Effect of Room-Temperature-Vulcanized Silicone Cure in Device

Chapter Effect of

43

Room-Temperature-Vulcanized

in

Silicone

Cure

Device Packaging

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 17, 2016 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch043

Ching-Ping Wong Engineering Research Center, AT&T, Princeton, NJ 08540

At AT&T, RTV silicone is widely used in all of our Bipolar, Metal Oxide Semiconductor (MOS) and Hybrid Integrated Circuitry (HIC) encapsulation. This RTV silicone has been proven to be one of the best encapsulants for alpha particle, moisture and electrical protection of these sensitive devices. The complete cure of the RTV silicone affects not only its chemical, physical and electrical properties, but also the packaging yield of these devices, especially in the RTV silicone-coated Gated Diode Crosspoint (GDX) - a super-high voltage, ultrafast switch used in AT&T No. 5 Electronic Switch System (ESS) and 256K Dynamic Random Access Memory (DRAM) device packaging. However, it is difficult, if not impossible, to detect the degree of cure of RTV silicone in the device manufacturing packaging process. Microdielectrometry, a recently developed technique which utilizes a miniature IC sensor and a wide range of frequencies to monitor the polymer cure, becomes a very attractive technique. This microdielectrometry, coupled with the time-dependent solvent extraction technique, provides a sensitive tool to quantify the degree of RTV silicone cure. This paper will describe the microdielectric measurement and solvent extraction experiments that we have used to investigate curing of the RTV silicone encapsulant systems to optimize the device packaging yields. The rapid development of integrated circuit (IC) technology from small-scale integration (SSI) to very large-scale integration (VLSI) has had great technological and economic impact on the electronic industry. * The exponential growth of the number of components per IC chip, the steady increase of the IC chip dimension and the exponential decrease of minimum device dimensions have imposed stringent requirements, not only on the IC physical design and fabrication but also in the IC device packaging. For bipolar, metal oxide semiconductor (MOS) and hybrid IC, Room Temperature Vulcanized (RTV) silicone has proven to be one of the best device encapsulants for protecting these devices. It is also important that the device encapsulant receives proper cure during packaging to ensure the long-term device reliability. It has been known for some time that the dielectric method was successfully used in monitoring the cure of epoxy resins.* The enormous change of the material dielectric properties during the transformation of the resin from a viscous liquid to a brittle solid reveals the crucial degree of cure of the polymeric material. These simple dielectric measurements have become widely used in material analysis and process control. The miniaturized integrated circuit sensor, with the on-chip sensor and wide frequency range, makes microdielectrometry a very attractive technique to monitor the polymer cure. This is especially important in the RTV 111

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0097-6156/87/0346-0511$06.00/0 © 1987 American Chemical Society

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

POLYMERS FOR HIGH T E C H N O L O G Y

512


silicone system where the complete cure of the material is critical in the nonhermetic and hermetic packaging processes. The R T V silicone-coated Gated Diode Crosspoint ( G D X ) , an ultrafast and super high voltage switch, and 256K Dynamic Random Access Memory ( D R A M ) devices are excellent current examples. Besides, we have developed a time-dependent solvent extraction technique that can couple with the dielectric measurement to quantify the degree of cure of the R T V silicone encapsulant. The combination of this new microdielectric measurement and time-dependent solvent extraction technique provides a definitive method which detects the degree of cure of this R T V material. In this paper, we will describe both of these dielectrometric and solvent extraction experiments that we have investigated for curing the R T V silicone system. EXPERIMENTAL 1.

Material A.

Preparations

Dielectric Measurement of RTV

Silicone

A thin layer (approximately 10 m i l thick) of R T V silicone was coated on the Micromet mini-dielectric sensor. The R T V silicone was first cured at room temperature, then in an oven at 120°C for a period of time, similar to certain R T V production cure schedules. The permittivity and loss factor of the R T V silicone were recorded during this cure cycle. B.

Solvent Extraction

of RTV

Silicone

R T V silicone samples were obtained from a thin coating which had been cured on a Teflon-coated steel or aluminum plate according to the standard cure schedule (room temperature and final oven cure). For R T V diluted to 50% by weight of the xylene, 20 grams of the material coated on the 5"x5" Teflon-coated aluminum plate will result in a 25 m i l uniformly thick sample. 2.

Experimental A.

