Reliability in the dormant condition

Microelectronics and Reliability, Vol. 20, pp. 33-44 (1980) Printed in Canada. All rishts reserved.

0026-2714/80/010033-12502.00/0 Copyright © 1980 Persamon Prc~,s Ltd.

"RELIABILITY IN THE DORMANT CONDITION" by A.P. Harris, R e l i a b i l i t y Consultant, A.P. Harris + Associates, Ottawa, Ontario.

I.

INTRODUCTION

The purpose of this paper is to examine the effects on r e l i a b i l i t y of equipment which spends a great deal of i t s time in the dormant condition. The paper w i l l l o o k at the effect on the f a i l u r e modes, the f a i l u r e rates, the f a i l u r e d i s t r i b u t i o n s and the possible r e l i a b i l i t y models. Equipment in the dormant state can be i d e n t i f i e d by two characteristics: a)

i t s connection to a functioning system so that i t ready to operate on demand; and

is immediately

b)

i t s non-operating condition where there is a reduction or elimination of most of the physical, e l e c t r i c a l and environmental stresses compared with the operating condition.

The dormant condition does not include equipment operating at very low levels of i t s function (e.g. 5% power output) or equipment which has been disconnected or is in storage. There is a remarkable amount of equipment which spends most of i t s time in the dormant condition and this condition may be the dominating factor in any appraisal of the equipment r e l i a b i l i t y and i t s maintenance support. Most equipment which can be classed as domestic or entertainment equipment spends most of i t s time in the dormant condition. Television sets for example are dormant 75% to 90% of the total calendar time; kitchen appliances more than go%; lighting appliances more than 80%. In the category of professional equipment, the figure for most office equipment located on desks is more than 75%; low-use copying machines more than 75%; electronic testing equipment in laboratories more than 90%. For industrial equipment the dormant mode can account for more than 90% of the time p a r t i c u l a r l y for safety and certain categories of control equipment. One relay manufacturer has advertised that his over-current relay is not l i k e l y to operate for a total of more than ten seconds in f o r t y yearg l i f e . I t should be remembered, however, that some protective relays are not normally used in a true dormant mode. Table l is a summary of well known equipment classes and typical values for the percentage of time spent in the dormant condition.

33

34

A.P. Harris 2.

EFFECT ON THE FAILURE MODES

As 'dormancy' usually indicates the removal or even e l i m i n a t i o n of most of the stress c o n d i t i o n s , the f a i l u r e modes of the equipment or part are l i k e l y to change considerably. Items of mechanical equipment, whose f a i l u r e modes are dominated by conditions of wear or physical damage to metal surfaces in contact, may encounter only those f a i l u r e modes which are s t a t i c in nature such as corrosion, contamination by dust and d i r t and various e f f e c t s of temperature and moisture. In an e a r l i e r study of telephone-type relays the p r i n c i p a l f a i l u r e modes and mechanisms of relays in the operating condition were found to be: l)

Short c i r c u i t of the c o i l or contacts due to accumulation of metal p a r t i c l e s from the manufacturing process over a period of time.

2)

Open c i r c u i t of the c o i l due to corrosion e f f e c t s from the presence of p o l a r i z i n g voltages, high humidity and i o n i z i n g core materials.

3)

Opens and misses due to contamination of the contact surfaces or s t i c k i n g of the contacts due to contact wear and arcing.

Relays which had spent long periods in the dormant mode s t i l l e x h i b i t e d the same p r i n c i p a l cause of f a i l u r e (short c i r c u i t due to metal p a r t i c l e s ) and, to a lesser degree, open c i r c u i t of the c o i l s and contact corrosion. In t h i s case only the l e a s t important mode of f a i l u r e contact erosion, changed from the operating to the dormant c o n d i t i o n . The f a i l u r e rates between the two conditions was, t h e r e f o r e , not g r e a t l y d i f f e r e n t . With most types of valves the p r i n c i p a l f a i l u r e modes are leaks or malfunction of the actuating mechanism. The p r i n c i p a l f a i l u r e mode is often removed i f the valve is in the dormant state although t h i s assumes that a l l pressures have been r e l i e v e d on the valve. 3.

