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 (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.
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 onto each of the pads to be soldered with great precision.
This is achieved by using a stencil to screen-print the solder 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 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%