Soldering Recommendations for Surface Mount and Multilayer Metal Oxide Varistors Application note

Soldering Recommendations for Surface Mount and Multilayer Metal Oxide Varistors Application note
Soldering Recommendations for Surface Mount
and Multilayer Metal Oxide Varistors
A pplication Note
J u ly 1 9 9 9
AN9211.2
Introduction
In recent years, electronic systems have migrated towards
the manufacture of increased density circuits, with the same
capability obtainable in a smaller package or increased
capability in the same package. The accommodation of
these higher density systems has been achieved by the use
of surface mount technology (SMT). Surface mount
technology has the advantages of lower costs, increased
reliability and the reduction in the size and weight of
components used. With these advantages, surface mount
technology is fast becoming the norm in circuit design.
The increased circuit densities of modern electronic
systems are much more vulnerable to damage from
transient overvoltages than were the earlier circuits, which
used relays and vacuum tubes. Thus, the progress in the
development of faster and denser integrated circuits has
been accompanied by an increase in system vulnerability.
Transient protection of these sensitive circuits is highly
desirable to assure system survival. Surface mount
technology demands a reliable transient voltage protection
technology, packaged compatibly with other forms of
components used in surface mount technology.
Harris Suppression Products has led the field in the
introduction of surface mount transient voltage suppressors.
These devices encompass voltages from 3.5VDC to 275VAC
and have a wide variety of applications. Their size, weight
and inherent protection capability make them ideal for use
on surface mount printed circuit boards.
There are two technologies of Littelfuse surface mount surge
suppressors. The CH Series metal oxide varistors which
encompass voltages from 14VDC to 275VAC and the ML,
MLE, MLN and AUML Series Suppressors which cover a
voltage range from 3.5VDC to 120VDC.
between the electrical terminals. The voltage drop across a
single grain in nearly constant and is independent of grain size.
I
V
FIGURE 1. V-I CHARACTERISTICS OF A MOV
The CH series of surface mount metal oxide varistors are of
a monolayer construction in a 5mm by 8mm package size.
They are fully symmetrical and are passivated both top and
bottom (Figure 2). The main advantage of this technology is
its high operating voltage capability (68VDC to 275VAC). The
CH Series of metal oxide varistors are supplied in both 7”
and 13” tape and reels.
ELECTRODE
PASSIVATION
ZINC OXIDE
MATERIAL
END
TERMINATION
FIGURE 2. CROSS-SECTION OF THE “CH” SERIES OF
METAL OXIDE VARISTORS
Metal Oxide Varistors
Multilayer Transient Voltage Suppressors
A metal oxide varistor (MOV) is a nonlinear device which has
the property of maintaining are relatively small voltage change
across its terminals while a disproportionately large surge
current flows through it (Figure 1). When the MOV is connected
in parallel across a line its nonlinear action serves to divert the
current of the surge and hold the voltage to a value that protects
the equipment connected to the line. Since the voltage across
the MOV is held at some level higher than the normal line
voltage while surge current flows, there is energy deposited in
the varistor during its surge diversion function.
All Littelfuse multilayers are constructed by forming a
combination of alternating electrode plates and
semiconducting ceramic layers into a block. This technology,
represents a recent breakthrough in its application to
transient voltage suppression. Each alternate layer of
electrode is connected to opposite end terminations (Figure
3). The interdigitated block formation greatly enhances the
available cross-sectional area for active conduction of
transients. This paralleled arrangement of the inner
electrode layers represents significantly more active surface
area than the small outline of the package may suggest. The
increased active surface area results in proportionally higher
peak energy capability.
The basic conduction mechanism of a MOV results from
semiconductor junctions (P-N junctions) at the boundaries of
the zinc oxide grains. A MOV is a multi-junction device with
millions of grains acting as a series parallel combination
10-13
1-800-999-9445 or 1-847-824-1188 | Copyright
© Littelfuse, Inc. 1999
Application Note 9211
INNER
ELECTRODES
SEMICONDUCTING
CERAMIC
END
TERMINATION
FIGURE 3. INTERNAL CONSTRUCTION OF THE HARRIS
MULTILAYER TRANSIENT VOLTAGE
SUPPRESSOR
A further advantage of this type of construction is that the
breakdown voltage of the device is dependent on the
thickness between the electrode layers (dielectric thickness)
and not the overall thickness of the device.