Dielectric

Measurements Measurements

The microdielectric sensor which incorporates an I C on-chip emiconductor diode thermometer for combined dielectric and temperature measurements is only 0.5 mm thick, 5 mm wide, with a flat Kapton ribbon package 35 cm long. A small sample of the R T V silicone dispersion (approximately 20 mg) was placed on the sensor electrodes (with interelectrode spacing fixed at 12 um) and its degree of cure was followed by the Micromet Instruments System II Microdielectrometer. A newly designed Fourier Transform Digital Correlator of the Micromet System II, with an accessible frequency range of 0.005-10,000 H z , was used to analyze the sensor response. Loss factors at frequencies as low as 0.01 H z can be measured, representing a typical cured resin to tangent value on the order of 0.003 . For elevated temperature measurement, the sensor and socket assembly were placed into an oven preheated to 120°C. The sample temperature, as measured by the on-chip temperature sensor, could reach an equilibrium within 10 minutes. The dielectric permittivity (dielectric constant), loss factors and temperature were recorded during the sample cure cycle. Results are shown in Figures 1-4. 181

B.

Time-Dependent Solvent Soxhlet Extraction

Measurements

A sheet of the R T V silicone sample, which was prepared by the method mentioned above, was weighed after being rolled up in a stainless steel wire gauge (250 mesh



43.

WONG


Silicone

Cure

513

F i g u r e 1. RTV s i l i c o n e c u r e s t u d y (A) P e r m i t t i v i t y measurement, (B) Loss f a c t o r measurement w i t h time a t ambient t e m p e r a t u r e .



514


®

PERMITTIVITY VS TIME

160

200 TIME (min)

F i g u r e 2. RTV s i l i c o n e c u r e s t u d y (A) P e r m i t t i v i t y measurement, (B) Loss f a c t o r measurement w i t h time a t 120°C a f t e r 2 h r . ambient c u r e .


WONG


®

Silicone

Cure

515

PERMITTIVITY VS TIME

3.0 2.8

1 «

2.4 20«C (AMBIENT)

§


100 Hz OTHERS (10.1,0.1 Hz)

2.2 240

280 320 TIME (min) LOSS FACTOR VS TIME

360

1000 100

1 Hz (OFF SCALE) 10 Hz (OFF SCALE) 0.1 Hz / 100 Hz (OFF SCALE)

400

440 TIME (min)

480

F i g u r e 3 . RTV s i l i c o n e c u r e s t u d y (A) P e r m i t t i v i t y measurement, (B) L o s s f a c t o r measurement a t ambient a f t e r 2 h r . ambient and 2 h r . a t 120°C c u r e . 1000 100 10

e

0.01 Hz

< u_ ρ

1 1

0.1 Hz

0.1 0.01

100 Hz

0.001 1 F i g u r e 4.

TIME (DAY)

RTV s i l i c o n e l o s s f a c t o r measurement a t c o m p l e t e

ambient c u r e .



516


size) to prevent the R T V from sticking to itself. This sample was placed inside the Soxhlet extractor which was attached below the water-cooled condenser. Freshly distilled Fréon T A was continuously extracted and overflowed from the top of the side-arm of the extractor. R T V silicone extractables were collected on the bottom of the boiling flask. After the standard two-hour extraction, the R T V stainless steel package was removed and dried at 120°C for one hour to remove the excess Freon. The oven-dried sample was cooled to room temperature and reweighed. The percent extractables was calculated. Results of the percent extractable of cured R T V with Freon T A are shown on Figure 5. C.

FT-IR

Analysis of Solvent

Extractables

Soxhlet extractables which were recovered from the Freon T A solvent were evaporated to a concentrated solution for the chemical analysis. A Nicolet Model 7199 F T I R was used to analyze the extractables. N a C l plates were used as sample holders. Standard procedures were used to obtain the F T - I R spectrum. Results are shown in Figure 6. D.

GC/MS

Analysis

Extractables were further identified by a Hewlett-Packard Model No.5993 G C / M S spectrometry. Solvent extractables were further diluted with Freon T A before G C / M S analysis. Six foot long, 0.3% SP2250 O V 1 7 type material was used as the G C column, with 20-30 cc/min flow rate of helium as the carrier gas. Mass/charge (m/e) units were scanned from 30-800 atomic mass units (amu). Threedimensional spectra G C / M S were printed out at the end of each run. Results of the G C / M S are shown in Figure 7. RESULTS A.

