IPC-D-279 EN.pdf - 第74页

Contamination The presence of contamination on the sur- face will increase the moisture absorption. The critical rela- tive humidity can be lowered by the presence of contami- nants. The nature of these contaminants will…

DC voltage gradients, since electronic circuitry is

most prone to CAF formation under these condi-

tions;

7) Avoid testing conditions which create failure mecha-

nisms and failure modes which will not be observed

during normal operating life and use conditions.

C-4.0 CRITICAL FACTORS FOR EMERGING ADVANCED

TECHNOLOGIES

The emerging advanced technologies are characterized by

denser packaging resulting in ever ﬁner conductor line

widths and spacings. Without changes in the material and

the operating environment, which for economical and prac-

tical reasons are not likely, ﬁner lines and spacings result

in reduced insulation resistance and increased threat of

CAF formation. The DfR principles listed in Section C-3.0

need to be kept in mind in the design and application of

these emerging technologies.

C-5.0 VALIDATION AND QUALIFICATION TESTS

C-5.1 SIR Test Procedures

The most commonly used

test vehicle for measuring SIR is an interdigitated comb

pattern. These patterns exist in a variety of conﬁgurations

with spacing between conductors ranging from 0.15 mm to

1.25 mm. SIR tests are carried out at elevated temperature

and humidity levels; however, some tests are performed

with a bias voltage applied throughout the duration of the

test, while others are performed without electrical bias

being applied.

Bias voltages applied during testing ranges from 10 V to as

much as 500 V. Periodically a test voltage of typically 100

V, for the electrically biased tests with reversed polarity, is

applied to measure the insulation resistance. In the case of

the electrochemical migration tests required by Bellcore

[Ref. C-7: 18] the bias voltage and the test voltage have the

same polarity.

SIR tests are normally performed at accelerated conditions

with elevated temperature and humidity levels. The test

conditions range from 35°C/95%RH to 85°C/85%RH with

test durations varying from 100 to 500 hours. Pass criteria

also vary from 100 MΩ to 200 MΩ. The electrochemical

migration test requires that any decline in insulation resis-

tance be less than a decade for the sample to pass. Table

C-1 compares the variation in the test conditions for the

IPC SIR test [Ref. C-7: 19] for soldering ﬂux and the

Bellcore SIR and migration tests [Ref. C-7: 14].

The variation in the test parameters, as illustrated in Table

C-1, results in a large variation in observed SIR data. As

indicated previously, the insulation resistance is an extrin-

sic property of the material sample under investigation.

This property will be affected by the test pattern, tempera-

ture, humidity, bias voltage and duration chosen for the test

as well as the contamination associated with prior process-

ing steps. This contamination may result in electrochemical

corrosion.

SIR readings are sensitive to and affected by a number of

factors.

The variation in the test parameters, as illustrated in Table

C-1, results in a large variation in observed SIR data. As

indicated previously, the insulation resistance is an extrin-

sic property of the material sample under investigation.

This property will be affected by the test pattern, tempera-

ture, humidity, bias voltage and duration chosen for the test

as well as the contamination associated with prior process-

ing steps. This contamination may result in electrochemical

corrosion.

SIR readings are sensitive to and affected by a number of

factors.

C-5.1.1 Factors Affecting SIR Readings

Geometry

The geometry of the test pattern is of primary

importance. When a bias voltage is applied, an interdigi-

tated comb pattern experiences a distributed resistance due

to the number of parallel traces over which the measure-

ment is taken. The length of the interacting conductors

divided by the separation between conductors is deﬁned as

the number of squares. In comparing data from two differ-

ent comb patterns, the readings are sometimes reported as

ohms/square.

Humidity When a monolayer of water is absorbed onto

the surface of an epoxy/glass printed board, the water mol-

ecules hydrogen-bond to the epoxy making them essen-

tially immobile. These hydrogen-bonded water molecules

can exist as either continuous coatings or as discrete

islands [Ref. C-7: 20]. As subsequent water layers are

added, thicker ﬁlms are formed allowing the dissolution of

contaminants and the formation of hydrated ions which can

move under the inﬂuence of an electric ﬁeld [Ref. C-7: 21].