These suppressors are often much smaller in size than the
components they are designed to protect. The present size
offerings are 0603, 0805, 1206, 1210, 1812 and 2220, with
voltage ranges form 3.5VDC to 120VDC. Its robust
construction makes it ideally suitable to endure the thermal
stresses involved in the soldering, assembling and
manufacturing steps involved in surface mount technology. As
the device is inherently passivated by the fired ceramic
material, it will not support combustion and is thus immune to
any risk of flammability which may be present in the plastic or
epoxy molded parts used in industry standard packages.
Substrates
There are a wide choice of substrate materials available for use
as printed circuit boards in a surface mount application. The
main factors which determine the choice of material to use are:
1. Electrical Performance
2. Size and Weight Limitations
3. Thermal Characteristics
4. Mechanical Characteristics
5. Cost
When choosing a substrate material, the coefficient of
thermal expansion of a Littelfuse surface mountable
suppressor of 6ppm/oC is an important consideration. Non-
organic materials (ceramic based substrates), like aluminum
or beryllia, which have coefficients of thermal expansion of
5-7ppmoC, are a good match for the CH and ML series
devices. Table 1 outlines some of the other materials used,
and also their more important properties pertinent to surface
mounting.
While the choice of substrate material should take note of
the coefficient of expansion of the devices. This may not be
the determining factor in whether a device can be used or
not. Obviously the environment of the finished circuit board
will determine what level of temperature cycling will occur. It
is this which will dictate the criticality of the match between
device and PCB. Currently for most applications, both the
CH and ML series use FR4 boards without issue.
Fluxes
Fluxes are used for the chemical cleaning of the substrate
surface. They will completely remove any surface oxides, and
will prevent re-oxidation. They contain active ingredients such
as solvents for removing soils and greases. Nonactivated
fluxes (“R” type) are relatively effective in reducing oxides of
copper or palladium/silver metallizations and are
recommended for use with the Littelfuse surface mount range.
Mildly activated fluxes (“RMA” type) have natural and
synthetic resins, which reduce oxides to metal or soluble
salts. These “RMA” fluxes are generally not conductive nor
corrosive at room temperature and are the most commonly
used in the mounting of electronic components.
The “RA” type (fully activated) fluxes are corrosive, difficult to
remove, and can lead to circuit failures and other problems.
Other non-resin fluxes depend on organic acids to reduce
oxides. They are also corrosive after soldering and also can
damage sensitive components. Water soluble types in
particular must be thoroughly cleaned from the assembly.
Environmental concerns, and the associated legislation, has
led to a growing interest in fluxes with residues that can be
removed with water or water and detergents (semi-aqueous
cleaning). Many RMA fluxes can be converted to water
soluble forms by adding saponifiers. There are detergents and
semi-aqueous cleaning apparatus available that effectively
remove most RMA type fluxes. Semi-aqueous cleaning also
tends to be less expensive than solvent cleaning in operations
where large amounts of cleaning are needed.
TABLE 1. SUBSTRATE MATERIAL PROPERTIES
MATERIAL PROPERTIES
GLASS TRANSITION
TEMPERATURE (oC)
XY COEFFICIENT OF THERMAL
EXPANSION (ppm/oC)
THERMAL CONDUCTIVITY
(W/MoC)
Epoxy Fiberglass-FR4
125
14-18
0.16
Polyamide Fiberglass
250
12-16
0.35
Epoxy Aramid Fiber
125
6-8
0.12
Fiber/Teflon Laminates
75
20
0.26
Not Available
5-7
21.0
SUBSTRATE STRUCTURE
Aluminium-Beryillia (Ceramic)
10-14
Application Note 9211
Land Pad Patterns
Land pad size and patterns are one of the most important
aspects of surface mounting. They influence thermal, humidity,
power and vibration cycling test results. Minimal changes (even
as small as 0.005 inches) in the land pad pattern have proven
to make substantial differences in reliability.
