IPC-D-279 EN.pdf - 第67页

with 10 being perfect, and is a measure of the quality of the plated copper deposit in terms of its material properties relative to those of a corresponding foil sample plated onto a plating mandrel. This index needs to …

the stiffness of the MLB structure surrounding the PTV

barrel. The degree of land rotation—and thus lower

stiffness—and any other stiffening structures, such as rein-

forcement weave, neighboring PTVs, components and

cooling plates, will have an impact on this stiffness. For

bare MLBs it was found that d

could vary from a rela-

tively small diameter of inﬂuence [Ref. B-7: 10]

≅ 3d (Eq. B-8)

for softer structures to possibly a very large diameter of

inﬂuence [Ref. B-7: 10]

≅ 2h (Eq. B-9)

for PTVs in MLB assemblies for which land rotation is

essentially prevented by large stiff components and heat

sink plates; the most probable representative value for bare

MLBs is [Ref. B-7: 10]

≅

+ 2d

(Eq. B-10)

It has been found [Refs. B-7: 2,7,10] that the average bar-

rel strains, ∆ε

avg

, thus calculated need to undergo a correc-

tion for the assumptions necessary for a closed form stress

and strain analyses. Further, stress concentrations can occur

due to the uneven PTV barrel geometries resulting from

inadequate drilling and/or plating processes. In addition,

localized differences in the resin content (B-stage layers)

and the inﬂuence of inner lands and power and ground

planes can cause non-uniformities in the stresses and

strains, and at temperatures above T

, the material proper-

ties of the polymeric dielectric materials change dramati-

cally and abruptly [Ref. B-7: 10].

Furthermore, PTV failures, as all failures due to wearout

mechanisms, have a statistical distribution. The available

data are not adequate to fully deﬁne this statistical distribu-

tion, but wearout mechanisms like fatigue typically follow

a Weibull distribution with a shape parameter or slope of

β≈3. Typical data are reported as the ﬁrst failure from a

number of daisy chains with upwards of 100 PTVs each.

An effective maximum strain range to be used in Eq. B-1

can be found from

∆ε

max

(eff) = K

eff

∆ε

avg

(Eq. B-11)

where K

eff

, the effective PTV strain coefficient, results

from a combination of discernible deviations from a uni-

form stress and strain distribution, such that

eff

= K

100

(Eq. B-12)

The coefficients in Eq. B-12 are the PTV strain distribution

factor, K

, the plating thickness ‘dog-boning’ coefficient,

, the PTV stress concentration factor, K

, and the PTV

plating quality index, K

. Initially, the last three coeffi-

cients had been combined in a general PTV quality index

[Ref. B-7: 2], but by separating the discernible quality

variations, the source of the reduced quality can be identi-

ﬁed and the impact less arbitrarily quantiﬁed. As a guide-

line it should be noted that the values for K

eff

in Reference

B-7:2 varied between about 1.2 and 10.

The PTV strain distribution factor, K

, corrects for the

model assumption of a uniform stress and strain distribu-

tion for a distribution that is in fact non-uniform. The non-

uniformity is a function of the MLB thickness, h, with

higher non-uniformities resulting from thicker MLBs. The

PTV strain distribution factor is also dependent on whether

or not the temperature excursions exceed T

, above which

not only the thermal expansion increases, but the materials

softens signiﬁcantly. Thus

1,T

max

> 200°C, any T

= 1+1.5

(

2.3 mm

)

max

− T

200 − T

max

> T

1+1.5

(

2.3 mm

)

, otherwise

(Eq. B-13)

The plating thickness ‘dog-boning’ coefficient, K

accounts for any non-uniform stress and strain distribution

in the PTV barrel due to the gradual thinning—‘dog-

boning’—of the copper deposit towards the barrel center.

This ‘dog-boning’ can result from plating conditions that

are slightly beyond the capability of the plating chemistry

used. The coefficient is given by [Ref. B-7: 10]

PTV shoulder

PTV center

The PTV stress concentration factor, K

, is a measure of

the stress concentrations caused by the localized abrupt

thinning of the copper deposit due to either drilling or plat-

ing defects. Its size may be taken from Figure B-3 using

the plating deposit narrowing to determine the local ‘reduc-

tion in cross-section’.

