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Capacitors > Ceramic capacitors > Flashcards

Ceramic capacitors Flashcards

1
Q

Basic components in MLCC

A
  • Metallic electrodes
  • Dielectric ceramic
  • Connecting terminals
  • Plating (Ni or Sn)
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2
Q

Classification of ceramic capacitors

A
  • Class 1: use paraelectric materials
  • Class 2: use ferroelectric materials
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3
Q

2 paraelectric materials used for class 1 ceramic caps and relative permittivity

A
  • Magnesium niobium oxide (MgNb2O6): 21
  • Magnesium tantalum oxide (MgTa2O6): 28
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4
Q

Example of 1 ferroelectric material used for class 2 ceramic caps and relative permittivity

A
  • Barium titanate (BaTiO3) + additives (aluminium, magnesium, aluminium oxide): 5000
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5
Q

Information specified by electrical industry alliance for class II MLCCs

A
  • Low T limit: X (-55°C), Y or Z
  • High T limit: 4 to 9 (200°C)
  • Change of C over T range: P,R,L,S,T,U,V
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6
Q

Materials used for internal and termination electrodes in MLCC

A

Internal:
- Noble metal: (Silver palladium) Ag-Pd alloy
- Base metal: Nickel (Ni)
Termination
- Substrate: Ag and Cu
- Barrier: Ni
- External: Sn

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7
Q

What is the most common termination style for MLCC today and why?

A
  • Base metal electrodes that uses Cu and Ni
  • It replaced Ag and Pd electrodes due to lower cost of Cu and Ni
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8
Q

Main steps of MLCC fabrication (Thomas pag 15)

A
  • Start from ceramic powder
  • Ceramic slurry
  • Tape casting
  • Sheet cutting
  • Screen printing
  • Stacking
  • Lamination
  • Dicing/cutting
  • Binder burnout and sintering
  • Termination
  • Termination firing
  • Test
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9
Q

Define ferroelectricity

A
  • Property of certain nonconducting materials that exhibit spontaneous electric polarization that can be reversed in direction by the application of an appropriate E field
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10
Q

Explain in more detail spontaneous electric polarization of ferroelectric materials

A
  • separation of the centre of positive and negative electric charge, making one side of the crystal positive and the opposite side negative
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11
Q

What happens in a ferroelectric material when the external E field is reversed?

A
  • It reverses the predominant orientation of the ferroelectric domains, though the switching to a new direction lags somewhat behind the change in the external E field.
  • This lag of electric polarization behind the E field is ferroelectric hysteresis
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12
Q

Under which condition, ferroelectricity of a material stops?

A
  • For high T (above Curie T) because heat agitates dipoles enough to overcome forces that spontaneously align them
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13
Q

Other characteristics of ferroelectric materials

A
  • Pyroelectricity (V is generated due to T changes)
  • Natural electrical polarization is reversible
  • spontaneous nonzero polarization (polarization even without E field)
  • Polarization dependent on current E field and history (hysteresis loop)
  • Saturation leading to decrease in permittivity as DC V increases
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14
Q

Classification of dielectric based on induced polarization vs E field (draw the curves) (Thomas pag 20)

A
  • Linear dielectric polarization
  • Paraelectric polarization (non-linear)
  • Ferroelectric polarization (non-linear)
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15
Q

Explain pyroelectricity and its main characteristics

A
  • Temporary V is generated when material is heated or cooled
  • T change modifies slightly the position of the atoms, changing the polarization
  • If T stays constant, V disappears due to leakage I (electrons moving in the crystal, ions moving through air, I leaking through voltmeter)
  • Property of crystal that are naturally electrically polarized, hence contain large E fields
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16
Q

Equation for capacitance of MLCC

A

C = ere0(n-1)*A/d
n: number of stacked inner electrodes

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17
Q

Explain temperature dependence of class 2 caps: X7R, Z5U, Y5V based on the curves (Thomas pag 30)

A

Capacitance change with T
- X7R decreases then increases and again decreases with T
- Z5U decreases with T
- Y5V increases then decreases with T

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18
Q

Explain frequency dependence of class 2 caps (Thomas pag 30)

A

Capacitance decreases with frequency (X7R and Y5V)

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19
Q

Explain T and frequency dependence of class 1 caps

A

Capacitance is almost constant with T and frequency (NP0)

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20
Q

Draw the change of capacitance against frequency and T for class 1 (NP0) and class 2 (X7R, Y5V) caps (Thomas pag 30)

A

Draw on paper

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21
Q

Variation of barium titanate dielectric constant vs T (draw on paper) (Thomas pag 31)

A
  • Overall dielectric constant increases with T
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22
Q

