IPC-D-279 EN.pdf - 第60页
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July 1996 IPC-D-279
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Appendix B
Design for Reliability (DfR) of Plated-Through Via (PTV) Structures
B-1.0 PLATED-THROUGH VIA (PTV) RELIABILITY
ISSUES
Plated-through-holes (PTHs) serve to electrically connect
different conductor layers in multilayer printed circuit
boards (MLBs). In conventional interconnection technol-
ogy employing through-mounted components, the PTHs
also serve the function of providing a structure that accepts
the component leads and to which these leads can securely
be solder attached. This structure consisting of the copper
PTH barrel containing a component lead and filled with
solder provides a very robust, multiple-redundant electrical
and mechanical connection between the component and the
MLB.
The continuing drive towards higher functionality, higher
density, and lower weight was brought about and made
possible by the development of surface mounted technol-
ogy. This reduced the purpose of the previously multi-
functional PTHs to providing only the electrical intercon-
nections between the MLB layers; the diameters of PTHs
could also be reduced, since they no longer needed to be
large enough for the component lead insertion operation.
To distinguish the two types of PTHs, the PTHs without
component leads are frequently referred to as ‘PTH-vias’ or
PTVs. At the same time, the increased functionality and
density of the components often brought about a need to
increase the number of layers in MLBs, and thus to
increase the MLB thickness.
The decreasing PTV diameters, particularly in combination
with the increasing MLB thicknesses, make copper plating
into the PTVs more difficult. This problem was first recog-
nized during an IPC round robin study [Ref. B-7: 1], which
led to a more detailed round robin study specifically
focused on this issue [Ref. B-7: 2]. PTVs having a small
diameter—less than ~0.5 mm—and/or a high aspect ratio
of MLB thickness to drilled PTV diameter—more than
4—were found to require special treatment for adequate
reliability.
During this study the material properties, processing
parameters, and environmental test and use conditions
important for the reliability of PTVs were identified. The
results of these studies together with prior and subsequent
work [Refs. B-7: 3-9] has been utilized to develop a prac-
tical methodology to aid in the DfR of PTVs, as well as to
permit the assessment of the reliability of PTVs given the
assembly and test procedures and the use environments in
the field [Ref. B-7: 10].
In the IPC round robin test program [Ref. B-7: 1] the IEC
(Hot Oil Test, IEC 362-2) test—designed to simulate solder
reflow thermal shock cycles—was used to evaluate the reli-
ability of small-diameter PTVs. It was found that assembly
processes involving large temperature excursions constitute
a significant reliability threat due to low-cycle fatigue for
PTV copper barrels with low ductility or large stress con-
centrations. It was concluded that failures, to the extent
they occur at all, occur typically in the first 10 cycles due
to overstress crack initiation followed by crack propaga-
tion. This conclusion was reinforced by the findings of
Oien [Refs. B-7: 3, 4], which showed that crack initiation
typically occurs during the first or second cycle of over-
stress. Unless failure occurs within 10 cycles of the over-
stress loading typical of solder reflow, solder reflow over-
stressing is not a problem. Additional cycles will
eventually lead to fatigue-induced failures. The failures
that occurred in the IPC study were observed in product
from vendors rated ‘poor’ to ‘good’ on an arbitrarily sub-
jective scale with some consistency in differentiation. No
superior-rated vendor product failed, but it has to be noted
that the testing had an arbitrary cut-off. This led to the
definition of a numerical quality index that is now utilized
in an improved more detailed form in the reliability mod-
eling shown in Reference B-7:2 [Ref. B-7: 10].
During the product use, the severity of the thermal use
environment has a great impact on the reliability of the
PTVs. The ‘‘MIL-T’’ and ‘‘COM-T’’ thermal cycling tests
[Ref. B-7: 2] were designed to simulate severe and rela-
tively benign use environments, respectively. While fail-
ures in the ‘‘MIL-T’’ test occurred depending on the con-
struction of the MLBs and the quality of the PTVs, failures
did not occur as the result of the ‘‘COM-T’’ test. It needs
to be noted however, that the ‘‘MIL-T’’ and ‘‘COM-T’’
thermal cycling tests unfortunately had arbitrary test cut-
offs at 400 and 1000 cycles, respectively.
B-1.1 Copper Plating Process
B-1.1.1 Acid Copper Plating
One finding of the IPC
round robin study [Ref. B-7: 2] was that PTVs with aspect
ratios larger than three and plated with standard electrolytic
acid copper show decreasing thermal cyclic fatigue life. It
was found that the copper plating process window narrows
as the PTV aspect ratio increases and that the standard
electrolytic plating processes become inadequate even with
optimum process controls. These findings agree with ear-
lier studies investigating the effects of plating current den-
sity and agitation level on copper deposit quality in PTVs
[Ref. B-7: 5]. In this study it was shown that inadequate
electrolyte replacement rates, which can clearly occur in
high-aspect-ratio PTVs, will lead to mass-transport limited
IPC-D-279 July 1996
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