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Transient induced failure of miniature resistors
the substrate, and can cause local hotspots that reach the
melting-point temperature of
the track material. Predicting
when and where such hotspots
will occur require a resistor
model that is more sophisticated than the traditional
‘ideal resistive layer’ model
that has normally been used.
For example, the equivalent
Although a great deal of attention is given to protecting ICs
from damage due to electrostatic discharge or current
surges, many design engineers
fail to realize that it is also possible for resistors to be damaged
by such transients. Resistors are
frequently concentrated in areas most likely to experience
transients, such as those adja-
For very short pulse durations in Region 1, which are
typically the result of transients
such as ESD or lightning
strikes, all the heat remains in
the resistive layer. There is no
time for it to penetrate either
the ceramic former or the outer
protective lacquer. For such
transients in cylindrical resistors, P peak is proportional to
d/ti where d is the track
layer
thickness and ti the
10
pulse duration.
10
For transients last10
ing between 1µs and
10ms (Region 2), the
10
heat generated at a
10
Region I
Region II
Region III
Region IV
hotspot has time to pen10
etrate the cylindrical
10
substrate but not time
enough to travel along
10
it. In this situation,
1
which is typical of dis10
10
10
10
10
10
10
10
10
10
10
1
10
10
10
charge pulses, P peak is
proportional to 1/√ ti.
Time (s)
For longer pulses of
Figure 1: Compared to traditional models, sophisticated models identify various types of hotspots
between 10ms and 1s
depending on the pulse duration.
(Region 3), the heat
has time to travel both
cent to power supplies. The circuit model must take into into and along the substrate,
small thermal mass of surface- account intrinsic inhomogene- and Ppeak becomes proportional
mount miniature resistors often ity in the resistive layer and the to 1/ti.
It is important to know the
makes them unable to absorb f ield strength at the laser
high-energy pulses without sus- trimmed end-points of the re- type of transients that a resistor
might experience in order to
taining damage. Even if the sistive material.
transients do not cause catastrophic failure, they may result
in an unacceptable shift in the
V = 0V
resistor’s ohmic value.
Virtually all of today’s metalfilm cylindrical resistors, including those from BC Components, are fabricated using high
aluminum content ceramic
substrates that have high thermal conductivities. Because
this allows heat to be transferred easily to the resistor’s
end caps and out onto the PCB,
these resistors tend to have less
pronounced hotspots and a
more uniform temperature disHelixing
Resistive layer
tribution than many older designs. For a given hotspot temV = 100V
perature, the overall power rating of the resistor can therefore
be increased. As a result, it is Figure 2: Hotspot generation through current crowding at resistor failure points.
now possible to produce very
This more sophisticated choose an appropriate power
small resistors with fine-line
resistance tracks that have con- model allows various types rating. Calculations based
tinuous power ratings higher of hotspot phenomena to be solely on normal dc power ratidentified depending on the ings will often lead to compothan expected.
The heat generated by very pulse duration and can be con- nent failure.
In addition to inhomogeneshort transients, however, has veniently divided into four
ities in a thin-film resistor’s heno time to dissipate itself into different regions.
8
7
6
Power (W)
5
4
3
2
1
10
9
8
7
6
5
4
3
2
1
2
3
lical tracking, there are also
potential failure points where
the helical track meets the end
rings. At these points, current
crowding can result in an uneven current density that leads
to a hotspot. This same effect
can be observed in thin-film
chip resistors that utilize a meander pattern to increase track
length, except that in these resistors, hotspots due to current
crowding can occur at any point
where the meander pattern
changes direction. However,
with careful track design, such
as that used in BC’s MCA 1206
chip resistors, the curve radius
of the meander pattern can be
chosen to reduce or eliminate
these current crowding effects.
Nevertheless, different manufacturers still use different meander patterns, which means
that equivalent resistors (in
terms of their dc characteristics) may have different survival
rates when subjected to similar
voltage transients.
Thick-film resistor performance
The pulse performance of thickfilm resistors is considerably
more difficult to predict than it
is for thin-film types, due to the
inherent inhomogeneity of current flow in thick-film materials and the less predictable nature of the manufacturing processes involved. Published
curves for these resistors are
therefore normally determined
from direct measurement. As
with conventional resistors,
heat energy concentrates at the
end of the laser groove, and the
type of laser cut influences this
behavior. For example, an
L-cut gives a lower heat flux at
these points than a single-cut.
The effect of laser trimming is
significant, with the result that
trimmed resistors may have
only one third or a quarter of the
pulse load capabilities of untrimmed resistors. In general,
thick-film resistors also have
lower pulse-load ratings than
cylindrical resistors.
By Joachim Von der Ohe
Applications Marketing Manager
BC Components
E-mail: joachim.von.der.ohe@
bccomponents.com