Low-Temperature Properties of Capacitors

Low-Temperature Properties of Capacitors Embedded into Printed Circuit Boards Andrzej Dziedzic, Tomasz Świetlik, Paweł Winiarski Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wroclaw, Poland Email: andrzej.d ziedzic@p wr.edu.pl

Abstract: This paper presents low-temperature properties of capacitors embedded in Printed Circuit Boards. Planar capacitors, differed in composition thickness and surface size, were fabricated from FaradFlex dielectric foil with copper plates laminated to FR-4 substrate. The dielectric tapes were polymer or BaTiO3 /polymer compositions with various dielectric constants. The investigated capacitors were covered with LDP 2×106 (Laser Drillable Prepreg) protective layers to achieve embedded structures. The temperature dependences of capacitance as well as dissipation factor (i.e. C(T) and tgδ(T) characteristics) were investigated for two values of frequency (1 and 10 kHz) in a wide temperature range - between -180 C and room temperature. Moreover durability of capacitors to low-temperature thermal shocks (between liquid nitrogen and room temperature) are reported and analyzed.

1. INTRODUCTION The large growth of high-advanced but simultaneously low-cost electronic products caused in the last years wider and wider interest in technologies of passive components’ embedding . There are two attempts in passives’ embedding into printed circuit boards (PCBs). The first is based on standard or ultrathin surface mount components attached to the laminate and building PCB around the components afterwards [1-3]. The second one uses special films and foils, where embedded component should be thinner than a distance between adjacent PCB layers and thus it does not increase the PCB thickness. Composites for embedded capacitors consist of a dielectric (eg. modified epoxy or modified epoxyfilled ceramic composition) with a thickness from several to 20-30 µm, and one or two Cu layers with a thickness of 18-70 µm [4-6]. The properties of such components are well described near the room temperature. But there is an increasing need for electronic devices and components operating in harsh environment including for example low-temperature application. Therefore this paper presents lowtemperature properties (C(T) and tgδ(T) characteristics) in a wide temperature range between -180C and room temperature as well as durability of test structures to low-temperature thermal shocks

(between liquid nitrogen and room temperature) of capacitors embedded in PCBs.

2. TEST STRUCTURES Planar capacitors were fabricated from special FaradFlex composite consisted of dielectric foil with two Cu layers deposited on both sides of dielectric. The fabrication process begins with etching capacitors plates, Cu paths and pads. Next foil is laminated to FR-4 substrate with one LDP 2×106 layer. At the end structure is covered with another LDP 2×106 coating to simulate embed-ding process and protect capacitors from atmospheric influence. For research FaradFlex composites with three different types of dielectric (12 μm thick BC12TM material with partially filled dielectric, 16 μm thick BC16T material with ceramic filler and 24 μm thick BC24M material without filler) were used [7]. Test structures were composed of capacitors embedded into printed circuit boards (PCBs). Each sample had planar capacitors with surface area 2.5×2.5; 5×5; and 10×10 mm2 (schematic cross section through capacitors is shown in Fig. 1 whereas topology of test coupon is shown in Fig. 2; such test coupon consist of 6 small, 4 medium and 3 large capacitors).

The highest dielectric constant has BC16T material and the lowest – BC24M. Chosen properties of such capacitors were presented earlier in [8,9]. Moreover similar components and their electrical as well as long-term stability properties were described in [10, 11]. Cu dielectric Cu Cu dielectric Cu Cu dielectric Cu

Fig. 1. Schematic presentation of BC12TM (top), BC16T (middle) and BC24M (down).

Fig. 2. Topology of test samples.

by the flow of liquid nitrogen mass which value was monitored by Pt100 sensor. All measurements were performed under NI Labview software and GPIB interface with as high accuracy as possible. To avoid parasitic effects short- and open-circuit corrections were made at the beginning of measurements. The measurements were made at 1 kHz and 10 kHz and 250 mV amplitude, using HP4263A LCR meter with 5½ digits accuracy. To minimize the measurement noises the result of measurement is calculated as average value of 64 measurements taken at one point. The capacitors were switched by Keithley 7001 scanner. The capacitance and dissipation factor versus temperature were measured in temperature range from -180°C to 20°C in steps of 10°C. Capacitors were also subjected to series of hundred low-temperature thermal shocks (between room temperature and -196°C) with 10 min soak at every temperature (measurements were made after every 10 shocks). Last measurements made for these capacitors were related with their DC current-voltage (I–V) characteristics for 10×10 mm2 structures. This relation was determined indirectly by measuring voltage drop on resistor (R = 1 kΩ) connected in series with embedded capacitor and supplied by regulated DC voltage (Fig. 4). The measurements were made in the temperature range between 25°C and 145°C for electrical fields from the range between 1.25 to 12.5 V/μm (because of various thickness of particular dielectric foils the voltages were from the ranges 15-150, 20-200 and 30300 V for 12TM, 16T and 24M dielectrics, respecttively). Capacitors were placed onto the PID-regulated heating table with temperature measured by Pt100 sensor and the measurements were made 60 seconds after switching on the circuits.