EFFECTS ON FAILURE RATES

As the stresses are r e l i e v e d or removed while the item is in the dormant state i t follows that the f a i l u r e rates in t h i s state are much less than that in the operating c o n d i t i o n . In the past a crude rule of thumb has been used that the r a t i o between operating and non-operating f a i l u r e rates was about lO to I . However, more precise information has subsequently become a v a i l a b l e which indicates that t h i s lO to l r a t i o is p e s s i m i s t i c . From studies conducted by the Rome A i r Development Center through the Martin Marietta Aerospace Corporation and others (see References l through 4) more precise data is now a v a i l a b l e . Table 2 is a l i s t of common equipment or parts showing the dormant f a i l u r e rates derived from f i e l d studies. When t y p i c a l assemblies of mechanical parts are compared in t h e i r operating and dormant c o n d i t i o n s , the r a t i o of the f a i l u r e rates is about 30 to l , although r a t i o s as high as 60 to l are possible. When assemblies of e l e c t r o n i c parts are compared a r a t i o of 80 to l is t y p i c a l . For integrated c i r c u i t devices t y p i c a l

dormant f a i l u r e rates are in

"RELIABILITY IN THE DORMANT CONDITION"

35

the range of l to 4 failures per b i l l i o n hours. For electromechanical components which may have b~th e l e c t r i c a l and mechanical f a i l u r e modes the difference between operating Bnd dormant f a i l u r e rates appears to be less pronounced. In the case of switches for example the r a t i o between operating and dormant f a i l u r e rates is in the order of lO to I . 4.

EFFECTS ON FAILURE DISTRIBUTION

I t is customary to regard most electronic and e l e c t r i c a l parts as possessing patterns of f a i l u r e which follow exponential d i s t r i b u t i o n s and mechanical components as following normal or other f a i l u r e d i s t r i b u t i o n s Which show increasing f a i l u r e rates with time. Exponential d i s t r i b u t i o n s imply constant f a i l u r e rates. There is evidence, however, that all classes of components have more or less constant f a i l u r e rates in the dormant condition. (Ref. l ) This follows from observations on f a i l u r e modes. Moving mechanical parts possess f a i l u r e modes due to wear, and phenomena which result in increasing f a i l u r e rates with time. In the dormant mode these failures do not occur and the remaining f a i l u r e modes tend to produce random f a i l u r e s or exponential d i s t r i b u t i o n s . For systems made up of mixed mechanical and e l e c t r i c a l . p a r t s there is f i e l d evidence that the system failures follow an exponential pattern. This effect has been observed in large missile systems during long-term storage in the dormant mode. (See Ref. 3) 5.

RELIABILITY MODELS

The most common r e l i a b i l i t y models for parts are found in MIL-HDBK217. These models usually are in the form of a base f a i l u r e rate modified by a series of environmental, e l e c t r i c a l .stress, quality or other factors. The MIL-HDBK-217 model for relays with 5 amp. contacts is shown in Fig. I . In this model the base f a i l u r e rate is dependent on temperature and e l e c t r i c a l stress and this rate is modified by environmental, contact, cycling rate and design factors. A model appropriate to the dormant condition can.be derived by eliminating from the equation a l l factors dependent on operation. This action reduces the model to a new ( d i f f e r e n t ) base rate modified p r i n c i p a l l y by design factors. The new base rate can be derived from the equation provided in the MIL-HDBK. Using the "operating" model, a f a i l u r e rate of 2.79 F/million hr. is calculated for a relay with a 5 amp. inductive load and a stress r a t i o of 50%. Using the reduced "dormant" model a f a i l u r e rate of .058 F/million hr. results. Figure 2 shows the same concept applied to a PC Board Connector. The same principle has been applied to a valve model developed by the author. The "operating" model has a base f a i l u r e rate depending on design, modified by size, environmental and pressure/temperature factors. To this is added the actuator f a i l u r e rates from IEEE - Standard 500. (Reference (7)) These models provide an operating

f a i l u r e rate of I0-20 f a i l u r e s /

36

A.P. Harris

million hr. and a dormant f a i l u r e rate about l.O f a i l u r e s / m i l l i o n hr. In considering dormant r e l i a b i l i t y models for equipment or systems the simplest approach is to: a)

Derive an environmental p r o f i l e or condition p r o f i l e over the time period under consideration. This p r o f i l e can be extended back to the factory door i f desired and can include all the various operating and dormant states.

b)

Estimate the probability of the equipment surviving each condition or state. The probability of surviving to the desired time is the product of the p r o b a b i l i t i e s of surviving a l l conditions up to that time, assuming no repair action.

c)

d)

Modify the model with intended diagnosis and repair actions and other circumstances.

