the Technology Interface / Spring97
Mark J. Hoban
MJHOBAN@CAEDM.BYU.EDU
Manufacturing Engineering Technology
Brigham Young University
and
Barry M. Lunt
LUNTB@aedm.byu.edu
Electronics Engineering Technology
Brigham Young University
I. Introduction |
There are two main classifications for the methods of soldering in use today: mechanical or non-electrical (using primarily acid flux), and electrical (using primarily rosin flux). This paper will address only the electrical applications of soldering.
While advances in transistors, resistors, capacitors, diodes, and especially integrated circuits have revolutionized the world, these devices are of very little value as individual components. For these devices to be of use, they must be electrically connected to each other and to mechanical devices. The majority of these electrical connections are made by soldering. Not only does solder make electrical connections, it is also used to provide a physical connection between the component and its supporting printed circuit board. The practice of soldering has been in existence for some time. While there is evidence to suggest that it was used even earlier, many different soldering techniques were widely used throughout the Greek and Roman Empires, as well as in Viking dominated Scandinavia. Archeologists have found jewelry, weapons, tools, and cutlery that have been very skillfully soldered [1]. Throughout the years solder has been used in various applications, however it was the invention of electronic devices in the latter part of this century that led to rapid advances in soldering technologies.
The advances in electronics would not have resulted in today's
mass production of electronic equipment without accompanying advances
in soldering technology, In order to understand how soldering
is used in the electronics industry, one must first become familiar
with the materials involved in soldering, how a solder bond actually
forms on the molecular level, and the process by which soldering
is accomplished.
Soldering is a method of making a permanent electrical and mechanical
connection between metals. Unlike glue, which forms a solely
physical adhesive bond, solder chemically reacts with other metals
to form a different alloy. While there are many different processes
utilized in soldering, virtually all of them involve four basic
elements: base metals, flux, solder, and heat.
A. Base Metals
A base metal is any metal that contacts the solder and forms
an intermediate alloy. When attaching electronic components to
a printed circuit board, the component's leads or pins and board's
metallic circuitry are the base metals that will contact the solder.
Many metals, such as copper, bronze, silver, brass, and some
steels, readily react with solder to form strong chemical and
physical bonds. Other metals, such as aluminum, high alloy steels,
cast iron, and titanium, range from very difficult to impossible
to solder. The fact that there are metals that do not react with
solder is important; these materials are used in the construction
of soldering machinery. These metals can also be used as temporary
covers for components that are not to be soldered. Also of importance
to the electronic industry is the fact that ceramics do not react
with solder. This allows a manufacturer to draw liquid solder
over a ceramic circuit board and not have any chemical reaction
between the solder and the board itself.
There is a direct relationship with the level of surface oxidation
on the base metal and how readily solder will react with it.
The more oxidation is present, the weaker the solder bond will
be. The fact that most metals oxidize at a very accelerated rate
when heated creates a particular problem, since the chemical reactions
associated with soldering require high temperatures. Flux is
the primary material used to overcome problems caused by oxidation.
B. Flux
Flux is often applied as a liquid to the surface of the base
metals prior to soldering. Though flux actually has a number
of purposes, the first and primary purpose of flux is to stop
the base metals from oxidizing while they are being heated to
the soldering temperature [2]. The flux covers the surface to be
soldered, shielding it from oxygen and thereby preventing oxidation
during heating. Most fluxes also have an acidic element that
is used to remove the oxidation already present on the base metal.
Using a strong acid, it would be possible to virtually completely
clean off the oxidation layer. However, the use of strong acids
presents a serious problem. The corrosiveness of acids desirable
to remove oxidation layers is not limited to the oxidation layer.
Very strong acids can be damaging to electronic components, and
even mild acids leave a residue that continues to corrode after
the soldering process is complete, leading to future failure.
There is a definite trade off between using a flux with a strong
acid that removes a lot of oxidation and is very corrosive, and
using a flux with a mild acid that is not as corrosive, but does
not do as good of a job removing the oxidation layer. In any
case, most fluxes in common use are corrosive enough that their
residue must be cleaned off after soldering.
When the liquid solder is applied, the flux must readily move
out of the way so the solder can come into direct contact with
the base metal. During this process some of the flux inevitably
combines with the solder. Flux designers typically take advantage
of this fact and design the flux to lower the surface tension
of the solder upon contact, thereby allowing a more efficient
wetting.
Fluxes can be divided into two basic parts, chemicals and solvents.
The chemical portion includes the active components, while the
solvent is primarily the carrying medium. The solvent determines
the cleaning method that must be employed to remove the flux residue.
While some fluxes can be removed with simple water treatments,
many require other cleaning agents such as organic solvents, alcohol,
terpenes, and chlorinated fluorocarbons. (Note: it is no
longer legal to use chlorinated fluorocarbons due to environmental
concerns).
C. Solder
There are many different metals and metal alloys that can be
used as solder. The decision to use a particular material is
largely based on its properties. Is it ductile or brittle? How
well does it conduct heat? Does it expand rapidly at high temperatures?
How much electrical resistance does it have? What is its tensile
strength? Is it toxic? What materials will it wet? And perhaps
most importantly, how much does it cost? Although it is by no
means the perfect alloy for soldering, the material most commonly
used in the electronics industry is a tin-lead alloy. Tin-lead
alloys have a relatively low melting point and can be produced
at a low cost in comparison with other alloys with similar properties.
Lead is a very cheap and abundant metal, so the cost of a tin-lead
solder is primarily controlled by the cost of the tin.
When an alloy is heated it typically goes thorough multiple phases.
It goes from a solid state to what is known as a pasty stage,
sort of halfway between a liquid and a solid, and then to a liquid
state. In soldering it is difficult to work with a substance
that goes through a pasty stage. Eutectic solder is often used
for this reason. A eutectic alloy is one that goes directly from
a solid state to a liquid state without a pasty stage. The eutectic
tin-lead alloy is made up of 63% tin and 37% lead. Eutectic tin-lead
solder can be applied as a liquid just above the melting point,
and then as it cools it will transform directly into a solid.
This makes it possible to form solid solder joints very quickly.
Sometimes a 60% tin and 40% lead alloy is used. This alloy exhibits
a nearly eutectic change from solid state to a liquid state and
can be produced at a lower cost [3].
It is very important to keep the solder free of impurities.
Not only is it important to produce a pure solder alloy, but it
is equally important to use a process that prevents metals from
the electronic components or circuitry from entering the solder
pot. The presence of even slight concentrations of other metals
in a tin-lead alloy results in drastic changes in surface tension.
Poor wetting of the base metals leading to a poor solder joint
is often the result. In addition, metal impurities often change
the melting temperature of the solder. Dust, oil, vapors, and
other non-metal impurities tend to weaken solder bonds.
Solder is typically transported and sold in solid form. Common
forms of solder include chips, bars, and wire (often with a core
of flux), each of which has advantages in different soldering
processes. A common process called reflow soldering calls for
a solder paste. Solder paste is a substance with a cream-like
consistency made up of solder, flux, and some carrying medium.
While putting the flux and solder together in the same mixture
has an obvious advantage when it comes to applying the substance
to the base metals, it also presents a problem. Highly corrosive
flux cannot be used in solder paste. The flux by nature is acidic
and corrosive to the metal solder, which means that solder paste
is inherently unstable. The shelf life of unused solder paste
ranges form about three weeks to three months. The time between
when the solder paste is applied to the base metal and when the
final heating is completed is limited to a maximum of a few hours [4].
When tin-lead solder is used, the tin reacts with the base metal
to form an intermetallic alloy. This intermediate layer of metal
ranges from a very high concentration of tin on the solder side
to a very high concentration of the base metal on the other side.
These intermetallic alloys are typically both brittle and weak.
This means that the solder joint is the "weak link",
and is susceptible to mechanical failure due to stress or vibration.
In order to reduce the likelihood of failure, the intermetallic
alloy is made to be as thin as possible. The intermetallic layer
grows at a negligible rate at room temperature, and its growth
rate accelerates as the temperature is increased. Therefore it
is advantageous to solder at the lowest possible temperature,
typically just above the solder melting point. A shorter time
of contact between the solder and base metal at an elevated temperature
results in a thinner intermetallic layer. This means that soldering
should be done as quickly as possible. Another way to get a
thinner intermetallic layer is to slow its growth. A lower tin
content in the solder results in slower growth [5]. This
creates another trade off; when the tin content is lowered from
its eutectic concentration, the advantages of having a eutectic
phase transition are lost.
Early attempts at soldering large numbers of electronic contacts
at the same time involved dipping the whole printed circuit board
into a pot of solder, or dragging the board across the upper surface
of solder in a pot [6]. These soldering techniques were vastly improved
when an Englishman by the name of Strauss came up with the idea
of wave soldering. He modeled wave soldering on a method used
to coat cookies with chocolate [7]. Liquid solder is pumped up through
a nozzle and out the end. Gravity then causes the solder to fall
back down, creating a parabola shaped "wave." The printed
circuit board, with the electronic components already inserted,
travels over the apex of the wave. As the wave of solder comes
in contact with the bottom side of the board, the already fluxed
metals chemically bond with the solder.
One of the advantages of wave soldering is that the process is
easily automated with the use of a conveyor system to move the
boards. Conveyor systems can move boards on flat pallets or by
the use of finger conveyors. In either case, the conveyor system
is constructed of a material that does not bond with solder.
The conveyor must move the board into the fluxing area, from the
fluxing area through a preheating process, and then over the solder
wave.
A. Applying Flux
Flux can be applied in a number of ways. Some early methods
involved dipping the board into a liquid flux or brushing the
flux onto the board with rotating bristles, similar to those used
in a car wash. The greatest problem with these methods was that
the amount of flux that is applied could not be controlled. Large
quantities of flux are very harsh on electronic components. In
addition, the brittle brushes had a tendency to dislodge the components
before they even arrived at the solder wave.
Another way of applying the flux is to have a flux wave. A flux
wave operates very similarly to a solder wave. The nozzle that
the flux is pumped through contains a mesh to help eliminate ripples
as the flux is propelled upwards. Flux is applied as the board
travels over the apex of the wave. An air knife is then used
to blow excess flux off as the board leaves the wave. While this
method is a very effective way of applying flux, it requires
continual cleaning and maintenance in order to keep operating.
Yet another way to apply flux is to spray it on. While spraying
has the inherent problem of depositing flux all around the target
area, the amount of flux applied can be very precisely controlled.
Flux can be sprayed by using a compressed gas process similar
that of industrial paint sprayers. However, most of the excess
flux is not recoverable when a compressed gas sprayer is used.
This can partly be overcome by using an "airless sprayer."
Though not widely used, this device compresses the liquid flux
and then sprays it out a nozzle without ever introducing a compressed
gas. Another type of flux sprayer utilizes a revolving drum and
compressed air. As the drum revolves, the underside comes into
contact with a tank of flux. The drum has a mesh like outer surface
which allows it to pick up small amounts of flux as it passes
through the liquid. Air then blows particles of the liquid flux
off the upper side of the drum onto the circuit board [8].
The most common way of applying flux in a wave soldering process
is foam fluxing. The flux is aerated with extremely fine bubbles
of compressed gasses, causing it to foam up. This foam is allowed
to climb up a chimney and spill out over the top, creating a foam
head over the chimney. The printed circuit board then moves across
the top of the foam head, picking us some of the flux. An air
knife is then used to help remove the excess foam as the board
exits [9].
B. Preheating
During preheating the printed circuit board, with its components,
is heated in order to raise the base metals to their soldering
temperature. If these metals are already at or near the required
temperature when the solder is introduced, the amount of time
that the solder, in its liquid form, must be in contact with the
board is minimized. This results in a much stronger intermetallic
layer. Heating the board up at a slow steady rate also works
to minimize thermal shock to the board and its components. Simply
subjecting a room temperature circuit board to a wave of liquid
solder without any preheating can cause extensive warping and
cracking.
There are three different ways to preheat that are in common
use. Electric heaters that work on the same principle of a household
toaster are used, as are convection and infrared heating processes.
Infrared heating has the advantage of being able to heat up the
assembly quickly, but the equipment employed is more expensive
than that used in other types of heaters. While there are some
inherent advantages and disadvantages of each method, each of
them accomplishes the desired result of preheating the board.
Therefore, the decision as to which one to employ is largely
based on cost, space, and personal preference.
The development and increased use of surface mount technology
has led to the use of other soldering methods. While surface
mount components can be wave soldered, they must first be attached
to the circuit board with some type of adhesive or cement in order
to keep them in place during soldering. This introduces another
step in the assembly process. Since the whole surface-mount component
is immersed in the wave, it must be constructed in such a manner
to withstand the high temperature of the liquid solder. There
is also a problem of gasses being trapped between the component
and the board [10]. Because of these difficulties with wave soldering,
re-flow soldering is the preferred method of soldering surface
mount components.
In a re-flow process, solder paste is put on the component sites
of the printed circuit board, and then the components are put
on the board on top of the solder paste. Often a separate adhesive
is used to hold the device in place until soldering takes place.
The board and attached components are then heated to activate
the flux, elevate the temperature of the base metals, and melt
(or "re-flow") the solder.
A. Applying Solder Paste
There is more than one method in use to apply the solder paste
to the circuit board. One way of doing this is to dispense a
slightly pressurized solder paste through the end of a tube.
This is very similar to the operation of a syringe. This type
of application has several advantages over other methods. First,
it employs a closed tank of solder paste that allows little opportunity
for solder contamination. The syringe can reach into odd shaped
places, which is of particular use in re-flow soldering surface-mount
components onto a board after through-hole components have already
been wave soldered on. Utilizing disposable syringes is also
a relatively inexpensive process. Notwithstanding these advantages
over other techniques, it is very difficult to control the precise
amount of solder paste that is applied to the board. Adequate
control can only be achieved with complicated, and expensive,
high tech control systems. Additionally, dispensing takes place
on only one pad at a time, making this method relatively slow.
Screen printing is a more common way of dispensing the solder
paste onto the circuit board. This is essentially the same process
that is used in applying paint to clothing and street signs.
A screen stencil is placed slightly above the board, and a squeegee
is manually drawn over the stencil, forcing solder paste through
the screen onto the board. The amount of solder deposited can
be quite accurately controlled with the density of the screen
and the shape of the squeegee. The alignment between the board
and screen is very important. If the either of them move even
slightly, or if the alignment is even slightly off, the solder
will not be deposited in the right place.
Another technique used is to dip dull pins into the solder paste,
and then dab the end of the pin onto the board. The amount of
solder applied is directly related to the size and shape of the
pin. An advantage to this method is that it is fast. A whole
array of pins can be lowered onto the board at the same time [11].
B. Heating
The assembly, the board and its components with solder sandwiched
between, is uniformly heated to a predetermined temperature.
It is then held at this temperature to give the solvents in the
solder paste time to evaporate and dry. This is the same temperature
that the flux becomes active, and begins to clean the base metals.
After a sufficient time at this temperature, the temperature
is raised above the melting point of the solder and held for a
time, usually between thirty and sixty seconds. The board is
then slowly cooled at a continuous rate. There is a trade off
here; cooling the board quickly results in a very strong solder
bond, but it also introduces stresses into the board.
Early heating devices used conduction heating similar to that
used in hand held soldering irons. As the component leads were
heated, energy was conducted to the solder. The heating device
never came in contact with the solder as in hand held soldering
irons. Today heating is accomplished with the use of either a
convection or radiation heating process or a combination of the
two.
A common type of convection heating is vapor phase heating.
A liquid is boiled, causing some of it to vaporize and saturate
the air within the vapor chamber. When the board is inserted
into the chamber, the vapor condenses onto it. Energy is transferred
from the vapor to the board during condensation, causing the board
to heat up. The temperature that the board is heated to is the
boiling point of the liquid. The fact that this is a very fast
heating method and that the increase in temperature is very uniform
makes it advantageous. Another advantage of this method is that
it is performed in a closed vapor chamber where no oxygen is present.
When oxygen is not present during the heating process, oxidation
does not occur. This allows the use of a very mild flux in the
solder paste. Sometimes the flux that is used is so mild that
cleaning is not required [12].
Radiation heating allows the assembly to be heated using electromagnetic
waves. Just as in a household microwave, these waves do not heat
up the air in-between their source and the board. Either infrared
or laser light is used. These processes allow precise control
of the amount and placement of the transferred energy. The drawback
of these methods is the slow rates at which they heat up the boards.
Scientific study has lead to an increased knowledge of materials
and their properties, and has made many advances in soldering
processes possible. This allows mass production of many different
electronic instruments. Yet soldering is still an evolving technology.
As advances in electronics continue to yield more efficient packages
and smaller components, soldering techniques must be developed
to meet the changing demand of the electronics industry.
[1] Rahn, Armin, The Basics of Soldering (New York: John Wiley & Sons, Inc., 1993), 2
[2] Rahn, Basics of Soldering, 20
[3] Rahn, Basics of Soldering, 52
[4] Manko, Howard H., Soldering Handbook for Printed Circuits and Surface Mounting Technology (New York: Van Nostrand Reinhold, 1995), 225
[5] Rahn, Basics of Soldering, 28-29
[6] Pecht, Michael G., Soldering Processes and Equipment (New York: John Whiley & Sons, Inc., 1993), 47
[7] Rahn, Basics of Soldering, 38
[8] Pecht, Soldering Processes, 52-55
[9] Pecht, Soldering Processes, 51
[10] Manko, Soldering Handbook, 194-197
[11] Pecht, Soldering Processes, 87-97
[12] Pecht, Soldering