IPC-D-279 EN.pdf - 第46页

and all leaded solder joints, show localized damage con- centrations with the damage shown in Figure A-1 preced- ing an advancing macro-crack. The solder joints frequently connect materials of highly disparate properties…

Appendix A

Design for Reliability (DfR) of Solder Attachments

A-1.0 SURFACE MOUNT SOLDER ATTACHMENT

RELIABILITY

The fatigue behavior of surface mount solder joints has

been investigated experimentally in numerous studies. The

results of the studies that were carried out in a manner to

assure the same damage mechanism as the mechanism

operative in typical electronic products have yielded a

mathematical solder fatigue model. This model has been

expanded and augmented to its current form, presented in

this section, as additional test results became available.

The model is for uncoated solder attachments. The com-

plexity and vast differences in conformal coatings make it

impossible to develop a generic model that considers all

the variables. Products with conformal coatings should be

evaluated using test vehicles having the same coating and

test vehicles without the coating in order to assess the

impact of the coating on reliability.

A-2.0 DAMAGE MECHANISMS AND FAILURE

The reliability of electronic assemblies depends on the reli-

ability of their individual elements and the reliability of the

mechanical thermal, and electrical interfaces (or attach-

ments) between these elements. One of these interface

types, surface mount solder attachment, is unique since the

solder joints not only provide the electrical interconnec-

tions, but are also the sole mechanical attachment of the

electronic components to the printed board and often serve

critical heat transfer functions as well.

A solder joint in isolation is neither reliable nor unreliable;

it becomes so only in the context of the electronic compo-

nents that are connected via the solder joints to the printed

board. The characteristics of these three elements - compo-

nent, substrate, and solder joint - together with the use

conditions, the design life, and the acceptable failure prob-

ability for the electronic assembly determine the reliability

of the surface mount solder attachment.

A-2.1 Solder Joints and Attachment Types

Solder joints are anything but a homogeneous structure. A

solder joint consists of a number of quite different materi-

als, many of which are only superﬁcially characterized. A

solder joint consists of:

(1) the base metal at the printed board

(2) one or more intermetallic compounds (IMC)—

solid solutions—of a solder constituent—

typically tin (Sn)—with the printed board base

metal

(3) a layer from which the solder constituent form-

ing the printed board-side IMC(s) has been

depleted

(4) the solder grain structure, consisting of at least

two phases containing different proportions of

the solder constituents as well as any deliberate

or inadvertent contaminations

(5) a layer from which the solder constituent form-

ing the component-side IMC(s) has been

depleted

(6) one or more IMC layers of a solder constituent

with the component base metal, and

(7) the base metal at the component.

The grain structure of solder is inherently unstable. The

grains will grow in size over time as the grain structure

reduces the internal energy of a ﬁne-grained structure. This

grain growth process is enhanced by elevated temperatures

as well as strain energy input during cyclic loading. The

grain growth process is thus an indication of the accumu-

lating fatigue damage. At the grain boundaries contami-

nants like lead oxides are concentrated; as the grains grow

these contaminants are further concentrated at the grain

boundaries, weakening these boundaries. After the con-

sumption of ~25% of the fatigue life micro-voids can be

found at the grain boundary intersections; these micro-

voids grow into micro-cracks after ~40% of the fatigue

life; these micro-cracks grow and coalesce into macro-

cracks leading to total fracture as is schematically shown in

Figure A-1.

Surface mount solder attachments exist in a wide variety of

designs. The major categories are leadless and leaded sol-

der attachments. Among the leadless solder joints a differ-

entiation has to be made between those without ﬁllets, e.g.,

Flip-Chip C4 (Controlled Collapse Chip Connection) sol-

der joints, BGAs with C5 (Controlled Collapse Chip Car-

rier Connection) solder attachments, BGAs with high-

temperature solder (e.g., 10Sn/90Pb) balls, and CGAs with

high-temperature solder columns; and solder joints with

ﬁllets, e.g., chip components, Metal Electrode Face compo-

nents (MELFs), and castellated leadless chip carriers. The

leaded solder attachments differ primarily in terms of their

compliancy and can be roughly categorized into compo-

nents with super-compliant leads {K

<~9 N/mm}, compli-

ant leads (~9 N/mm<K

<~90 N/mm), and non-compliant

leads {(K

>~90 N/mm}.

The different surface mount solder attachment types can

have signiﬁcantly different failure modes. Solder joints

with essentially uniform load distributions, e.g., Flip-Chip,

BGA, CGA, show behavior as illustrated in Figure A-1.

Solder joints with non-uniform load distributions, e.g.,

those on chips components, MELFs, leadless chip carriers,

IPC-D-279 July 1996

and all leaded solder joints, show localized damage con-

centrations with the damage shown in Figure A-1 preced-

ing an advancing macro-crack.

The solder joints frequently connect materials of highly

disparate properties, causing global thermal expansion mis-

matches [Refs. A-9: 1-6], and are made of a material, sol-

der, that itself has often properties signiﬁcantly different

than the bonding structure materials, causing local thermal

expansion mismatches [Refs. A-9: 4,7].

