IPC-D-279 EN.pdf - 第18页

3.1.7 Environmental Stresses The operating life of a surface mounted assembly is dependent upon a number of factors which include intended usage, usage environments, strength of the materials and components to withstand …

This damage should be minimized by keeping the number

of excursions to a minimum; and the damage needs to be

considered in the overall reliability estimates for the assem-

blies. See IPC-PE-740 for trouble-shooting information.

3.1.3 Burn-In and Environmental Stress Screening

(ESS)

Burn-in tests and ESS have the potential of identi-

fying latent defects that may cause early failures in prod-

uct, but they also have a negative impact on the good

assemblies. The degree of the detrimental impact on reli-

ability depends on the severity of the burn-in and/or ESS

procedures.

Burn-in testing generally should be a complete environ-

mental test involving perhaps worst case but still realistic

operational environments.

ESS should never be employed routinely. ESS needs to be

speciﬁcally designed to cause the failure of ‘‘weak’’ ele-

ments in the assemblies for which a strong suspicion of

processing defects exists.

The assembly elements that are typically most affected by

these procedures are the surface mount solder attachments.

The effect of extended solder joint temperature cycling can

use up a signiﬁcant amount of solder joint life.

3.1.4 Transport While transport conditions like vibra-

tion, mechanical shock and moisture are routinely consid-

ered and accommodated, little is done about the thermal

conditions. Electronic product can sit on loading docks or

in warehouses, or be in cargo holds of ships, airplanes

and/or trucks, in temperatures ranging from −40 to +70°C.

For some applications, e.g. medical implants, these trans-

port conditions would be signiﬁcantly more severe than the

operational environments.

3.1.5 Storage For some product applications, the envi-

ronmental conditions during storage become signiﬁcant in

the total life cycle environment. In particular, military

applications, such as munitions (proximity fuses, etc.), and

space applications can require long storage periods, fre-

quently in uncontrolled environments, before ﬁnal use.

The user should consult with the vendor to determine the

shelf life and special storage conditions.

3.1.6 Use Environments The use environments are

highly dependent upon the product application. In Table

3-1, typical worst case use environments for 9 product cat-

egories are given. These use environments should be

regarded as guidelines only; the actual use environment as

well as the environmental conditions of the SM assembly

being designed may be signiﬁcantly different.

Table 3−1 Realistic Representative

(1)

Use Environments, Service Lives, and Acceptable Failure Probabilities for Surface

Mounted Electronics Attachments by Use Categories

Worst-Case Environment

Use Category

Tmin

°C

Tmax

°C

∆T

(2)

°C

Dwell Time

hrs Cycles/Year

Typical Years

of Service

Accept.

Failure

Risk

(3)

Consumer 0 +60 35 12 365 1-3 ~1

Computers +15 +60 20 2 1460 ~5 ~0.1

Telecom -40 +85 35 12 365 7-20 ~0.01

Commercial Aircraft -55 +95 20 12 365 ~20 ~0.001

Industrial

and Automotive

Passenger

Compartment

−55 +96 20

&40

&60

&80

185

100

~10 ~0.1

Military

Grounds and

Ship

−55 +95 40

&60

100

265

~10 ~0.1

Space leo

geo

−55 +95 3 to 100 1

8760

365

5-30 ~0.001

Military a

Avionics b

Maintenance

−55 +95 40

&20

365

~10 ~0.01

Automotive

under hood

−55 +125 60

&100

&140

1000

300

~5 ~0.1

& = in addition

1 Does not cover all possible use environments, but only most common.

2 ∆T represents the maximum temperature swing, but does not include power dissipation effects for components; for reliability estimations the actual local tem-

perature swings for components and substrate, including power dissipation should be used.

3 The ‘Acceptable Failure Risk’ is the percentage of product in the ﬁeld that has failed, due to wearout mechanisms, at the end of the ‘Typical Years of Service.’