3. M EASUREMENT PROCEDURES

V

The measurements of capacitance and dissipation factor versus temperature measurement were performed in modified KSE-95 cryosystem [12] in a wide temperature range between -180°C and +20°C. To measure C(T) and tgδ(T) characteristics the capacitor arrays were placed in a special probe holder with beryllium-copper gold plated pins mounted inside the vacuum insulated cryochamber supplied with nitrogen from the Dewar vessel and connected with measuring instruments by appropriate cables and connectors (Fig. 3). The temperature was controlled

R UD C

C

RC

Fig. 4. Circuit diagram fo r measuring DC current-voltage characteristics.

.

Control of channels’ sw itching

Scanner Keithley 7001

LCR 4263A HP 1689B

(2x7011S QUAD 1x10) Measuring wires

F Vapors’ outlet

Measurement

Lewar

Flowmeter Safety valve Manometer

Vacuum valve

Measuring head

P

Vacuum pump

Pressure reductor

Vapors

Nitrogen

Nitrogen Vacuum

Vacuum

Cryochamber

Dewar vessel

Fig. 3. Schema of measurement system.

4. R ESULTS AND DISCUSSION 4.1. Basic properties of capacitors Ten test samples with every dielectric foil was measured in the temperature range between -180°C and 20°C. This means that the be low results are mean values for thirty 10×10 mm2 , forty 5×5 mm2 and sixty 2.5×2.5 mm2 capacitors of every dielectric type. The mean values of capacitance and loss tangent for temperature 0°C are collected in Table 1. Capacitance and dissipation factor decreased with increase of measuring frequency capacitance. The decrease in capacitance was equal to about 2.5% for frequency decade. Values of both parameters were characterized by rather small distribution. The distributions were smaller for larger structures.

Table 1. Mean values of capacitance and loss tangent for T = 0°C. Dimensions [mm2 ]

f [kHz] 1

10×10 10 1 5×5 10 1 2.5×2.5 10

12TM

16T

24M

680 pF 0.015 663 pF 0.013 193 pF 1 189 pF 10 52.2 pF 1 51.0 pF 10

1877 pF 0.026 1800 pF 0.032 513 pF 0.015 492 pF 0.014 143.7 pF 0.017 137.8 pF 0.015

193.1 pF 0.0257 184.7 pF 0.0323 45.7 pF 0.026 43.3 pF 0.032 12.5 pF 0.025 11.8 pF 0.031

4.2. Low-temperature characteristics of capacitors Mean normalized characteristics of capacitance and loss tangent for 12TM and 16 T capacitors are show in Figs. 5 and 6. It is seen that the kind of dielectric layer is more important than capacitor dimension. The relative decrease of capacitance between -180C and 0C is equal to about 10% for 12TM dielectric foil and 25% for 16T. It is worth to notice that capacitance changes below -60C are much smaller than above this temperature. Dissipation factor is decreased with temperature decrease and independently on dielectric type its value at -180C is about ten times smaller than at 0C.

(2) C (T )  C0  B exp  T   TB  (3) C (T )  C0  B exp T   D exp T   TB   TD  where T – temperature (in C), C0, B, D (all in pF), A (in pF/C), TB , TD (in C) – constants.

Fig. 6. Normalized temperature characteristics of capacitance and dissipation factor for 16T capacitors.

Fig. 5. Normalized temperature characteristics of capacitance and dissipation factor for 12TM capacitors.

