Electronic (THM) is the process by which components are

Electronic
Construction Skills Report

 

This report will cover the various steps that are taken in
the assembly of Printed Circuit Boards (PCB) using both through hole and
surface mount technology.  Through Hole Mounting
(THM) is the process by which components are kept in place by wires that are
placed through drilled holes on a PCB and soldered on the other side of the
board.  It was the more common method
until the rise of Surface Mount Technology (SMT) in the 1980s.  SMT is the method in which components are
placed directly on to board and soldered on the same side.  SMT is much more popular now than PCB as SMT
components are smaller thus able to fit more components on to a Printed Circuit
Board.  SMT is also less time consuming
are holes do not need to be drilled on to the board.

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Difficulties in manual assembly and soldering
of surface mount technology

Manual assembly is more time consuming and
expensive as human’s wouldn’t be able to go as fast as machines.  Machines also don’t need breaks, don’t get
tired and don’t make mistakes from lack of concentration.  Operator skills in manual soldering is also
very important.  It would be much harder
to produce the same boards over and over again if there are different people
making them.  Hand soldering operations vary from operator-to-operator,
therefore hand soldering is less favourable than an automated assembly method.

 

Describe
the transport of boards within assembly facility

 

In Assembly facilities PCBs are transported
usually over conveyor belts through machinery designed for different parts of
the manufacturing process.  Once off of
machines PCBs can be stored on racks/trolleys with wheels so they can be easily
manoeuvred around the facility safely and in many numbers.

 

Describe
Solder paste deposition

Solder paste is the material used to connect
components to pads on the Printed Circuit Board. The paste initially keeps
components in place by being sticky, then it is heated (along with the rest of
the board) melting the paste and forming a mechanical bond as well as an
electrical connection. When solder paste is used in mass PCB assembly as well as prototype PCB
assembly there are a number of stages that are undertaken. Solder paste is
first applied to the PCB. Then the paste applied only to where solder is
required. A solder paste stencil that allows only the solder paste through in
certain areas is used to solder where required.  Solder paste has an expiry date and must be
used within that timeframe.

 

Pick
and Place operation

                      

Pick and place machines are used widely
in the manufacture of today’s surface mount technology SMT electronics circuit
boards. Using these pick and place systems or pick and place machines, it is
possible to accurately place large numbers of small, or large components
quickly and accurately onto circuit boards.

With
some electronic circuit boards using a lot of surface mount technology (SMT)
components, many of which are very small, most of the components therefore
require very accurate placement, it is not feasible to place them manually. Pick
and place machines are used that can place all the components accurately and in
a repeatable fashion.

Pick and place machines are used in the manufacture if electronic
circuit boards for the placement of Surface Mount Technology (SMT) components
onto boards. They pick the components, typically from specially prepared reels
or other packaged forms of components, and place them onto printed circuit
boards. The pick and place machines are pre-programmed with the information
about component positions so that they know where to place the components. This
programme is normally developed directly from the printed circuit board design
information.

 

Reflow Soldering

Reflow soldering is the most widely used method
of attaching surface mount components to printed circuit boards (PCBs) because
it enable mass production at a very fast rate.  The aim of the process is
to form acceptable solder joints by first pre-heating the components/PCB/solder
paste and then melting the solder without causing damage by overheating.

 

Cleaning
and Fluxing

Fluxing is needed for good soldering practices in an electronic
assembly, preparing a circuit board with flux is
critical to the soldering process. Solder flux prepares the copper soldering
pads of PCBs and the leads of components so the applied solder will bond
properly. 

When melted, solder attaches very well to bare copper,
but attaches poorly or not at all to the various oxides of copper. The trouble
is that exposed to air, copper tarnishes, or forms oxides on its surface. At normal
temperatures, this is a slow process. At high temperatures though, the speed of
tarnishing increases. 

During the soldering process fluxing has a couple functions.
The first is that it dissolves the oxides on the metal surface, which allows
the molten solder to flow over the metal. 
The second is that it blocks oxygen from combining with the metal while
the metal is hot for soldering. 

 

When we talk about cleaning a PCB we are usually talking about unwanted
debris – or matter, residues and corrosion of any kind on the circuit board. Such
things not only make the assembly look untidy, but could be damaging the
functionality or long term reliability of the product. So most of the time,
when we’re talking about cleanliness it’s about cleaning off the flux residues.

To get the boards clean again we load batches
into a high powered washing machine. This high temperature, high pressure, all
stainless steel dishwasher uses deionized water to remove any residue from the
manufacturing process.  Deionised
water is very pure water that lacks any ions. Deionised water doesn’t
damage the circuit as it’s the ions that allow electricity to short various
parts of the device.  If a washer
is too grand or if there is just a few boards that need cleaning a tooth brush
with the bristles dipped in deionised water would work aswell.

Component lead
forming

 

Lead Forming machines come in manual and motorised forms.  These machines are used to cut and bend
components such as diodes and resistors to the length desired.  This saves a lot of time as the machines are
able to cut work at a faster rate than a human.  It also has the advantage of accuracy as all
the components will now be the same length and bent at the same angle.

 

Wave soldering

 

Wave
soldering is a process by which large printed circuit boards can be soldered
quickly and reliably during PCB assembly. 
The wave soldering process gains its name from the fact that the process
passes the printed circuit boards to be soldered over a wave of solder.

In this way
a complete board can be soldered in a matter of seconds producing joints that are
reliably, both mechanically and electrically. Apart from being much faster than
manual soldering, wave soldering is also produces joints with a much higher
degree of reliability.

Wave
soldering can be used in PCB assembly for both conventional through hole
mounted components as well as surface mount components. 

A wave soldering machine consists of a flux
applicator, a pre-heat tunnel and solder pot with a pump.  A wave soldering machine eliminates hand
soldering leading to a faster production and higher quality.  Boards are placed on to a pallet and ran
through the machine.  Flux is added to
the boards, and the boards travel through the heat tunnel to pre-heat the
boards and activate the flux.  After
exiting the heat tunnel the boards travel over the wave pump and as it does so,
the wave solders the connections as it passes. 
The boards exit the machine and cool down as they await to be moved on.

Wave soldering is not as widely used for PCB assembly as it was at one
time. It is not suited to the very fine pitches required by many of the boards
in manufacture today. However it is ideal for the many boards still
manufactured with conventional leaded components and some surface mount boards
that use larger components.

 

In Conclusion I have looked at the many steps involved in the process of
automated manufacturing of Printed Circuit Boards including the difficulties of
manual soldering compared with automated soldering, the machinery used in developing
the circuit boards and the processes of the machines being used on the
boards.  It’s clear to see that
automation is a much more cost/time effective and accurate way to build circuit
boards, so it’s no wonder that it has become the most popular form of
production.

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