ACCELERATED HUMIDITY TESTS FOR PLASTIC ENCAPSULATED DEVICES

Byron L. Bair and Erwin A. HerrSemiconductor Products Department

General Electric CompanyElectronics Park

Syracuse, New York

Abstract

This late news paper presents the results of theexamination of the screening effectiveness of steampressure and boiling water stresses for rapidlyevaluating potential failures on long term, accele-rated humidity life testing. Studies such as theseshould allow the development of standard acceleratedevaluation tests for assessing device design capabili-ties on a periodic basis and for acceptance of groupsof devices. Also, it could be possible to determinethe relationship between these accelerated tests andapplication environments.

IntroductionMany significant improvements have been made in

plastic encapsulated semiconductor devices since theirintroduction in 1962. An area of concern in militaryand high reliability applications has been the cap-ability of the devices to withstand high humidityenvironments. Several accelerated stresses such as;steam pressure, boiling water and high temperatureplus humidity have been used to evaluate deviceresponse. High temperature plus humidity tests havebeen run with and without electrical bias. Theresults reported in this paper are those of humiditytests without bias. This was demonstrated to be theset of conditions producing the greatest deviceresponse in some tests which were run for 5000 hours.

An area of great interest to the users andmanufacturers of plastic encapsulated devices is thedevelopment of a standard humidity plus elevatedtemperature test for evaluating package designs. Itis very desirable to run these humidity stresses asaccelerated tests so that it is economical in termsof direct costs and in terms of the time involved toobtain the results.

Test ProgramA program was started to investigate these

various types of tests and to find any correlation be-tween the tests. These tests were performed on NPNand PNP General Purpose Amplifiers and Switches. Thetests were completed and the results compiled inMarch 1970. The test matrix involved over 1,900devices and over 10 million unit hours of temperatureplus humidity stress were accumulated. Discretetransistors were used in this particular investiga-tion of the analysis of device response. The deviceswere electrically tested to a family of specificationsand no special preconditioning or screening was used.

It was also decided to determine the-screeningeffectiveness of the boiling water and steampressure tests. The experimental design consisted of:

- Read the electrical parameters,- Stress in boiling water or steam pressure,- Read the electrical parameters,- Bake for 24 hours at 150°C,- Read the electrical parameters,- Stress at 85°C and 85% relative humidity with-

out electrical bias for 5000 hours, withelectrical parameter readouts at 168, 1000,

3000 and 5000 hours.

Experimental control units were included in thedesign. These devices were stored at room ambientand were measured at each time the experimentaldevices were readout. To eliminate one source ofvariability, a consistent readout procedure wasadopted for the stressed devices. This was:

- Remove the devices from stress,- Remove moisture on the outside of the devices,- Store devices at room ambient,- Read the electrical parameters between 4 and

8 hours after removal from stress.

Test ResultsAfter an analysis of the test results, an attempt

was made to model the response from the pressurecooker, boiling water, and humidity life tests. Afit was found to the Arrhenius model and the developedmodel is shown in Figure 1. This model is describedin general by the equation X = eA-B/T where X is theresponse parameter of interest such as the failurerate, A and B are constants and T is absolutetemperature in degrees Kelvin. If the logarithm of Xplotted against a linear reciprocal scale of T, thenthe constants A and B represent the intercept and theslope, respectively,of the resultant straight lineplot.

The data points shown in the figure were spreadenough and there were enough units involved in thetotal complex of tests so that the response model wasverified. At the time the information was beingcollected a check was made for the length of time anyinduced response would remain visible in the unit. Itwas found that there was no significant recovery, atroom temperature, of any induced device response forperiods of time up to 50 hours.

The results, and the model analysis, showed thatboth boiling water or the steam pressure type of testwould give response in less than 100 hours of stress.Therefore, these appear to be attractive for rapidand economical evaluation tests of the devices.

It was found, as expected, that all of the unitsrecovered to approximately the initial values, afterthe bake. There was very little device parameterresponse observed except in ICBO' Typical responsecurves after the boiling water stress screen are shownin Figure 2 and another typical curve after thepressure cooker initial stress is shown in Figure 3.Any device with an ICBO of over 100 nanoamperes afterthe initial stress was labeled a "failure" but thedevice was not removed from the lot. After therecovery bake it was desired to increase the contrastof the units that would "fail" again, so the definit-ion of the failure limit was tightened to 50 nA. Theresults of this stress test were then analyzed todetermine the screening effectiveness and to estimatethe failure rates of the devices.