5 failure modes of ceramic caps

A
  • Cracking
  • short-circuiting at low V in high impedance circuits. Appears as micro-cracks in the ceramic
  • Due to moisture and polarizing V, electrolytic material transportaion from one electrode to the other happens (ion bridge) (ionic migration), thus leakage I increases and insulation R is reduced leading to breakdown
  • Corrosion damages electrodes surfaces thus increasing ESR and reducing C
  • Very thin conductive path is easily burnt away if V exceeds rated values
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23
Q

List the 4 different defects in ceramic caps

A
  • Manufacturing defects
  • Flex crack (formed by excess bending of an MLCC)
  • Thermal shock crack
  • Placement cracks
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24
Q

Why is flex crack a severe problem?

A
  • The defect only turns into failure (short circuit) in the field after the cap is exposed to humidity
  • cannot always be detected electrically or visually
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25
Q

How the conductive path is formed at the flex crack? draw an schematic

A
  • Due to the combination of flex cracks, humidity, applied voltage and time
  • moisture penetration reduces insulation R of dielectric
  • Eventually breakdown or short circuit occurs
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26
Q

Types of delamination of MLCCs based on the used material

A
  • base metal electrodes BME (Cu and Ni)
  • precious metal electrodes PME (Ag and Pd)
27
Q

1 advantage and applications of PME delamination

A
  • TCE compatible with glass dielectric, lowering the stress under T changes
  • Used in space and defense
28
Q

Problems of reducing the thickness of dielectric in MLCC (3 ideas)

A
  • Failures are more frequent
  • High sensitivity to dust and particles in the air
  • very controlled clean rooms are needed
29
Q

Which process might be harmful for nickel barrier designs? (Thomas pag 44)

A
  • Soldering process, as nickel layer reacts with its thermal expansion
  • Specially critical for large sizes, as it might lead to cracks (some manufacturers avoid nickel)
30
Q

Equation of E force

A

F = Eq
F = Vq/d

31
Q

How can ESL and ESR of MLCC be reduced? and 1 advantage

Not related to fabrication

A
  • By changing the orientation of the MLCCs
  • Losses are reduced
32
Q

Draw MLCCs in standard and low loss orientation (Thomas pag 55)

A

Draw on paper

33
Q

Advantages of low loss orientation

A
  • Lower ESL and ESR
  • Better thermal distribution
  • Lower thermal R
  • Higher ripple I with frequency
34
Q

Method to reduce cracking in ceramic caps

A
  • Soft termination
35
Q

Explain soft termination concept

A
  • Standard termination uses only Cu, Ni, Sn layer
  • Soft uses Cu, conductive resin, Ni, Sn layer
  • The resin layer absorbs stress accompanying expansion or shrinkage of a solder joint due to thermal shock or flex stress on the board.
36
Q

7 Applications of ceramic caps (cera link caps)

A
  • DC link
  • Filter
  • Snubber
  • EVs
  • DC/DC converters
  • Drives
  • Charging systems
37
Q

Disadvantage of using integrated caps with low ESL and ESR? (Thomas pag 60)

A
  • Oscillations during double pulse tests due to lack of damping from ESR
38
Q

7 Main features of cera link caps

A
  • Compact design: due to anti-ferroelectric PLZT ceramic high capacity density
  • Ultra low ESL: 2nH
  • High ripple I
  • High operating T (150°C)
  • High voltage ragins (900V)
  • Thermal stabilization (lowest I through hottest cap)
  • Robust: can absorb mechanical stress better
39
Q

6 Advantages of using silicon caps

Consider factors that affect class II, III capacitors

A
  • High T stability (250°C)
  • Signal stability over f
  • No V dependance
  • Minimum lifetime of 10 years
  • Low thickness
  • V range: 6 to 150V
40
Q

What is high T used for during fabrication of ceramic caps? (Thomas pag 68)

A
  • to evaporate binder and have contact between electrode and dielectric
41
Q

2 challenges associated with silicon caps?

A
  • Packaging and thermal management
  • deposition of high V dielectric
42
Q

1 Advantage and 1 disadvantage of using MLCCs in EVs

Advantage related to class I ceramic caps

A
  • High stability
  • Vibrations in EVs can cause cracking
43
Q

Why are MLCC limited to small sizes? 3 reasons

A
  • Number of defects increases with the size
  • No homogeneous oxide
  • Cost
44
Q

7 main disadvantages of MLCC

General and includes 1 from class II ceramic caps

A
  • Low C value
  • Fragile
  • Limited volume
  • High cost
  • High tolerances
  • Capacitance dependence on voltage due to hysteresis and saturation effects
  • No self-healing
45
Q

Explain dependence of C against V for class 2 ceramic caps

A

-C decreases with applied DC V
- Permanent E dipoles interact with applied field, so polarizability and hence permittivity is V dependent
- Initially domain polarizations are randomly oriented when DC V increases dipoles become aligned leading to increase polarization (permittivity).
- As V increases, saturation occurs and dipole movement becomes more restricted, leading to reduced charge change dq. So C=dq/dV decreases.

