SMD Fuse
for Space Applications
Introduction
Fuses for over-current circuit protection
have been used in space applications for
a considerable period of time. With the
MGA-S and as a manufacturer of passive
components, SCHURTER is not only able
to provide a fuse product, which meets all
requirements for this industry but is also
qualified according to ESCC Generic Spe-
cification no. 4008. As a listed supplier at
ESCC and as a European fuse supplier,
SCHURTER offers a product that is made
using thin-film technology. The standard
version for industrial applications has been
on the market since 1990 and has annual
sales of more than one million pieces. This
clearly demonstrates a stable manufacturing
process together with operation at a high quality level. For the space version, where only narrow
tolerances in the manufacturing process are accepted, 8000 fuses have been manufactured and
passed through the screening procedure. The customer TESAT in Germany has reported no failures
to date. This article gives an insight into the fuse’s technical data, its design, manufacturing process
and application information.
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Technical Data • Consistent minimum and maximum pre-arcing time
limits for over-currents (Fig. 3) are also relevant for a
fuse that is independent from the mode of operation
For its type (Fig. 1) the fuse is
(e.g. in a vacuum). The super-quick-acting characte-
very small (3.2 x 1.55 x 1.55
ristic ensures a rapid fuse disconnection in the event
mm or a 1206 footprint) and
of a short circuit.
is intended for use in both
direct (DC) and alternating
Rated Current In 1.43 x In 3.58 x In 3.58 x In 5.71 x In 5.71 x In 8.57 x In 8.57 x In
(AC) current circuits. They min. min. min. min. min. min. min.
are capable of operation 0.14 A - 3.5 A 4.0 h 2.0 ms 5.0 s 0.5 ms 10.0 ms 0.05 ms 2.0 ms
Fig. 1: MGA-S Fuse
in an ambient temperature
range of -55 °C to 150 °C including high vacuum envi- Fig. 3: Pre-Arcing Times
ronments (< 50 mTorr).
• The current range extends from 140 mA to 3.5 A • The overload operating I2t-value of a fuse defines the
for continuous operation. Based on the rated cur- energy level of an over-current pulse that is needed
rent, the recommended derating curve is shown for fuse interruption. This is important when inrush
(Fig. 2). The fuse can be operated at its rated current pulses that are well in excess of the fuse’s rated cur-
indefinitely, this being in accordance with the IEC rent occur regularly. These forms of pulses stress the
definition of rated current. fuse and accelerate the aging process. Calculation
on this can be made, but go beyond the content of
110 this article.
100
90 100% In / 23°C • The breaking capacity / interrupt rating define the
80 amount of current and voltage level the fuse is able
Current Derating (%)
70
to interrupt in the event of an over-current failure.
60
A breaking capacity of 300 A @ 125 VDC defines
50
that a fuse is able to interrupt a prospective over-
40
current of up to 300 A at 125 VDC and a max. time
30
constant of 1 ms. The power factor is defined as
20
followed: (R/X) = Real Power (W) / Apparant Power
10
0
(VA).
-50 -25 -0 25 50 75 100 125 150
Ambient temperature (°C) • Environmental tests according to ESCC Generic
Specification no. 4008, Chart F4 have shown that
Fig. 2: Derating Curve
the fuse is able to withstand a broad range of tests
such as Rapid Change of Temperature, Mechani-
In contrast, one of the requirement of UL 248-14 is cal Shock, Damp Heat Steady State, Resistance to
that a fuse must operate at its rated current (1.0 Soldering Heat or Thermal Vaccum. It can be said
x In) for at least four hours. After this time, the fuse that the fuse is humidity and shock resistant. The
should be able to open and disconnect the circuit. robust design permits soldering of the fuse in either
This means, therefore, that a UL type fuse should a reflow or wave soldering process.
not be operated continuously above 70% of its rated
current. The previous definition of the current range
which is not in use anymore was from 200 mA to
5.0 A which were in accordance with the UL defini-
tion of rated current. For the standard model MGA,
the UL definition of rated current still applies.