The boiling water initial stress experimentalresults are summarized in Figure 4. At the time of

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preparation of paper only 3000 hours of stress in the85°C, 85% relative humidity were available. Thistest presently is continuing. The initial stress asshown in the illustration, consisted of 150 hours ofboiling water and then there was a 100% recovery bakeat 150°C for 24 hours. The screening and life testfailure criteria are also shown in the illustration.When the chart in the illustration is analyzed it maybe seen that of the 39 units that eventually wereconsidered failures on the humidity life test, 35 ofthese could have been located by the initial stressscreen. In a similar fashion the number of unitsthat would have been rejected which later were foundto be good is shown in the chart. The screeningeffectiveness, as indicated, was 90% in removingrejected units or units that would later fail, and82% effective in passing units that would not berejected later. When the estimated failure ratesare examined, the screening effectiveness can be seento be 3 times.

The results of this same test program using 6psig of steam as the initial stress are suTmmarized inFigure 5. In this case, after 5000 hours of humiditylife testing, it can be seen that the screeningeffectiveness was 100% in removing potential rejects,and 93% in passing non-rejects. The screeningeffectiveness in this case was 10 times. The summaryof the data for the 15 psig steam stress is shown inthe same manner in Figure 6. In this case theeffectiveness of the screen in removing rejects was

71% and in passing non-rejects 91%. The screeningeffectiveness was 5 times. It should be noted thatthese results are quite good in terms of the lowfailure rates estimated, which can be expected sincethe line is a very high volume line and is in verygood process control.

Conclusions

The initial success of this recent investigationshows that these techniques offer a definite poten-tial for future work and application. The possibleapplications include not only economical and accele-rated reliability evaluation tests, but groupevaluation types of tests. However, since thestresses in this program were not followed by operat-ing power life tests, the relationship of theaccelerated humidity and temperature tests to normaloperating conditions has not been completelydetermined.

It is planned to extend this work and obtainsome of these types of results for integrated circuits.It is expected that different results will be seenparticularly from the thinner and more complexmetallization patterns. The monitoring that will bedone on the devices will also have to includeinvestigations of any potential corrosion effects.It is also planned to assess the effects of barriersand glassivation against ion migration and corrosioneffects.

More of this type of work should be done andother encapsulating materials and devices should beexamined. The devices used on this program were

passivated with silicon dioxide and no additionalcoating was added to the chip before application ofthe epoxy encapsulation. The induced failuremechanisms were related to surface effects so thatleakage currents and current gains were logicalparameters to monitor. The electrical parameters thatwould have to be monitored for different devices mightwell change.

is expected to gain even more favor particularlywhen complex packages such as MOS memories areconsidered. As the number of leads emanating fromthe package increases, it becomes increasinglydifficult to obtain sealed packages with extremelylow leak rates. Work will be required to overcomethis difficulty. One approach that is being studiedfor plastic encapsulated devices is to provideprotection by sealing the junctions with additionalpassivation. Work must be continued to assess theadequacy of any protection added as a passivation onthe device surface and to assess the potentialadvantages of present encapsulants as well as any new

encapsulants.

Acknowledgment

Some of the information contained in this paper

was included in the paper by E. A. Herr and A. Fox,Reliability in Plastic Encapsulated Semiconductors,presented at the 28th Annual Technical Conference ofthe Society of Plastics Engineers, Inc., May, 1970.The referenced paper is to be published in theConference Proceedings.

EPOXY ENCAPSULATED TRANSISTORSACCELERATED HUMIDITY LIFE-ARRHENIUS FAILURE MODEL

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TEMPERATURE IN DEGREES CELSIUSILINEAR RECIPROCAL ABSOLUTE TEMPERATURE SCALE)

FIGURE 1 ACCELERATED HUMIDITY LIFE - ARRHENIUS FAILURE MODEL

The use of plastic as an encapsulating material

111

PRESSURE COOKER 15 PSIG

LEAST SQUARE MODEL o PRESSURE COOKER i 6 PSIGX X

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BOILING WATER

FAILURE CRITERIA

I'Co. 100 nA MAXhFE 25% CHANGE HUMIDITY LIFE

85'C - 65 °/.RH

FOR T * 40eC) - .001% / K HRS

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EPOXY TRANSISTOR PANRMETER RESPONSE ON HUMIDITY - TEMPERATURE STRESSING NPN NERNPAL PURPOSERMPLIFIER & SWITCHi 0028CONDITIONS: A PRE-STRESSB BOILING WATER

