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
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.
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
4. Pick-and-place machines are robots that are
used to place surface mount devices on a printed circuit board. These
robots are used for high speed and precision placing of a wide variety of
electronic components, like resistors, capacitors and integrated circuits onto
the PCBs which then in turn can be used in computers, consumer electronics
as well as medical, automotive, military
and telecommunications equipment. The robots 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
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%
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.