The equipment or system r e l i a b i l i t y under specific conditions can be estimated using conventional techniques. A hypothetical

system and the model is shown in Fig. 3.

To confirm the presence of r e l i a b i l i t y or safety hazards a f a u l t tree analysis can be performed in each of the operating or dormant conditions using appropriately restricted f a i l u r e modes for the latter. The preceeding paragraphs presented some actual f i e l d data for parts in the dormant condition and some methods of deriving f a i l u r e rates from MIL-HDBK models or by simple ratios. I t is interesting to compare the answers produced by these methods and this is done below: RELAYS~ 5 AMP Source

Dormant Failure Rate

Field Data

.032

Ratio Method - 1:60 1:30

.046 .092

Model

.058

CONNECTORS~ PCB Field Data

.0015

Ratio Method - 1:30 1:60

.0014 .0007

Model

.042

CONCLUSIONS Much industrial equipment spends i t s l i f e

in a dormant condition.

Parts and equipment which spend most of t h e i r time in a dormant condition require special treatment for r e l i a b i l i t y analysis and reconsideration of f a i l u r e modes, d i s t r i b u t i o n s and modelling.

"RELIABILITY IN THE DORHANT CONDITION"

Failure rate data for parts in the dormant condition is available from field studies, modified MIL-HDBK models or simple ratios of operating failure rates. Such analysis is essenti&l to demonstrate the true usefulness of the equipment in the intended appli~atlon.

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A.P. Harris

REFERENCES

(1)

D.F. Cottrell et a l . , Effects of Dormancy on Non-electronic Components and Materials, Martin Marietta Space Corp., NTIS AD/A-O02 838.

(2)

J. Bauer, D. Cottrell et a l . , Dormancy and Power On-Off Cycling Effects on Electronic Equipment and Part R e l i a b i l i t y , Martin Marietta Aerospace, NTIS AD-768 619, (Aug 1973).

(3)

S.M. Charkasky, Lon9 Term Storage and System R e l i a b i l i t y , Singer-General Precision, Kearfott Division, New Jersey.

(4)

Northern Electric Co., R e l i a b i l i t y Bulletin 9, Shelf Life of Electronic Components, (1969).

(s)

T. Wesolowski and G. Fletcher, System R e l i a b i l i t y Calculation When Some Components are Subject to Only Intermittent Short Demands, Second National R e l i a b i l i t y Conference (U.K.), page 6c/3/I to 6c/3/9, (1979).

(6)

MIL-HDBK-217B, R e l i a b i l i t X Prediction, U.S. Dept. of Defence, (1974).

(7)

ANSI/IEEE STANDARD 500-1977, Guide to the Collection and Presentation of E l e c t r i c a l , Electronic and Sensin9 Component R e l i a b i l i t y Data for Nuclear Power Generating Stations~ IEEE Standards Board, (1977).

"RELIABILITY IN THE DORMANT CONDITION"

TABLE l

39

Typical Values f o r Percentage of Calendar Time For Equipment in the Dormant Conditio.n

DOMESTIC APPLIANCES Television Sets Kitchen E l e c t r i c a l Appliances

75% 97%

Personal Use Taxis~

93% 38%

CARS

PROFESSIONAL EQUIPMENT Personal Calculators Small Copying Machine Electronic Test Equipment

98% >90%

INDUSTRIAL EQUIPMENT Safety Equipment Standby Power Valves (most) A i r Conditioning B u i l t - i n Test Equipment (MIL)

98% >90% >75% 50-80% 99%

40

A.P. Harris

TABLE 2

Typical Dormant Mode Failure Rates '(Ground Environment) FAILURES PER MILLION HR.