LOT SAMPLE 40 C 150% RAKED 85°C - 8S RH

PERCENTILELEGEND

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CONOITIONS A B C - D---K CONDIOTONS A B C k -o-D

FIGURE 2 TRANSISTOR RESPONSE AFSER BOILING WATER AND BAKE STRESS

EPOXY TRANSISTOR PRAMETER RESPONSE ON HNXMIRITY - TEMPERATURE STRESSING NPN GENERAL PURPOSECONIDITIONS: A PRE-STRESS AMPLIFIER & SWITCH D32BLOT SNMPLE: S0 B 121% PRESSURE COOKER

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FIGURE 3 TRANSISTOR RESPONSE AFTER STEAM PR0SSU01E AND BAKE STRESS

DEMONSTRATION OF SCREENING EFFECTIVEESSAND EFFICIENCY OF SHORT TIDE

ACCELERATED TEMPERATURE - HUMEDITY STRESSING(Epoxy Encapsulated Transistors)

STRESS SCREEN: 150 Hours of Boiling Waterfollowed with 100% Recovery Bake@ 150°C, 24 hours

LIFE TE1ST EVALUATION: 3000 Hours of 850C/85% RHHumidity Life

SCREEN FAILURES CRIERIA: ICBO exceeding 100 nALIFE TEST FAILURE CRITERIA: ICBO exceeding 50 nA

LIFE3TEST

SCRETESTFAIL

PASS

ToTAL

IRWECTS NON-REJECTS35 81- _- -I -

1439

360

441

TOTAL

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48o GRANDTOTAL

SCREEN EFFE CTIVENESS MEASURE IN:Removing Rejects P90Passing Non-Rejects = 82%

BEST ESTIMATE FAILURE RATE:SCREENED PRODUCT = 0.4%/1000 hoursUNSCRENED PRODUCT AT 3000 HOURS = 1.3%/1000 hoursSCRE EFECTIVENESS = 3X

FIGURE 4 SCREEN EFFECTIVENESS - BOILING WATER

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DEMONSTRATION OF SCREENING EFFECTIVENESS

AND EFFICIENCY OF SHORT TIME

ACCELERATED TEMPERATURE - HUMIDITY STRESSING(Epoxy Encapsulated Transistor)

STRESS SCREEN: 64 Hours of Steam Pressure6 PSIG - 110°C

LIFE TEST EVALUATION: 5000 Hours of 85°C/85% RHHumidity Life

SCREEN FAILURE CRITERIA: ICBO exceeding 100 nALIFE TEST F'AILURE CRITERIA: ICBO exceeding 50 nA

LIFE

SCTE REJECTS NON-REJECTS TOTAL

TEST I\ _

FAIL 2 11 130

2

147

158

147 160 GRAND

TOTAL

DEMONSTRATION OF SCREENING EFFECTIVENESS

AND EFFICIENCY OF SHORT TIM

SCREEN EFECTIVENESS MEASURE INRemoving Rejects = 100%Passing Non-Rejects = 93%

BEST ESTIMATE FAILURE RATE:

SCREENED PRODUCT - 0.1%/1000 hours

UNSCREEN PRODUCT AT 5000 HOURS = 1.1%/1000 hours

SCREEN EFFECTIVENESS = lOX

FIGURE 5 SCREEN EFFECTIVENESS - STEAM PRESSURE 6 PSIG

ACCELBERATED TEMPERATURE - EUMIDITY STRESSING(Epoxy Encapsulated Transistors)

STRESS SCREEN: 64 Hours of Steam Pressure15 PSIG - 121°C

LIE TEST EVALUATION: 5000 Hours of 850C/85% RHHumi,dity Life

SCREEN FAILURE CRIERIA: ICBO exceeding 100 nALI TEST FAILURE CRITERIA: ICBO exceeding 50 nA

LIFEEST RECTS NON-REJECTS TOTAL

TEST \

FAIL 5 27 32

PASS 2 _ 286 288

TOTAL 7 313 320 GRAND

TOTAL

SCREEN EFFECTIVENESS MEASURE IN:Removing Rejects = 71%Passing Non-Rejects - 91%

BEST ESTIMATE FAILURE RATE:

SCREENED PRODUCT = 0.2%/1000 hours

UNSCREEIED PRODUCT AT 5000 HOURS = 1.0%/1000 hours

SCREEN EFFECTIVENESS: 5X

FIGURE 6 SCREEN EFFECTIVENESS - STEAM PRESSURE 15 PSIG

113

PASS

TOTAL

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