Soldering is a group of joining processes which produces coalescence of
material by heating them to a suitable temperature and by using a
filler metal having a liquidus not exceeding 450oC and below the
solidus of the base metals.
The filler metal is distributed between the closely fitted surfaces of
the joint by capillary attraction. Solder is a filler metal used in
soldering which has a liquidus not exceeding 450oC. It is normally a
nonferrous alloy.
The temperature of 450oC is the temperature that differentiates
soldering from brazing. This arbitrary number was selected many years ago
and is universally accepted. Most of the factors involved with brazing
apply to soldering.
Like brazing and other joining processes, soldering involves several fields
of science, including mechanics, chemistry and metallurgy. Soldering is a
simple operation, consisting of the relative placements of the parts to be
joined, wetting the surfaces with molten solder, and allowing the solder to
cool until it has solidified.
The mechanism for joining by soldering involves three closely related
factors: 1) wetting, 2) alloying, and 3) capillary attraction.
Soldering in the field of electronics is in many respects different from
soldering in other branches of industry. Although the physical principles
of all soldering (and brazing) processes are the same, the features
specific to their use in electronics are so numerous that it is possible
to speak of soldering in electronics as a separate subject.
Soldering has several clear advantages over competitive joining techniques:
- The solder forms itself by the nature of the flow, wetting, and
subsequent crystallization process, even when the heat and the solder
are not directed precisely to the places to be soldered. Because the solder
does not adhere to insulating materials, it often can be applied in excess
quantities, in contrast to conductive adhesives. The soldering temperature
is relatively low, so there is no need for the heat to be applied locally as
in welding.
- Soldering allows considerably freedom in the dimensioning of joints,
so that it is possible to obtain good results even if variety of components
is used on the same product.
- The soldered connections can be disconnected if necessary, thus
facilitating repair
- The equipment for both manual soldering and machine soldering is
relatively simple
- The soldering process can be easily automated, offering the possibility
of in-line arrangements of soldering machines with other equipment
Soldering Methods
Iron soldering (INS) was the earliest soldering method. The hand-held
soldering iron used in this process continues to be popular. It is the most
cost-effective tool for:
- prototypes and short production jobs
- rework of defective solder joints and/or components
- soldering of components that are too delicate or specialized to
solder using mass-production techniques
Furnace and Infrared Soldering. Furnace soldering (FS) encompasses
a group of re-flow soldering techniques in which the parts to be joined
and pre-placed filler metal are put in furnace and then heated to the
soldering temperature.
Dip Soldering. Dip soldering (DS) is accomplished by submerging
parts to be joined into a molten solder bath. The molten bath can be
any suitable filler metal, but the selection is usually confined to the
lower melting point elements. The most common dip soldering operations
use zinc-aluminum and tin-lead solders.
Resistance Soldering. Resistance soldering (RS) is a soldering
process in which the heat needed to melt the solder is developed by the
resistance of the material when a large electrical current is supplied.
Resistance soldering can be applied to electrically conductive materials
that allow the passage of electric current. The process can be used for
selective spot soldering of small components, for the soldering of closely
placed parts on an assembly, or for heat restriction when necessary. It is
similar in many ways to resistance brazing.
Laser Soldering (LS). Industrial lasers are able to deliver large
amounts of heat with great precision and without contact, making them
ideal for application that have either a destructive nature, such as
cutting or drilling, or a constructive nature, such as soldering or
annealing. Laser soldering uses the well-focused, highly controlled
beam to deliver energy to a desired location for a precisely measured
length of time. The main advantage of laser soldering is a non-contact
procedure.
Hot Gas Soldering. Hot gas soldering is a process that is commonly
used in applications where the work-piece thermal mass is small and the
melting temperature of the solder is relatively low. The electronics
industry utilizes hot gas soldering to reflow or melt solder in localized
areas on circuit assemblies.
Induction Soldering. Induction or radio frequency heating are
versatile means of providing heat to the joint area. Heating is caused
by electrical resistance to eddy currents induced in the work piece.
These currents are induced by the rapidly changing magnetic field generated
by a coil supplied with an alternating current. The eddy current is
generated at the surface of the work-piece (skin effect) and diminishes
toward the interior. The advantages of induction heating include the
ability to supply heat uniformly over the entire joint area while
maintaining a localized temperature rise so heat sensitive materials
or devices neighboring the joint area are not damaged.
Soldering of Aluminum and Aluminum Alloys
Soldering of aluminum and aluminum alloys is relatively simple. Compared
with brazing, soldering presents advantages such as little loss of base
metal temper, minimal distortion, and easy removal of flux. Aluminum is
soldered at a minimum 110oC below the solidus temperature of the
base metal.
Wetting and spreading are affected by the presence of an oxide at the
surface of the joint. The nature of the aluminum oxide is different from
alloy to alloy. The heat-treatable alloys present a more tenacious oxide
that is more adherent and more difficult to remove. The oxides of the
non-heat-treatable alloys are less tenacious and easier to remove. The
action of a corrosive flux is enough to disrupt and displace the oxide
layer in a non-heat treatable aluminum alloy. However, the oxide layer
of a heat-treatable alloy must be removed by chemical or mechanical means
before soldering.
Solderability of aluminum alloys is influenced by alloying elements of
the base metal. Generally, purer aluminum alloys are easily soldered.
Analysis shows that alloys of groups 1xxx, 3xxx and 6xxx have good
solderabilities.
Solderability is primarily affected by two alloying elements, magnesium
and silicon. The presence of magnesium in an aluminum alloy not only
reduces the wettability (more than 1.0 wt%) but also increases the
intergranular penetration (more than 0.5 wt% Mg). Magnesium content
up to 1.0 wt% does not reduce the flux effectiveness, so it does not
affect wetting and spreading of the molten filler metal. Alloys with
less than 1.0 wt% Mg can be soldered with all flux types. Between 1.0
and 1.5 wt% Mg the low-temperature organic-type flux is not effective
to remove the surface oxide of the faying surface. When the amount of
magnesium exceeds 1.5 wt%, the corrosive flux does not work either.
Silicon plays the same role. Thus, if the amount of silicon is higher
than 4.0 wt%, all flux types are ineffective. To solve this problem, a
fluxless technique such as abrasion or ultrasonic soldering should be
utilized.
Typical solders used for aluminum are Zn, ZnCd, SnZn, SnPb, SnCd, SnZn.
Solders that melt below 260oC are called low-melting-point solders. Those
that melt between 260 and 370oC are called intermediate-melting-point
solders. Those that melt between 370 and 440oC are called
high-melting-point solders.
Aluminum alloys can be soldered to all usual metals and nonmetallic
materials. However, loss of corrosion resistance should be expected.
The high-melting-point solders are suitable for soldering mild steel,
stainless steel, nickel, copper, brass, zinc, and silver directly to
aluminum. Magnesium, titanium, zirconium, niobium, tantalum, molybdenum
and tungsten may be soldered if they are plated with a solderable metal
coating such as silver.
Soldering of Copper and Copper Alloys
Copper and copper alloys are among the most frequently soldered engineering
materials. The copper oxide is easily disrupted and displaced by most flux
types. The presence of alloying elements such as beryllium, chromium,
silicon, and aluminum modifies the nature of the oxide, making it more
tenacious. For these alloys, a special flux is recommended to remove the
oxide from the surface and enhance the solderability of these base metal
groups.
The most common solders for copper are tin- or lead-base solders. Tin can
react with copper and form two intermetallic phases, Cu6Sn5
and Cu3Sn, at the solid-liquid interface.
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