Surface Mount Soldering Techniques Thermal Shock in Multilayer Ceramic Capacitors

Surface Mount Soldering Techniques Thermal Shock in Multilayer Ceramic Capacitors
TECHNICAL
INFORMATION
SURFACE MOUNT
SOLDERING TECHNIQUES
AND THERMAL SHOCK IN
MULTILAYER CERAMIC CAPACITORS
John Maxwell
AVX Corporation
Abstract:
All components used in surface mount
assemblies have temperature processing
limitations that must be adhered to for
maximum reliability. This paper discusses
multilayer ceramic capacitors in detail and
soldering process considerations that are
valid for all SMT components.
SURFACE MOUNT
SOLDERING TECHNIQUES
AND THERMAL SHOCK IN
MULTILAYER CERAMIC CAPACITORS
John Maxwell
AVX Corporation
Surface mount manufacturing promises tremendous
advantages in product density, automated assembly and
improved electrical performance but the components are
directly exposed to soldering temperatures where the
thru hole counterparts are not. This direct exposure
causes reliability problems when the rate of rise in
temperature is too rapid due to the inability of
mechanical stress to be spread throughout the
component. This is compounded by differences in
coefficients in thermal expansion (CTE) and thermal
conductivities (T) of materials used in the construction
of electronic parts. Multilayer ceramic capacitors (MLC)
which are one of the most commonly used SMD, are
among those components sensitive to thermal shock and
will be discussed in detail. Figure 1 shows a cross
section of an MLC with typical CTE and T listed for
the termination, ceramic body and electrodes.
Figure 2. Forces Exerted by the Termination
on the Ceramic Body
Figure 3. Temperature Forces that Stress
an MLC’s Structure
Figure 1. MLC Structure with CTE and T Listed
The termination and electrodes heat up more quickly
than the ceramic body, exerting forces on the ceramic
which cause cracking if the temperature rate of rise is
too rapid. This is due to a large difference in CTEs and
T between the ceramic and termination. As the
termination heats up, it is an expanding rectangular
annulus pulling on the ceramic while the expanding
electrodes act as wedges forcing the ceramic apart at
the electrode/termination interface as shown in Figures
2 and 3.
Different soldering techniques impact temperature
rates of rise by the heat transfer mechanisms that are
used. Wave soldering uses liquid metal which has the
highest heat transfer rate and is the hardest soldering
method to use without shocking any SM component.
Vapor phase reflow (VPR) soldering uses latent heat of
vaporization of the condensing vapor as the heat
transfer method. Thermal shock of components is not
obvious when VPR soldering is used but it can still be
present. Hot belt reflow and infrared (IR) reflow have
lower heat transfer rates because conduction, convection
and radiation are the heat transfer methods used.
Wave soldering has the highest heat transfer rates
and puts the greatest thermal shock stress on
components. When extreme thermal shock is present, it
is very obvious with visible cracks on the surface and
sides of the MLC. These cracks start at or near the
termination and ceramic interface extending from the
termination down along the MLC edge. This surface
crack can become elliptical or circular shaped in the
larger MLC sizes (1812 or larger). It is even seen in the
1210 size (120 mils x 100 mils) in very severe cases.
Thermal shock has two manifestations, obvious
visible cracks and the more insidious, invisible micro
crack. Micro cracks are formed at or just under the
termination and ceramic interface along isothermal lines
(constant temperature lines) at slower temperature
rates of rise. Maximum shear occurs along these lines at
the termination and ceramic interface during the fastest
temperature increases of soldering.
Figure 4. Stress Risers Caused by the Termination
on the MLC Body
Figure 4 shows the stress risers generated by the
termination on the ceramic body. Once a crack is
initiated, it will propagate away from these stress risers
giving rise to classical thermal shock cracks. Figures 5
and 6 show this physical manifestation of extreme
thermal shock. Full elliptical surface cracks due to
thermal shock are sometimes mistaken as damage
caused by the pick and place machine bit smashing the
MLC onto the substrate. If an MLC has been smashed
or crushed, the surface crack will not be smooth but
will have a rough or powdered edge. In fact it is quite
difficult to cause surface cracks with pick and place
machines, the parts being generally crushed or broken
into pieces.
