Chosen electrical, noise and stability characteristics of passive components embedded in Printed Circuit Boards
Andrzej Dziedzic1), Adam Kłossowicz1), Paweł Winiarski1), Adam W. Stadler2), Wojciech Stęplewski3)
1)Faculty of Microsystem Electronics and Photonics, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland, [email protected]
2) Rzeszów University of Technology, Powstańców Warszawy 12, 35-959 Rzeszów, Poland 3) Tele and Radio Research Institute, Ratuszowa 11, 03-450 Warsaw, Poland
Abstract
This paper presents systematic studies of electrical, noise and long-term stability parameters of resistors (thin-film or polymer thick-film) and capacitors embedded in Printed Circuit Boards (PCBs). The temperature dependence of resistance or capacitance were determined in a wide temperature range (from -180oC to 130oC) and analyzed as a function of geometry of passives and cladding process. The in-situ accelerated ageing process (basic properties of passives measured directly at ageing conditions) was carried out to perform long-term behavior analysis. Low frequency noise measurements were made in room temperature using noise spectra measurements in dc bridge configuration.
The R(T) characteristics are linear with almost constant, negative value of differential TCR (of about -60 ppm/K for 100 Ω/sq Ni-P resistors). Both groups of investigated resistors revealed similar range of relative resistance changes after ageing processes but the results showed the quite different behavior of both groups versus time. It means that the dynamics of ageing changes was different. Only positive resistance changes were observed for Ni-P resistors, whereas the shape of characteristics for polymer ones were much more complex, exhibited increase as well as decrease in resistance under environmental exposure. 1/f noise generated by resistance fluctuations was found as the main noise component but the significant difference of noise level was observed for both groups of investigated resistors.
The C(T) characteristics are nonlinear with larger capacitance changes at higher temperature. Capacitors exposed to elevated temperature exhibited capacitance and dissipation factor decrease. The relative changes were from the range from -12% to -2% for capacitance and up to -60% for dissipation factor. The value of relative drift of parameters was dependent strongly on dielectric composition and size. Moreover the results revealed non-linear characteristics in temperature domain as well.
1. Introduction Significant part of Printed Circuit Boards’ area is
consumed by passives. The continuous growth of de-mands for more efficient, functional, reliable and miniatu-rized electronic circuits determines PCBs producers to look for new solutions which will permit manufacturing PCBs with a higher density of connections. The increase of component packaging density is possible thanks to multilayer technology which makes possible to embed passives into PCBs after lamination process.
In the last years the large growth of high-advanced but simultaneously low-cost electronic products, e.g. mobile phones, laptops, etc caused the wide interest in technolo-gies of passive components’ embedding. The first trials of embedded capacitors and resistor started in the 60’s or early 70’s of the last century [1-5]. The embedding of passives into PCB allows further miniaturization, has the potential to reduce cost and moreover exhibits superior electrical behavior with respect to the minimization of parasitic effects [2,6].
There are two attempts in embedding of passives into PCBs. The first is based on standard or ultrathin surface mount components attached to the laminate and building PCB around the components afterwards [7]. The second one uses special films and foils where embedded component ought to be thinner than a distance between adjacent PCB layers and thus it does not increase the PCB thickness. The commonly used prepreg 106 with high resin content (69%), so perfectly filling the mosaic of the inner PCB’s layer, has a thickness of 50 µm after lamination. In practice, the application of two prepreg layers permits to embed into the inner layers electronic components with a thickness up to about 75 µm. Additionally, such components have to be durable to chemical and thermo-mechanical exposures (e.g. during the lamination process the pressure up to 30 bars and temperature of 180°C are used) applied in the PCB manufacturing process.
