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Electrocomponent Science and Technology, 1981, Vol. 8, pp. 151-1570305-3091/81/0804-0151 $06.50/0

(C) 1981 Gordon and Breach Science Publishers, Inc.Printed in Great Britain

FUNDAMENTAL CHARACTERISTICS OFSOLVENT-SOLUBLE POLYIMIDE RESIN AS A POLYMER

FOR HYBRID APPLICATIONS

EIKI WATANABE

Department of Electrical Engineering, Faculty of Technology, Tokyo Metropolitan University, Tokyo,Japan

KYOICHI KANOU

Reliability Section, Electronic Device Division, Electrotechnical Laboratory, Ibaraki Prefecture, Japan

TADASHI KUBOTA

Japan Technological Research and Development Laboratories, Co., Kokubunji-shi, Tokyo, Japan

Polyimide resin of solvent-soluble type, which is called "X-resin" in short, was discovered lately. It is soluble indimethylacetoamid (DMAc), and new extensive applications are expected.Some physical, chemical and electrical characteristics are studied from the viewpoint of polymer material for hybrid

application. Particularly comparison of the characteristics of X-resin specimens before and after long term exposureto humid and hot environments were made, remembering that electronic devices often operate under such conditionsin Japan.From the results obtained up to now, no conspicuous difference seems to exist between the characteristics of X-resin

and those of the conventional (i.e., solvent-insoluble) resins, as published in various literature. It should be noted,however, that some unusual effects, i.e., the remarkable increase in surface resistivity a, the evident decrease in tan 6,etc., were found in X-resin specimens after going through the above-mentioned damp-hot test. All those effects arecontrary to the usual concepts of deterioration of polymer materials, but are rather advantageous for hybridapplication. It is also worth noticing that silver migration into X-resin seems scarcely to occur.

Furthermore, new prospective uses of X-resin for printed wiring boards, packaging materials, binder for pastematerials, etc., are investigated.

1. INTRODUCTION

Polyimide resins have outstanding physical, chemicaland electrical characteristics combined with highthermal stability over a wide temperature range.Nevertheless their applications have been restricted tosome particular uses in extreme environments, and theyhave scarcely been applied to hybrid ormicroelectronics devices. Manufacturers will encountertwo serious problems when they attempt to usepolyimide resin. One is the extreme expensiveness andthe other is the awkwardness of forming andprocessing, since polyimides have been consideredneither to melt nor to have any organic solvent.However, the former may be tolerated if the latter issolved, and extensive uses are developed. In

151

consequence, the demands for polyimide resins risesteeply.A new type of polyimide resin, which was called

"X-resin" in short, found very recently. A remarkablefeature of X-resin is that it has "specific"solvent-solubility similar to such usual linear polymersas polystyrene, etc., i.e., it can dissolve in a specificorganic solvent even if it is once cured. Since it can beformed or processed far more easily than conventionalpolyimides, various new and extensive applications arelooked forward to unless it has any fatal disadvantagein its properties.Thus the physical, chemical and electrical

characteristics of X-resin began to be studied as apossible polymer material for hybrid application. Firstof all, various characteristics of X-resin are compared

152 E. WATANABE, K. KANOU AND T. KUBOTA

with those of conventional polyimides. Damp-heat testsare also achieved for film specimens of X-resin,considering that in Japan electronic devices are oftenoperated under humid conditions. In fact, moisture isthe cause of some kinds oftroubles in various electronicdevices. Among these, a typical and serious one ismetal migration into polymer materials.

From the results obtained by means of infra-redspectra studies, no conspicuous difference has beenfound between chemical structures of X-resin andconventional polyimide resin.

3. SPECIMENS AND CONDITIONS FOREVALUATION TESTS

2. CHEMISTRY OF X-RESIN

Conventional polyimides are formed typically by threestages as shown in Figure 1. Chemical reactions ofX-resin formation are considered to be substantiallysimilar to those in Figure 1, except for a few modi-fications of conditions. Forming and processing of con-ventional polyimide resins are usually made for solvent-soluble intermediate products at stage (II) (polyamicacid), because the final product at stage (III) (i.e.,polyimide resin) has no solvent-solubility. On theother hand, X-resin dissolves in a specific solvent.

dimethylacetamide (DMAc), though it is considered to

be equivalent to a final product at stage (III).

