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SMD Plastic Film Capacitors for High Temperature Applications

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					CARTS 2004: 24th Annual Capacitor and Resistor Technology Symposium, 29 March – 1 April

SMD Plastic Film Capacitors for High Temperature Applications

K. Saarinen ¹, E. Matero ², J. Perälä ²
¹ Evox Rifa Group Oyj Lars Sonckin kaari 16, 02600 Espoo, Finland Tel: + 358 9 5406 5006, Fax: + 358 9 5406 5010, E-mail: kimmo.saarinen@evoxrifa.com ² Evox Rifa Oy Siikarannantie 3, 89600 Suomussalmi, Finland Tel: + 358 8 747 0200, Fax: + 358 8 747 0222, E-mail: esa.matero@evoxrifa.com, jouni.perala@evoxrifa.com

INTRODUCTION
Today the high temperature limit for electronics is very often given as 125 ºC. More and more often applications call for at least temporarily higher temperatures especially in automotive, but also in some other industrial areas, like aerospace and oilfield electronics. Characteristic for automotive applications is that the electronics has to work in very varying conditions, and can experience e.g. temperature shocks from – 40 ºC to + 175 ºC, high random mechanical vibration up to 40g, and shock up to 50g, not to mention the moisture variations (See ref. 1). In oil field applications the electronics has often to withstand long times in high temperatures up to 225 ºC. The electronic components as the building blocks of the electronic circuits have to be developed and characterised to meet the requirements of these applications. This paper describes the possibilities to use Metallized Plastic Film Capacitors in high temperature applications based on the development of dielectric films. The paper targets in the development of SMD capacitors, especially. European Commission is financing a PROCURE project to develop high temperature passive components. The work presented in this paper is done as part of this project. The overall project has been presented in CARTS Europe 2003 (See ref 1.).

PLASTIC FILM DIELECTRICS SELECTION
The selection of possible plastic film dielectric materials was made based on their temperature characteristics and commercial availability. The candidates were Polyethylene terephthalate (Polyester) (PET), Polypropylene (PP), Polyethylene naphthalate (Polyester) (PEN), Polyphenylene sulphide (PPS), Polyimide (PI), and Polytetrafluoroethylene (PTFE). PP has the drawback of low melting point (160 – 170 ºC), which rules it out from high temperature applications. PI and PTFE -films are not commercially available as thin films (< 6 µm), and are very difficult to metallize properly. This is why they were not considered as viable candidates, although their high temperature characteristics are very good. In the Table 1. the selected characteristics of the remaining PET, PEN and PPS films are given. Table 1. Film Dielectric Properties
Film Material Dielectric constant Dissipation factor (%/1kHz) Insulation resistance (MΩ*µF) Dielectric absorption (%) Melting point (°C) Min commercial thickness (µm) PET 3,3 0,5 >25k 0,5 254 0,9 (0,7) PEN 3 0,4 >25k 1,2 266 0,9 PPS 3 0,05 >50k 0,05 285 1,2

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PET –film, often known according to one brand name as Mylar, has been widely used in the industry already for many decades, and its high temperature behaviour is known. Capacitors made of metallized PET are today used e.g. in automotive applications, where the continuous service temperature is limited to 125 ºC, but short excursions to 150 ºC are allowed. Because one target was to have the capacitors in Surface Mounting configuration (as SMDs), the soldering process stress to the capacitors has to be taken into consideration. Today PET capacitors are successfully used as SMD components when special care is taken to adjust the reflow soldering conditions not to over-heat the capacitor element. In near future the soldering conditions are anyhow due to the ban of lead (Pb) going to change, and unfortunately towards higher process temperatures. PET capacitors will have problems to meet the increasing temperatures, and therefore the development focus has been in PEN and especially in PPS capacitors. Figure 1. shows typical recommended process conditions for Pb-free reflow soldering (SnAgCu – solder alloys) taken from a committee draft document IEC 61760-1, Ed. 2. (Ref 2) The test specifications and standard proposals for resistance to soldering heat have often higher peak temperatures than in the Figure 1.. The Pb-free processes are anyhow still under development, especially the soldering machine temperature controls, and it is still to be seen what the status quo will be. It may be that several component
300

CONSTRUCTION OF SMD FILM CAPACITORS
In addition to high temperatures the capacitors should be able to survive in the harsh environment of automotive applications. This has been the motivation to develop an encapsulated construction. To have maximum stability, at this time wound elements were chosen as capacitor elements. The basic construction of a winding is in Figure 2., where two metallized films are wound together to form a winding. The metallizing material is typically Aluminium, and the round winding is normally flattened in a heat treatment process to form a suitable and inert package for further processing.