Currently, the number of available materials for em-bedded passives manufacturing is growing. The thin-film resistors materials generally have a thickness below 1 µm. Several materials (alloys), characterized by a relatively good chemical resistance and constant surface resistivity (dependent on the layer thickness and composition) are used. The following ones can be mentioned among them: • NiP alloy electroplated onto the copper foil permit-
ting for resistors’ manufacturing in subtractive process (Ohmega-Ply RCM from Ohmega Technologies, sheet resistance from 10 to 250 Ω/sq),
• NiP plated directly onto the inner layer through an additive process (M-Pass from MacDermid, sheet resistance 25 to 100 Ω/sq),
• NiCr alloy, Nichrome-Aluminum Silicon and CrSiO film (sheet resistance between 25 and 1000 Ω/sq) from Ticer Technologies,
• doped-platinum deposited on copper foil (InSite from Rohm and Haas (Shipley), sheet resistance from 50 Ω/sq to 1000 Ω/sq). Thick-film resistors are usually made of polymer
pastes with carbon black, graphite or carbon-silver filler
served as functional phase and proper solvents. Optionally they contain powder insulating filler for proper rheological properties. Their thickness after curing is equal to about 10-25 µm and they permit to fabricate resistance from several ohms to several tens Mohms [8].
Materials for resistors manufactured by InkJet techno-logy are available as inks with carbon-silver nanopowders (the film thickness in this technology is equal to several hundred nanometers).
Composites for embedded capacitors consist of a die-lectric with a thickness from several to 20-30 µm, and one or two Cu layers with a thickness of 18-70 µm [9,10]: • modified epoxy or modified epoxy-filled ceramic bet-
ween Cu layers (FaradFlex from Oak Mitsui, capa-citance density up to 1700 pF/cm2),
• thin film paraelectrics SiOx (10-15 nF/cm2) and thin-film ferroelectrics (200 nF/cm2) named InSite (Rohm and Haas and Energenius),
• anodized Ta2O5 (capacitance density 200 nF/cm2, Stealth Capacitor from Xanodics),
• BaTiO3 thin film on copper (Interra from DuPont), • polymer film with copper deposited thereon (capa-
citance density about 216 pF/cm2, Gould Electronics), • high Tg (170°C) epoxy resin with 106 fiberglass
(capacitance density about 143 pF/cm2, BC2000 from Sanmina-SCI),
• epoxy dielectric filled with BaTiO3 (capacitance density from 0.78 to 4.65 nF/cm2, C-Ply from 3M). A next generation material, allowing the formation of
both resistors and capacitors within the PCBs, appeared on the market recently [10]. Ohmega/FaradFlex is a com-bined product of an OhmegaPly thin-film NiP alloy elec-trodeposited on copper foil that is laminated to a Farad-Flex dielectric material and subtractive processed to produce embedded RC networks.
This paper presents systematic studies of electrical, noise and long-term stability parameters of resistors (thin-film or polymer thick-film), capacitors, RC filters and inductors embedded in Printed Circuit Boards (PCBs). The temperature dependence of resistance (from -180oC to 130oC), capacitance (from 15oC to 100oC) were determined and analyzed as a function of geometry of passives and cladding process. The in-situ accelerated ageing process (basic properties of passives measured directly at ageing conditions) was carried out to perform long-term behavior analysis. Low frequency noise measurements of thon-film and polymer thick-film resistors were made in room temperature using noise spectra measurements in dc bridge configuration.
2. Test sample fabrication Thin-film resistors were made of nickel–phosphorus
alloy (Ohmega–Ply material with 25, 100 or 250 Ω/sq) [11]. Polymer thick-film resistors were fabricated of 10 Ω/sq or 1 kΩ/sq inks. These resistors had decidedly different thickness of resistive layers – 0.4, 0.1 or 0.05 μm thick NiP alloy and approximately 12 μm for polymer resistive films. NiP samples had Cu contacts whereas polymer thick-film resistors were printed on bare Cu or Cu contacts with Ni/Au coating. Both resistors were
covered with Resin Coated Copper (RCC) protective coating (the cross section of resistors is shown in Fig. 1, whereas exemplary topology of test structures is visible in Fig. 2).
Fig. 1. Cross-section of thin-film NiP (top) and polymer
thick-film resistor (not in scale).
Fig. 2. Topology of test structure (an example).