Conductivity measurements made on aluminum oxide

revealed that for ﬁlms with thicknesses of less than three

(3) monolayers, the surface conductivity is two orders of

magnitude below that of bulk water [Ref. C-7: 22]. The

surface conductivity increased asymptotically with the

increase in the number of monolayers with equilibrium

being reached above 20 monolayers. Evidence indicates

that there is a critical relative humidity at which a com-

pound exhibits a surge in moisture absorption [Ref. C-7:

23,24]. For example, it has been demonstrated that den-

dritic growth of gold on alumina surface is dependent upon

the relative humidity and that there existed a threshold for

gold migration to occur [Ref. C-7: 25]. It has been shown,

that the critical relative humidity for epoxy coatings is 70%

and that the epoxy degrades over time when exposed to

humid environments [Ref. C-7: 26].

IPC-D-279 July 1996

Contamination The presence of contamination on the sur-

face will increase the moisture absorption. The critical rela-

tive humidity can be lowered by the presence of contami-

nants. The nature of these contaminants will determine how

much moisture is absorbed at a given humidity level. If

these contaminants are ionic in nature, they can enhance

electrochemical reactions that occur in the presence of a

bias voltage.

Voltage The bias voltage applied across the insulator will

set-up a response in the dipolar polymer substrate. In per-

forming SIR testing, it is important that the bias voltage

chosen is realistic as it relates to the actual operating and

use conditions of the electronic assembly. Typical test

methods require 45-50 V bias because this represents a

moderate accelerating condition relative to the +/- 15 V

circuits common in telecommunication hardware. Exces-

sively high voltage tests for routine circuits can lead to

damage mechanisms and failure not representative of pro-

duct use.

C-6.0 SCREENING PROCEDURES

For the threats to reliability from low SIR and CAF forma-

tion no effective screening procedure exists. The best that

can be done is following the DfR recommendations in Sec-

tion C-3.0 and the testing of representative samples using

the test procedures discussed in Section C-5.0.

C-7.0 REFERENCES

1. Engelmaier, W., ‘‘On the Parametric Temperature/

Humidity Dependence of Insulation Resistance of

Covercoated Fine-Line Flexible Printed Wiring,‘‘ Proc.

Nat, Electronic Packaging and Production Conf,

(NEPCON West), Anaheim, CA. February 1976, p. 87.

2. ‘‘D-C Resistance or Conductance of Insulating Materi-

als,’’ ASTM D 257-78. Annual Book of ASTM Stan-

dards, ASTM, Philadelphia, PA.

3. Gorondy, E. J., ‘‘Surface Insulation Resistance Part I:

The Development of an Automated SIR Measurement

Technique,‘‘ IPC Technical Paper IPC-TP-518, The

Institute for Interconnecting and Packaging Electronic

Circuits, Lincolnwood, IL, September 1984.

4. Zado, F. M., ‘‘Effects of Non-Ionic Water Soluble Flux

Residue,’’ Western Electric Engineer, Vol. 1, No. 1,

1983.

5. Kawanobe, T., and K. Otsuka, ‘‘Metal Migration in

Electronic Components,’’ Proc. 32nd Electronic Com-

ponents Conf., San Diego, CA, May 1982, pp. 220-

228.

6. Uhlig, H. H. ‘‘Corrosion and Corrosion Control,’’ 2nd

ed., John Wiley & Sons, Inc., New York, NY, 1971,

p.322.

7. Lando, D. J., J. P. Mitchell, and T. L. Welsher, ‘‘Con-

ductive Anodic Filaments in Reinforced Polymeric

Dielectrics: Formation and Prevention,’’ Proc. 17th

Ann. Reliability Physics Symp., April 1979, pp. 51-63.

8. Augis, J. A., D. G. DeNure, M. J. LuValle, J. P. Mitch-

ell, M. R. Pinnel, and T. L. Welsher, ‘‘A Humidity

Threshold for Conductive Anodic Filaments in Epoxy

Glass Printed Wiring Boards,’’ Proc. 3rd Int, SAMPE

Electronics Conf.. June 20-22, 1989, pp. 1023-1030.

9. Ready, W. J., S. R. Stock, and L. J. Turbini, unpub-

lished results.

10. Turbini, L. J., unpublished work.

11. Cotter, M., and W. Engelmaier, unpublished results.

12. LuValle, M. J., T. L. Welsher, and J. P. Mitchell, R. J.,

‘‘A New Approach to the Extrapolation of Accelerated

Life Test Data,’’ Proc. 5th Int. Reliability and Main-

tainability Conf., Biarritz, France, 1986, pp. 630-635.

