IPC-D-279 EN.pdf - 第87页
(molded-in) or external mechanical stress and particu- larly in the presence of some solvents (such as con- densing alcohol vapor) • mechanical stressing of components containing mate- rials with mismatched coef f icient…
gas ambient, hydrolyzable dust and sand, solvents and
residual hydrolyzable contaminants from the fabrication
and assembly processes and handling. Mechanical stresses
include sand and dust, mechanical shock, vibration,
connection/removal of connectors, and unrelieved strain
due to warped printed circuit boards. Other stresses include
low atmospheric pressure/high altitude/vacuum, electro-
magnetic radiation, ionizing radiation such as X-rays and
alpha particles. A special but not uncommon case of a com-
bined stress is that of temperature- humidity- bias.
E-7.0 IMPORTANCE OF TEMPERATURE AS A COMPO-
NENT STRESS FACTOR
Temperature is the greatest single contributing stress to
component failure.
E-7.1 Temperature-Related Reversible/Temporary
Changes in Component Parameters
Examples of revers-
ible changes include:
• increased resistance in metallic resistors
• changes in capacitance, dielectric constant, power fac-
tor, and equivalent series resistance (ESR) of capaci-
tors
• decreased inductance of inductors and filters, particu-
larly near Tc (Curie temperature)
• increased gain of bipolar transistors
• decreased breakdown voltage and increased saturation
(leakage) current of P-N junctions
• increased output resistance of bipolar and field effect
transistors
• decreased viscosity and load carrying ability of lubri-
cants.
High temperature exposure during SM assembly process-
ing (particularly solder reflow) or service also results in
reversible
• expansion of materials (particularly of polymers above
T
g
) with the possibility of the jamming of moving
parts and the ‘‘bi-metallic’’ bowing of materials
• loss of control and a long recovery period in tempera-
ture stabilized components such as crystal oscillators.
E-7.2 Temperature-Related Irreversible/Permanent
Changes in Component Parameters
Examples of irre-
versible effects of exposure to high temperatures include
the following:
• oxidation (particularly of lubricants and contacts)
• corrosion (particularly in the presence of moisture and
hydrolyzable contaminants)
• Intermetallic compound (IMC) formation particularly
in the case of SMT and Tape Automated Bonded sol-
der joints and other bi-metallic joints.
• grain growth in multiphase alloys such as eutectic
lead-tin solder
• diffusion of alkali metals in semiconductor devices
resulting in device instability
• diffusion of halogenated solvents through rubber seals
of non-solid electrolytic capacitors resulting in internal
corrosion and subsequent component failure
• evaporation and subsequent loss of high vapor pres-
sure fractions of silicone compounds, greases and flu-
ids
• evaporation and subsequent loss of plasticizers in plas-
tics
• evaporation and transportation of silicone and plasti-
cizer compounds to the mating surfaces of separable
contacts such as those in relays and connectors and
cold flow of materials such as polytetrafluoroethylene
(PTFE).
• evaporation and subsequent loss of fluid in non-solid
electrolytic capacitors has caused losses on compo-
nents and instruments which have been in storage at
room temperatures. Evaporation is accelerated at
higher temperatures and higher altitudes. Some alumi-
num electrolytic capacitors rated for operation from
−55°C to +85°C or +105°C contain fluids such as dim-
ethyl formamide (DMF) which has a boiling point of
67°C. Replacement of the DMF with dimethyl aceta-
mide (BP = 74°C) or gamma butyrolactone (GBL) (BP
= 97°C) reduces the susceptibility of the component to
high temperatures. Because these solvents have flash-
points very close to their boiling point, the solvent
change also reduces the fire hazard. Changing from
DMF to dimethyl acetamide or GBL also reduces the
toxicity of any leaking electrolyte.
High temperature exposure during SM assembly process-
ing (particularly solder reflow) also results in irreversible
• internal and external SM integrated circuit package
delamination and cracking, mechanical damage to the
surface of the die and its microinterconnections as well
as and potential failure due to corrosion of the die
metallization. Passive component networks are also
susceptible to this delamination, cracking, and corro-
sion. See appendix D for a summary and IPC-SM-786
for details.
• temporary softening and a degradation in resistance to
cut-through of insulating polymers (such as those in
capacitors) with a permanent loss of dielectric with-
stand strength and mechanical strength and a long
period to recover some of the lost properties.