ACTUATORS, LINEAR HYDRAULIC BATTERIES, LEAD ACID BEARINGS, BALL BLOWERS, AXIAL BOARDS, P.C. CAPACITORS, GENERAL CAPACITORS VARIABLE, AIR CAPACITORS, FIXED, FILM COMPUTER PERIPHERALS MAGNETIC CORES MEMORY DISC CONNECTIONS, SOLDER CONNECTORS, GENERAL DIESEL ENGINES FILTERS, MECHANICAL FITTINGS, HYDRAULIC GASKETS 0 RINGS PACKING HEATERS, ELECTRICAL HOSES, FLEXIBLE INTEGRATED CIRCUITS, GENERAL MANIFOLDS MECHANISMS, POWER TRANSMISSION GEARBOXES - COUPLINGS MOTORS, ELECTRICAL -D.C. GENERATORS,

A.C.,

3 H.P.

A.C. PUMPS, FUEL PUMPS, HYDRAULIC RECTIFIERS, SILICON CONTROLLED (SCR) RELAYS, GENERAL RELAYS, LATCHING, XTAL CAN RELAYS, THERMAL RESISTORS, VARIABLE, W.W. RESISTORS, VARIABLE, GENERAL SLIP RING ASSEMBLIES SOLENOIDS SWITCHES, GENERAL SWITCHES, PRESSURE SWITCHES, PUSHBUTTON SWITCHES, SENSITIVE SWITCHES, STEPPING SWITCHES, TOGGLE TRANSISTORS, SILICON, GENERAL

.030 .006 .011 .125 .00083 .00048 .025 .45 .00002 .148 .000026 .0015 ,898 .035 2.77 .011 O78 001 355 1 74

0015 1 85

.146 .441 220 499 796 114 043 017 032 021 2.00 .163 .020 .II0 .300 .021 .083 1.51 .557 .400 .907 .OOll

"RELIABILITY IN THE DORMANT CONDITION"

TRANSFORMERS, GENERAL TRANSFORMERS, POWER VALVES, GENERAL VALVES, SOLENOID VALVES, FUEL, GENERAL VALVES, CHECK, FUEL VALVES, HYDRAULIC, GENERAL VALVES, RELIEF, HYDRAULIC VALVES, PNEUMATIC, GENERAL

41

0005 0012 017 194 127 235 99 l 26 214

42

A.P. Harris

RELAY OPERATING MODEL

~p :~b

(ITE XT/c X-TI'cyc XlTF)

where : b = base f a i l u r e rate and s t r e s s r a t i o .31 f o r

:

depending

50% r a t i n g ,

ITE : e n v i r o n m e n t a l -TI- c = c o n t a c t

inductive

stress

on t e m p . ,

load type

load.

factor

factor

~Tcyc : c y c l i n g

(operations)

factor

"~F = design f a c t o r

X

P

= 2.79 F/106 HR f o r armature r e l a y , DPDT, 5 amp c o n t a c t r a t i n g , at 50% s t r e s s , ground e n v i r o n m e n t and I0 c y c l e s per hour or l e s s .

DORMANT MODEL ~p = ~bd

(II'E x11-C x I I ' F )

where: bd = base f a i l u r e :

~

P

rate

.0065

= .058

Fig.

F / I O 6 HR

1

Operating and Dormant R e l i a b i l i t y Models for 5A Relays

"RELIABILITY IN THE DORMANT CONDITION"

PCB CONNECTOR OPERATING MODEL ~p = ~b (-ITg XTFp) + NTTcyc vlhere: b = base f a i l u r e rate depending on ambient temp. and i n s e r t material 11"E = environmental stress f a c t o r -rlp = f a c t o r depending on number of pins and ~IL spec quality ;~ = number of pins -ITcy c = frequency of connect/disconnect

P

= .043 F/lO 6 HR f o r a 14 pin connector operating at 40° in a ground environment disconnected I / l O 0 hr. (Type A m a t e r i a l )

DOR~IANT MODEL pd = ~b (TFE x'Irp) = .028

Fig. 2

F / I O 6 HR

Operating and Dormant R e l i a b i l i t y Models f o r PCB Connector

43

44

A.P.

SYSTEM

MODELLING

-

Harris

STANDBY

GENERATOR

A

w Dt

j..

W

6 i

1

F

T~ where A = s h i p p i n g C :

checkout

D = dormant E-G : H : Assuming

Reliability

R

=

period

period

operational

periodic

operating

test

periods

reliability

between t e s t

Fig.

3

period

period

RA x RB x Rc . . . . .

.Q/~DTD

test

periods

no r e p a i r ,

R:

E

stress

B = installation

G

X~_-

at

operating

x RH

periods:

A E TE

S i m p l e System Model f o r Hypothetical System

period: