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| Ceramic Dielectric Capacitors Classes I, II, III and IV - Part I: Characteristics and Requirements |
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EIA-198-1-F - Ceramic Dielectric Capacitors Classes I, II, III and IV - Part I: Characteristics and Requirements
Foreword
This EIA Standard is a revision and update of EIA-198-E covering ceramic dielectric capacitors. Formulated under the cognizance of the EIA P-2.1 Subcommittee on Ceramic Capacitors.
Clarification statement
The purpose of this clarification statement is to advise the reader that it is EIA procedure when revising a standard to include all of the outstanding addendums of the standard being superseded in the revision.
Accordingly, previously published addendums to EIA-198-B; EIA-198-B-1: EIA-198-B-2; EIA-198-B-3A and EIA-198-C, EIA-198-D, and EIA-198-E have been incorporated into EIA-198-F.
EIA-198-1-F of this standard provides means to characterize ceramic capacitors electrically and mechanically by use of type designators. In addition, this section outlines dielectric classifications, marking specifications and test sequences.
Dielectric classification
There are four major classifications of ceramic dielectrics, with class I being the least variable with temperature and voltage, and class IV being the most variable. Class I dielectrics are typically used in applications requiring the tightest tolerance.
Class I
Components of this type are temperature compensating ceramic dielectrics, fixed capacitors of a type suited for resonant circuit applications or other applications where high Q and stability of capacitance characteristics are required. (See table 1.)
Class II
Components of this classification are fixed, ceramic dielectric capacitors of a type suited for bypass and decoupling application or for frequency discriminating circuits where Q and stability of capacitance characteristics are not of major importance. This classification is further defined as those capacitors having temperature characteristics A through S (see table 3). Class II ceramic dielectrics exhibit a predictable change with time and voltage. Compensation for the aging effect is made by referencing capacitance limits to a future time deemed to be most useful to the buyer; 1,000 hours is normally chosen, but other arrangements may be negotiated between the buyer and seller. Voltage will also cause a temporary capacitance change, and the test sequence should be such that capacitance measurements are not affected by previous voltage tests.
The aging rate of a dielectric is essentially constant over many decades of time, i.e., 10 h to 100 h, 100 h to 1,000 h, 1,000 h to 10,000 h, etc., when measured from the time of the last heat of depolarization in manufacture.
Restoration of the original capacitance at time of manufacture will occur on heating to 150 C for one hour, after which normal aging will again commence. Capacitors measured prior to 24 hours may exhibit temporarily high capacitance values that will age downward.
Class III
Components herein standardized are fixed ceramic dielectric capacitors of a type specifically suited for use in electronic circuits for bypass, decoupling or other applications in which dielectric losses, high insulation resistance and capacitance stability are not of major consideration. This classification is identical to that of class II, except that it is restricted to those capacitors having temperature characteristics T through V (table 3).
Class IV
This classification is restricted to those components utilizing reduced titanate or barrier layer type construction. While basically fitting the descriptions of class II and class III, certain other electrical differences can be noted, as described in EIA-198-3-F of this specification.
Mechanical classifications
Unleaded multilayer ceramic capacitors
Unleaded ceramic chip capacitors are available in a variety of physical sizes and shapes lending themselves to a wide variety of specialized applications. Generally, the unit consists of an unencapsulated fired capacitor element with metallized terminations. A range of end metallizations is available to match the variety of possible bonding techniques. The absence of leads and any encapsulating material makes them extremely space efficient and well suited to hybrid circuit use in decoupling, bypassing, timing, tuning, etc.
Scope
Generally, chip capacitors are available in dielectric classifications, I, II and III, suiting them for a wide range of hybrid applications where space is a prime consideration. The absence of lead inductance enables operation at considerably higher frequencies than comparable leaded units of the same capacitance value. Since these units are unencapsulated, they require an environment that minimizes the effects of humidity and contamination. Care must be taken to ensure that units are kept free or cleaned of ionizable residues deposited by handling or fluxing during manufacture. Care must also be exercised in the selection of substrate materials to minimize possible stresses due to differences in thermal expansion coefficients.
Soldering methods, especially wave soldering, can cause thermal shock failures in unleaded surface-mounted capacitors. Susceptibility to cracks caused by substrate thermal expansion, flexure, and thermal shock increases with body size. CC1210 bodies and larger, and CC0402 bodies and smaller, ceramic capacitor arrays, and any chips thicker than 1.7mm in height are generally not recommended for wave solder assembly, especially on the bottom of PC boards