• stress cracking of susceptible polymers, such as trans-
parent nylon optical components under internal
July 1996 IPC-D-279
75
(molded-in) or external mechanical stress and particu-
larly in the presence of some solvents (such as con-
densing alcohol vapor)
• mechanical stressing of components containing mate-
rials with mismatched coefficients of thermal expan-
sion (CTE) such as joint stress between a polymer-
glass based SM substrate and rigid ceramic
components such as large multilayer capacitors, power
resistors, ceramic based hybrids, and inductors
• melting and opening of soldered connections internal
to capacitors, inductors, crystals, and resistor/capacitor
networks
• melting or softening of the polymeric capacitor dielec-
trics such as polystyrene, polycarbonate (PC), polypro-
pylene, polyethylene terepthalate (PET) with dielectric
thinning as a result of the relaxation of winding
stresses at high temperature. The thinned dielectric
results in uncontrolled increase in capacitance and
decrease in dielectric breakdown voltage and a long
period to recover some of the lost properties
• expansion of elastomeric materials such as silicone or
RTV used for ‘‘potting’’ components (such as optocou-
plers, pulse transformers, inductors, and delay lines)
with subsequent breaking of components, wires and
joints
• softening and relaxation of elastomeric materials such
as O-rings in variable resistors, with subsequent loss of
seal and initiation of corrosion
• softening and cracking of the low T
g
conformal coat-
ings of axial and radial capacitors, with subsequent
corrosion
• softening and weakening of internal epoxy connections
in assemblies such as crystals and hybrid oscillators
• softening, distortion, or deformation of plastic compo-
nents (such as surface mount connectors and light
emitting diode [LED] display scramblers) with loss of
dimensional accuracy
• overcuring of polymers used for insulation with a
decrease in insulation resistance (IR)
• boiling and evaporation of the fluids in non-solid elec-
trolytic (such as aluminum and wet slug tantalum)
capacitors with subsequent loss of capacitance and
increased Equivalent Series Resistance.
Identify the maximum allowable internal body temperature
(defined by the boiling point of the electrolyte, the soften-
ing point of the plastic dielectric), or the softening range of
the internal solder joints and match that temperature con-
straint with the solder reflow profile to prevent component
degradation.
See also the comments in this design guide, Appendix A,
on the effects of temperature and temperature cycling on
solder joint reliability and solder joint fatigue and methods
of computing the reliability impact. See also the comments
on process and rework temperatures in this design guide,
particularly the effects on plastic surface mount compo-
nents in Appendix F.
E-7.3 Effects of Low Temperature Low temperatures
result in:
• loss of flexibility and decreased impact resistance in
polymers
• liquid water film formation below dewpoint with sub-
sequent opportunity to induce corrosion
• ice formation with subsequent delamination or melting
of the ice
• viscosity increase particularly in lubricants and liquid
electrolytes
• contraction of materials with subsequent jamming of
moving parts or bi-metallic bowing of materials
• thermomechanical stress of components containing
materials with mismatched coefficient of thermal
expansion (∆) joined by SM reflow
• decreased bipolar transistor gain and increase FET
transconductance
• stress and rupture of some SM solder masks and other
coatings
• loss of control in temperature stabilized components
such as crystal oscillators
• increased dissipation factor in ‘‘hi-k’’ ceramic capaci-
tors
• increased stress in encapsulants and molding com-
pounds with subsequent damaged IC passivation, dam-
aged IC metallization, cracked silicon, or induced dark
line defects and loss of light output in LEDs.
E-7.4 Effects of Temperature Changes Temperature or
thermal cycling can result in:
• repeated stressing of structures and material systems
with mismatched thermal expansions resulting in
repeated ‘‘bi-metallic’’ bowing and possible fatigue.
Particularly susceptible are systems of ceramic and
organic components affixed to organic and ceramic
substrates, respectively (for instance, where ceramic
LCC are affixed to organic FR-4 boards or plastic
encapsulated components are affixed to ceramic sub-
strates). The most severe cases can result in compo-
nent cracking or solder joint failure due to overload.
Thermo-mechanical fatigue effects are worse with
lower cycling frequencies due to relaxation effects.
• jamming of moving parts
• repeated condensation of moisture
• repeated evaporation of moisture
IPC-D-279 July 1996
76
With significant power dissipation in the component, the
effects of the cycling of power dissipation within the com-
ponent (power cycle) may be more influential than the
cycling of ambient temperature, even when the CTE of the
component and substrate are matched.
