Mitsubishi 20-L Specifications download pdf (Page 10)

Dec. 20

Mitsubishi IPM-series Application Note

Reliability

7.Reliability

7-1. Introduction

It has only been somewhat over 30 years since

semiconductor devices such as rectifier diodes,

thyristors, and transistors gained widespread

acceptance for use in industrial machinery and

consumer appliances, but during that period the

reliability standards for these devices have made rapid

advances.

In equipment where high reliability is a must, failure

rate of the semiconductor devices must range from 10 to

100 FIT (1 FIT=10

-9

/hours). Of course, to achieve such

reliability in the equipment itself, not only must each

individual device be reliable, but also it is also extremely

important to match the specific characteristics of the

device with its application within the piece of equipment.

In fact, information obtained in field studies show that for

semiconductor devices manufactured using identical

procedures, failure rates in the field could vary by a

factor of 10 depending simply on how the device was

used.

The following information covers device reliability with

regards to how a device is used. An introductory

discussion is also presented on quality-control

procedures, and some examples of reliability testing

data are given.

7-2. Basic Concepts of Semiconductor Device

Reliability

7-2-1. Semiconductor Device Failure Rate Varied with The Lapse of Time

The failure rate of devices used in an average piece of equipment can be expressed by using the bathtub curve shown in Fig.7.1,

line (a). Taken from the standpoint of time, device failures can be classified as an early failure, random failure and wear out failure

period.Three points must be considered regarding the service life of a device; early and random failures rate, and lifetime before

wear out.

But the failure rate of semiconductors is illustrated by line (b) in the graph, where failure rate is shown to gradually diminish as a

factor of time. In other words, a notable feature of semiconductor devices is that the longer a particular device has been used, the

more stable it will be. Viewed from a different perspective, even though random failure rate has been reduced to virtual stability,

the failure distribution pattern shows early failures to still be prevalent. As shown by Fig.7.2 where failure rate versus time is given

for an actual device, the highest failure rate occurs immediately after manufacture, but the process of ageing and debugging

gradually lowers this failure rate.

The next step is with the user, who assembles, adjusts, and takes the device aging. Failure rates continue to decline during this

period also. Generally, the rate for major defect during this period drops to less than 0.1%, and if this rate is exceeded by a

substantial margin, one must look for a fault in the circuit design, assembly procedure, or the device itself. Unless the problem is

found and corrected, frequent field failures will be the likely result. In most cases, the field failure rate can be correlated to major

defect during this period, so this is an important aspect of device reliability.

Upon transferring the equipment to field service, the stress level is reduced further, with a corresponding drop in failure rates.

Failure rates normally range from several FIT to several hundred FIT during this period.

On the other hand, the user must design greater margins. For example, diodes and thyristors should be operated at 50 ~ 60% of

their maximum voltage ratings or lesser, and junction temperatures should not exceed 70 ~ 80% of maximum rating. It is also

important to remember that a device must be in working harmony with other components in the circuit for maximum reliability

standards can be assured.

When designing a piece of equipment for reliable service, device selection must be considered from a standpoint of performance,

reliability, and economy. Since it is not easy to achieve high performance/reliability and economy at the same time, a balance must

be struck on the side of practical value. In other words, device selection should be based on the user’s expectations for the

machine he is designing.

Fig.7.1 Failure Rate Versus Time

Fig.7.2 Semiconductor Device Failure Rate Versus Time

ado

m F

Wearout Failure

Tim

Time

O-A-B-C Initial Failure (Factory)

C-D Early Failure (Field)

D-E Random Failure (Field)

E-F Wearout (Field)

(0-A-B-C-D Debugging Period)

Failure Rate

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 ... 42 43

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