0 0

oo +0 0

P,yrome t cdianhydride

(PMDA)

AT(25C to 50C)

H2N"-O’-NH2

4,4 Diamino phenyl ether(DOP)

in a solvent such asdimeth acetami de(DMAc

(II)

Polyamic acid(solvent-soluble)

(IO0C to 300C) Catalyst(metalc salts

ro o

Polyimide(solvent-insoluble)FIGURE Chemical reactions of polyimide formation.

X-resin specimens for various evaluation tests areprepared as follows: X-resin powder dissolves inDMAc at a ratio of approximately 1 vs. 10 in weight tobecome a paste-like disposition. Then it is formed insheets or films on a suitable flat plate. They are cured at100C for 30 minutes, and further heat-treated at200C for one hour for the purpose of sufficientvolatilization of solvent. In addition, a sampleheat-treated at 300C is added for thermal analysis.Thickness of the specimens is between 100 and 200 #m.Temperature and relative humidity for damp heat

tests are usually determined respectively at 40 +_ 2Cand 90-95% according to JIS (Japan IndustrialStandards) C5023 "Damp Heat (Steady State) TestingMethod for Electronic Components". The term ofexposure can be selected from five steps between 48and 1,000 hours. As for temperature, however, asomewhat severer condition, i.e., 70C, has beenemployed in this study, in order to facilitate theenvironmental deterioration of polymer specimens.

4. TEST RESULTS AND EVALUATIONS

TGA and DTA Thermogravimetric analysis (TGA)and differential thermal analysis (DTA) are achievedfor three kinds of X-resin samples, and also for twokinds of conventional polyimide on the market for thepurpose of comparative evaluation of the thermalresistance of X-resin. The specifications of those five

TABLESpecifications of polyimide samples for thermal analysis.

Sample Trade name Condition

A heat-treated at 200CB X-resin heat-treated at 300CC powder sample (before cured)D Kaptnb

heat-treated at 300CE Torayneece

aTentative name: Japan Technological Research &Development Laboratories, Ltd.bDuPont, Inc.CToray Industries, Inc.

SOLVENT-SOLUBLE POLYIMIDE RESIN FOR WIRING BOARDS 153

kinds of samples (sample A-E) are put together inTable I. The resultant TGA and DTA curves forX-resins are shown in Figure 2, and those forconventional polyimides are shown in Figure 3. FromTGA curves in Figure 2, it is observed that for sampleA and B the first apparent decreases in weight beginfrom the temperatures a little over 200 and 300Crespectively. Note that those temperatures roughlycoincide with those of heat treatments for each sample(cf. Table I). Moreover, from DTA curves in Figure 2,it is found that the above-mentioned decreases inweight are accompanied with endothermic chemicalprocesses. The slight decrease in weight is alsoobserved in sample C at a temperature as low as lessthan 100C, and is surely due to the desorption of gasesabsorbed on the surfaces of powder particles of X-resinsample. As for the conventional polyimide sample Dand E, neither a decrease in weight nor a chemicalreaction is observed up to as high as near 500C, asshown on TGA and DTA curves in Figure 3. Thus thefirst decreases in weight of samples A and B shouldbe caused by volatilization of residual solvent.

It is obvious that for all X-resin samples (A-C) thesecond abrupt decreases in weight begin from thecoincident temperature, i.e., about 450C, as shown in

TGA curves in Figure 2. Those second decreases are allaccompanied with somewhat complicated exothermicand/or endothermic reactions, as also shown in DTAcurves in Figure 2. Thus those reactions are consideredto correspond to thermal crackings of X-resin.

In consequence the thermal resistance of X-resin is alittle lower than that of conventional polyimides, but itcan be improved remarkably by means of heattreatment at a relatively high temperature so that theresidual solvent volatilizes completely.

