Aging Kinetics for Temperature Loads of z-Conductive Adhesives In-Situ Monitoring of the Contact Resistance of Heat Seal Connectors

J.F.J.M.Caers and F.J.H.Kessels

Philips Centre for Manufacturing Technology PO Box 218/SFK

5600 MD Eindhoven, The Netherlands

ABSTRACT

In-situ contact resistance measurements are shown to be a very powerful experimental tool to determine the ageing kinetics of new types of interconnections. As an example of a z-conductive adhesive interconnection, heat seal connectors to a printed board are taken; these are a low-cost option to interconnect liquid crystal displays (LCDs) and printed boards.

The approach consists of the measurement with a high resolution of the contact resistance during a hot storage test. Based on a lOOOh test, a first estimate of the activation energy for ageing in static temperature conditions is given, and an extrapolation to user conditions is made. The strength of the approach is illu- strated by showing the effect of a different board layout on the eventual life time of the interconnection.

I. INTRODUCTION

Going back to the basics of accelerated testing, there should be a clear correlation between the accelerated test and test conditions and the life cycle profile of a product: the same failure mechanisms are evoked in a shorter time. For new types of interconnections, such as conductive adhesives, this correlation has to be established: one cannot use the result of a test to assess the reliability of solder connections to predict the expected lifetime for conductive adhesive interconnects! Although a lot of interesting work is being done on electrical conduction models and failure mechanisms of conductive adhesives, only very limited amounts of useful data are available at this moment [1,2,3,4].

In this study, a heat seal connector to a printed board is taken as an example of z-conductive adhesive interconnections. In a first step to asses the reliability of this type of interconnection, the ageing kinetics for static temperature loads are derived in a hot storage test. A new method is used to get the experimental data: a test structure has been developed to enable continuous monitoring of the contact resistance during the test. The aimed measuring resolution is typically 100ppm. This requires a tempera- ture stability better than 0.1 "C during the hot storage test.

The power of this method is the high accuracy of the experimental data, low overstress and a short test time, typical 1000 hours. This is illustrated with results for different heat seal interconnect designs.

This new approach can be generalised to generate transforms for other kind of load for new types of interconnections based on short test results.

11. EXPERIMENTAL

Materials

The tested heat seal connector is supplied by Shin-Etsu (J-type). The foil width is 30". Fourteen carbon tracks are screen printed on the supporting foi1,The tracks are 10 to 15pm thick, 250pm wide at a pitch of lmm. The adhesive is a thermoplastic material, 30pm thick, with a carbon conductive filler. The sup- porting foil is polyester, 25pm thick. According to the speci- fication of the supplier, the temperature during use is limited to 80°C. The heat seal foils are bonded to a FR4 type PCB.

The tracks on the PCB are Ni/Au plated Cu (Au-flash) and have a thickness of =35pm. At one side of the board, dummy tracks are interposed between the contact pads.

A dedicated test layout on the printed board is used to enable a four-wire contact resistance measurement during accelerated testing. The principle is shown in Fig. 1. To be able to contact the foil permanently without stressing the heat seal interconnection, a triple bond is made: the ends of the heat seal foil are bonded to pads on the printed board as well. A top view of the test board, including a heat seal interconnection to this board, is shown in Fig.2 (see next page).

Heat seal bonder A heat seal bonder from Weld-Equip with a pulsed heated thermode is used. Typical bonding conditions are 5-10s at 130-160°C. The bonding pressure is typical 3-5MPa.

heat seal connector

printed board 'fin lout

Fig.1 The principle of a four-wire contact resistance measurement

0-7803-3857-X/97 $4.00 01 997 IEEE 153 1997 Electronic Components and Technology Conference

To eliminate thermocouple effects, the contact resistance is measured with reversed current:

vthemo = (V+ + v-) 1 2 [VI (2)

with V+ and V- voltage drops across the interconnection, measured at +lmA and -1mA

Evaluation methods For the contact resistance, an arbitrary failure criterion of 25Qm has been applied. The cumulative failure distributions are plotted and fitted with a monomodal lognormal distri- bution. The activation energy for temperature degradation is estimated assuming an exponential Arrhenius type of equation.

