Quality Soldering Basics
This procedure covers the basic concepts for high quality soldering.
Printed Board Type: R/F/W/C | Skill Level: Intermediate
| Conformance Level: N/A | Rev.: D |
Rev. Date: Jul 7, 2000
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
Figure 1: Wetting occurs when
molten solder penetrates a copper
surface forming an intermetallic bond.
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.
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
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
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 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.
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 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.
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
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
Figure 2: Minimal thermal linkage
due to insufficient solder between
the pad and soldering iron tip.
A third factor to consider is thermal linkage. This
is the area of contact between the iron tip and the
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
Figure 3: A solder bridge provides thermal linkage
to transfer heat into
the pad and component lead.
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
The cleaning solvents, solutions and methods used should
not have affected the parts, connections, and materials
being cleaned. After cleaning, boards should be adequately
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
Figure 4: Solder blends to the
soldered surface, forming a small
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.
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