Microelectron. Reliah. Vol. 19, pp. 279 Io 280 © Pergamon Press Ltd. 1979. Printed in Great Britain
0026 2714/79/0601 0279502.00/0
STEP-STRESS A C C E L E R A T E D LIFE T E S T I N G OF D I O D E S J. S. BORA Reliability Evaluation Laboratory, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India
(Received fi~r publication 27 November 1978)
1. INTRODUCTION
a variable auto transformer. According to manufacturer's specification, the peak inverse voltage is 50 V and the maximum forward current is 150 mA for CD-22 diodes. Wire wound resistors of 350f~ + 5%, 10 W had been connected in series with all the diodes as shown in Fig. 1, firstly to limit the current and secondly in the event of failure of any diode, the electrical stress of surviving diodes is not disturbed. Three diodes, type No. 1N4005, were connected in parallel as shown in Fig. 1, to share the electrical load. Two performance parameters, reverse current and breakdown voltage, were measured initially and at the end of each test level. Then the stress was increased to the next higher level• The initial electrical stress was fixed at the rated level. The duration of test at each level and incremental step height are arbitrary. The term "breakdown voltage" is somewhat a misnomer because it does not at all mean permanent performance damage (in its real sense) as the measurement of breakdown voltage is a nondestructive test. It is to be noted that the current flowing through each diode of group 1 and group 2 is almost the same. Similarly, the voltage across each diode of group 1 and group 3 is also almost the same. This means the diodes of group 1 are experiencing the stresses which the diodes of group 2 and group 3 are experiencing separately.
Silicon diodes, type No. CD-22, have been tested at ambient by the method of step-stress under the following conditions: (a) Sinusoidal voltage and current testing, in which in the first half cycle voltage appears across the diodes and in the next half cycle current flows through the diodes. (b) Sinusoidal current testing, in which current flows through the diodes during half cycle only. (c) Sinusoidal voltage testing, in which voltage appears across the diodes during half cycle only. A total of 30 diodes were taken and divided equally into three groups to be tested under three conditions as mentioned above. The frequency of the electrical stress was 50 Hz. On the average, diodes were tested for 9 hours a day, 280 days a year, for 5 years. The total operating time and the ratio of operating time to the total time come out to be 13,125 hr and 0.3, respectively. The objectives of the test programme are as follows: (1) To verify the correctness of the specified ratings. (2) To assess the capability of the diodes to withstand accelerated electrical stress. (3) To determine the severity and relative effectiveness of voltage-current, current and voltage tests. (4) To study the failure modes.
3. TEST R E S U L T S
The successive levels of peak inverse voltages, maximum currents, test durations, number of failures under each group of test and failure modes have been shown in Table 1. The important test results are as follows:
2. E X P E R I M E N T A L SET U P A N D TEST P R O C E D U R E
Experimental set up is shown schematically in Fig. 1. The alternating voltage is adjusted with the help of
lN4005 Group
0
Group 2
'!0
ID I
230 V OHz
R
Group 3
IR
rR I I0
D =Type C D - 2 2 ,
20
R=350g, * 5 % , lOW
Fig. 1. Electrical life testing scheme for diodes. 279
21
30
280
J. S. BORA Table 1. Step-stress accelerated life testing of CD-22 diodes Voltage and current testing
Step No.
Peak inverse voltage (V)
Maximum current (mAt
Test duration Ihr)
1 2 3 4 5 6 7
50 57 64 71 78 85 99
143 163 183 203 223 243 283
300 400 350 300 260 460 240
8
113
323
9
119
10 11 12 13 14
No. of failures
Failure mode
l
open
215
1 1 1 l 1 2
open short short short short open
340
580
1
short
122 125 127 130 134
348 357 363 37l 383
515 425 1065 870 900
1
open
15
156
446
415
16 17 18 19
170 184 198 212
486 526 566 606
1260 1270 1800 1500
(1) The ratio of highest operating electrical stress to rated electrical stress is about 4 for both voltage and current tests. Comparing the test results of group 2 and group 3, it is quite obvious that current test is less severe than voltage test as in the former case six diodes are still surviving after the termination of tests. (2) Reverse current at 35 V d.c. was found to be less than I nA throughout the test programme for all diodes except during the rainy season. During the rainy season the reverse current was found to be about 30 nA at 2 V d.c. This high reverse current is attributed to encapsulation surface leakage due to high relative humidity in the ambient. (3) Breakdown voltage of all the diodes lies in the range between 70 V and 120 V. Breakdown voltage was found to be consistent throughout the test programme until failure. Even rainy season did not have any influence on the value of breakdown voltage. Out of 24 failures, 21 failures were sudden and complete. Breakdown voltage of three
Current testing No. of failures
Failure mode
Voltage testing No. of failures
Vailure mode
1
open
2
open
I 1
open short
3
almost short (BV = 5lOV)
1 1
open short
2
open
2
short
diodes was measured to be between 5 and 10 V in case of voltage test at the time of failure. This can be considered either degradation type of failure at the fag end or tendency towards short failure. 4. CONCLUSIONS (1) Current test is less severe than voltage test for the same ratio of operating to rated stress. (2) In general, time dependent degradation type of effects are not noticeable as most of the failures are sudden and complete. However, in case of voltage test sudden decrease in breakdown voltage for three diodes has also been observed. (3) The probability of open and short failure is almost equal irrespective of the type of electrical stress and its level.
Acknowledqements--I am thankful to Shri Y. Chellam and Shri D. Kiro for discussions and to Shri S. B. Bhave for the tracing.