to apply the right
amount of pressure across the entire length of the blade to ensure a clean wipe
of the stencil.  Too little pressure can cause the paste to smear on the
stencil and can cause an incomplete transfer of the paste to the PCB.  Too
much pressure can scrape up too much paste from the board causing excess wear
on the stencil and squeegees, and may cause the paste to seep out between the
PCB and the stencil. 

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.



 Pick-and-place machines are robotic machines
which 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 of
electronic components, like resistors s, capacitors, integrated circuits onto
the PCBs which in turn are used in computers, consumer electronics as well
as medical, industrial, automotive, military and telecommunications equipment. The
machines are programmed with the information of where to place certain
components and can work in groups pacing multiple components together or in an assembly
line fashion where one piece is placed and is then moved along to the next machine


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. There are 5 basic stages to this
process. Stage 1: preheating. This should get rid of any solvents and activate the
flux.   Stage 2: bring all components to
the same temperature. The soak zone
brings the temperature of all components and board areas to an equal level. Components
do not heat up at the same speed. This is especially the case with infra-red
heating, due to uneven absorption of infra-red energy by components. Stage 3:  Heating.  In the reflow zone the temperature is increased
to above the melting points of the solder tis causes it to fuse with the copper
and wets the pads and component pins better when it is hotter, this  creates better joints.  Stage 4: Cooling. The components need to cool fast
but also at a rate which does not cause thermal stress. Room temperature cooling
is usually fine


Solder needs flux to help with the fusing between metals at
high temperatures. For example tin and copper fuse well together but at high
temperatures, like those that occur during soldering the oxides of copper do
not fuse well with tin. The flux helps to reduce the oxides and help bond the
two metals.

In order for the boards to be produced at a high standard
the stencils and equipment has to be cleaned regularly. Boards are often
cleaned with a cleaning chemical such as IPA (Isopropyl Alcohol). This rubbing alcohol is used to remove the flux from
the stencil to allow for it to be clean ready for it to be used again.


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%