Planar capacitors were fabricated from special FaradFlex composite consisted of dielectric foil with two Cu layers deposited on both sides of dielectric. The fabrication pro-cess 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 embedding process and protect capacitors from atmospheric influ-ence. For research FaradFlex composite with three differ-rent types of dielectric were used [12] varied in form of BaTiO3 dielectric, its thickness and dielectric constant: • BC12TM material, 12 μm thick with partially filled
dielectric, • BC16T material, 16 μm thick with ceramic filler, • BC24M material, 24 μm thick without filler. The highest dielectric constant has BC16T material and the lowest - BC24M. The microscopic view of tested structures is shown in Figs. 3 and 4.
Fig. 3. Microscopic view and schematic presentation of
BC24M dielectric material.
dielectric
Cu
Cu
Fig. 4. Microscopic view and schematic presentation of BC16T
dielectric material.
3. Electrical, stability and noise properties of embed-ded thin-film and polymer thick-film resistors
3.1. Resistance of test structures – influence of resistor dimensions and cladding material
The real value of sheet resistance for surface PCB resistors is somewhat (a few %) larger form nominal one (please see Table 1). This could be connected with thickness or resistivity of Ni-P foil, but also can be connected with roughness resistive foil. The roughness of these layers is dependent directly on roughness of used substrates. The NiP layer is deposited on the oxide treatment copper foil and while the lamination is mapped in the resin. The average surface roughness (Ra) is at the level of 0.4-0.6 µm, and is slightly smaller than the FR4 laminate roughness (0.5-0.7 µm). The outer unoxidized Cu layer had a surface roughness of about 0.2 microns (Fig. 5). The roughness may be different for each lot of material as a result of different levels of copper layer oxide treatment on which NiP alloy is deposited.
The distribution of this parameter is equal to 3-7% of mean sheet resistance and is independent on dimensions of resistive layer. The same the dimensional effect for thin-film resistors is insignificant (much smaller than for polymer thick-film resistors). But covering of resistors by RCC or LDP 2×106 cladding leads to significant chan- ges of sheet resistance. For example, 14% increase of resistance is observed for 25 Ω/sq foil.
Table 1. Influence of resistor dimensions and cladding material on mean sheet resistance (for 25 Ω/sq film)
Fig. 5. The roughness of Cu, NiP and FR4 layers for 100 Ω/sq
RCM Ohmega-Ply material.
3.2. Resistance vs. temperature dependence The Keithley 2000 Multimeter interfaced to personal computer for data acquisition and presentation was used in measurements of resistance versus temperature in the range from -180oC to 130oC. The examples of normalized temperature dependence of resistance and differential TCR, where )( dTR
dRTCRdiff ⋅= , are shown in Fig. 6.
Hot (HTCR) and cold (CTCR) temperature coefficient of resistance were calculated based on resistance values at temperatures -55oC, 25°C and 125°C (Eq.(1) and (2) ) – the data are presented in Table 2.
Table 2. Comparison of cold and hot TCR for various versions of 100 Ω/sq Omega-Ply resistors
Mark CTCR [ppm/oC] HTCR [ppm/oC] 100 – 1 – 1 – A 100 – 1 – 1 – B 100 – 5 – 1 – A 100 – 5 – 1 – B
-61 -56 -65 -60
-66 -55 -68 -61
The presented characteristics are linear with almost constant, negative value of differential TCR (about -60 ppm/oC). One should note that longer resistors exhibit in-significantly larger negative TCR (by about 3-4 ppm/oC). Laminated (embedded) resistors exhibit less negative TCR in comparison with unlaminated ones (by about 5 ppm/oC) – this can be caused by stresses induced into laminate during second lamination process.
Ra= 226 nm
Ra= 635 nm
Ra= 576 nm
Cu
NiP
FR4
mm
µm
dielectric
Cu
Cu
Fig. 6. Normalized temperature dependence of resistance and
differential TCR for 1×1 and 5×1 mm2 surface (A) and embedded (B) 100 Ω/sq. thin-film resistors.