AND

DISCUSSION

Dielectric Study of RTV

Silicone

Figure 1 shows the first two-hour, room-temperature (ambient) cure, R T V silicone results. The permittivity (dielectric constant) and loss factor of the R T V silicone were recorded with the curing time and measured at frequencies of 0.1, 1, 10, 100, 1000 H z . The frequency dependence of the loss factor with cure time indicates the superposition of two components, an ionic conductivity and a dipole relaxation . The permittivity starts out with frequency at 0.1, 1, 10, 100 H z around 2.5 and rapidly increases to about 2.7 within the first 30-minute room temperature cure. This phenomenon is due to the evaporation of a low dielectric constant xylene solvent, leaving behind the higher dielectric constant silicone. During the same period (30 minutes) the loss factor of all frequencies (such as 0.1, 1, 10, 100, 1000 Hz) decreases by two orders of magnitude. Again, this is most likely due to loss of solvent, thus decreasing the ionic conductivity significantly. After 45 minutes ambient cure, the permittivity of the R T V is fairly steady, indicating no vitrification or loss of dipoles. However, the loss factor keeps decreasing but at a moderate rate. This is probably due to either slower system loss or a tightening of the network by crosslinking of the silicone matrix. Figure 2 shows the permittivity and loss factor change after 120 to 240 minutes. Upon heating at 125 minutes from room temperature to 120°C, the permittivity at all frequencies experienced a sudden decrease to approximately 2.5. This observation of decreasing permittivity with increasing temperature is usually attributed to thermal randomization of the dipoles. Due to thermal activation of ionic conductivity, the loss factor during the same period decreased, and then steadily decreases with further reaction with 120°C oven cure. A tightening of the silicone network is a reasonable explanation (please note the log scale of the loss factor). 191


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WONG


Silicone

517

Cure

100 90 80

g

70

CO LU

60 50 1

EXTRAi


ω

40 j-

a*

30

:

20 r 10 0

: :

0

1

2

3

4

5

6

7

8

RTV CURE TIME - (DAYS)

Figure

RTV s i l i c o n e p e r c e n t

5.

solvent

extractables

i n Fréon T A .

97.00

Ί

QQ-j

r

' 4000

Γ

3600

3200

,

2800

1

1

2400

2000

1

1600

1

1200

1

800

I

400

WAVENUMBERS (cm ) -1

Figure

6.

F T - I R spectrum o f RTV s i l i c o n e s o l v e n t

extractables.



POLYMERS FOR HIGH T E C H N O L O G Y Finally, in Figure 3, after two hours (240 minutes total curing) of heating at 120°C, the sample was cooled back to room temperature. The permittivities went back to approximately 2.7 due to the removal of the thermal randomization of dipoles in R T V . However, the loss factors were so low that only the 0.1 H z was observed, indicating it was definitely cured further than before heating (compare Figures 1,2,3). The continuous decrease of loss factor at low frequency (such as 0.1 Hz) after 360 minutes of cure time is a good indication of the continuous further cure of the R T V silicone but at a much slower rate. In order to further examine the R T V silicone cure, we have rerun the R T V silicone at 0.01 H z and followed with a longer period of cure time. The R T V silicone was cured at room temperature for 3 days. During this cure process the permittivities show very little change. However, the loss factors in Figure 4 continue to show the decrease with curing time. This loss factor change indicates that the R T V silicone is still undergoing a curing process at room temperature prior to the first 16 hrs. The change is so small that we can only detect this change at the very low frequency (0.01 Hz) of the measurement. This result correlates with our solvent extraction data in estimating the complete R T V cure schedule. In normal R T V silicone material, under normal % relative humidity (50%) and room- temperature conditions, it takes approximately 1 day for the R T V to achieve the complete cure of a 20 mil thick sample (see Figure 6). Heating of the R T V could speed up the cure by removing the excess xylene solvent, unreactive cyclics which are present in the R T V . In addition, the heating of R T V increases the crosslinking rate of the R T V silicone. However, the moisture-initiated, catalyst-assisted R T V cure system does take a longer cure time than normal heat curable silicone. (See Figure 8 ) . 1,01