IPC-D-279 July 1996

3.1.7 Environmental Stresses The operating life of a

surface mounted assembly is dependent upon a number of

factors which include intended usage, usage environments,

strength of the materials and components to withstand the

stresses imposed by the usage and the environments, mate-

rial (variables), etc. In a surface mounted assembly, the

most critical element from the life cycle viewpoint are the

solder joints and PTVs. Cyclic (or fatigue) displacements

experienced during various phases of the product’s life

cycle are responsible for consumption of useful life of

material elements.

One of the major contributors to the cyclic loading is ther-

mal cycling due to the internal power cycling and external

environmental changes. Another important contributor is

vibration during the operational use, transportation, han-

dling, etc. An estimate of the fatigue life of the solder

assembly that will be consumed by these fatigue cycles

during the product’s life cycle can be obtained by perform-

ing a cumulative damage analysis.

It should be noted that the cumulative damage analysis

requires the knowledge of fatigue characteristics of the

materials involved. Also, a thorough understanding of how

the product will be used, handled and maintained, by the

user and under what environments, is necessary. See

Appendix E.

3.1.8 Temperature/Thermal Temperature is one of the

most important parameters in the use environment that

must be considered in the SM PWA design process. Tem-

perature history is the most signiﬁcant parameter affecting

the reliability of SM solder joints. For some product appli-

cations, the use environment consumes the most significant

portions of the required fatigue life; this is typically the

case where the product development cycle includes produc-

ing a prototype prior to the ‘‘ﬁnal build.’’

Therefore, DfR depends to a large extent on the thermal

design for the assembly, as well as the external thermal

environment. Temperature is also important, since many

materials in electronic assemblies have properties which

change signiﬁcantly with temperature.

3.1.9 Cyclic Temperature Swings Cyclic thermal excur-

sions, ∆T, cause thermal expansion mismatches due to dif-

ferent parts of the assemblies having materials with differ-

ent CTEs and/or being at different temperatures. Thus, the

size of the cyclic temperature swing is proportional to the

resulting loading. The larger the ∆T, the larger the threat to

reliability.

During cyclic temperature excursions, these cycles can

have different proﬁles, e.g., sinusoidal, trapezoidal, saw-

tooth, square-wave, etc. These differences are important in

understanding time-dependent and rate-dependent pro-

cesses. The transient parts of the proﬁle give the ramping

rates of the temperature change which can cause transient

temperature gradients and over-stress conditions. The

steady-state, or near steady-state, parts of the proﬁle deter-

mine the duration of the temperature dwells which are

important for time-dependent processes like creep and

stress relaxation.

3.1.10 Thermal Shock In thermal shock, the extremely

rapid temperature changes (∼30°C/minute and above)

result in warping of the surface mount assembly. The

warpage is caused by large transient thermal gradients

induced by the rapid temperature change when the boards

are plunged into a new thermal environment. The warpages

result in tensile and shear stresses where the tensile load-

ing dominates over the steady state expansion mismatch.

Thus, even assemblies with matched coefficients of thermal

expansion will exhibit solder joint failures when subjected

to thermal shock. The thermal shock loading mechanism is

summarized in Figure 3-1.

Thermal shock conditions can arise from several sources.

Examples of these are:

1) Rapid changes in external environment, e.g., sun-to-

shade in space, missile launch, wing-mounted avion-

ics, automobile start-up from very cold.

2) Rapid changes in power dissipation.

3) Various manufacturing/repair processes, e.g., reﬂow,

vapor degrease, rework, etc.

The distinction between thermal shock and thermal cycling

is not always addressed in designing reliability experi-

ments. There is a fundamental difference between thermal

shock and thermal cycling. The primary differences arise

from the mechanism of loading. Thermal shock tends to

result in multiaxial states of stress dominated by tensile

overstresses and tensile fatigue. On the other hand, as pre-

viously discussed, thermal cycling results in shear loads

and failure occurs from an interaction of shear fatigue and

stress relaxation.