Fig.2 Top side of the test board to measure the contact resistance of heat seal connectors Cross-sections are used to study the failure mechanism.

111. EXPERIMENTAL RESULTS

Ageing Ageing and in-situ monitoring of the contact resistance is done in a Destin type EM test system. A lOOOh test has been done at three temperature levels: 80°C, 75°C and 70°C. The temperature stability during the test was better than 0.1 "C.

An example of the contact resistance of some tracks of a heat seal interconnect to the test board, measured at 80"C, is shown in Fig.3. Track 1 is the outer track. The average median time to failure (TIRO%) for five samples with fourteen tracks each is shown in Table1 for a failure criterion of 25mQ.

10.0-

7.5-

5 0-

2 5 -

0 0-

-2 5 - I l ' l l 1 1 1 1 1 1 ~ 1 1 1 l 1 1 1 1 1 1 1 , T

0 20 40 60 80 100 I20 140 160 180 200 220 240 260 280 300 320 340 360 380 4M) 420 440 460 480 500

time [hours] Figure 3 Contact resistance of some tracks of a heat seal connector in a hot storage test at 80°C

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Table 1 Average median time to failure, TFFgo%, of heat seal connectors for three temperature levels

465 212

From the data for the individual tracks, cumulative failure distributions for the three test conditions are obtained (see Fig.4). An Arrhenius plot of the results is given in Fig.5. From Fig. 5, the estimated effective activation energy for the temperature dependency is =lev. This yields for the temperature degrada- tion transform:

99

90

75

5 0

30

20 3 2? 10

5

2 1

0.1

0.0

1 1

Tlife Trest TTF(Ttes t ) / TI'F(T1ife) = exp. [ 11628~(- - -)I

300 1000 7000 4e+4

Time (hours]

Figure 4 Cumulative failure distribution for 70°C. 75°C and 80°C

(3)

Based on this equation, an extrapolation to the expected life time at user conditions can be made for 0.01% failure probability:

~ 0 . 0 1 % ( 4 0 " c , Rcontact I 25mSZ) = 2180h

~ 0 . 0 1 % ( 2 5 " C , Rcontact <: 25mQ) = 14150h

Fig.6 shows a cross-section after the lOOOh hot storage test at 80°C. The increased contact resistance is caused by a gap of 4pr1-1 between the tracks on the heat seal connector and the tracks on the board.

loooo 1 1 1000

100

10

*- 4 -x- 5 -e -6 +7 -0 -9 -0- U 1 1 +I2 *-- 13

1 1 I+-14

0.012 0.0125 0.013 0.0135 0.014 0.0145 0.015

Temp [I/K]

Figure 5 Arrhenius plot of the hot storage test results

IV. DISCUSSION

Based on the calculations shown in the experi- mental part, a first estimate of the activation energy and hence of the transform for static temperature loads can be derived. However, Fig.3 and Fig.5 hide several phenomena. In the ageing curves of the tracks (Fig.3), we can distinguish two different regimes: the first shows a slowly increasing contact resistance; the second shows a sharply increasing contact resistance and has an unpredictable character. An infinite contact resistance is not observed during the test time. The failure criterion of

I 25mR is clearly in the first regime for all the test conditions used. For a robust product, it I

? , -

is necessarv to desien the useful life within v

1 polyester foil 3 copper track on printed board 2 carbon track of heat seal connector 4 printed board

the first regime of the ageing curves, although for the application a higher contact resistance could be allowed. Figure 6 Cross-section of a heat seal connector exposed to 80°C for 1000 hours