Figure 7. Isothermal Line Shortly After Exposure
to Solder Temperatures
The micro crack will propagate along isothermal lines
where the capacitor structure received maximum stress.
This propagation takes a long time and is dependent on
the physical size of the capacitor, CTE difference
between the substrate and capacitor and actual
temperature excursions during power cycling. Crack
propagation is minimized with small components, small
differences in CTE and low temperature swings,
unfortunately the real world is not ideal and large
differences do exist.
Figure 5. Extreme Thermal Shock Cracks in MLCs
Figure 8. A Micro Crack at the Termination
Ceramic Interface
Figure 6. Severe Thermal Shock Cracks in Large MLCs
Large CTE differences or physically large
components1 will have increased forces on the part
during power cycling due to the linear displacement of
size or CTE difference. When the substrate’s CTE is
larger than the component such as PC boards and
MLCs, the part is held in tension during power cycling
and micro cracks propagate more quickly than if they
were in compression. When an MLC is mounted on
alumina the opposite is true (now compression) and the
micro crack will propagate more slowly. (The reverse
would be true for cooling cycles.)
MATERIAL
Alumina
Barium Titanate Capacitor Body
FR-4/G-10 PC Boards (X, Y)
Polyimide/Glass PCB (X, Y)
Polyimide/Kevlar PCB (X, Y)
Copper Clad Invar
Copper
Tin Lead Alloys
CTE (ppm/ºC)
≈7
9.5-11.5
≈18
≈12
≈7
6-7
17.6
≈27
Table 1. CTEs of Typical Components and Substrates
Increased electrical and mechanical failures are not
acceptable alternatives for increased density,
automation and improved electrical performance. Once
the problem is understood and controls are instituted,
any soldering technique can be used reliably.
Unfortunately, once a user has experienced problems
with thermal shock, they tend to go overboard with illconsidered or irrelevant tests that only prove that they
can indeed exceed the mechanical strength of the parts.
Figure 9. A Propagated Micro Crack After Power Cycling
These tests include plunging the part into a solder pot
with tweezers with unknown pressure, position or size
or they solder components to a test board with
exaggerated solder temperatures. Different lots from
different vendors might pass such tests but the main
problem is that surface mount manufacturing requires
process modification to insure reliable assemblies. In
particular, wave soldering is the biggest problem
because many manufacturing groups do not want to
change solder temperatures or flux. They insist that the
vendor must comply with a process that will not work
reliably and will cause long term failures that show up
many months later as field service problems. Remember
all soldering processes must be component and material
vendor independent.
Surface mount manufacturing requires discipline;
a reliable process must be developed and adhered
to, solder profiles run each shift and solderable
components used. This will be a new experience for
some manufacturing groups that use the “crank the
temperature up” philosophy (remember if you crank it
up field service must recrank it up).
The following process guidelines allow proper
soldering with minimum thermal shock and component
degradation with maximum yield.
WAVE SOLDER: This is the most critical process. The
actual solder wave temperature should be reduced from
250ºC-260ºC to 232ºC ±2ºC for 60/40 solder and the
preheat temperature of the assembly bottom should
exceed 140ºC with the rate of rise limited to 4ºC/sec. The
total wave dwell time for components should not exceed
10 seconds with 5-7 sec as an optimum time allowing
adequate soldering with obtainable preheat
temperatures. The higher the preheat or smaller
difference between the preheat and solder wave
temperature the better. The absolute maximum
difference between preheat and solder wave should be
less than 100ºC, 70-80ºC is a better number eliminating
any possibility of micro cracks.
