Ceramic capacitors Flashcards
Basic components in MLCC
- Metallic electrodes
- Dielectric ceramic
- Connecting terminals
- Plating (Ni or Sn)
Classification of ceramic capacitors
- Class 1: use paraelectric materials
- Class 2: use ferroelectric materials
2 paraelectric materials used for class 1 ceramic caps and relative permittivity
- Magnesium niobium oxide (MgNb2O6): 21
- Magnesium tantalum oxide (MgTa2O6): 28
Example of 1 ferroelectric material used for class 2 ceramic caps and relative permittivity
- Barium titanate (BaTiO3) + additives (aluminium, magnesium, aluminium oxide): 5000
Information specified by electrical industry alliance for class II MLCCs
- 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
Materials used for internal and termination electrodes in MLCC
Internal:
- Noble metal: (Silver palladium) Ag-Pd alloy
- Base metal: Nickel (Ni)
Termination
- Substrate: Ag and Cu
- Barrier: Ni
- External: Sn
What is the most common termination style for MLCC today and why?
- Base metal electrodes that uses Cu and Ni
- It replaced Ag and Pd electrodes due to lower cost of Cu and Ni
Main steps of MLCC fabrication (Thomas pag 15)
- Start from ceramic powder
- Ceramic slurry
- Tape casting
- Sheet cutting
- Screen printing
- Stacking
- Lamination
- Dicing/cutting
- Binder burnout and sintering
- Termination
- Termination firing
- Test
Define ferroelectricity
- Property of certain nonconducting materials that exhibit spontaneous electric polarization that can be reversed in direction by the application of an appropriate E field
Explain in more detail spontaneous electric polarization of ferroelectric materials
- separation of the centre of positive and negative electric charge, making one side of the crystal positive and the opposite side negative
What happens in a ferroelectric material when the external E field is reversed?
- 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
Under which condition, ferroelectricity of a material stops?
- For high T (above Curie T) because heat agitates dipoles enough to overcome forces that spontaneously align them
Other characteristics of ferroelectric materials
- 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
Classification of dielectric based on induced polarization vs E field (draw the curves) (Thomas pag 20)
- Linear dielectric polarization
- Paraelectric polarization (non-linear)
- Ferroelectric polarization (non-linear)
Explain pyroelectricity and its main characteristics
- 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
Equation for capacitance of MLCC
C = ere0(n-1)*A/d
n: number of stacked inner electrodes
Explain temperature dependence of class 2 caps: X7R, Z5U, Y5V based on the curves (Thomas pag 30)
Capacitance change with T
- X7R decreases then increases and again decreases with T
- Z5U decreases with T
- Y5V increases then decreases with T
Explain frequency dependence of class 2 caps (Thomas pag 30)
Capacitance decreases with frequency (X7R and Y5V)
Explain T and frequency dependence of class 1 caps
Capacitance is almost constant with T and frequency (NP0)
Draw the change of capacitance against frequency and T for class 1 (NP0) and class 2 (X7R, Y5V) caps (Thomas pag 30)
Draw on paper
Variation of barium titanate dielectric constant vs T (draw on paper) (Thomas pag 31)
- Overall dielectric constant increases with T
5 failure modes of ceramic caps
- 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
List the 4 different defects in ceramic caps
- Manufacturing defects
- Flex crack (formed by excess bending of an MLCC)
- Thermal shock crack
- Placement cracks
Why is flex crack a severe problem?
- 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
How the conductive path is formed at the flex crack? draw an schematic
- 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
Types of delamination of MLCCs based on the used material
- base metal electrodes BME (Cu and Ni)
- precious metal electrodes PME (Ag and Pd)
1 advantage and applications of PME delamination
- TCE compatible with glass dielectric, lowering the stress under T changes
- Used in space and defense
Problems of reducing the thickness of dielectric in MLCC (3 ideas)
- Failures are more frequent
- High sensitivity to dust and particles in the air
- very controlled clean rooms are needed
Which process might be harmful for nickel barrier designs? (Thomas pag 44)
- 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)
Equation of E force
F = Eq
F = Vq/d
How can ESL and ESR of MLCC be reduced? and 1 advantage
Not related to fabrication
- By changing the orientation of the MLCCs
- Losses are reduced
Draw MLCCs in standard and low loss orientation (Thomas pag 55)
Draw on paper
Advantages of low loss orientation
- Lower ESL and ESR
- Better thermal distribution
- Lower thermal R
- Higher ripple I with frequency
Method to reduce cracking in ceramic caps
- Soft termination
Explain soft termination concept
- 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.