155 1997 Electronic Components and Technology Conference

0.016

0.014 -- occur in the outer tracks. This is clearly

to d. These figures show

+ 200

illustrated in Fig. 7 a 0.012 _. E

the average time to 5; N

failure for each track 0.0°8 -- 2

c 0.01 -- 150

2 100 z ii for the three tempera- 0.006 --

ture levels (Fig. 7b, c, 0.004 -- + + * + + + + + + + L and d). For the 80°C o,oo2 + + 50 test, the 0-hour results + are given as a com- 0 , 0

Fig.8 shows that in a fact the temperature-

* + --

* -- * * -- + + --

, , , , 7 7

track # track # b

CFT Heat Seal 70 C

ing in the temperature CFT Heat Seal 75 C 700

range between 70 and 80°C cannot really be 350

described by a simple Arrhenius law: the outer

temperature sensitivity at high temperatures compared with the 100 --

that at high tempera-

600 --

$ 500 -- * * 0 * * 8

* * * + 400 -- * * 300 - -

5 250 .. + tracks show a higher 2oo 6 4 II 300 -- * = + , E 200 --

150 -- e

100 -- I- inner tracks. This means 50 -- , ) . , , , . . , , 0 . : , . . . , , , , , , , , , , , I . I , , , . . 0 ; ~ ~ , , I I I I I , , / .

4 )

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Time [hours1 a

99

90

7 5

50

E 30

3 20 d s 10

5

2 1

0.

b

/ / ,

Time [hours1

Figure 8 Arrhenius plot of hot storage test results for heat seal connectors

v. CONCLUSION / SUMMARY

In-situ contact resistance measurements during accelerated tests are very powerful in assessing the reliability of new inter- connection types.

The ageing curves during a hot storage test for the tested heat seal connectors show clearly two different regimes: in the first, the contact resistance increases slowly, the second is characterised by a steep increase and an unpredictable behaviour. For a robust product design, therefore, the useful life should be within the first regime of the ageing curves.

The average observed activation energy -lev.

At high temperatures (>75”C) the Arrhenius equation is not good enough to describe the temperature-dependence of the contact resistance of all the tracks of a 30mm-wide connector. Additional failure mechanisms are activated. As a result, at elevated temper- atures, the outer tracks will fail earlier than the inner tracks.

Optimising the board layout can increase the useful life dramatically.

ACKNOWLEDGEMENTS

The authors wish to thank Dr. L. Tielemans from Destin N.V. for performing the contact resistance measurements and for the valuable discussions.

REFERENCES

R.R. Reinke, “Interconnection method of Liquid Crystal Driver LSIs by Tab- on-Glass and Board to Glass using Anisotropic Conductive Film and Monosotropic Heat Seal Connectors” Proc. 41” ECTC Con$ 1991, pp

J.F.J.M. Caers, J.N.J.M. van den Reek, F.J.H. Kessels, “Criteria to assure the reliability of heat seal inter- connections - a critical review” Proc. Adhesives in Electronics ’94, 1’‘ Int. Con$ on Adhesive Joining Tech- nology in Electronics Manufacturing, Berlin November 1994 J.E. Morris, Li Li, ‘‘Elel trical con- duction models for isotropically con- ductive adhesives” Proc. Adhesives in Electronics ‘96, 2nd Int. Con$ On

355-361

Adhesive Joining Technology in Electronics Manufacturing, Stockholm June 1996, pp126-133

[4] K. Gustafsson, J. Lui, 2. Lai, “Surface characteristics, reliability and failure mechanisms of Tin, Copper and Gold metallisations” Proc. Adhesives in Electronics ‘96, 2nd Int. Con$ On Adhesive Joining Technology in Electronics Manufacturing, Stockholm June 1996, pp141-154

[5] B. Vanhecke, J. Roggen, E. Eeyne, W. De Ceuninck, L. De Schepper, L. Stals, “On the interconnection interface study in microsystems” Verbindungstechnik in die Elektronik, Heft 4, December ‘92 pp164-170

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