Document Sample
SOLDERING Powered By Docstoc
					Electronics and
Computer Systems Engineering

                   BASIC SOLDERING


Electronics and
Computer Systems Engineering


Soldering is the oldest metallurgical technique known for joining two metals and is
called 'soft soldering, because it uses lower temperatures than welding, and the
process can easily be reversed by reheating and removal of the solder.

Soldering is not a gluing process. It is the joining of two metals by a third metal
(solder) as the bonding agent, using heat as the bonding medium, via a soldering

SOLDERING IRONS: - There are many different types of soldering irons ranging
from small hand-held constant temperature irons available from the supermarket, to
large expensive multi-function units used by professionals in the industry.

Temperature controlled irons offer significant advantage over constant heat irons,
providing rapid initial heat-up, better heat transfer, more control over the soldering
process and improved tip life. They should be cleaned and maintained prior to and
during use.

Temperature controlled irons can be categorised as either thermostatically controlled
or electrically controlled.

The Weller iron (figure 1) is thermostatically controlled and has a preset tip
temperature as set in the factory. Temperature is controlled by a thermo-magnetic
switch within the barrel.

The Hakko iron (figure 2) is electrically controlled using a temperature sensor to
control the amount of heat generated. The temperature can be set for different
applications by adjusting the temperature preset control on the soldering station.

      figure 1                                                    figure 2

Electronics and
Computer Systems Engineering

The primary purpose of the tip is to transfer heat from the iron to the component lead,
PCB pads and solder. Tips are made of copper or copper compounds, including
nickel and iron and are plated with chromium to protect them from oxidation. Tips
come in many shapes including conical, screwdriver and chisel, and different lengths
and widths.

Selection of the appropriate size and width is determined by the size of the
connection to be made, ideally 2/3 to equal the diameter of the pad.

Tips should be correctly 'tinned’ at all times, by adding a small amount of solder to
the hot tip. This is particularly important before use and prior to turning the iron off.
Tinning the tip reduces the possibility of oxidation when the soldering iron is
not in use.

Solder is made up of a mixture of tin and lead and sometimes silver. The most
common for commercial hand soldering has a composition of Tin (Sn60) and Lead
Pb40, with a diameter of 0.71 mm. Another called eutectic solder, is Sn63 (tin) / Pb37
(lead), with a diameter of between 0.35 mm and 0.56 mm, is commonly used in
soldering surface mount devices.

The wetting temperature of tin-lead solder is approx. 245-275C, although the iron
temperature is set at approx. 350 for proper wetting to occur and to minimise the
time spent on the PCB pad. The iron temperature is set higher than the wetting
temperature as it will cool slightly as it moves through the air, when it is wiped across
the damp sponge when cleaning the tip and when contact is made with the
component lead and PCB pad.

The 60/40 solder has a ‘plastic’ stage between the melting and solidifying
stages, so it is important not to allow any movement of the joint during this
time. Any movement may cause poor contact between lead and pad, resulting
in a ‘dry joint’. Eutectic solder transforms sharply from solid to liquid at 183,
minimising the chance of dry joints.

Electronics and
Computer Systems Engineering

                                                                           figure 3

Solder is usually supplied on a thin wire roll. The solder has several hollow cores
containing resin flux (figure 3). The resin flux melts during the soldering process and
helps conduct the heat, cleans the surfaces to be soldered and helps stop the
oxidisation of the surfaces during soldering.

It is important to never break or stretch solder wire, as this alters the ratio of
flux/solder in the stretched area. Insufficient flux will make the soldering process
more difficult. Always cut the solder from the spool; never break or melt it.


Often called printed circuit boards (PCB) figure 4, they are etched with a
pattern of electrically conductive tracks on one or both sides. These tracks
provide the connection between individual components whose leads are
generally inserted through holes in pads (or lands). The boards are laminates
and some are made of fibreglass composition making them strong and
resistant to heat, cold, moisture and corrosion. The tracks are bonded to the
boards and are made from copper or solder alloy coated copper. Other metals
can also be used depending on the application.