• An important feature of a fuse is the value of its vol-
tage drop and cold resistance. For both values the
minimum and maximums are given. With the follow-
ing formula, heat dissipation can be calculated:
PHeat Dissipation = UVoltage Drop * IOperating Current
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Fuse Design Sputtering Fuse Links on Glass Stick
and Insertion
Fig. 4 illustrates the design of the fuse. The core is
One of the key manufacturing process steps is the
a glass stick, which is sputtered with different layers
sputtering of the glass stick with different layers of ma-
of materials for the fuse-link (red) and pads (orange).
terials such as copper. As shown in Fig. 5, the narrow
Different layer thickness and the choice of different
part in the middle shows the fuse link that will melt and
materials permit different current ratings of fuses to be
interrupt the circuit when an inadmissible over-current
occurs. The pads on both ends connect the fuse-link
Terminal Plate to the terminal plates.
Ceramic Housing
Ceramic tubes, which include fifty fuse units per tube,
Solder Past Pads are placed on a board, and a wax is used to fix the
tubes. Following on from this, silicon is injected into the
ceramic tubes. The sputtered glass stick with the fifty
units is now inserted into the tube and brought to an
exactly defined position. Through the octagonal cera-
mic tube hole and the square glass stick, positioning
is ensured and the silicon is well spread around the
Silicon Filler
glass stick.
Glass Stick
Fuse Link
Solder Past
Terminal Plate
Fig. 4: Construction of the fuse
made. As an important feature of the fuse, the breaking
capacity or interrupt rating is achieved with the silicon
filling (yellow). The terminal plates (grey), which are sol-
dered with a high temperature solder past (Pb content
> 80%) (dark grey) to the glass stick, finalises the elec-
trical connection to the outside. The ceramic housing Fig. 5: Sputtering on glass stick
construction is environmentally sealed, very robust and
ideally suited for long-term operation in space. The
sealing ensures that in the event of an disconnection,
no arcs or gasses can escape.
Manufacturing Process
The manufacturing of thin-film technology based fuses
is a complex process and contains many stages. The-
se stages can be loosely grouped as follows:
1. Sputtering fuse links on glass stick and insertion
3. Cutting and cauterisation
4. Sputtering end metals and soldering of terminal
plates
5. Seal test and conditioning
6. Final terminal plating and testing
These are described in more details in the following
sections.
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Cutting and Cauterisation The conditioning
process lasts a
After inserti- total of 84 hours
on, the board Ceramic Housing
and the tempe-
is now ready rature is incre-
for cutting. An ased to 150 ˚C.
Silicon Filler
automatic cut- This ensures that
ting machine the acid solvent
Glass Stick
ensures that shows those
the ceramic fuses that have
tubes are cut not been sealed
(Fig. 6) at the and where the
right point. Fuse after cutting
electrical cha-
The de-waxing racteristic of the
process sepa- Fig. 6: Cutting fuse has chan-
rates the fuses ged. These can
into single pi- Ceramic Housing be then be se- Fig. 8: Fuse are filled in the “bomb”
eces with flat parated out with
front surfaces. a cold resistance measurement. The fuses are only
Continuing on Pad
placed into stock if the test has shown them to be
from this, the sealed.
single fuse ele- Glass Stick
ments are cau-
terised (Fig. 7)
or etched so Final Terminal Plating and Tests
as to remove Fuse after cauterisation
This final process step is completed after receipt of a
some silicon Fig. 7: Cauterisation
customer’s order. The terminals of the fuse-links are
and thus to
still a copper based surface (Fig. 9). Using a galvani-
provide access to the solder pads of the fuse-link.
sing process, an SnPb layer is applied to the surface.
The fuses are finally tested 100% both visually and
electrically (cold resistance) before forwarding on to a
Sputtering of End Metals and Solde- further screening process.
ring of Terminal Plates
The fuses are placed on another board where the
front surfaces face upwards. The board is now in-
serted into the sputter machine where, among other
materials, copper is applied to the forward surfaces
making them solderable. Using a high temperature
solder, which means a solder paste having Pb con-
Fig. 9: Before and after tin-plating
tent > 80%, copper terminal plates are soldered to
the fuse ends (see Fig. 4).