46
Q

Why are sharp edges rounded in ceramic caps?

A

To improve performance

47
Q

How are disc ceramic caps built?

A
  • Two sides of small porcelain or ceramic (usually BaTiO3) are coated with Ag
  • Only 1 disc is needed for very low C values
48
Q

4 Common applications of ceramic caps (Deshpande pag 188)

A
  • RF
  • Audio applications
  • XY caps
  • Coupling and decoupling (
49
Q

4/5 main advantages of ceramic caps

A
  • C values from pF to 0.1uF
  • wide range and suitability for RF
  • Cheap (in small sizes) and reliable
  • Low loss factor (depends on type of ceramic)
  • non polar
50
Q

2 Types of ceramic caps based on shape

A
  • SMD multilayer (rectangular)
  • Capacitors with through hole leads
51
Q

Advantages of low dielectric constant C0G ceramic cap

A
  • high stability and low loss
  • Virtually no ageing
  • No dependance on T, f and DC V
  • High tolerance and T performance
52
Q

Ceramic caps (specify the code) with high dielectric constant and at least 2 disadvantages

A
  • X7R, Z5U: poor stability, high loss factor
53
Q

3 Common materials of ceramic dielectrics

A
  • Titanium oxide
  • Barium titanate
  • Strontium titanate
54
Q

How can we interpret the class number of ceramic caps according to EIA?

Refers to class I, II, III

A
  • Lower class number means better overall characteristics, but larger size for given capacitance
55
Q

7 Main characteristics of class I ceramic caps (NP0 or C0G) (Deshpande 190)

A
  • Negligible dependance on: T, V, f, time
  • Used in circuits requiring very stable performance
  • 1pF-0.1uF
  • tolerance 5%
  • large size and expensive
  • dielectric constants: 5-150
  • low dissipation factor: 0.15%
56
Q

8 Main characteristics of class II ceramic caps (X7R)

Include 1 point with applications, and type of material used

A
  • predictable change of properties with: T, V, f, time
  • smaller size
  • lower accuracy and stability
  • used for decoupling, coupling and bypass
  • Tolerances from +-10% down to 20% to -80%
  • Higher dissipation factor i.e. 2.5%
  • Uses ferroelectric material
  • High capacitance: 100pF to 2.2uF
57
Q

At least 8 characteristics of class III ceramic caps (Z5U, Y5V)

A
  • very high dielectric constant
  • high variations of properties with T
  • High C values: 1nF-10uF
  • tolerance: +-20%, +22% to -56% (Z2U), +22% to -82% (Y5V)
  • Cannot withstand high V
  • often made of BaTiO3
  • used in bypass and coupling
  • low price and small size
  • low accuracy
  • Not suitable for apps with V spikes
58
Q

Can ceramic caps compete with Al El caps? if so give 3 reasons

A

Yes, because
- C values of ceramic caps are increasing
- Offer better electrical performance
- Prices continue falling

59
Q

Describe the structure and draw a MLCC cap, 4 ideas

A

Consists of
- thin dielectric layers interleaved with staggered metal-film electrodes
- leads are connected to alternate ends of electrodes
- ceramic acts as dielectric and as encapsulation
- ink of precious materials (platinum, palladium, gold (cost)) are used as electrodes are fired above 1000°C along with ceramic to develop desired ceramic properties

60
Q

Explain and draw graph of DC bias effect on capacitance of C0G, X5R, X7R, Y5V (Deshpande 194)

A
  • C0G does not vary with DC bias
  • The rest (high dielectric constant) decreases with DC bias
61
Q

General tradeoff of high dielectric constant ceramic caps

A
  • We have larger values of C and smaller sizes
  • We will have change in C due to ageing (less stable)
62
Q

List the 8 ceramic caps tolerance codes (not for T-compensated caps)

A
  • C: +-0.25pF
  • D: +-0.5pF
  • J: +-5%
  • K: +-10%
  • M: +-20%
  • P: +100-0%
  • Y: -20+50%
  • Z: -20+80%
63
Q

2 Main advantages of ceramic caps regarding HF applications compared to film and Al El caps

A
  • They are fairly non-inductive
  • resonance occurs at very high frequencies

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