Polyterm® Flexible SMD Termination
Christopher England - Applications Engineer, HVS Products
February 1, 2008
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Cracked capacitors can manifest themselves as latent defects including increased leakage
current, intermittent opens, shorts or no problem found when field returned assemblies
are analyzed. In some cases these defects can lead to catastrophic failure depending on
the application energy available. Cracks could be introduced at virtually any point in the
surface mount assembly process from pick and place, soldering and reflow, handling after
assembly, de-paneling, and board level testing or equipment assembly (see below
example of standard termination after flexing). Board flexure and handling are the
predominant source of capacitor cracking. Polyterm® flexible SMD termination was
developed to provide a flexible bond capable of withstanding higher physical strain than
standard terminations. Standard terminations are more susceptible to cracking during
processing, which may not be discovered by visual means. Over time these cracks can
propagate into opposing electrodes within the capacitor possibly causing the capacitor to
fail short and subsequent catastrophic failure. Polyterm®, in addition to proper board
layout practices, provides added protection and greatly reduces the possibility of the
capacitor failing due to these board stresses.
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Performance verification of the Polyterm® flexible SMD termination was performed on
the R15 (0805), R18 (1206), S41 (1210), S43 (1812), R29 (1808), S47 (2220), X43
(1812) and X44 (1410) size capacitors. These tests were conducted based on industry
standards. JIS-C-6429, IEC-384-1, IEC 60068-2-21.
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TESTING PROCEDURE
Group 1)
80 samples and were subjected to gradual bending measured incrementally in 1
mm steps from 1mm to 10mm. A failure would be indicated by loss of capacitance >10%
(JIS-C-6429) and >5% for X43 and X44 X2Y capacitors. These samples were then
removed from the board and the terminations were etched from the capacitor body. The
ceramic body was then physically inspected for damage. Both Polyterm® and Standard
terminations were subjected to this experiment. Each different part size is represented by
the summary of test results (Figure 1) and a graph showing cumulative failure of both
Polymer and standard terminations (Figures 2-9)
Group 2)
Samples were bent to varying degrees of deflection to provide photographic
representation of how the polymer terminations perform and illustrate how failures
manifest themselves. This test was performed using the S47 (2220) sample group (figures
10-14). One board for each mm of deflection was used.
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SUMMARY OF TEST RESULTS
COMPARISON OF STANDARD TERMINATION vs. POLYTERM®
Figure 1
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POLYTERM® CROSS SECTION ANALYSIS
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At 3mm only partial separation of the polymer was visible. The silver layer is clearly
separating from the polymer with no ceramic damage visible.
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The photo in Figure 12 shows almost complete separation of the silver termination from
the polymer layer. Although the polymer layer is partially separated the capacitor still
operates normally. When returned to the original state all capacitors regardless of failure
operated within specification.
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Complete separation of the polymer layer may cause open capacitor, but ceramic failure
was not observed. This will protect against the capacitor failing short causing
catastrophic failure.
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TEST BOARD AND FLEX TESTING EQUIPMENT
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CONCLUSION
Polyterm® flexible SMD termination is useful in reducing or eliminating MLCC failures
due to PCB flexure stresses related to PCB assembly operations and/or harsh operating
environments. Although all components may exhibit some termination separation there
was no degradation observed that would interfere with the capacitors operation or
performance. Polyterm® in addition to proper board layout and handling can greatly
reduce or eliminate failures associated with MLCC cracking.
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| Notice: Specifications are subject to change without notice. Contact your nearest Johanson Dielectrics,
Inc. Sales Office for the latest specifications. All statements, information and data given herein are
believed to be accurate and reliable, but are presented without guarantee, warranty, or responsibility 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. |