Seal Test and Conditioning
The finished single fuses are put in a chamber (Fig. 8)
called a “bomb” where with high pressure and an acid
solvent tests whether the fuses are sealed. The test
lasts 72 hours with cyclic tests up to 1500 PSI.
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Screening Process After „Burn-In“, cold resistance and voltage drop are
measured again for each fuse. The two sets of values
are permitted to show a maximum deviation of 10%;
Following the final test after production, cold resistance otherwise the fuse has failed the test. Should there be
and voltage drop are measured for each individual 5% or more from a production series that lies outside
fuse. this tolerance, the entire production lot is disposed off
and a new series of fuses is produced.
The „Burn-In“ is a combination of a current and tem-
perature test (Fig. 10 + 11) carried out for every fuse Every fuse undergoes a strict visual check so as to ex-
under the following conditions: clude material faults.
• Duration: min. 168 hours
Additional test fuses are manufactured with each pro-
• Current: 95.7% of rated current
duction lot, which have also been subjected to Burn-
• Ambient temperature: 80 ºC
In and relevant tests. Time-current characteristics and
• Continual monitoring of current during
solderability tests are carried out to ascertain that the-
the entire test
se properties are still fulfilled. Depending on customer
requirements further qualification tests can be carried
CHART F2 -
PRODUCTION CONTROL
out.
External Visiual Inspection
according to Paragraph 5.2.1
As a result of this extensive procedure, an absolute mi-
Dimension Check
Cold Resistance nimum failure rate is achieved as well as a complete
Initial Measurement
according to Paragraph 5.2.2
according to Paragraph 5.2.1 guarantee of the electrical properties. A detailed test
report is made for each order and supplied with the
Current Carrying Capacity and fuses. The test report and the sequence with which the
Overload Operation Test
according to Paragraph 5.2.4/5.2.5
fuses are placed into the blister tape correspond with
each other. This permits retraceability of each single
CHART F3 - fuse back to the screening process.
Voltage drop
SCREENING TESTS Initial Measurement
according to Paragraph 8.1.1.2
Burn-In (168h)
according to Paragraph 8.3
Voltage drop
Final Measurement
according to Paragraph 8.1.1.2
Cold Resistance
Final Measurement
Test Equipment for Burn-In according to Paragraph 8.1.1.1
External Visual Inspection
according to Paragraph 8.2
Solderability
according to Paragraph 8.4
Current Carrying Capacity and
Fig. 11: Burn-in equipment with fuses
Check for Lot Failure
Overload Operation
according to Paragraph 6.4
according to Paragraph 8.5/8.6
Delivery with Test Report to Customer
Flight Model
Approved according to
ESCC Generic Specification no. 4008 +
ESCC Detail Specification no. 4008/001
Fig. 10: Procedure for the screening process
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Applications
Satellites are very often equipped with a multitude of
electronic modules, for a variety of functions, which are
fed from a central supply unit. An example of this is
the transmitting unit that is manufactured among other
items by TESAT in Germany. This module amplifies the
data signals that are transmitted via the antenna back
to earth. The existence of numerous channels leads
to a multiple redundant system. With an over-current
caused by a fault in a channel, the fuse interrupts the
supply in a secure and controlled manner. In a case
such as this, the system switches over to another
channel. The disconnected fuse thus ensures that no
unnecessary current flows in the defective channel.
Should an applica-
tion require a higher
rated current than a
single fuse can co-
ver, a parallel swit-
Fuse Fuse
ching (Fig. 12) of
two or more fuses is
possible. As a result
of current/tempe-
rature equalisation
between the fuses, Fig. 12: Parallel circuit of fuses
simultaneous dis-
connection is ensured. It is to be noted that the fuses
have to have the same rated current values, that they
are from the same production series and that they do
not influence each other with their operating tempera-
ture.
In future, further R&D efforts will be made to increase
current ratings by putting two or more fuses in parallel
in one unit. This will also allow meeting the demand
for higher current application, since satellite technolo-
gy gets more sophisticated and therefore power con-
sumption will increase.
Lucerne, 22nd November 2007
Author:
Thomas Hubmann
Product Manager for Fuses
SCHURTER AG, Switzerland
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