Table C−1 SIR Test Parameters for Some Industry Tests

Parameters

IPC-TM-650

Solder Flux

Bellcore

SIR

Bellcore

Electromigration

Test Voltage 100 V 100 V 45 to 100V

Bias Voltage 50 V 50 V 10 V

Polarity Reverse Reverse Same

Environment 85°C/85%RH 35°C/85%RH 85°C/85%RH

Duration 7 days 4 days 500 hours

Lines/Spacing 0.4/0.5 mm 0.64/1.27 mm

0.32/0.32 mm

Number of Squares ~1000 ~100

~500

Failure Criteria 100 MΩ 10

MΩ

2x10

MΩ

SIR less than 1 decade

decline

July 1996 IPC-D-279

13. Klein Wassink, R. J., ‘‘Soldering in Electronics,’’ 2nd

ed., Electrochemical Publications, Ltd., Isle of Man,

UK, 1989, pp. 215-216.

14. Sinnadurai, F. N. ‘‘Handbook of Microelectronics

Packaging and Interconnection Technologies,’’ ed.,

Electrochemical Publications, Ltd., Ayr, Scotland,

1985, p. 271.

15. Lahti, J. N., R. H. Delany, and J. N. Hines, ‘‘The Char-

acteristic Wearout Process in Epoxy-Glass Printed Cir-

cuits for High Density Electronic Packaging,’’ Proc.

17th Ann. Reliability Physics Symp., April 1979, pp.

39-43.

16. Welsher, T. L., J. P. Mitchell, and D. J. Lando, ‘‘CAF

in Composite Printed-Circuit Substrates: Characteriza-

tion, Modeling and a Resistant Material,’’ Proc. 18th

Ann, Reliability Physics Symp., April 1980, pp. 235-

237.

17. Welsher, T. L., J. P. Mitchell, and D. J. Lando, ‘‘Con-

ductive Anodic Filaments (CAF): An Electrochemical

Failure Mechanism of Reinforced Polymeric Dielec-

trics,’’ Proc. Ann. Electrical Insulation and Dielectric

Phenomena Conf., 1980, pp. 234-239.

18. ‘‘Generic Physical Design Requirements for Telecom-

munications Products and Equipment,’’ Bellcore Tech-

nical Reference TR-NWT-000078, Issue 3. December

1991.

19. ‘‘Surface Insulation Resistance, Fluxes,’’ Test Method

2.6.3.3. Test Methods Manual IPC-TM-650, The Insti-

tute for Interconnecting and Packaging Electronic Cir-

cuits, Lincolnwood, IL.

20. Krumbein, S. J., ‘‘Metallic Electromigration Phenom-

ena,’’ IEEE Trans. Components, Hybrids, and Manu-

facturing Technology, Vol. 91, CHMT-11, No. 1,

March 1988, pp. 5-15.

21. Cvijanovich, G. B., and A. R. Baily, ‘‘Technical Report

on the Conductivity and Electrolytic Properties of

Adsorbed Layers of Water,‘‘ Advanced Product and

Manufacturing Technologies Dept., AMP Inc., Harris-

burg, PA.

22. Howard, R. W., ‘‘Electrochemical Model for Corrosion

of Conductors on Ceramic Substrates,’’ IEEE Trans,

Components, Hybrids, and Manufacturing Technology.

Vol. CHMT-4, No. 4, December 1981, pp. 520-525.

23. Jennings, C. W., ‘‘Effect of High Moisture Environ-

ments of Printed Wiring Board Insulation,’’ Proc.

Printed Circuit World Convention II (IPC), June 1981.

24. Sinclair, J. D., ‘‘An Instrumental Gravimetric Method

for Indexing Materials, Contaminants and Corrosion

Products According to Their Hygroscopicity,’’ J. Elec-

trochemical Society. Solid State Science and Technol-

ogy, May 1978, pp. 734-742.

25. DerMarderosian, A., and C. Murphy, ‘‘Humidity

Threshold Variation for Dendritic Growth on Hybrid

Substrates,’’ Proc, 15th Ann. Reliability Physics

Symp., April 1977, pp. 92-100.

26. Lefebvre, D. R., K. M. Takahashi, A. J. Muller, and V.

R. Raju, J. Adhesion Science and Technology, Vol. 5,

No. 3, 1991, pp. 201-227.

IPC-D-279 July 1996