Electrical Characteristics DC and AC measurementswere made to obtain fundamental electricalcharacteristics of X-resin. The resultant values forsample A are shown in Table II, together with those ofconventional polyimides and, for ready comparison,some other resins which are often used in hybriddevices. Note that frequencies for AC measurementsare different among various specimens.

The insulation resistance of X-resin has nosubstantial difference from conventional polyimides,and is roughly equal to or rather higher than those ofepoxy resins. Taking the difference in measuringfrequency into consideration, tan 6 of X-resin does notseem to be so large.

I00

8O

6O

4O

2O

0

-0

-400 I00 200 300 400 500 600

Temperature (C)

FIGURE 2 Results of TGA and DTA for X-resin samples.


io0

80TGA ---iq,, O0

60 =90 ’g

40 ’

80

0 70

-2060

-400 I00 200 300 400 500 600

Temperature (C)FIGURE 3 Results of TGA and DTA for conventional polyimides

TABLE IIFundamental electrical characteristics

Polymer p(f.cm) a(f) ’ tan 6(x10-4)

X,resin (sample A) 1016-1017 1015 3.9(10 MHz) 370(10 MHz)conventional 3.0(60 HZ) 10-50(60 Hz)polyimide (Kapton) 1016-1017 3.5(1 KHz) >30(1 KHz)phenol resin 1011-1012 5-15 50-500epoxy resin 1016 3.5-5 100-800

TABLE IIIResults of damp-heat tests for electrical characteristics

Condition of p(2.cm) a()samples 10 MHz 50 MHz

before exposure 1016-1017 1015 3.94 3.95after exposure 1016-1017 1019 3.94 3.97

tan 6( x 10-4)

10 MHz 50 MHz

366 308186 151


The results of damp-heat tests for the electricalcharacteristics of X-resin are tabulated in Table III.There are few differences in volume resistivity p andpermittivity e’ between the values before andafter the exposure to testing environment. On theother hand, surface resistivity tr increases remarkablyand, at the same time, dielectric loss tangent tan 6decreases almost to a half, contrary to usual tendencyon deteriorated polymer materials due to moistureabsorption and/or heat. The mechanisms of thoseunusual phenomena are now under investigation.

Moisture Absorption and Permeability Moistureabsorption isotherms of X-resin specimens before andafter exposure to the environment for damp heat testsare shown in Figure 4, where typical values of moistureabsorption coefficient of conventional polyimides andsome other practical polymers quoted from literaturesare dotted together.

Generally speaking, the moisture absorptioncoefficient of X-resin is nearly equivalent to that ofconventional polyimides, and is not so small amongother resins.

It can be observed also in Figure 4 that moistureabsorption of X-resin increases fairly after exposure tothe damp-heat test environment.

Moisture permeabilities of X-resin are shown inFigure 5 with regard to the thickness of specimen. The

X-resin

0 "before exposure

"after exposure

jX

oJ Kapton

"" epoxy .,|

0 20 40 60 80 I00

relative humidity %

FIGURE 4 Moisture absorption isotherms of X-resinspecimens.

typical values for conventional polyimides and someother practical resins quoted from literature are alsodotted together. Permeability of X-resin is a little largerthan that of conventional polyimides. It also increasesafter exposure to the environment for damp-heat tests.

It is worth noticing again that unusual behaviours oftr and tan 6 occur though moisture absorption andpermeation of X-resin are comparatively large.

3OPVC

X-resin

0 "beforeexposure

0 "afterexposure

Kapton

0 I00 200 300

sample thickness mFIGURE 5 Moisture permeabilities of X-resin specimens.

Silver Migration Among other characteristics, metalmigration is a matter of concern from viewpoint ofhybrid applications, since it is often one of the seriousproblems in the humid climate such as Japan. Silvermigration tests are being made for the samples of printedwiring board of X-resin. No conspicuous migration hasbeen found until now though the tests are not finished,

yet.

5. AVAILABILITY FOR HYBRIDAPPLICATIONS

In view of the test results so far achieved, thecharacteristics of X-resin are considered notunfavourable compared with those of conventional


polyimides and also some other resins. It is expectedthat in manufacturing processing the versatility ofX-resin owing to the solvent-solubility will more thanmake up for a few deficiencies, and will further open upnew and extensive uses. Some instances of prospectiveapplications are described below.