7 Applications of ceramic caps (cera link caps)
- DC link
- Filter
- Snubber
- EVs
- DC/DC converters
- Drives
- Charging systems
Disadvantage of using integrated caps with low ESL and ESR? (Thomas pag 60)
- Oscillations during double pulse tests due to lack of damping from ESR
7 Main features of cera link caps
- 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
6 Advantages of using silicon caps
Consider factors that affect class II, III capacitors
- High T stability (250°C)
- Signal stability over f
- No V dependance
- Minimum lifetime of 10 years
- Low thickness
- V range: 6 to 150V
What is high T used for during fabrication of ceramic caps? (Thomas pag 68)
- to evaporate binder and have contact between electrode and dielectric
2 challenges associated with silicon caps?
- Packaging and thermal management
- deposition of high V dielectric
1 Advantage and 1 disadvantage of using MLCCs in EVs
Advantage related to class I ceramic caps
- High stability
- Vibrations in EVs can cause cracking
Why are MLCC limited to small sizes? 3 reasons
- Number of defects increases with the size
- No homogeneous oxide
- Cost
7 main disadvantages of MLCC
General and includes 1 from class II ceramic caps
- Low C value
- Fragile
- Limited volume
- High cost
- High tolerances
- Capacitance dependence on voltage due to hysteresis and saturation effects
- No self-healing
Explain dependence of C against V for class 2 ceramic caps
-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.
Why are sharp edges rounded in ceramic caps?
To improve performance
How are disc ceramic caps built?
- Two sides of small porcelain or ceramic (usually BaTiO3) are coated with Ag
- Only 1 disc is needed for very low C values
4 Common applications of ceramic caps (Deshpande pag 188)
- RF
- Audio applications
- XY caps
- Coupling and decoupling (
4/5 main advantages of ceramic caps
- 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
2 Types of ceramic caps based on shape
- SMD multilayer (rectangular)
- Capacitors with through hole leads
Advantages of low dielectric constant C0G ceramic cap
- high stability and low loss
- Virtually no ageing
- No dependance on T, f and DC V
- High tolerance and T performance
Ceramic caps (specify the code) with high dielectric constant and at least 2 disadvantages
- X7R, Z5U: poor stability, high loss factor
3 Common materials of ceramic dielectrics
- Titanium oxide
- Barium titanate
- Strontium titanate
How can we interpret the class number of ceramic caps according to EIA?
Refers to class I, II, III
- Lower class number means better overall characteristics, but larger size for given capacitance
7 Main characteristics of class I ceramic caps (NP0 or C0G) (Deshpande 190)
- 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%
8 Main characteristics of class II ceramic caps (X7R)
Include 1 point with applications, and type of material used
- 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
At least 8 characteristics of class III ceramic caps (Z5U, Y5V)
- 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
Can ceramic caps compete with Al El caps? if so give 3 reasons
Yes, because
- C values of ceramic caps are increasing
- Offer better electrical performance
- Prices continue falling
Describe the structure and draw a MLCC cap, 4 ideas
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
Explain and draw graph of DC bias effect on capacitance of C0G, X5R, X7R, Y5V (Deshpande 194)
- C0G does not vary with DC bias
- The rest (high dielectric constant) decreases with DC bias
General tradeoff of high dielectric constant ceramic caps
- We have larger values of C and smaller sizes
- We will have change in C due to ageing (less stable)
List the 8 ceramic caps tolerance codes (not for T-compensated caps)
- C: +-0.25pF
- D: +-0.5pF
- J: +-5%
- K: +-10%
- M: +-20%
- P: +100-0%
- Y: -20+50%
- Z: -20+80%
2 Main advantages of ceramic caps regarding HF applications compared to film and Al El caps
- They are fairly non-inductive
- resonance occurs at very high frequencies