Figure B-3 contains a curve [Ref. B-7: 10] which quanti-

ﬁes the large impact stress concentrations due to localized

thinning of plated copper conductors on ﬂexible printed

wiring have on increasing the stress—and thus the strain—

locally. PTV copper barrels, however, due to their three-

dimensional geometric structure are less susceptible to

stress concentrations that occur as localized features visible

on two-dimensional cross-sections. Figure B-3 also con-

tains a curve which is an attempt to quantify the impact of

these localized stress concentrations, which do not affect

the whole PTV barrel cross-section, in terms of the portion

of the basic material ductility that is required to accommo-

date these stress concentrations. From Figure B-3 a local-

ized reduction in plating thickness by 50% would result in

a value for K

of about 82, raising the effective strain due

to the stress concentration by about a factor of 1.22.

The PTV plating quality index, K

, is on a 10-to-1 scale

July 1996 IPC-D-279

with 10 being perfect, and is a measure of the quality of the

plated copper deposit in terms of its material properties

relative to those of a corresponding foil sample plated onto

a plating mandrel. This index needs to be established by

experience with PTVs in coupons or MLBs fatigued to

failure.

B-3.0 DfR-PROCESS

A successful ‘Design for Reliability’-process requires that

a number of issues be addressed at the design stage. The

generally applicable guidelines for the DfR-process are;

1) Keep PTV diameters as large as possible and the

MLB thickness/PTV diameter aspect ratio as small

as possible;

2) Require a nominal copper deposit thickness of 30 µm

to obtain actual plating thicknesses in the range of 25

to 40 µm;

3) Use E3 copper foil for the signal, power, and ground

layers for aspect ratios larger than 3:1;

4) Tent PTVs for applications with severe operational

loading conditions (see Table B-2) with solder mask

to prevent solder from partially ﬁlling the PTVs and

causing stress concentrations.

It is much more difficult to plate consistent high quality

copper deposits into small-diameter PTVs using standard

electrolytic processes. Also, smaller diameter PTV barrels,

especially in thicker MLBs, are subjected to higher loading

conditions.

A plating thickness of ~25 µm has been found to be the

minimum thickness which gives good reliability; a plating

thickness of ~40 µm is optimum from a reliability perspec-

tive. Plating thicknesses greater than that tend to promote

shoulder fractures (see Fig. B-2).

The quality of the copper foil for the signal, power, and

ground layers is of importance for aspect ratios larger than

about 3:1. Standard E1 copper foil [Ref. B-7: 14] has a

coarse columnar grain structure with the grain boundaries

perpendicular to the foil surfaces and has an elongation

requirement of only 2%. Thus, brittle E1 vendor foil can

lead to signal layer fractures and shoulder cracks as illus-

trated in Figure B-2. ‘High Temperature Elongation’ -E3

copper foil is recommended for PTVs with aspect ratios

larger than about 3:1.

Tenting the PTVs is a prudent and pragmatic decision.

PTVs entirely ﬁlled with solder certainly are more robust

and reliable than PTVs without solder; the problem is that

it cannot be guaranteed, that all the PTVs will be entirely

ﬁlled with solder. Partially solder-ﬁlled PTVs have stress

concentrations where the transition from fully ﬁlled to par-

tially ﬁlled occurs; these stress concentrations reduce the

Figure B−3 Reduction of Available Copper Ductility Due to Localized Nicks Reducing the Width of the Flex Circuit

Conductors [Ref. B-7: 25] and PTV Stress Concentration Factor, K

IPC-D-279 July 1996

reliability of these PTVs signiﬁcantly. Therefore, it is best

to avoid the possibility of these stress concentrations all

together by tenting the PTVs. However, it needs to be

emphasized, that this issue is important only for severe use

conditions with temperature cycles of about ∆T≥50°C, as

can be seen in Table B-2.