This design/reliability relationship has been shown to exist
for all types of designs such as in J lead, quadpacks, chip
resistors, capacitors and small outline integrated circuit
(SOIC) packages. Recommended land pad dimensions are
provided for some surface mounted devices along with
formulae which can be applied to different size varistors.
Figure 4 gives recommended land patterns for the direct
mount ML and CH series devices.
either platinum, palladium or a mixture of both, which have
the benefit of significantly reducing any leaching effects
during soldering. To further ensure that there is no leeching
of the silver electrode on the varistor, solders with at least
2% silver content are recommended (62 Sn / 36 Pb / 2 Ag).
Examples of silver bearing solders and their associated
melting temperatures are as follows:
260
TEMPERATURE (oC)
For the Harris Suppression Products range of surface mount
varistors, nonactivated “R” type fluxes such as Alpha 100 or
equivalent are recommended.
250
240
230
220
210
200
5
10
TIME (SECONDS)
M
L - (M x 2)
W
W + 0.010
OR 0.020W
L
FIGURE 5. RECOMMENDED MAXIMUM TIME AND SOLDER
TEMPERATURE RELATIONSHIP OF LITTELFUSE
MOVs
TABLE 3. SILVER BEARING SOLDERS (ALPHA METALS)
T+M
MELTING TEMPERATURE
T
FIGURE 4. FORMULA FOR SURFACE MOUNTABLE
VARISTOR FOOTPRINTS
ALLOY
oF
oC
62 Sn / 36 Pb / 2 Ag
355
179
96.5 Sn / 3.5 Ag
430
221
95 Sn / 5 Ag
430-473
221-245
20 Sn / 88 Pb / 2 Ag
514-576
268-302
536
280
TABLE 2. RECOMMENDED MOUNTING PAD OUTLINE
DIMENSION
SUPPRESSOR
FAMILY
T+M
L-(M X 2)
0.020W
(W + 0.010)
5 X 8 CH Series
2.21
(0.087)
5.79
(0.228)
5.50
(0.216)
0603 ML/MLE Series
1.12
(0.044)
0.56
(0.02)
1.62
(0.064)
0805 ML/MLE Series
1.48
(0.058)
0.69
(0.027)
2.13
(0.084)
1206 ML/MLE Series
1.65
(0.065)
1.85
(0.073)
2.62
(0.103)
1210 ML/AUML Series
1.85
(0.073)
1.85
(0.073)
3.73
(0.147)
1812 AUML Series
1.85
(0.073)
3.20
(0.126)
4.36
(0.172)
2220 AUML Series
1.84
(0.073)
4.29
(0.169)
6.19
(0.240)
Solder Materials and Soldering
Temperatures
No varistor should be held longer than necessary at an
elevated temperature. The termination materials used in
both the CH and ML series devices are enhanced silver
based materials. These materials are sensitive to exposure
time and peak temperature conditions during the soldering
process (Figure 5). The enhanced silver formulation contains
10-15
5 Sn / 92.5 Pb / 2.5 Ag
Soldering Methods
There are a number of different soldering techniques used in
the surface mount process. The most common soldering
processes are infrared reflow, vapor phase reflow and wave
soldering.
With the Littelfuse surface mount range, the solder paste
recommended is a 62/36/2 silver solder. While this
configuration is best, other silver solder pastes can also be
used. In all soldering applications, the time at peak temperature
should be kept to a minimum. Any temperature steps employed
in the solder process must, in broad terms, not exceed 70oC to
80oC. In the preheat stage of the reflow process, care should
be taken to ensure that the chip is not subjected to a thermal
gradient of greater than 4 degrees per second; the ideal
gradient being 2 degrees per second. For optimum soldering,
preheating to within 100 degrees of the peak soldering
temperature is recommended; with a short dwell at the
preheat temperature to help minimize the possibility of
thermal shock. The dwell time at this preheat temperature
should be for a time greater than 10T2 seconds, where T is
the chip thickness in millimeters. Once the soldering process
has been completed, it is still necessary to protect against
further effects of thermal shocks. One possible cause of
Application Note 9211
thermal shock at the post solder stage is when the hot printed
circuit boards are removed from the solder and immediately
subjected to cleaning solvents at room temperature. To avoid
this thermal shock affect, the boards must first be allowed to
cool to less than 50oC prior to cleaning.
homogenous and to level, but not enough to cause leaching
of solder, metallization or flux charring.