Figure 2. Winding

The flattening and heat treatment process is the most important one to prepare the capacitor especially for the soldering process, and for the long time service in high temperatures. The flattening and heat treatment are not discussed in detail here. It is essential by correct treatment to guarantee that the process temperature / time in the reflow soldering are not affecting the capacitor element. At the same time it should not by no means deteriorate the film’s polymer structure, which could cause long term reliability problems. The electric contacts to the flattened winding are made by spraying molten metal to the ends of the winding. This metal makes contact with the metallized electrodes on both films separately. There are normally at least two different sprayed metal layers on both ends: contact layer to winding, typically Aluminium, and solderable layer towards outer electrode. The high process temperatures in Pb

SnAgCu Reflow
250 220 °C Temperature (°C) 200 250 °C …… 245 °C ..……....... 235 °C .......................... 180 °C

Preheating
150 150 °C typical 100

ca. 45 ... 90 s > 220 °C

ramp down rate < 6 K/s ramp up rate < 3 K/s

50

0 0 30 60 90 120 150 180 210 240 270 300 330 360 Time (s)
Continuous line: typical process (term inal tem perature) Dotted line: process lim its; Bottom process lim it (term inal temperature) Upper Process lim it (top surface tem perature)

groups require special handling. Figure 1. Recommended reflow soldering curve for SnAgCu solders taken from Draft IEC 61760-1, Ed 2

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–free reflow soldering have put their own requirements on the solderable layer, and the material tests to optimally cope with these requirements are still going on. The capacitor element is encapsulated in pre-moulded box, which is of glass reinforced PPS material. The potting of the element in the box is made with specially selected halogen-free epoxy. Both the box and the epoxy are self extinguishing materials. The Figure 3. shows end-sprayed capacitor element, box, element with outer electrodes attached, and the ready made capacitor without epoxy potting.

The intended capacitance and voltage (at 125 °C) ranges are: PPS: Capacitance 1 nF – 3,3 µF Voltage 400 VDC – 50 VDC PEN: Capacitance 1 nF – 4,7 µF Voltage 630 VDC – 50 VDC The voltage at 150 °C and 175 °C respectively will be determined with high temperature testing.

PARAMETRIC CHANGES OVER TEMPERATURE
The major capacitor types on the market are Single and Multilayer Ceramic Capacitors (SLCCs and MLCCs), Tantalum and Aluminium Electrolytic Capacitors and Film Capacitors. The motivation to use plastic film capacitors in competition with other capacitor types is often the better stability of the major electrical parameters over temperature and time, and the good level of these parameters. The Figures 4., 5. and 6. show capacitance, dissipation factor (1 kHz) and insulation resistance as a function of temperature . These are example measurements made for 2220 size 100 VDC (at 125 ºC) capacitors of PPS and PEN dielectric.

Figure 3. Capacitor construction The sizes, which have been designed at this time are shown in the Table 2.

DC/C from -50 deg C to +175 deg C
20

15 PPS 33nF-100V PEN 47nF-100V DC/C (%) 10

5

0 -50 -5 T(degC) -25 0 25 50 75 100 125 150 175

Size code 2220 2824 4036 5045 6560

L(mm) 5,7 7,3 10,2 12,7 16,5

B(mm) 5,0 6,0 9,1 11,5 15,0

H(mm) 2,5-4,0 2,5-4,5 5,5 6,5 7,0

Figure 4. ∆C/C vs. temperature

Table 2. Sizes of capacitors

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Dissipation Factor at 1 kHz from -50 deg C to +175 deg C
0,016 0,014 0,012 Tan delta 0,01 0,008 0,006 0,004 0,002 0 -50 -25 0 25 50 75 100 125 150 175 T(degC) PPS 33nF-100V PEN 47nF-100V

In Figures 6., 7., 8. and 9. the capacitance and dissipation factor changes are shown up to 2000 h. The insulation resistance has stayed on the original level of ca. 100 TΩ to 1000 TΩ depending on capacitor for both capacitor types through the test.
Endurance Test at 150 deg C / 0,625 * U rated dC/C for PPS Capacitors
0,1 0 0h -0,1 dC/C (%) -0,2 -0,3 -0,4 1 nF - 250 V -0,5 33 nF - 250 V 220 nF - 50 V 680 nF - 50 V -0,7 Time (h) 1000h 2000h

Figure 5. Tan δ vs. temperature

Insulation Resistance from 25 deg C to 175 deg C
1000000

-0,6

100000

Figure 6. ∆C/C of PPS capacitors It is noticeable that the capacitance change is clearly smaller for lower voltage PPS capacitors, which us thinner dielectric films. This has most probably its origin in larger moisture absorption in high voltage capacitors made of thicker films.
100 T(degC) 125 150 175

10000 IR(MOhm)

1000

100 PPS 33nF-100 10 PEN 47nF-100V

1 25 50 75

Figure 6. Insulation resistance vs. temperature
0,0025

Dissipation Factor at 100kHz for PPS Capacitors in 150 degC / 0,625 * U rated Endurance Test

The design of electronics for very high temperatures needs special knowledge of the components. E.g. with capacitors the strong decrease of insulation resistance, which is similar behaviour for all capacitor types, must be taken into consideration.