5.1. Printed Wiring Board

Printed wiring boards heretofore in use are mostlymade of either phenol or epoxy resin reinforced withpaper or glass cloth. They cannot be laminated as thepolymers do not dissolve in any organic solvent afteronce being cured.

Supposing that printed wiring boards are made fromX-resin, they can be laminated to make multilayersafter thick film devices are formed on each board, sincesurfaces of the boards are soluble in DMAc, and henceable to adhere tightly with each other. This means thata greater number of devices can be integrated withinlimited sizes.Thermal stresses on printed wiring boards are

necessarily exerted when they are going through thesoldering processes for component-attachment, andoften give rise to such troubles as warp of the board,exfoliation of conductor (metal) foil, etc. Thermalresistance of the boards is therefore one of the seriousproblems in order to ensure high reliability for hybriddevices.Supposing again that X-resin is utilized as a practical

material for printed wiring boards, many kinds ofthermal restrictions will be removed from component-attachment processes. Moreover, it may well be thatnew manufacturing methods can be developed sinceprocessing can be done at far higher temperature.

5.2. Binder for Thick Film Materials

X-resin may well be utilized as binder of paste materialsfor thick film conductors, dielectrics and resistors. If theX-resin paste is used in combination with theaforementioned X-resin wiring boards, the fitnessbetween them should be the best owing to their mutualsolubility. Various heat treatments and processing willbecome possible with high accuracy in dimensionsowing to high thermal stability and with almost thesame thermal properties, e.g., thermal expansioncoefficient, of both printed wiring boards and thickfilm devices upon them.

ponent

printedwiring boa lead wire

(metal foil)

FIGURE 6 An example of hooked lead wires of componentfor the convenience of attachment processing.

purpose of making all components of uniform heightand also preventing the lead wires from rattling orcoming out during the attachment process. In this case,however, the lead wires should be made a little thickerso that the hooked portions cannot be distorted easilyby mechanical forces. This means that heattransmission can occur easily through lead wires to theinternal devices of electronic components. In fact, it hasbecome one of the serious problems at present thatcapacitor dielectrics such as myler are often melted ordeformed thermally at the connecting points ofcapacitor electrodes with lead wires during soldering.Thus higher thermal resistance is required for capacitordielectrics.

X-resin may be favourably used for such capacitordielectrics as mentioned above.

5.4. Packaging Materials

Polymer materials have been widely used as mouldingand coating materials for hybrid packaging. It goeswithout saying that X-resin can be used similarly. Itmay well be expected also that some new packageconstructions are developed making the most of thefeature of X-resin i.e., the solvent solubility.

5.5. Other Applications

X-resin may possibly have a capability of preclusivematerial for metal migration, though experimentalsupports are not sufficient yet.

5.3. Capacitor Dielectrics

Lead wires of discrete components are often hooked to

form semicircular portions as shown in Figure 6 for the

6. CONCLUSION

The fundamental characteristics of X-resin are notsynthetically inferior to those of conventional


polyimides and other practical resins. The results ofdamp heat tests are noticeable, i.e., surface resistityand dielectric loss tangent tan fi change contrary tousual concepts of deterioration of polymers. Theseeffects seem to be favourable for hybrid andmicroelectronic applications if they last a long period oftime equal to the expected life spans of electronicdevices.The solvent-solubility will make it possible to

develop various new methods of manufacturingprocessings and constructions of hybrid devices, inconnection with their essential thermal resistance.

Polyimide resins have been used only for particularpurposes mainly because of their extreme expense. It isexpected, therefore, the new features of X-resin willdevelop new and extensive uses accompanied with newmarkets, and in consequence, bring about reduction ofa cost in the near future.

ACKNOWLEDGEMENTS

The authors are indebted to Professor A. Kakimoto and Mr.K. Ogawa who kindly undertook the measurements ofdielectric characteristics, and to Professor Y. Awakuni for hishelpful discussion.

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