Two different resistance to solder heat tests are routinely
performed by Harris Suppression Products to simulate any
possible effects that the high temperatures of the solder
processes may have on the surface mount chip. These tests
consist of the complete immersion of the chip in to a solder
bath at 260oC for 5 seconds and also in to a solder bath at
220oC for 10 seconds. These soldering conditions were
chosen to replicate the peak temperatures expected in a
typical wave soldering operation and a typical reflow
operation.
A fast heating rate may not always be advantageous. The parts
or components may act as heat sinks, decreasing the rate of
rise. If the coefficients of expansion of the substrate and
components are too diverse or if the application of heat is
uneven, fast breaking or cooling rates may result in poor solder
joints or board strengths and loss of electrical conductivity. As
stated previously, thermal shock can also damage components.
Very rapid heating may evaporate low boiling point organic
solvents in the flux so quickly that it causes solder spattering or
displacement of devices. If this occurs, removal of these
solvents before reflow may be required. A slower heating rate
can have similar beneficial effects.
Reflow Soldering
InfraRed (IR) Reflow
There are two major reflow soldering techniques used in
SMT today:
InfraRed (IR) reflow is the method used for the reflowing of
solder paste by the medium of a focused or unfocused
infrared light. Its primary advantage is its ability to heat very
localized areas.
1. InfraRed (IR) Reflow
2. Vapor Phase Reflow
The only difference between these two methods is the process
of applying heat to melt the solder. In each of these methods
precise amounts of solder paste are applied to the circuit board
at points where the component terminals will be located.
Screen or stencil printing, allowing simultaneous application of
paste on all required points, is the most commonly used
method for applying solder for a reflow process. Components
are then placed in the solder paste. The solder pastes are a
viscous mixture of spherical solder powder, thixotropic vehicle,
flux and in some cases, flux activators.
During the reflow process, the completed assembly is heated to
cause the flux to activate, then heated further, causing the
solder to melt and bond the components to the board. As reflow
occurs, components whose terminations displace more weight,
in solder, than the components weight will float in the molten
solder. Surface tension forces work toward establishing the
smallest possible surface area for the molten solder. Solder
surface area is minimized when the component termination is
in the center of the land pad and the solder forms an even fillet
up the end termination. Provided the boards pads are properly
designed and good wetting occurs, solder surface tension
works to center component terminations on the boards
connection pads. This centering action is directly proportional to
the solder surface tension. Therefore, it is often advantageous
to engineer reflow processes to achieve the highest possible
solder surface tension, in direct contrast to the desire of
minimizing surface tension in wave soldering.
In designing a reflow temperature profile, it is important that
the temperature be raised at least 20oC above the melting or
liquidus temperature to ensure complete solder melting, flux
activation, joint formation and the avoidance of cold melts.
The time the parts are held above the melting point must
belong enough to alloy the alloy to wet, to become
10-16
The IR process consists of a conveyor belt passing through
a tunnel, with the substrate to be soldered sitting on the belt.
The tunnel consists of three main zones; a non-focused
preheat, a focused reflow area and a cooling area. The
unfocused infrared areas generally use two or more emitter
zones, thereby providing a wide range of heating profiles for
solder reflow. As the assembly passes through the oven on
the belt, the time/temperature profile is controlled by the
speed of the belt, the energy levels of the infrared sources,
the distance of the substrate from the emitters and the
absorptive qualities of the components on the assembly.
The peak temperature of the infrared soldering operation
should not exceed 220oC. The rate of temperature rise from the
ambient condition to the peak temperature must be carefully
controlled. It is recommended that no individual temperature
step is greater than 80oC. A preheat dwell at approximately
150oC for 60 seconds will help to alleviate potential stresses
resulting from sudden temperature changes. The temperature
ramp up rate from the ambient condition to the peak
temperature should not exceed 4oC per second; the ideal
gradient being 2oC per second. The dwell time that the chip
encounters at the peak temperature should not exceed 10
seconds. Any longer exposure to the peak temperature may
result in deterioration of the device protection properties.