3.3. Long-term stability To determine the long-term stability of resistors the
in-situ method [13] was used with measurements done during ageing process. In this case test samples were placed on the HP-801 hot plate treated with a constant temperature – 70, 100, 130 or 160°C for NiP resistors and 100 or 130°C for polymer ones. A Pt100 sensors placed at the test structure surface (to compensate thermal capacity of the substrate) were used to monitor and control temperature on the plate. Structures were equipped with gold plated needle probes connected to the Agilent 34970A multimeter. The selector collected data every 10 minutes and sent them to personal computer for data acquisition and presentation.
The stability of resistors was determined based on re-lative resistance changes during ageing tests. A norma-lized percentage resistance changes were calculated by:
ΔR(t)/R0 [%] = ((Rt – R0)/ R0)∙100 [%] (3)
where: R0 – resistance at the beginning of ageing process, Rt – resistance measured in time t.
The single ageing mechanism was found for exposure below 160°C, which according to [14,15] was described by the following equation:
ΔR/R0 = Atn∙exp(-E/kT) (4)
where A is the pre-exponential constant characteristic for particular ageing mechanism, t – time, n is the index describing time dependence, E is the activation energy, k is the Boltzmann constant and T is the temperature.
Fig. 7. Relative resistance changes for NiP 250 Ω/sq. resistors
in log-log scale.
The dependence on time, determined by index n, was from the range between 0.3 and 0.6. Activation energy of ageing process can be calculated using the approximation of results presented on Arrhenius plots
(5)
where activation energy E is a slope of linear function which fitting results. For example, the value of E was from the range between 0.27 and 0.42 eV for NiP 25 Ω/sq resistors with various geometry and cladding material.
Polymer thick-film resistors revealed a quite different behavior from NiP ones. Depending on sample parameters and ageing condition resistance drift goes to negative or positive value. For example, changes observed for 1 kΩ/sq. resistors (Fig. 8) indicate the presence of two different ageing mechanisms. The first leads to decrease and the second to increase in resistance. The overall changes in resistance did not exceed ±4% but the analysis of results for PTF resistors is very complex and it is difficult to find a proper physical function to describe the observed data. However, confining oneself to purely mathematical analysis, the results of 1 kΩ/sq. structures can be described with quite good accuracy using the following equation:
ΔR/R0 = A1tn +A2exp(1 – exp(-t/τ)) (6)
The first part of this equation can be referred to formula (4) thus 𝐴1 = 𝐴 exp (−𝐸/𝑘𝑇).
-200 -150 -100 -50 0 50 100 1500,990
0,995
1,000
1,005
1,010
1,015R(
T)/R
(25°
C) 100 - 1 - 1 - A 100 - 1 - 1 - B 100 - 5 - 1 - A 100 - 5 - 1 - B
-100
-75
-50
-25
0-200 -150 -100 -50 0 50 100 150
TCR
[ppm
/°C]
100 - 1 - 1 - A 100 - 1 - 1 - B 100 - 5 - 1 - A 100 - 5 - 1 - B
Temperature [°C]
0,1 1 10 1000,01
0,1
1
10
∆R/R
0 [%
]
t [h]
100°C 130°C 160°C
Fig. 8. Relative resistance changes for PTF resistors.
3.4. Low-frequency noise Low-frequency noise measurements were made in
room temperature using two different measurement setups. One of them has been used for acquiring spectra of voltage fluctuations and kind of noise identification while the second one based on standard meter of current noise index (CNI) - parameter describing noise properties which is commonly used by manufacturers of passives [14]. Both methods give consistent data, which allow for evaluation of parameters describing noise intensity of material that are then used for comparison noise properties of studied resistors with other previously investigated resistive materials applied in film technolo-gies. Tested resistors were made of carbon based ED7100_200 Ω/sq resistive ink screen-printed onto con-ductive contacts (Cu contacts, Cu contacts with protective Ni/Au coating and Ag contacts) and cured at temperature of about 170oC and next covered by RCC cladding. Thin-film embedded resistors were prepared from 100 Ω/sq foil coated by RCC or LDP 2×106 cladding.
Collections of typical noise spectra measured in studied resistors for different sample voltages, U, are shown in Fig. 9. Spectra of excess noise SVex ≡ SV – SU=0 have been depicted while corresponding sample voltages have been listed in the legend. System (background) noise, SU=0, and spectrum of ideal 1/f noise have been also shown for reference. One can see that 1/f noise compo-nent is dominant. Hence, product of frequency f and SVex averaged in specified frequency band Δf, S = <f∙SVex>Δf, is very convenient measure of noise intensity. It was checked that all noise components depend linearly on U2, what means that the noise is caused by resistance fluctuations.
Dependence of <f∙SVex>Δf on sample voltage is the basis of determining quantities describing noise properties. Parameter C ≡ SU-2Vol (called noise intensity), where Vol is sample volume, is often used in comparative studies as the one which describes properties of material with respect to 1/f noise. It is independent of sample voltage and its volume. Another parameter describing noise properties is CNI (Current Noise Index). It relates to C by equation [15]
VolCUfSsCNI
fVex =>=<
⋅=−
Λ2
12
where
; ln10)s10log(10 (7)
Further, for comparison of noise properties of materials of different resistivity, parameter K = C/ρ (ρ - material resistivity) is used [16]. Parameter K is considered as the figure of merit with respect to 1/f noise and also as index of material and quality indicator of its technology. The most important noise parameters of both group of tested resistors is collected in Table 3. Assuming small contribution of the contact noise, we may conclude that noise intensity of Ni-P foil resistors is close to that observed in the Au, poly-Si and poly-SiGe thin films, for which similar values of parameter K were found [16]. Typical polymer and cermet thick-film resistors are characterized by K from the range from 2·10-11 to 2·10-8 μm2/Ω [17], ie. like ED7100/Ag and ED7100/Au structures. The noise properties of ED7100 resistors with Cu contacts are very poor.
1 10 100 1k10-20
10-19
10-18
10-17
10-16
SV=0
1/f
sample voltage 0.00 V 0.62 V 0.68 V 0.80 V 0.91 V 1.00 V
S Vex, S
V=0 [V
2 /Hz]
frequency [Hz]
samples 1-5, 2x106 cladding
R1 = 119.7 ΩR5 = 117.9 Ω
b)
Fig. 9. Spectra SVex and SV=0 for a) ED7100/Au polymer thick-film, b) NiP thin-film embedded resistors. Sample voltages are
listed in the legend. The solid or dashed lines, added for reference, show ideal 1/f noise.
4. Electrical and long-term stability properties of capacitors embedded in Printed Circuit Boards
4.1. Insulation resistance Insulation resistance of capacitors was determined
indirectly by measuring voltage drop on discrete resistor (R = 1 kΩ) connected in series with embedded capacitor to voltage supply. The voltage was set to such value to achieve the same electric field (8.3 V/µm) in every sample regardless the dielectric thickness. The tested samples were placed on heater in order to determine the influence of temperature on insulation resistance. The temperature was changed in range from 25°C to 150°C.
Investigation of insulation resistance showed that BC16T structures with ceramic filler have the lowest resistance whilst BC24M structures without the filler - the highest. For temperature increase the increase of leakage current was observed. While the temperature is increased the insulation resistance is decreased and reached stable level when the temperature was sustained. This situation was repeated until temperature reached certain value after which a resistance breakdown was observed. Only BC24M structure endured the test without the breakdown. The fastest insulation resistance decrease was observed for structure BC16T (Fig. 10).
Fig. 10. Insulation resistance of tested structures for four steps
of temperature change, structures 10×10 mm2.
4.2. Temperature characteristics To measure the C(T) and tgδ(T) characteristics as well
as to determine the long-term stability of capacitors the measurement system comprised of personal computer, LCR 4263A meter, and HP34970A data selector switch was used. Test samples (embedded capacitors) were connected to the switch by plated needle probes and were subjected to elevated temperature in range from 15°C to 100°C. The frequency of the test signal was 1 or 10 kHz and the amplitude – 250 mV.