Time-Dependent Solvent Extraction

Experiments

The Fréon T A Soxhlet extraction of the R T V silicone sample at different curing times with Freon reveals the degree of curing of the material. A t time zero, when the R T V material was first coated for room temperature cure, almost 100% extractables were obtained (except residue of fillers). This 100% extractable indicates a 0% cure of the material. After 16 hrs. of room temperature cure at 50% relative humidity, most of the R T V material (>90%) was cured. After the second day, almost all of the R T V s studied were fully cured, and they seem to reach an extraction equilibrium. Further curing time shows no noticeable change in amount of extractables. For the fully cured R T V silicone, the level of extractables at their equilibrium extraction was a good indication of the unreactive cyclics present in this material. In Figure 5, the R T V silicone extractable is approximately 4%. F T - I R measures the vibrational or rotational absorption for an organic molecule. It provides a fingerprint of regions of absorption for organic functional group identification. The collected extractables show a strong S i - O - S i stretching vibration between 1100 and 1000 c m " . This distinctive absorption is attributed to silicone compounds. Although the lack of absorption around the 3500 c m " region has indicated the absence of O H terminated silicone (silanol) compounds, it does not distinguish the type of extracted silicones. Figure 6 clearly shows this characteristic feature of the silicone compounds. G C / M S is one of the most useful analytical instruments to separate and identify mixtures of extracted organic compounds. Gas chromatography ( G O separates each of the components from the mixture by their differences in retention time. Mass spectrometry ( M S ) identifies each component by its fragmentation pattern. The fragmentation results from the ionization of the parent molecule into the radical cations. Each radical cation fragment has its own characteristic pattern and mass units which are used to identify the component. Low molecular weight volatiles were collected from the Soxhlet extraction using Freon and evaporated to a viscous oil. Figure 7 shows the components of 1

1

1111



43.

WONG


Silicone

519

Cure

'/////////////////////////////////////A Substrate

CH I CHj-O-Si3

CH I O-Si I CH

CH

CH,

3

-O-Si-O-fiiCH -oVSi3

ί

3

n

I CH

— ι

3

'A///////////////////////////////// Figure

8.

3

RTV s i l i c o n e

0 I

cure

CH I O-Si 3

CH

3

CH

3

-0-Si-0-CH

3

I 0 CH

3

mechanism.



520


the gas chromatography spectrum of the silicone extractables. Each extractable component was further identified by its mass spectrum. Cyclic siloxanes were identified by this M S analysis. The absence of peaks at m/e values of 58, 62, 76 or 78 suggested that there is no linear siloxane present. The observed mass spec fragments of m/e 73, 147, 133, 221, 286, 327, 355, 357, 415, 429, 503, 577 have structures of the silicone cyclic compounds and agree well with their G C / M S assignments. Prior to the completion of the R T V cure, there are unreactive O H fluids remaining in the solvent extractables, however, when the R T V silicone cure is completed, only unreactive cyclics are observed in the extractables. CONCLUSIONS Microdielectrometry, utilizing a miniature I C sensor to perform the dielectric measurement, is a sensitive technique to detect the R T V silicone degree of cure. This real time dielectric measurement technique and instrument, coupled with the time-dependent solvent extraction experiment, could be used to monitor incoming materials as well as packaging configurations using R T V when critical applications require a careful definition of "optimum cure." These methods appear to be very useful in monitoring curing or aging effects of the R T V silicone encapsulant in I C device packages. ACKNOWLEDGMENT The author would like to express his gratitude to David Day of Micromet Instrument Company for the helpful dielectric measurement discussion.

Literature Cited

1. Wong, C.P., "Polymeric Encapsulants", Encyclopedia of Polymer Science and Enginee Vol. 5, p. 638, Second Edition, John Wiley & Sons, Inc., NY, NY (1986). 2. Sze, S.M., ed., "VLSI Technology", McGraw-Hill Inc., NY, 1983 and references therein. 3. Wong, CP., Rose, D.M., IEEE Trans. Components Hybrids Manufacturing, CHMT -6 (4), p. 485 (1983) and references therein. 4. Delmonte, J., J. Appl. Polym. Sci., 2 (4), 108 (1959).

5. Warfield, R.W., Petree, M.C, J. Polym. Sci., 37, 305 (1959). 6. Baumgartner, W.E., Ricker, T., SAMPE Journal, 19 (4), 6 (1983). 7. Dragatakis, L.K., Sanjana, Z.N., Insulation/Circuits, 27 (Jan. 1978). 8. Senturia, S.D., Sheppard, N.F., Lee, H.L., Marshall, S.B., SAMPE Journal, 19 (4), 22 (1983). 9. Day, D.R., Lewis, T.J., Lee, H.L., Senturia, S.D., 1984 Adhesion Society, Jacksonville, Florida, Feb. (1984). 10. Wong, CP., "Improved RTV Silicone Elastomers as IC Encapsulants", in Polymer Materials for Electronic Applications, edited by E.D. Feit, C. Wilkins, Jr., ACS Symposium Series, Vol. 184, 171 (1982). 11. Wong, CP., "Thermogravimetric Analysis of Silicone Elastomers as IC Device Encapsulants," in Polymers in Electronics," edited by T. Davidson, ACS Symposium Series, Vol. 242, 285 (1984). RECEIVED

March 2, 1987


Effect of Room-Temperature-Vulcanized Silicone Cure in Device

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