Cooling of the substrate assembly after solder reflow is
complete should be by natural cooling and not by forced air.
The advantages of IR Reflow are its ease of setup and that
double sided substrates can easily be assembled. Its biggest
disadvantage is that temperature control is indirect and is
dependent on the IR absorption characteristics of the
component and substrate materials.
On emergence from the solder chamber, cooling to ambient
should be allowed to occur naturally. Natural cooling allows a
gradual relaxation of thermal mismatch stresses in the
Application Note 9211
solder joints. Forced air cooling should be avoided as it can
induce thermal breakage.
dwell time is a function of the circuit board mass but should
be kept to a minimum.
The recommended temperature profile for the IR reflow
soldering process is as Table 4 and Figure 6.
On emergence from the solder system, cooling to ambient
should be allowed to occur naturally. Natural cooling allows a
gradual relaxation of thermal mismatch stresses in the
solder joints. Forced air cooling should be avoided as it can
induce thermal breakage.
TABLE 4. RECOMMENDED TEMPERATURE PROFILE FOR IR
REFLOW SOLDER PROCESS
INFRARED (IR) REFLOW
TEMPERATURE (oC)
TIME (SECONDS)
25-60
60
60-120
60
120-155
155-155
155-220
220-220
220-50
The recommended temperature profile for the vapor phase
soldering process is as Table 5 and Figure 7.
TABLE 5. RECOMMENDED TEMPERATURE PROFILE FOR
VAPOR PHASE REFLOW PROCESS
30
60
60
10
60
VAPOR PHASE REFLOW
TEMPERATURE (oC)
TIME (SECONDS)
25-90
8
90-150
13
220
200
3
10
222-80
80-25
7
10
160
140
225
120
225
100
200
TEMPERATURE (oC)
TEMPERATURE (oC)
180
150-222
222-222
80
60
40
20
0
1
2
3
4
TIME (MINUTES)
5
6
175
150
125
100
75
50
FIGURE 6. TYPICAL TEMPERATURE PROFILE
25
5
Vapor Phase Reflow
Vapor phase reflow soldering involves exposing the assembly
and joints to be soldered to a vapor atmosphere of an inert
heated solvent. The solvent is vaporized by heating coils or a
molten alloy, in the sump or bath. Heat is released and
transferred to the assembly where the vapor comes in contact
with the colder parts of the substrate and then condenses. In
this process all cold areas are heated evenly and no areas can
be heated higher than the boiling point of the solvent, thus
preventing charring of the flux. This method gives a very rapid
and even heating affect. Further advantages of vapor phase
soldering is the excellent control of temperature and that the
soldering operation is performed in an inert atmosphere.
The liquids used in this process are relatively expensive and
so, to overcome this a secondary less expensive solvent is
often used. This solvent has a boiling temperature below
50oC. Assemblies are passed through the secondary vapor
and into the primary vapor. The rate of flow through the
vapors is determined by the mass of the substrate. As in the
case of all soldering operations, the time the components sit
at the peak temperature should be kept to a minimum. The
10-17
10
15
20
25
30
TIME (SECONDS)
35
40
45
50
FIGURE 7. TYPICAL TEMPERATURE PROFILE
Wave Solder
This technique, while primarily used for soldering thru-hole
or leaded devices inserted into printed circuit boards, has
also been successfully adapted to accommodate a hybrid
technology where leaded, inserted components and
adhesive bonded surface mount components populate the
same circuit board.
The components to be soldered are first bonded to the
substrate by means of a temporary adhesive. The board is
then fluxed, preheated and dipped or dragged through two
waves of solder. The preheating stage serves many
functions. It evaporates most of the flux solvent, increases
the activity of the flux and accelerates the solder wetting. It
also reduces the magnitude of the temperature change
experienced by the substrate and components.
