Soldering Process
Soldering is the process of joining two metals by the use of a solder
alloy, and it is one of the oldest known joining techniques. Faulty
solder joints remain one of the major causes of equipment failure and
thus the importance of high standards of workmanship in soldering cannot
be overemphasized.
The following material covers basic soldering procedures and has been
designed to provide the fundamental knowledge needed to complete the
majority of high reliability hand soldering and component removal
operations.
Properties of Solder
Solder used for electronics is a metal alloy, made by combining tin and
lead in different proportions. You can usually find these proportions
marked on the various types of solder available.
With most tin/lead solder combinations, melting does not take place
all at once. Fifty-fifty solder begins to melt at 183 C -361 F, but it's
not fully melted until the temperature reaches 216 C - 420 F. Between
these two temperatures, the solder exists in a plastic or semi-liquid
state.
The plastic range of a solder varies, depending upon the ratio of tin
to lead. With 60/40 solder, the range is much smaller than it is for
50/50 solder. The 63/37 ratio, known as eutectic solder has practically
no plastic range, and melts almost instantly at 183 C -361 F.
The solders most commonly used for hand soldering in electronics are
the 60/40 type and the 63/37 type. Due to the plastic range of the 60/40
type, you need to be careful not to move any elements of the joint
during the cool down period. Movement may cause what is known as
disturbed joint. A disturbed joint has a rough, irregular appearance and
looks dull instead of bright and shiny. A disturbed solder joint may be
unreliable and may require rework.
Wetting Action
When the hot solder comes in contact with a copper surface, a metal
solvent action takes place. The solder dissolves and penetrates the
copper surface. The molecules of solder and copper blend to form a new
alloy, one that's part copper and part solder. This solvent action is
called wetting and forms the intermetallic bond between the parts. (See
Figure 1) Wetting can only occur if the surface of the copper is free of
contamination and from the oxide film that forms when the metal is
exposed to air. Also, the solder and work surface need to have reached
the proper temperature.
Although the surfaces to be soldered may look clean, there is always a
thin film of oxide covering it. For a good solder bond, surface oxides
must be removed during the soldering process using flux.
Flux
Reliable solder connections can only be accomplished with truly cleaned
surfaces. Solvents can be used to clean the surfaces prior to soldering
but are insufficient due to the extremely rapid rate at which oxides
form on the surface of heated metals. To overcome this oxide film, it
becomes necessary in electronic soldering to use materials called
fluxes. Fluxes consist of natural or synthetic rosins and sometimes
chemical additives called activators.
It is the function of the flux to remove oxides and keep them removed
during the soldering operation. This is accomplished by the flux action
which is very corrosive at solder melt temperatures and accounts for
flux's ability to rapidly remove metal oxides. In its unheated state,
however, rosin flux is non-corrosive and non-conductive and thus will
not affect the circuitry. It is the fluxing action of removing oxides
and carrying them away, as well as preventing the reformation of new
oxides that allows the solder to form the desired intermetallic bond.
Flux must melt at a temperature lower than solder so that it can do
its job prior to the soldering action. It will volatilize very rapidly;
thus it is mandatory that flux be melted to flow onto the work surface
and not be simply volatilized by the hot iron tip to provide the full
benefit of the fluxing action. There are varieties of fluxes available
for many purposes and applications. The most common types include: Rosin
- No Clean, Rosin - Mildly Activated and Water Soluble.
When used, liquid flux should be applied in a thin, even coat to
those surfaces being joined and prior to the application of heat. Cored
wire solder and solder paste should be placed in such a position that
the flux can flow and cover the joints as the solder melts. Flux should
be applied so that no damage will occur to the surrounding parts and
materials.
Soldering Irons
Soldering irons come in a variety of sizes and shapes. A continuously
tinned surface must be maintained on the soldering iron tip's working
surface to ensure proper heat transfer and to avoid transfer of
impurities to the solder connection.