See also the comments in this design guide in Appendix A
on the effects of temperature and temperature cycling on
solder joint reliability and solder joint fatigue and methods
of computing the reliability impact.
E-7.5 Thermal Shock Thermal shock is defined by a
rapid temperature change.
Thermal shock can occur during SM solder reflow, wave
solder or hand solder. Without adequate preheat, the result
is cracked laminated or multilayer ceramic and ferrite com-
ponents such as Multilayer Ceramic Capacitors (MLCC),
inductor and filter networks. Recommended process param-
eters to avoid thermal shock:
•a∆T/∆t < 4°C/second
•a∆T of < 100°C from preheat to peak process tem-
perature; noted by most manufacturers for most of
their products.
The literature reports variations from supplier to supplier
and within a supplier on these ‘‘rules.’’
E-8.0 POWER
The primary consideration of power is the resulting tem-
perature. Roughly
(P x θ
ja
)+T
a
»T
j
Where P is the power dissipation, θ
ja
the thermal resistance
from junction to ambient, T
a
the ambient temperature and
T
j
the junction or active film temperature of the compo-
nent.
θ
ja
is a parameter value generally averaged over the com-
ponent package.
Hotspots can occur in semiconductors as a result of delami-
nation or large voids due to poor die attach material or poor
die attach process. Hotspots can also occur in semiconduc-
tors due to avalanche voltage breakdown and during sec-
ond breakdown. θ
ja
= θ
jc
+ θ
ca
, the thermal resistance from
junction to case added to the thermal resistance from case
to ambient. θ
jc
is a component package parameter value
strongly dependent upon the conditions under which it is
evaluated (air flow speed and orientation, resulting laminar
or turbulent air flow, etc.).
Cyclic power dissipation conditions result in cyclic tem-
perature fluctuations. Power devices with solder die attach
can suffer fatigue of the solder after power cycling, with
resulting cracks and delamination of the solder and subse-
quent increase in thermal resistance and progressive fail-
ure.
Excessive ripple voltage or current in non-solid electrolytic
capacitors, in conjunction with the equivalent series resis-
tance (ESR), results in internal heating or unexpected
power dissipation; this increase in internal temperature
increases the evaporation of the electrolytic fluid, increases
ESR and results in subsequent failure.
High frequency currents in ferrite inductors also results in
internal heating; this effect is exacerbated by high levels of
DC current.
See also the comments in this design guide (Appendix B)
on surface mount thermal design.
E-9.0 VOLTAGE
Solid dielectric breakdown, a bulk effect, affects the oxides
used in integrated circuit dielectrics, MOS device gates,
and other junction bipolar passivation; also affected are the
polymers used in capacitor, inductor and optocoupler insu-
lation systems. Solid dielectric breakdown also occurs at
bipolar junctions such as the collector-base of transistors.
The derated breakdown voltage should not be exceeded.
Worst case power transient conditions and other such ‘‘low
probability’’ occurrences must be considered.
Gas dielectric breakdown occurs between closely spaced
unpassivated or uncoated conductors and can result in
melting and fusing of conductor materials; the accompany-
ing arcing can also degrade, melt or burn adjacent insulat-
ing materials and spray molten materials on those insulat-
ing materials. Conduction through a gas may be initiated in
such different forms as corona, glow discharge, spark and
arc.
Surface breakdown occurs between conductors or between
conductors and generally underlying semiconductor and
‘‘around’’ interposed insulating materials; surface break-
down can result in melting and fusing of conductor and
semiconductor. In addition to the breakdown mechanisms,
voltage bias can result in metal ion migration in dielectrics,
with subsequent instabilities in semiconductors (where
light alkali metals and ions are particularly notorious) and
voiding/shorting in thick film components and assemblies
(where silver and palladium shorts have been noted). When
the energy release during the surface breakdown event is
high enough, the surface may be degraded and the residues
may be electrically conductive. Where the surface is the
printed board, a parameter such as comparative tracking
index (CTI) may be required by regulations.
Cyclic voltage stresses introduce the effects of ‘‘equivalent
series resistance’’ (ESR) in dielectric systems; some frac-
tion of the energy delivered to the capacitor expresses itself
as heat (Joule heating + dielectric loss) and must be con-
sidered in the thermal derating of the component. ESR
effects are a function of the peak to peak value of the
applied voltage and the frequency of the applied voltage.
In ceramic dielectric capacitors (MLCC) such as those used
July 1996 IPC-D-279
77