In Table B-1 in Section B-1.2.2 the minimum fatigue duc-

tilities resulting from two accelerated fatigue tests of PTVs

in MLBs are given. These estimates of the copper deposit

properties in the PTV barrels are used in Table B-2 to esti-

mate the minimum fatigue lives for a number of typical

electronic use environments. The fatigue lives are given

together with the pertinent information on the use condi-

tions and the resulting stresses and strains.

The results in Table B-2 indicate that the PTVs of good

quality do not constitute a reliability threat to most product

applications in the ﬁeld. Only for the more severe use envi-

ronments would premature failures be anticipated. How-

ever, the results in Table B-2 would change drastically for

PTVs of low quality.

The DfR-process needs to emphasize a physics-of-failure

approach. The process might involve the following steps:

A. Identify Reliability Requirements—

expected design life and acceptable cumulative fail-

ure probability at the end of this design life;

B. Identify Loading Conditions—

use environments (e.g., IPC-SM-785) and thermal

gradients due to power dissipation;

C. Identify/Select Assembly Architecture—

substrate selections, material properties (e.g., CTE),

PTV diameter, aspect ratio;

D. Assess Reliability—

determine reliability potential of the designed assem-

bly and compare to the reliability requirements using

the approach shown here; this process may be itera-

tive;

E. Balance Performance, Cost and Reliability Require-

ments.

B-4.0 CRITICAL FACTORS FOR EMERGING ADVANCED

TECHNOLOGIES

The emerging advanced technologies are characterized by

denser packaging resulting in ever smaller structures. Thus,

the temptation exists to drive the PTV diameters ever

smaller and the aspect ratios higher. The DfR principles

detailed in Section B-3.0 need to be kept in mind in the

design and application of these emerging technologies.

B-5.0 VALIDATION AND QUALIFICATION TESTS

Validation and qualiﬁcation tests have not been established

for PTVs. However, the test procedures used in the IPC

round robin program reported in IPC-TR-579, Round

Robin Reliability Evaluation of Small Diameter Plated

Through Holes in Printed Wiring Boards [Ref. B-7: 2],

could be utilized for this purpose.

Efforts are underway within the IPC via a round robin test

program to establish both qualitative and quantitative cor-

relation for a number of promising test methods.

B-6.0 SCREENING PROCEDURES

The crucial task is the elimination of the MLBs with thin-

plated PTVs without signiﬁcantly affecting the remainder

of the MLBs. The fact that the defects not only involve

very thin plating (<10 µm), but occur in conjunction with

substantial stress/strain concentrations, makes this task pos-

sible.

An Environmental Stress Screening (ESS) could employ

the same test setup as the Hot Oil Test (IEC Speciﬁcation

362-2, Test C) [Ref. B-7: 2], for three (3) to ﬁve (5) cycles.

Thus, together with the solder reﬂow operations necessary

for production, the MLBs would experience between eight

(8) to ten (10) such temperature excursions.

Given the result, based on standard IEC test criteria, that

the life under these loading conditions is 32 cycles, this

would consume between 25 and 30 % of the MLBs lives.

Considering the results in Table B-2, that still would leave

adequate life for most use environments.

B-7.0 REFERENCES

1. ‘‘Leading Edge Manufacturing Technology Report,’’

IPC Technical Report IPC-TR-578, The Institute for

Table B−2 Estimates of the Fatigue Life and Time to Failure of PTVs in Some Typical Use Environments from Table A-1

Used

Environment

∆T

[°C]

Estimated

Maximum

Annual

Cycles

Barrel Stress

[MPa/ksi]

Strain Range

∆ε

[%]

Effective

Strain Range

∆ε

max

(eff)

[%]

Minimum

Fatigue Life

[cycles]

Estimated

Time to First

Failure

[years]

Computers 20 1460 67/9.7 0.08 0.20 8.0X10

5 500

Telecomm 35 365 117/16.9 0.14 0.35 75 000 205

Industrial 60 250 173/25.1 0.28 0.71 2 900 12

Automotive 80 365 174/25.2 0.38 0.95 1 200 3.3

July 1996 IPC-D-279