The severity of these thermal expansion mismatches, and

thus the severity of the reliability threat, depends on the

design parameters of the assembly and the operational use

environment. In Table A-1 guidelines as to the possible use

environments for nine of the more common electronic

applications are illustrated [Refs. A-9: 8,9]. However, it

needs to be emphasized, that the information in Table A-1

should serve only as a general guideline; for some use cat-

egories the description of the expected use environment

Figure A−1 Depiction of the Effects of the Accumulating Fatigue Damage in Solder Joint Structure

Table A−1 Realistic Representative

(1)

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

Mounted Electronics Attachments by Use Categories [Ref. A-9: 10]

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.’

July 1996 IPC-D-279

can be rather more complex [Ref. A-9: 9].

A-2.2 Global Expansion Mismatch

The global expansion mismatches result from differential

thermal expansions of an electronic component or connec-

tor and the printed board to which it is attached via the

surface mount solder joints. These thermal expansion dif-

ferences result from differences in the CTEs and thermal

gradients as the result of thermal energy being dissipated

within active components.

Global CTE-mismatches typically range from ∆α~2

ppm/°C (1 ppm=lx10

-6

) for CTE-tailored high reliability

assemblies to ~14 ppm/°C for ceramic components on

FR-4 printed boards. CTE-mismatches of ∆α <2 ppm/°C

are not achievable in reality as a consequence of the vari-

ability of the CTE values of the materials involved on both

components and printed boards.

Global thermal expansion mismatches typically are the

largest, since all three parameters determining the thermal

expansion mismatch—the CTE-mismatch, ∆α, the tem-

perature swing, ∆T, and the acting distance, L

—are large.

This global expansion mismatch will cyclically stress, and

thus fatigue, the solder joints. The cyclically cumulative

fatigue damage will ultimately cause the failure of one of

the solder joints, typically a corner joint, of the component

causing functional electrical failure that is initially intermit-

tent.

A-2.3 Local Expansion Mismatch

The local expansion mismatch results from differential

thermal expansions of the solder and the base material of

the electronic component or printed board to which it is

soldered. These thermal expansion differences result from

differences in the CTE of the solder and those of the base

materials together with thermal excursions [Refs. A-9: 4,7].

Local CTE-mismatches typically range from ∆α~7 ppm/°C

with copper to ~18 ppm/°C with ceramic and ~20 ppm/°C

with Alloy 42 and Kovar

. Local thermal expansion mis-

matches typically are smaller than the global expansion

mismatches, since the acting distance, the maximum wetted

area dimension, is much smaller—in the order of hundreds

of ~µm.

A-2.4 Internal Expansion Mismatch

An internal CTE-mismatch of ~6 ppm/°C results from the

different CTEs of the Sn-rich and Pb-rich phases of the

solder. Internal thermal expansion mismatches typically are

the smallest, since the acting distance, the size of the grain

structure, is much smaller than either the wetted length or

the component dimension—in the order of less than 25 µm

[Ref. A-9: 11].

A-2.5 Solder Attachment Failure

The failure of the solder attachment of a component to the

substrate to which it is surface mounted is commonly

deﬁned as the ﬁrst complete fracture of any of the solder

joints of which the component solder attachment consists.

Given that the loading of the solder joints is typically in

shear, rather than in tension, the mechanical failure of a

solder joint is not necessarily the same as the electrical

failure. Electrically, the mechanical failure of a solder joint

results, at least initially, in the occasional occurrence of a

short-duration (<1 µs) high-impedance event during either

a mechanical or thermal disturbance. From a practical point

of view, the solder joint failure is deﬁned as the ﬁrst obser-

vation of such an event.

For some applications this failure deﬁnition might be inad-

equate. For high-speed signals with sharp rise times signal

deterioration prior to the complete mechanical failure of a

solder joint might require a more stringent failure deﬁni-

tion. Similarly, for applications which subject the electronic

assemblies to signiﬁcant mechanical vibration and/or shock

loading, a failure deﬁnition that considers the mechanical

weakening of the solder joints as the result of the accumu-

lating fatigue damage might be necessary.

A-3.0 RELIABILITY PREDICTION MODELING

A-3.1 Creep-Fatigue Modeling

It has been experimentally shown [Refs. A-9: 2,4,12,13]

that the fatigue life of surface mount solder joints can be

described by a power law similar to the Coffin-Manson

low-cycle fatigue equation [Ref. A-9: 14] developed for

more typical engineering metals. For practical reasons and

as the direct consequence of the time-dependent stress-

relaxation/creep behavior of the solder at typical use envi-

ronments (see Table A-1), the specialized case of the

Coffin-Manson equation requires reversion to the more

general strain-energy relationship of Morrow [Ref. A-9:

15]; it also requires that the cyclic strain energy be based

on the total possible thermal expansion mismatch and that

the exponent is a function of temperature and time to pro-

vide a measure of the completeness of the stress-relaxation

process. The Engelmaier-Wild solder creep-fatigue equa-

tion [Refs. A-9: 1-6,9,12], subject to some caveats listed

later, relates the cyclic visco-plastic strain energy, repre-

sented by the cyclic fatigue damage term, ∆D, to the

median cyclic fatigue life for both isothermal-mechanical

and thermal cycling [Ref. A-9: 16]

(50%)=

[

2e’

∆D

]

−1

[Eq. A-1]

where

e’

= fatigue ductility coefficient, =0.325 for eutectic and

60/40 Sn/Pb solder (for other solders the value of e’

is expected to be somewhat different).

IPC-D-279 July 1996