0,002 Tan Delta 0,0015 0,001 0,0005 1 nF 250 V 0 0h 33 nF 250 V 220 nF 50 V 680 nF 50 V 1000h Tim e (h) 2000h

ACCELERATED TESTING 150 °C Tests:
Important test is the high temperature endurance test. For PPS capacitors there exists an international standard IEC 60384-20, which gives for 150 °C (155 °C) a so called category voltage 0,5 * rated voltage. We have followed that, and tested PPS capacitors in 150 °C with 1,25 * 0,5 * U rated. Because for PEN capacitors the international standard is only existing as draft (Draft IEC 60384-23), we have used the same voltage rule for them.

Figure 7. Tan δ of PPS capacitors

Endurance Test at 150 deg C dC/C for PEN Capacitors
0 0h -0,5 -1 dC/C (%) -1,5 -2 -2,5 -3 -3,5 Time (h) PEN 15 nF - 250 V PEN 47 nF - 400 V PEN 68 nF - 63 V PEN 1 uF - 50 V 1000h 2000h

Figure 8. ∆C/C of PEN capacitors
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Dissipation Factor at 100kHz for PEN in 150 degC/0,625xUr Endurance Test 0,01 0,009 0,008 0,007 0,006 0,005 0,004 0,003 0,002 0,001 0 0h

T an D elta

1,5 nF 250 V 47 nF 400 V 68 nF 63V 1 uF 50 V 1000h Test tim e (h) 2000h

In 175 °C tests after 1000 h the 50V (at 125 °C) PPS capacitors start to show low insulation resistance, and after 2000 h some of the capacitors have dropped from 100 TΩ -level to under 1 MΩ -level. This result is not anymore acceptable, and the tests have to be repeated. It is most probably so that the voltage should be derated more than is made in these first test. New tests are running with various voltage levels from 0 V to the nominal voltage to be able to derive the voltage acceleration factor for these capacitors. The higher voltage PPS and all PEN capacitors show good stability and have very small parametric change.

Figure 9. Tan δ of PEN capacitors

175 °C Tests:
The tests have been performed also at 175 °C. For that temperature no test voltage has been given in any standard. To start with, the voltage has been selected to be the same as in 150 °C tests. In Tables 3., 4., 5. and 6. the capacitance change and dissipation factor (100 kHz) change is given.

SUMMARY
The Metallized Plastic Film Capacitors seem to be very possible products to be used in high temperature applications up to 175 °C. More testing is still needed to determine the correct voltage specification, and also the end-of life conditions. Development work will be still done also to maximise the temperature durability of these capacitors to meet the requirements of Pb-free soldering.

1 nF-250V 33 nF-250V 220 nF-50V 680 nF-50V 0h 1000h 2000h 0 -0,4 -0,54 0 -0,05 -0,02 0 0,24 0,66 0 0,19 0,35

Table 3. ∆C/C of PPS capacitors in 175 °C test 1.
47nF-400V 68 nF-63V 0h 1000h 2000h 0 -1,8 -1,8 0 -2,2 -2 1,5 nF-250V 1 uF-50V 0 -2 -2,1 0 -0,28 0,35

REFERENCES
A. Dehbi et.al., “Passive Components for High Temperatures: Application Potential and Technological Challenges”, CARTS 2003 Procedings of 23rd Capacitor and Resistor Technology Symposium, 27-30 October, 2003, Stuttgart, Germany Committee Draft for Voting of Edition 2 of IEC Publication 61760-1: Standard Method for the specification of surface mounting components (SMDs), International Electrotechnical Commission, 2004, Geneve, Switzerland

Table 4. ∆C/C of PEN capacitors in 175 °C test
1 nF-250V 33 nF-250V 220 nF-50V 680 nF-50V 0h 1000h 2000h 0,0007 0,0004 0,0008 0,0007 0,0006 0,0008 0,0012 0,0019 0,0024 0,0005 0,0006 0,0014

2.

Table 5. Tan δ change of PPS capacitors in 175 °C test
47nF-400V 68 nF-63V 0h 1000h 2000h 0,008 0,006 0,006 0,008 0,006 0,007 1,5 nF-250V 1 uF-50V 0,008 0,006 0,006 0,008 0,008 0,008

ACKNOWLEDGEMENTS
This work has been performed within the PROCURE project G1RD-CT-2001-00539, supported by the European Commission.

Table 6. Tan δ change of PEN capacitors in 175 °C test

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Description: SMD Plastic Film Capacitors for High Temperature Applications