The first wave in the solder process is a high velocity
turbulent wave that deposits large quantities of solder on all
Application Note 9211
wettable surfaces it contacts. This turbulent wave is aimed at
solving one of the two problems inherent in wave soldering
surface mount components, a defect called voiding (i.e.
skipped areas). One disadvantage of the high velocity
turbulent wave is that it gives rise to a second defect known
as bridging, where the excess solder thrown at the board by
the turbulent wave spans between adjacent pads or circuit
elements thus creating unwanted interconnects and shorts.
The second, smooth wave accomplishes a clean up
operation, melting and removing any bridges created by the
turbulent wave. The smooth wave also subjects all previous
soldered and wetted surfaces to a sufficiently high
temperature to ensure good solder bonding to the circuit and
component metallizations.
In wave soldering, it is important that the solder have low
surface tension to improve its surface wetting characteristics.
Therefore, the molten solder bath is maintained at
temperatures above its liquid point.
On emergence from the solder wave, cooling to ambient
should be allowed to occur naturally. Natural cooling allows a
gradual relaxation of thermal mismatch stresses in the
solder joints. Forced air cooling should be avoided as it can
induce thermal breakage.
The recommended temperature profile for the wave
soldering process is as Table 6:
TABLE 6. RECOMMENDED TEMPERATURE PROFILE FOR
WAVE SOLDER PROCESS
WAVE SOLDER
TEMPERATURE (oC)
25-125
125-180
TIME (SECONDS)
60
60
180-260
260-260
60
5
260-180
180-80
80-25
60
60
60
Termination Options
Littelfuse offers two types of electrode termination finish for
the Multilayer product series:
1. Silver/Platinum (standard)
2. Silver/Palladium (optional)
Cleaning Methods and Cleaning Fluids
The objective of the cleaning process is to remove any
contamination from the board, which may affect the
chemical, physical or electrical performance of the circuit in
its working environment.
There are a wide variety of cleaning processes which can be
used, including aqueous based, solvent based or a mixture
of both, tailored to meet specific applications. After the
soldering of surface mount components there is less residue
10-18
to remove than in conventional through hole soldering. The
cleaning process selected must be capable of removing any
contaminants from beneath the surface mount assemblies.
Optimum cleaning is achieved by avoiding undue delays
between the cleaning and soldering operations; by a
minimum substrate to component clearance of 0.15mm and
by avoiding the high temperatures at which oxidation occurs.
Littelfuse recommends 1, 1, 1 trichloroethane solvent in an
ultrasonic bath, with a cleaning time of between two and five
minutes. Other solvents which may be better suited to a
particular application and can also be used may include one
or more of the following:
TABLE 7. CLEANING FLUIDS
Water
Acetone
Isopropyl Alcohol
Fluorocarbon 113
Fluorocarbon 113 Alcohol Blend
N-Butyl
1, 1, 1 Trichloroethane Alcohol Blend
Trichloroethane
Toluene
Methane
Solder Defects
Non-Wetting:
This defect is caused by the formation of oxides on the
termination of the components. The end termination has been
exposed to the molten solder material but the solder has not
adhered to the surface; base metal remains exposed. The
accepted criterion is that no more than 5% of the terminated
area should remain exposed after an immersion of 5 seconds in
a static solder bath at 220oC, using a nonactive flux.
Leaching:
This is the dissolving of the chip termination into the molten
solder. It commences at the chip corners, where metal
coverage is at a minimum. The result of leaching is a weaker
solder joint. The termination on the Littelfuse surface mount
suppressors consist of a precious metal alloy which
increases the leach resistance capability of the component.
Leach resistance defined as the immersion time at which a
specified proportion of the termination material is visibly lost,
under a given set of soldering conditions.
De-Wetting:
This condition results when the molten solder has coated the
termination and then receded, leaving irregularly shaped
mounds of solder separated by areas covered with a thin
solder film. The base metal is not exposed.
References
For Littelfuse documents available on the web, see
http://www.littelfuse.com/
[1] “Transient Voltage Suppression Devices”, Harris
Suppression Products DB450.
[2] CANE SMT 2588, Syfer Technology Limited, UK.
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