The boards can be single-sided, double-sided or multi-layered. Always handle
boards by the edges, or with cotton gloves, to avoid contaminating the pads
and tracks with oils and grease from your fingers. This can prevent the solder
from properly wetting the pads and making a good solder connection .

Is the process of removing contaminates before
and after soldering to ensure good
mechanical/electrical connection and increase the
 life of the connection, component and PCB. It is performed
using three different methods.                                               figure 4

   1. Mechanical - scourer or bristle brush.

   2. Chemical - solvents eg. Isopropyl Alcohol is applied to the completed solder
      joint to remove excess flux and other contaminants. It evaporates quickly and
      should not leave any residue.

Electronics and
Computer Systems Engineering

   3. Thermal - solder pot. This contains molten solder used to pre-tin component
      leads. The heat thermally shocks any oxides and contamination off the
      component leads.


Axial-leaded components are usually loaded with the body of the component
resting on, or very close to the PCB, except where they are large and produce
excessive heat. Axial leaded components should be mounted centrally
between the holes. Resistors should be mounted so that the colour code can
be read and orientated in the same direction, either vertically or horizontally
(figure 5). All polarised components should be orientated so that polarity
symbols (+ or - ) and vital information such as component value are visible.

                                    figure 5

Glass diodes can be mounted off the board to allow for expansion and
minimise stresses which could crack them. An alternative is to sleeve the
glass body with close fitting clear plastic tubing, or they can be mounted flush
to the board with stress relief in the form of stress relief loops (camel humps)
as in figure (figure 6)

Electronics and
Computer Systems Engineering

                                   figure 6

Resistors with wattage ratings greater than 1 watt are usually mounted off the board to
allow heat to dissipate freely around the component (figure 7).

                                   figure 7

Radial leaded Components are usually loaded slightly off the PCB (0.3 -
3.0mm) to allow for stress relief and inspection between component body and
board surface figure 8.

                                   figure 8

Electronics and
Computer Systems Engineering


The minimum distance from the end of the component body to the start of the
lead bend is usually 2 lead diameters or 1.5mm for hand bending methods,
and 1mm for automatic bending equipment.

       Correctly bent resistor                   Bend to close to body

The component lead must extend straight out from the end of the component
and enter the board at right angles to the surface of the PCB.

Component bending

Without undue pressure hold the component lead with the pliers (figure 9) and
bend the leading protruding beyond the pliers with a finger until a 900 angle is
formed. Do not bend the lead with pliers as this places stress on the
component body. Inspect for any nicks or cuts to the lead. If damaged discard
component. A nick or scrape which exposes the base metal renders the
component susceptible to contamination and may reduce its operational life.

                                    figure 9

Electronics and
Computer Systems Engineering


TYPE 1 FULLY CLINCHED - bend the lead flat against the pad. Normal spring-back
should not exceed 15. Trim the lead to a length of between 1-2 lead diameters. It
should not overhang the pad area more than 0.4mm and should not interfere
electrically with other connections. Used in High Quality/High Reliability military or life
support applications. Not recommended for commercial assembly as they are more
difficult to rework.

TYPE 2 SEMI-CLINCHED - bend the lead to an angle of between 30-60 (usually
45). Cut to a minimum of 0.5mm and a max. of 1.5mm, or between 1-2 lead
diameters. Used for commercial applications and preferred, as it retains the
component prior to soldering, yet allows ease of desoldering.

TYPE 3 RIGID LEAD TERMINATION - or straight through termination. Cut to a
length of min. 0.5mm and max. of 1.5mm, or between 1-2 lead diameters. Used in
cheap electronic equipment, with short time guarantees.

Electronics and
Computer Systems Engineering

BASIC SOLDERING PROCEDURE: - Check that your soldering iron tip is
suitable for the Project. (no larger than the diameter of the pad).