Before using the soldering iron the tip should be cleaned by wiping
it on a wet sponge. When not in use the iron should be kept in a holder,
with its tip clean and coated with a small amount of solder
Note
Although tip temperature is not the key element in soldering you should
always start at the lowest temperature possible. A good rule of thumb
is to set the soldering iron tip temperature at 260 C - 500 F and
increase the temperature as needed to obtain the desired result.
Controlling Heat
Controlling soldering iron tip temperature is not the key element in
soldering. The key element is controlling the heat cycle of the work.
How fast the work gets hot, how hot it gets, and how long it stays hot
is the element to control for reliable solder connections.
Thermal Mass
The first factor that needs to be considered when soldering is the
relative thermal mass of the joint to be soldered. This mass may vary
over a wide range.
Each joint, has its own particular thermal mass, and how this
combined mass compares with the mass of the iron tip determines the time
and temperature rise of the work.
Surface Condition
A second factor of importance when soldering is the surface condition.
If there are any oxides or other contaminants covering the pads or
leads, there will be a barrier to the flow of heat. Even though the iron
tip is the right size and temperature, it may not be able to supply
enough heat to the joint to melt the solder.
Thermal Linkage
A third factor to consider is thermal linkage. This is the area of contact between the iron tip and the work.
Figure 2 shows a view of a soldering iron tip soldering a component
lead. Heat is transferred through the small contact area between the
soldering iron tip and pad. The thermal linkage area is small.
Figure 3 also shows a view of a soldering iron tip soldering a
component lead. In this case, the contact area is greatly increased by
having a small amount of solder at the point of contact. The tip is also
in contact with both the pad and component further improving the
thermal linkage. This solder bridge provides thermal linkage and assures
the rapid transfer of heat into the work.
Applying Solder
In general, the soldering iron tip should be applied to the maximum
mass point of the joint. This will permit the rapid thermal elevation of
the parts to be soldered. Molten solder always flows from the cooler
area toward the hotter one.
Before solder is applied; the surface temperature of the parts being
soldered must be elevated above the solder melting point. Never melt the
solder against the iron tip and allow it to flow onto a surface cooler
than the solder melting temperature. Solder applied to a cleaned, fluxed
and properly heated surface will melt and flow without direct contact
with the heat source and provide a smooth, even surface, filleting out
to a thin edge. Improper soldering will exhibit a built-up, irregular
appearance and poor filleting. For good solder joint strength, parts
being soldered must be held in place until the solder solidifies.
If possible apply the solder to the upper portion of the joint so
that the work surfaces and not the iron will melt the solder, and so
that gravity will aid the solder flow. Selecting cored solder of the
proper diameter will aid in controlling the amount of solder being
applied to the joint. Use a small gauge for a small joint, and a large
gauge for a large joint.
Post Solder Cleaning
When cleaning is required, flux residue should be removed as soon as
possible, but no later than one hour after soldering. Some fluxes may
require more immediate action to facilitate adequate removal. Mechanical
means such as agitation, spraying, brushing, and other methods of
applications may be used in conjunction with the cleaning solution.
The cleaning solvents, solutions and methods used should not have
affected the parts, connections, and materials being cleaned. After
cleaning, boards should be adequately dried.
Resoldering
Care should be taken to avoid the need for resoldering. When
resoldering is required, quality standards for the resoldered connection
should be the same as for the original connection.
A cold or disturbed solder joint will usually require only reheating
and reflowing of the solder with the addition of suitable flux. If
reheating does not correct the condition, the solder should be removed
and the joint resoldered.
Workmanship
Solder joints should have a smooth appearance. A satin luster is
permissible. The joints should be free from scratches, sharp edges,
grittiness, looseness, blistering, or other evidence of poor
workmanship. Probe marks from test pins are acceptable providing that
they do not affect the integrity of the solder joint.
An acceptable solder connection should indicate evidence of wetting
and adherence when the solder blends to the soldered surface. The solder
should form a small contact angle; this indicates the presence of a
metallurgical bond and metallic continuity from solder to surface. (See
Figure 4)
Smooth clean voids or unevenness on the surface of the solder fillet
or coating are acceptable. A smooth transition from pad to component
lead should be evident.
Source :
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