Introduction can be difficult, requiring tweezers, unlike

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Introduction

This report with be a comparison of the benefits and
drawbacks of both through hole and surface mount technology and I will look to
establish which one is more effective as a method for producing printed Circuit
boards.

1. 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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2. 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.

3. 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.

 

4. 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

5. 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

6. 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.

7. Lead forming is used in through hole technology can
either be done manually or by a machine. Lead forming is the cutting, forming and
bending of axial components to get them into the desired shape

Manual lead forming requires special tools and requires
quite a bit of precision. Machine lead forming requires specialist machines but
is more accurate and quicker than manual lead forming.

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%
antimony.

In conclusion both through hole and surface mount technology
are both effective ways of producing PCBs; however they are more effective for
different things. Surface mount technology is accountable for a higher and
quicker manufacture whereas through hole technology caters for small and
precise projects where a large number of units are not required. 

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