Engineering Considerations for the Substitution of Solid Tantalum and Tantalum
Polymer Capacitors for Surface-Mount MLCCs
Due to current bottlenecks in the procurement of surface-mount ceramic capacitors,
designers are looking for substitutes to keep their production lines running smoothly,
and to find long-term replacements for hard to find ratings. This article will examine key
performance characteristics that will help streamline the evaluation process of
alternative capacitor technologies for the replacement of multilayer ceramic chip
capacitors (MLCCs). The most likely MLCC candidates for successful replacement by
tantalum technology are the higher capacitance values in case sizes from 0402 to1210.
Applications that require high capacitance for filtering or voltage stabilization are good
candidates for replacement.
There are two primary capacitor technologies that are most often considered for
surface-mount applications: electrostatic and electrolytic. The most common
electrostatic types are MLCCs and film capacitors. The most common electrolytic types
are aluminum and tantalum (including both solid and polymer tantalum technologies).
When looking to substitute for high capacitance MLCCs, it makes sense to pick
tantalum electrolytic devices to get the broadest overlap in size, surface mount ability,
capacitance values, voltage ratings, and reliability.
Although top-level comparisons of capacitance, voltage, tolerance, and size are useful
as a starting point, MLCCs and surface-mount tantalums utilize different designs and
materials in their construction. As a result, they have different electrical and mechanical
properties. Instead of throwing massive amounts of performance data at the design
team, this paper will attempt to look at the key parametric differences that relate to the
performance of the capacitors. In addition, some helpful hints and suggestions for
testing will be offered with the goal of successfully substituting solid tantalum or
tantalum polymer capacitors for MLCCs with maximum efficiency.
The Equivalent Circuit of a Capacitor
To simplify and organize our investigation, we will utilize the capacitor equivalent circuit
as a model and discuss how the different elements of the circuit vary between MLCCs
and tantalums. Figure 1 shows the universal equivalent circuit of a capacitor:
Figure 1
��� R
ESR
= equivalent series resistance in ohms. This is the real part of the
impedance that produces losses via heat generation
• C = capacitance value in Farads. The reactance of this component is X
C
= 1 /
2πfC
• L = inductance in Henrys. The reactance of this component is X
L
= 2πfL
• R
IR
= insulation resistance (in an ideal capacitor this would equal infinity, but in
an actual capacitor it’s a finite resistance value that can be used to calculate the
DC leakage current value I
DCL
)
The overall impedance of the circuit is: Z=√(R
ESR
)
2
+ (Xc – X
L
)
2
in ohms.
Figure 2
Let’s take the components of the equivalent circuit one at a time and consider their
effect on the overall circuit performance.
Capacitance (C)
Most MLCCs used in today’s applications are "Class II" types. This means that the
capacitance value varies over temperature (temperature coefficient of capacitance
(TCC)) in the following ways:
• ± 15 % from -55 °C to +125 °C for X7R dielectrics
• ± 15 % from -55 °C to +85 °C for X5R dielectrics