Ensure the solder sponge is damp. A dry sponge will not clean the tip
effectively, and one that is too wet will lower the temperature of the tip making
for an ineffective solder joint.

Check the tip is clean and shiny. If not, tin it. Tinning the iron tip with solder
will encourage heat transfer from the tip to the work piece.

When ready, hold the soldering iron at a 45 angle, and heat both the lead
and the pad simultaneously. Touch the solder wire in the space between the
iron tip and the lead, thus forming a heat bridge (figure 10).

Keep the soldering iron still while moving the solder around and allow it to
melt around the join.

Do not press hard with the iron tip or rub it back and forth across a
connection. This does NOT improve heat transfer and wears out the tip, it can
damage the PCB.

Remove the solder and the iron simultaneously, (prevents spiking).

Identifying the correct amount of solder to apply is a matter of observing the
soldering process and continual practice. The soldering process should not
exceed approx. 2 – 3 seconds. Any PCB pad will delaminate (break away
from the PCB) if excessive heat is applied for extended times.

Wipe the tip frequently as you solder. This removes burnt fluxes etc. and
prevents contamination of further fillets.

Thoroughly clean your board, using Isopropyl Alcohol, and a bristle brush, to
remove the flux residue and other contaminants. Wipe or pat dry with a lint
free tissue to remove traces of residue.

                                                       Figure 10

Electronics and
Computer Systems Engineering

The solder fillet should be concave in shape, feathering out smoothly to the
edge of the pad. Allow to cool naturally and undisturbed, do not blow on the
solder to cool it.

Inspect for a good solder connection. The solder joint should be clean,
smooth and shiny.

Re-tinning the tip often will assist in rapid transfer of heat. The tip should
always be shiny.

Leave a large blob of solder on the tip when switching the iron off as this will
protect the tip from oxidationand contamination.

Constructing a good solder joint

Figure a) the amount of solder applied is minimal and may result in a poor
electrical connection over time.

Figure b) shows the optimal solder joint that has good wetting between
component lead and PCB pad.

Figure c) indicates an excessive amount of solder has been applied to the

Electronics and
Computer Systems Engineering

Are usually categorised as either axial leaded, radial leaded, multi leaded or
mounted devices.

                                             Axial-leaded components are
                                             one of the most common types.
                                             They are two leaded PCB
                                             mounted devices, having the
                                             leads exiting from either end of
                                             the component. The most
                                             common are resistors, tubular
                                             capacitors and diodes.

Radial leaded
components have two or
more leads which all exit
from the one side of the
component. They are
capacitors, transistors,
LEDs single in-In-line
package devices, and
various PCB connectors.

Electronics and
Computer Systems Engineering

                                    Multi-leaded components
                                    are commonly integrated
                                    circuits    (ICs).     They
                                    contain many individual
                                    encapsulated within a
                                    plastic package. ICs are
                                    marked to show the
                                    position of pin 1 by an
                                    indentation next to pin 1
                                    or a notch at the end of
                                    the IC, with pin1 to the left
                                    of the notch. IC leads
                                    should never be cut!

Surface          mounted
components are soldered
and mounted to the same
side of the PCB. They are
leadless components and
usually have end-caps
which are soldered to
small pads.

Electronics and
Computer Systems Engineering

Vices and clamps
Used to hold wires, components and assemblies during the soldering process.
They should be of a type which will not damage or deform the components
and PCB.

Wire and lead cutting tools
Should be either full flush or shear cutting types designed to cut squarely
without leaving burrs or sharp points.

Flush cutting types are
commonly used. When using
them the flat side of the
cutters should be facing the
board, so no sharp points
remain on the component
after cutting.

Other useful tools are needle
nose pliers and wide nose
pliers, used for bending
component        leads   and
handling the component when
mounting it in the PCB.


Shared By: