Section 1:
Solder Reflow Basics
Introduction
Electronics assembly is the general term for
joining electrical components to printed
circuit boards (PCB). Surface mount
technology (SMT) is the electronics
assembly process where electrical
component leads are joined to the PCB via
individual pad connections located on the
board surface.
applied and the components are placed on
the board, the only way to create a
functioning circuit board is an effective
solder reflow process.
The basic SMT process consists of the
following steps:
1.
2.
3.
4.
Solder paste is applied to the PCB using
a screen printer. Stencils designed with
holes over individual pads control the
solder application to the board.
Electronic components are positioned on
the PCB using placement equipment
(pick-and-place machines, chip shooters,
etc.). Component leads are placed in
direct contact with the solder-pasted
pads.
The solder paste is heated until liquidus
(reflowed) then cooled until the solder
hardens and creates permanent
interconnection between the component
leads and the PCB. This process is
performed in a SOLDER REFLOW
OVEN.
After reflow, the assembled circuit
board can be cleaned, tested or
assembled into a final product.
High-volume SMT lines use automated
equipment to perform these steps. These
lines can typically produce a completed
circuit board in less than 20 seconds, with
placement machines that can position
upwards of 40,000 components per hour on
the PCB. However, once the solder paste is
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A high-volume SMT assembly line.
Reflow Know-How: Prevent and Resolve
SMT Process Problems
The reflow oven is the key to the soldering
process. Properly working ovens should be
“invisible” to line operators. Yet if process
problems occur during board production, the
reflow oven is often the first place
manufacturing engineers look to find
answers. Knowing the reflow process and
ways the oven effects soldering results is
critical to consistent SMT production.
The purpose of the Reflow Technology
Handbook is to explain the reflow process
in a way that answers basic soldering
questions. The handbook also helps qualify
the performance factors you should consider
to find a reflow oven to best suit your
process needs.
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Solder Alloys
The vast majority of electronic
interconnections are accomplished with tinlead alloy solders.
Tin
Pure tin is a soft, shiny metal that is most
often found in ore form rather than metallic
form. It is easily shaped and molded
without breaking.
The essence of the solder process is the
ability of molten tin to dissolve nearly any
other metal. Copper is one of those metals,
and copper is used extensively in the
manufacture of printed circuit boards. In the
next subsection we will discuss the role of
compounds formed when surface copper is
dissolved by tin.
without going through a “pasty” stage. This
is called the eutectic temperature.
When eutectic tin/lead is heated to a
temperature higher than 183°C it is said to
be in its liquidus stage. The relatively low
melting temperature has made eutectic
tin/lead the solder alloy of choice for printed
circuit board assembly because circuit
boards made of FR4 substrate or similar
materials can be safely processed at this
temperature.
In addition, the low melting temperature
means that the equipment that solders the
boards does not have to operate at higher
temperatures. Finally, a benefit most
managers and accountants appreciate is the
low cost of tin/lead compared with other
alloys.
Intermetallic Compounds
Lead
Lead, when exposed to air, has a dull gray
appearance. Like tin, lead is an easy metal
to work with because it is both soft and
flexible.
Lead does very little to aid the bonding of
metal to metal during soldering. However,
when lead is combined with tin at a ratio of
63% tin to 37% lead, the melting point of
the resultant alloy becomes lower than that
of either pure tin or pure lead. The
abbreviation for tin in chemistry is Sn and
for lead is Pb, so this alloy is usually written
Sn63Pb37.
This type of alloy is called a eutectic
composition. Pure tin melts at 232°C
(449°F) and pure lead melts at 327°C
(621°F). The melting temperature of
Sn63Pb37, however, is 183°C (361°F). At
this temperature, the alloy goes from a
completely solid to a completely liquid state
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As mentioned above, molten tin dissolves
most metals. During the solder process, the
primary metals in the solder form
compounds with the metals in component
leads and circuit board pads at boundaries
between the solder and the pad or lead.
(Figure 1-1).
Sn63\Pb37 solder
One to two
micron
intermetallic
layer: Cu3Sn
and Cu6Sn5
Copper
pad
Figure 1-1. The intermetallic layer at the
boundary of the solder and copper surfaces.
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These intermetallic compounds form a
boundary layer that is extremely strong
when it is no thicker than one to two
microns. At this thickness the natural
brittleness of the intermetallic layer is not a
problem.
As the intermetallic layer thickens, it
becomes more susceptible to cracking. The
cracking is caused by expansion and
contraction of the circuit board substrate as
it heats and cools during normal operation of
the electrical device in which it operates.
The thickness of the intermetallic layer is
controlled primarily by controlling the
liquidus time of the solder. Most solder
manufacturers recommend a liquidus time
between 45 and 60 seconds. (See Section 3
– Profiling for additional information.)
Other Solder Compounds
Some special applications require alloys
other than eutectic tin/lead solder. A
common application for other solder
compounds is double sided surface mount
assembly. To solder on both sides of the
board requires bottom side components be
soldered with an alloy that has a melting
temperature higher than eutectic tin/lead.
Then, the board is flipped over and top side
components are soldered with an alloy that
has a melting temperature lower than that of
eutectic tin/lead. The higher melting
temperature allows the solder used for the
bottom side components to stay solid during
the top side reflow, reducing the risk of
intermetallic growth.
Bismuth and Low Temperature Soldering
The addition of bismuth to tin/lead solder
reduces the melting temperature
significantly without adding serious
solderability problems. A typical
tin/bismuth/lead alloy is Sn43/Bi14/Pb43.
This alloy has a pasty stage from 143°CPage 3
163°C (289°F-325°F). The pasty stage
means the solder is no longer completely
solid at 143°C, but not yet completely liquid
until it reaches 163°C.
The lower melting temperature of this and
similar alloys means that the flux used in the
solder paste must also be different from that
used in eutectic tin/lead. This may change
the shape of the thermal profile. Consult
your solder paste supplier for the
recommended thermal profile for the
particular low temperature solder you use.
Another matter to consider when you use
low temperature solder is that the melting
temperature of the solder is affected by any
metals dissolved from leads or pads that
were plated to prevent oxidation. Usually
these contaminant metals raise the melting
point of the solder. If this becomes a
problem in your process, it might be
necessary to strip the plating just prior to the
low temperature soldering operation.
Silver
Some printed circuit boards have
components with silver plated leads. As
noted in the section on tin above, molten tin
dissolves most metals, including the silver in
silver plated leads.
One way to reduce the amount of plated
silver that dissolves in the molten tin/lead is
to add a small quantity of silver to the solder
itself. The alloy Sn62/Pb36/Ag02 (62% tin,
36% lead, 2% silver) is good for this
application.
Another popular silver alloy Sn96/Ag04
melts at 221°C (430°F) and is sometimes
used for double sided reflow.
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Lead Free Solder
Among the most discussed topics in the late1990s is the effort to remove lead from
electronic products. Concerns about ground
water lead contamination are forcing the
debate. Many governments are demanding
that Tin/Lead alloys be phased out and
replaced with lead-free solders. Solder paste
manufacturers are working to create new
alloys to meet what is likely to become a
new regulatory requirement for SMT
assemblers.
According to Phil Zarrow of ITM
Consulting, the most promising lead-free
alloys are in the Tin-Silver family with
variations incorporating relatively low
amounts of copper of bismuth. (Circuits
Assembly – August 1999). These alloys cost
about the same as Sn63/Pb37, but the 221°C
melting temperature means that electronic
components must be exposed to reflow
temperatures as high as 240°C for effective
soldering.
Another reflow alloy being considered is
Sn91.8/Ag3.4/Bi4.8. Adding bismuth
results in a melting range between 208°C
and 215°C (Phil Zarrow: Circuits Assembly
– August 1999).
Lead-free alloys are being evaluated to
verify their performance to standard TinLead compounds. Anyone involved in SMT
production should be prepared to convert
their lines to lead-free soldering by the mid2000s.
Flux
Oxygen in the air combines with metal on
circuit board pads and surface mount
component leads and forms oxides. This
oxidation blocks the molecular attraction
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between the solder and the metal at the
surface of the pad or lead.
Flux removes oxides from pad and lead
surfaces, as well as from the surfaces of the
solder particles themselves. Molten solder
then can flow evenly over the surface of
pads and leads and form intermetallic bonds
with them.
Surface Tension and Wetting
Surface tension is the attraction that
molecules at the surface of a drop of liquid
have for each other. If that attraction is
greater than the attraction for the material
which the liquid touches, the liquid will not
spread, but will remain in drop form.
Surface tension explains why a drop of
rainwater on a waxed car stays in drop form.
The wax surface is intended to repel water
and keep it from bonding to the car’s
surface.
The earth’s gravity works against surface
tension and tries to flatten the raindrop into
an oval. If the attraction of the molecules of
rainwater for each other is greater than the
pull of gravity and the attraction of the car’s
surface, the drop will remain somewhat
round and self contained. If gravity is
stronger and the wax is worn down, the drop
will disperse.
Wetting is the word used to describe the
extent that solder flows over the surfaces to
be bonded. Poor wetting is the result of
solder particles in the paste bonding to
themselves to form a sphere, so the edges of
a poorly wetted solder joint are rounded
rather than flat.
Wetting is measured by the angle made
where the edge of a layer of solder paste
meets the pad on a circuit board. Poor
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wetting leaves a thick edge and a large angle
(Figure 1-2).
Strong
Attraction
Weakened
Attraction
Poor wetting:
>90° angle to
pad
Acceptable
wetting: <90°
angle to pad
Ideal wetting:
<45° angle to
pad
Figure 1-2. The effect of surface tension on
wetting.
What Flux Cannot Do
Even the strongest flux does not remove
thick layers of oxidation. At most, flux
removes the oxide a few molecules thick
from a metallic surface.
Flux Chemistry
The basis of flux is usually a solid that has
been dissolved by a solvent. The most
commonly used solid for many years in PCB
assembly was rosin. Rosin is derived from
pine trees, and is an inert substance, which
does not conduct electricity at room
temperature. Rosin becomes liquid between
125°C and 130°C (257°F -266°F).
A substance known as water white flux
(abbreviated w/w) is pure rosin dissolved in
isopropyl alcohol. Water white is mild and
able to remove only the thinnest layers of
oxide during soldering.
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To make rosin based flux more aggressive
when removing oxides, activator substances
are added. Many different substances are
used as activators, but it is not as important
to know exactly what the activator is, as to
know:
•
at what temperature the activator
begins to work, because this affects
the thermal profile in the reflow
oven, and
•
how corrosive the activator is, which
determines whether the board needs
to be cleaned and what cleaning
solution is necessary. Your solder
paste supplier can give you this
information.
RMA and RA are the most common
activator fluxes. RMA is mildly activated
rosin based flux, and is much stronger in
removing oxidation than water white. RA is
activated rosin based flux, and is more
aggressive than RMA.
Flux Residue and Cleaning
Water white and RMA fluxes leave a
residue after soldering that is not corrosive
or electrically conductive at levels high
enough to affect the function or life of a
circuit board. Residue from these fluxes has
often been removed by washing, however,
for aesthetic reasons, or because it is nonconductive it interferes with bed of nails
testing devices.
The residue from RA fluxes can be more
corrosive, and usually needs to be cleaned
from the board. RA fluxes require two types
of cleaning:
1. A nonpolar solvent to remove the
rosin, as well as any oils or waxes
that may have contaminated the
board during handling. Nonpolar
solvents such as chlorofluorocarbons
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(CFCs) were used for years, but
environmental concerns eliminated
them from consideration. Semiaqueous cleaning systems use a
terpene as the nonpolar solvent. The
terpene is then removed by a
surfactant, which makes the terpene
soluble in water.
2. A polar solvent, which is usually
very pure water, removes the
activator, as well as salts and other
water-soluble contaminants that may
have gotten onto the circuit board
during handling.
A substance called a saponifier can be used
to clean PCBs also. One way to understand
and remember what a saponifier does is to
know that the root word is sapo, which is
Latin for soap. A saponifier acts like soap in
reacting with substances that are not water
soluble (oils, for example) so that they
become water soluble and can be washed
off.
Water Soluble Flux
Water soluble fluxes usually are based on
the oxidation removing qualities of an
organic acid such as citric or glutamic acid.
Organic acid fluxes are more aggressive
than RMA, so they provide good cleaning,
but they are usually corrosive as well.
Because of the corrosiveness of these fluxes,
a thorough water wash following solder
reflow is necessary. Boards stored without
washing for more than an hour after exiting
the reflow oven may have serious oxidation
buildup.
No Clean Fluxes
Millions of printed circuit boards are
manufactured that are never cleaned
following solder reflow. This is especially
true in consumer product electronic
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assemblies, primarily to save money. Inert,
non-corrosive flux residues are left on the
boards and do not cause problems.
By virtue of the fact that the boards are not
cleaned, it can be said the fluxes used are no
clean fluxes. However, in recent years the
term no clean has come to refer to a type of
flux specifically designed so flux residues
are minimized. No clean fluxes generally
have the same aggressiveness as RMA
fluxes.
Lower residue is achieved in no clean fluxes
by using a lower solids content than in other
fluxes. Solids content refers to the ratio of
solvent thinner to solid component in the
flux. A typical no clean flux now has less
than 15% solids, compared with 50% in
other types of flux.
Since the solids in a flux are the basis for its
effectiveness, reducing the solids content
tightens the process window for no clean
fluxes. Flux density control thus is more
important than with other fluxes.
No clean fluxes can be either rosin or resin
based. The most noticeable difference is
that rosin based no clean flux produces a
thin layer of sticky residue that can be
removed if desired. Resin based no clean
flux produces a thin residue that is hard and
not sticky, but cannot be removed by water
or other solvents. This can occasionally be a
consideration if rework is necessary.
Nitrogen gas inert atmosphere solder reflow
is often helpful when no clean flux is being
used, especially when it is used with fine
pitch surface mount devices. Nitrogen gas
eliminates oxygen from the heat chamber of
the reflow oven so that oxidation is
minimized. (See Section 4 – Inert
Atmosphere Soldering for additional
information.)
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