Temperature characteristics were measured for samples before and after ageing process and were determined based on relative capacitance and dissipation factor changes related to value of these parameters at 25°C. Temperature characteristics of capacitance changes are linear only in temperature range from 25 to 75ºC or for structures with the smallest dimension. The smaller capacitance change was observed with temperature for smaller structures. For the same material, dimension or ageing temperature larger capacitance changes were observed when test signal frequency was 1 kHz than 10 kHz. In the case of dielectric material the highest changes were noticed for BC16T and the smallest for BC24M.
The ageing process led to reducing the characteristics of relative capacitance change and TCC (Temperature Coefficient of Capacitance) calculated from equation:
]/[1050)25(
)25()75( 6 CppmCCC
CCCCTCC °⋅°⋅°°−°
= (8)
The higher the ageing temperature was the higher TCC changes were observed. Additionally after ageing C(T) characteristics became more non-linear. Ageing process caused permanent capacitance decrease which was larger for higher ageing temperature for structures BC16T and the contrary tendency was noticed for BC24M (Table 4 and Fig.11).
Table 4. Relative capacitance changes and changes of TCC after ageing, structures 10×10 mm2.
Fig. 11. Normalized temperature dependence of capacitance for
as-made and thermally aged 10×10 mm2 BC16T embedded capacitors (measurement frequency f = 10 kHz).
4.3. Long-term stability Long-term stability of capacitors was determined at
elevated temperature for 70, 100 or 130°C during 170
0 20 40 60 80 100100
10k
1M
100M
10G
Insu
latio
n re
sista
nce
R C [Ω]
Time [h]
12TM 16T 24M
20
40
60
80
100
120
140
160
Tem
p [°C
]
Temp
20 40 60 80 100-2
0
2
4
6
8
10
12
∆C/C
25 [%
]
Temp [°C]
before ageing after ageing at 70°C after ageing at 100°C after ageing at 130°C
hours. It was determined based on relative capacitance changes during ageing tests – Eq. (9):
[%] 0
0
CCC
CC t −=
∆ (9)
where: Ct – capacitance value measured in time t, C0 –capacitance value at beginning of ageing process.
Ageing process leads only to capacitance decrease. Characteristics shape revealed fast capacitance change just after the beginning of the process and next reaches some stable level after about 100 hours. Capacitance changes are higher for larger structures. For example, relative capacitance change was nearly two times higher for structures 10×10 than for 2.5×2.5 mm2.
In the case of dielectric material the largest changes were noticed for BC16T and the smallest for BC24M (Fig. 12). However for ageing at 130ºC the tendency is reversed and the highest relative changes are observed for BC24M structures.
Fig. 12. Long-term stability of test capacitors, signal frequency
10 kHz, structures 10×10 mm2, ageing temperature – 100ºC.
The ageing process itself leads to capacitance and dissipation factor decrease. The higher the temperature was the larger decrease was observed. Capacitance changes were higher for larger structures or at lower frequencies.
5. Conclusions All the measurements, advanced characterizations and
analyses presented above for various passive components embedded in Printed Circuit Boards as well as other our investigations not discussed in this paper (characterization of multiterminal thin-film resistors embedded in PCBs [18], determination and comparison of transient and steady-state thermal properties of thin-film, polymer and cermet thick-film as well as LTCC resistors [19], characterization of pulse durability of thin-film and polymer thick-film embedded PCB resistors [20], determination of chosen electrical and long-term stability properties of embedded capacitors and RC filters, especially with the aid of impedance spectroscopy [21,22], electrical and long-term stability characterization of planar PCB inductors [23]) confirmed that such passives with small and medium values of resistance, capacitance or inductance exhibit basic electrical properties and long-term thermal stability level acceptable for many consumer applications.
Acknowledgments The work has been supported from Grant DEC-2011/01/B/ST7/06564 funded by National Science Centre (Poland).
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