JOHANSON DIELECTRICS INC.
15191 Bledsoe Str eet, Sylmar, Ca. 91342 Phone (818) 364-9800 Fax (818) 364-6100
28 February 2006 Rev. 1.01
1
Understanding Ceramic Capacitor Terminations
John Maxwell, Director of Product Development
The relationship between termination and ceramic body is critical in understanding stress
concentration, crack formation due to heat, mechanical flexure and solder joint stress and
resulting leakage current conduction. Capacitor hermetic performance is also dictated by
the relationship between termination and ceramic body.
Termination System
A ceramic capacitor termination is not solid metal but a composite of structures built
upon one another. During manufacturing after a capacitor is fired or sintered each end is
dipped in metal glass frit thick film ink consisting of metal particles or flakes and doped
borosilicate glass. That ink is then dried and when both ends are coated it is fired or
sintered to the ceramic body. During this secondary sintering process the glass particles
fuse along contacts with one another creating a matrix or sponge like structure. The glass
also diffuses into the ceramic capacitor body to insure adhesion creating a thin zone that
has undergone a phase change. The ceramic capacitor body is polycrystalline with the
exception of NPO dielectrics so glass diffusion creates an amphorus boundary layer with
different material properties other than either the glass or ceramic. Depending on a
number of capacitor manufacturing process parameters this boundary layer may be thick
or thin, cracked or not. Thick layers tend to crack during capacitor manufacturing
increasing cracking in PWB assembly operations or during normal product use. After the
termination has been fired or sintered it is then electroplated with a nickel layer to
minimize dissolution during soldering and is finally plated with a solder or tin layer to
preserve solderability of the nickel layer. Photograph 1 show ceramic, termination thick
film metal, nickel-plated layers and solder.
Photograph 1. Capacitor Termination Components and Solder Joint
The function of the glass is to also hold termination precious metal (Ag or PdAg)
particles in contact with capacitor electrodes that slightly protrude from the ceramic
Ceramic
Solder Joint
Capacitor
Electrodes
Nickel Barrier
Thick Film Ag
Glass Matrix
JOHANSON DIELECTRICS INC.
15191 Bledsoe Str eet, Sylmar, Ca. 91342 Phone (818) 364-9800 Fax (818) 364-6100
28 February 2006 Rev. 1.01
2
body. In the case of base metal terminations either copper or nickel metal glass thick film
ink is used in conjunction with nickel electrodes.
The termination structure is slightly porous allowing air and moisture in the form of
humidity to penetrate through that layer. Nickel plating operations and chemistry do not
typically allow nickel to be plated directly on the ceramic capacitor surface leaving a
small gap between the nickel layer and ceramic body and is shown in Photograph 2.
Photograph 2. Gap between Nickel Barrier and Capacitor Ceramic Body
This is the path for moisture to enter the porous termination structure as the ceramic
capacitor body is not porous. This path extends into the ceramic capacitor along electrode
surfaces trough the gap between electrode and ceramic. If there is a microscopic crack
between opposing electrodes then moisture can completely penetrate the structure and
provide the necessary initial conductive path. Water hydrolyzes at .86V breaking down
into H+ and OH- ions that conduct current. The free hydrogen atom can easily rob an
oxygen atom out of the ceramic along a small crack of flaw surface leaving behind a
permanent titanium rich surface. This is critical in understanding long term ceramic
capacitor failures in that a part may seem fine during burn in and test but then fail later in
the field. More frustrating is when a part is sent out for analysis in that a no problem
found is reported. Some times a part can be “Fixed” by touching it with a soldering iron.
Once there is a small crack it is not fixed with a soldering iron but over time leakage
current increases to the po int of permanent capacitor failure.
Notice: Specifications are subject to change without notice. Contact your nearest Johanson Dielectrics
Sales Office for the latest specifications. All statements, information and data given herein are believed
to be accurate and reliable, but are presen ted without guarantee, warranty, or respon sibility of any kind,
expressed or implied. Statements or suggestions concerning possible use of our products are made
without representation or warranty that any such use is free of patent infringement and are not
recommendations to infringe any patents. The user should not assume that all safety measures are
indicated or that other measures may not be required. Specifications are typical and may not apply to all
applications.