It’s not uncommon to see solder wave temperatures
exceeding 300ºC with low or no preheat in “crank it up”
manufacturing groups because the solder wave controls
have gone into thermal run away. There is a fear of
using high preheat in wave soldering because of low
melting point plastics used in some thru hole
components but only the board bottom is being heated.
Actual measured thru hole component temperatures are
well within any maximum limits. Some manufacturing
groups are now using wave temperatures as low as
225ºC with higher preheat to achieve zero soldering
defects with sub ppm long term return rates. Simply
stated they control the solder process to eliminate field
returns.
VAPOR PHASE REFLOW SOLDERING: VPR
soldering does not produce visible cracks but can cause
micro cracks if improper preheat is used. A preheat of
100ºC is recommended for VPR soldering. This will
eliminate micro cracks while drying and activating the
solder paste, improving soldering and minimizes solder
balls or splatter. Component termination temperature
rates of rise in excess of 50ºC/sec have been measured
when no preheat is used in a VPR system. These rates
of rise will induce micro cracks because the component
cannot heat uniformly or quickly even with the uniform
heat source.
Dwell time in the saturated vapor zone needs to be
less than one minute which yields excellent soldering
with a minimum of solder migration. At longer dwell
times, degradation of epoxy resin molded parts such as
transistors, ICs, tantalum capacitors, etc., is accelerated
due to these components maximum temperature
capabilities and the epoxy resin’s low glass transition
temperature, Tg. This is the temperature where the long
molecular chains in the resin become very active with
CTE increasing from 18-25ppm/ºC to over 200ppm/ºC.
Tg for most epoxies is in the 80 to 125ºC range and over
200ºC for polyimides. Some complex PC boards with
high thermal mass will require longer times in the vapor
zone but care must be taken to absolutely minimize this
time.
HOT BELT REFLOW AND IR REFLOW: Any
thermal shock is unusual when IR reflow is used
because of the lower heat transfer rate. The maximum
temperature rate of rise should be less than 4ºC/sec
(1-2ºC/sec typical) which allows uniform substrate
heating and minimum stress on the components.
Hot belt reflow is used mainly for hybrids or
assemblies on alumina substrates and can cause
problems if high temperature solders such as
goldgermanium are used because of very high
temperature rates of rise due to short heat zones.
A micro crack will form and propagate through the
capacitor very quickly during rapid heat up and cool
down and can actually pull the termination right off of
the component. Temperature rates of rise should be
limited to 4ºC/sec maximum for hot belt reflow. Most
surface mount assemblies use 63/37 eutectic solder or
low silver bearing solder such as 62/36/2 which
minimizes solder migration and termination leaching.
These solders have melting points near 186ºC. An ideal
profile for IR or hot belt reflow with these solders will
have a peak temperature of 215-219ºC with 45-60
seconds above the melting point.
These guidelines allow reliable assemblies to be built.
The multilayer ceramic capacitor is sensitive to thermal
shock but all electronic components share this problem
MATERIAL
Alloy 42
Alumina
Copper
Filled Epoxy (<Tg)
Nickel
Silver
Steel
Tantalum
Tin Lead Alloys
CTE (ppm/ºC)
5.3
≈7
17.6
18-25
15
19.6
15
6.5
≈27
T (W/mºK)
17.3
34.6
390
.5
86
419
46.7
55
34
Table 2. CTEs and Ts of Component Materials
especially tantalum capacitors. Table 2 lists CTEs and T
for common materials used to make chip resistors, SOTs
(small outline transistors), SOICs (small outline
integrated circuits) and other epoxy resin molded
components.
All surface mount components are built with different
materials that expand and conduct heat at different
rates. If a part is heated up too rapidly, it will crack or
internal seals are lost so the part will fail more quickly
because the stress cannot spread throughout the
component body. Proper processing eliminates these
problems but users and vendors must understand the
component limitations and not exceed those limits.
Reference
1. B.S. Rawal, etc., “Factors Responsible for Thermal
Shock Behavior of Chip Capacitors.”
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apply to all applications.
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