Thermal shock is usually performed in dual chamber

arrangements or with liquid nitrogen injection whereas

thermal cycling is performed in single chamber cycling

equipment. Dual chamber arrangements will produce tem-

perature transition rates in excess of 50°C/minute.

Single chambers generally do not produce transition rates

even close to 30°C/minute which is the rate necessary to

induce thermal shock. The results of these two types of

testing are generally incompatible. Finally, thermal shock

testing for purposes of evaluating surface mount solder

joint reliability is only appropriate if thermal shock is

indeed a ﬁeld condition encountered by the product.

In some speciﬁcations, the deﬁnitions of thermal cycling

and thermal shock are not fully differentiated; the rates of

change are more closely associated with what we are call-

ing thermal shock.

July 1996 IPC-D-279

3.1.11 Electrical

3.1.11.1 ESD Susceptibility and Damage Prevention

All electronic components containing thin conducting or

insulating ﬁlms are susceptible to electrostatic discharge

(ESD) damage. These components include those fabricated

in high speed technologies (MOS, bipolar, GaAs), thin film

technologies (resistors, integrated circuits, magnetic heads,

MOS capacitors), and in future, wafer scale integration and

multichip modules.

3.1.12 EMC/EMI The electromagnetic spectrum is usu-

ally divided into categories ranging from the long-

wavelength radiation from power lines through radio, infra-

red, visible, ultraviolet, and x-rays, to gamma rays at the

short-wave end. All electromagnetic waves consist of an

electric ﬁeld and a magnetic ﬁeld. The relative magnitude

of these ﬁelds depend on the emitter (EM source), wave

propagation medium, and the proximity of the emitter to

the subject assembly.

Many electronic circuits are susceptible to electromagnetic

radiation and must be shielded to ensure proper operation.

One of the most important effects of the electromagnetic

radiation in the environment is electromagnetic interfer-

ence (EMI). EMI is the electro-magnetic disturbances that

impair the desired signal. In practice, EMI is often divided

into two categories: conducted EMI and radiated EMI.

Conducted EMI is an interfering signal resulting from an

undesirable voltage or current coupled into a signal or

other pertinent conductor. Radiated EMI is an interfering

signal resulting from an electric and/or magnetic ﬁeld

amplitude and frequency spectra intentionally or uninten-

tionally radiated by an electrical source. Examples of radi-

ated emission sources are radio and TV transmitters, light-

ning, digital system noise from electronic control systems,

etc. In military applications, an important effect is the inter-

action of electromagnetic radiation with electroexplosive

devices used as detonators. Improper EMI could acciden-

tally initiate the explosion.

EMC is the ability of electronic systems to operate in the

intended electromagnetic environment at designed levels of

performance and efficiency. The most direct approach to

protection is, in most cases, to avoid the limited region in

which high radiation levels are found. When exposure can-

not be avoided, shielding is the important protective mea-

sure. The material selected for shielding can be an impor-

tant factor. Ideal materials include steel, copper and nickel

coating. In the design process, apertures for cooling venti-

lation and cable connections on the shielding box should be

properly designed so that the EMI will have no inﬂuence

inside the shielded space.

3.1.13 Mechanical Shock and Vibration Shock and

vibration are common accelerators of failure in electronic

packaging. The most frequent vibration-induced failures in

surface mount are:

1. Flexing of leads and interconnects.

2. Dislodging or damaging of parts and structures.

Methods have been developed to counter the destructive

effects of shock and vibration. Generally, isolation of a

printed board against the effects of shock and vibration

requires that the natural frequency of the printed board be

substantially lower than the undesired frequency of vibra-

tion to avoid the resonance.

The basic system level isolators available are:

1. Natural or synthetic rubber, used to damp the vibra-

tion.

IPC-279-03

Figure 3−1 SMT Assembly Response to Thermal Shock

THERMAL

STEADY

STATE

THERMAL

STEADY

STATE

+ 125°CAT – 65°C

+ 125°C

– 65°C

IPC-D-279 July 1996