It is difficult to find physical explanation of the shape of characteristics presented in Figs. 5 and 6. Nevertheless this is possible to observe certain temperatures where characteristic bends or maxima/minima appear on such curves. Therefore it is possible to fit these characteristics (both C(T) and tgδ(T) as well as normalized capacitance and loss tangent characteristics) in some temperature subranges using the following equations: C (T )  C0  AT

(1)

Below, in Table 2, one can find values of parameters of Eq. (2) which gives satisfactory fit of experimental C(T) characteristics for 12TM capacitors in two subranges: from -180C to 0C and from 0C to 20C. Table 2. Values of parameters fro m Eq.(2) for best fit of C(T) characteristics for 12TM capacitors (f = 1 kHz) Dimensions Temp. ranC0 [pF] B [pF] TB [C] [mm2 ] ge [C] -180 ÷ 0 598.18 83.44 56.63 1010 0 ÷ 20 670.45 9.95 16.87 -180 ÷ 0 169.26 24.17 53.63 55 0 ÷ 20 190.62 2.52 15.24 -180 ÷ 0 45.89 6.36 49.34 2.52.5 0 ÷ 20 51.86 0.39 11.37

4.3. Durability of capacitors to low-temperature thermal shocks The durability of tested structures were determined by relative changes in resistance C

C0



(C ( n)  C 0 )

C0

 100 %

(4)

The measured characteristics in logIL – logUDC scale are shown in Fig. 8. One can see that behavior of 12TM and 24M components is very similar. The leakage current starts to increase exponentially with temperature for temperatures higher than 90-100°C – it is increased by about 2 orders within 40°C range. T [C]

-6

10

25 45 65 85 105 125 145

-7

10

IL [A]

where C0 – initial capacitance (before shocks), C(n) – capacitance after n shocks. The example results are presented in Fig. 7. Generally in literature there is very few information about such tests. Of course such behavior is important for cryogenic resistance thermometers – please see e.g. [13]. Moreover durability of various kind of thick- and thin-film resistors were reported and compared recently in [14]. The low-temperature thermal shocks lead to small changes in capacitance and slight increase of dissipation factor after 100 shocks between liquid nitrogen and room temperature. The observed changes are strongly related with capacitor area.

4.4. DC current-voltage characteristics of capacitors The leakage current IL was equal: U IL  R (5) R where UR – voltage drop on serial resistor and R – resistance of serial resistor.

0,5

-8

10

C/C0 [%]

0,0

-9

10

-0,5

10

-1,0

100

UDC [V] 2

Dimensions [mm ] 10x10 5x5 2,5x2,5

-1,5

-2,0

T [C]

-2

10

25 45 65 85 105 125 145

-3

10

0

20

40

60

80

100

Number of shocks

-4

10

C/C0 [%]

IL [A]

2

-5

10

0

10

-2

10

-6

-7

-8

10

-4

10

100

UDC [V] -6

-8

2

Dimensions [mm ] 10x10 5x5 2,5x2,5 0

20

Fig. 8. Influence of temperature on IL = f(UDC) characteristics of 12TM (top) and 16T (bottom) capacitors 40

60

80

100

Number of shocks

Fig. 7. Influence of number of low-temperature thermal shocks on fractional capacitance changes of 12TM (top) and 16T (bottom) capacitors

The increase of electrical field causes increase of leakage current. Such changes can be described by power function , where a is constant and index n is increased with temperature in the range 0.81.13 for 12TM and in the range 0.7-1.2 for 24M, respectively. Based on value of n one can say that I-V

characteristics and the same insulation resistance of is slightly nonlinear. Similar behavior was observed previously for dielectrics based on modified epoxy resin or modified epoxy resin-ceramic powder composites [11]. Quite different and much more complicated behavior was observed for 16T capacitors. The increase of leakage current appears from very low temperatures. Moreover the dielectric breakdown is observed for these structures (for T higher than 125°C and electric field higher than 11 V/μm. The above results are compatible with those presented in [9].

5. CONCLUSIONS The results presented in this paper point at very interesting low-temperature properties of capacitors embedded in PCB, which are based on composite dielectric from modified epoxy or modified epoxyfilled ceramic composition. In general capacitance is decreased as temperature comes down. But it is worth to notice that capacitance changes below -60C are much smaller than above this temperature. Also dissipation factor is reduced at low-temperatures independently on dielectric type its value at -180C is about seven to ten times smaller than at room temperature. The capacitance of structures with large surface area (10×10 mm2 ) are almost insensitive for low-temperature thermal shocks. Changes in capacitance of small capacitors (2.5×2.5 mm2 ) take about -1% for 12TM dielectric and -6% for 16M one. DC current-voltage characteristic strongly depends on dielectric type. Leakage current for 12TM and 24M dielectric foil is rather small and increases gradually with temperature. The relation between leakage current and electric field can be described by power function with power exponent not far from 1. This means small nonlinearity of insulation resistance versus voltage. But 16T dielectric foil possesses 1÷2 orders larger insulation resistance. Moreover the dielectric breakdown appears for this foil at temperature higher than 125C and electric field stronger than 11 V/μm.

ACKNOWLEDGMENTS This work was supported by statutory activities of the Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology.

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