10
TIME (seconds)
T
MAX
T
°
C
0
0
50
100
150
200
250
60
Preheat
Zone
Cool Down
Zone
Reflow
Zone
120
180
240
300
Figure 23. Surface Mount Assembly Profile.
Diode Burnout
Any Schottky junction, be it an RF
diode or the gate of a MESFET, is
relatively delicate and can be
burned out with excessive RF
power. Many crystal video
receivers used in RFID (tag)
applications find themselves in
poorly controlled environments
where high power sources may be
present. Examples are the areas
around airport and FAA radars,
nearby ham radio operators, the
vicinity of a broadcast band
transmitter, etc. In such
environments, the Schottky diodes
of the receiver can be protected by
a device known as a limiter
diode.
[6]
Formerly available only in
radar warning receivers and other
high cost electronic warfare
applications, these diodes have
been adapted to commercial and
consumer circuits.
Hewlett-Packard offers a complete
line of surface mountable PIN
limiter diodes. Most notably, our
HSMP-4820 (SOT-23) or HSMP-
482B (SOT-323) can act as a very
fast (nanosecond) power-sensitive
switch when placed between the
antenna and the Schottky diode,
shorting out the RF circuit
temporarily and reflecting the
excessive RF energy back out the
antenna.
Assembly Instructions
SOT-363 PCB Footprint
A recommended PCB pad layout
for the miniature SOT-363 (SC-70
6 lead) package is shown in
Figure 22 (dimensions are in
inches). This layout provides
ample allowance for package
placement by automated assembly
equipment without adding
parasitics that could impair the
performance.
0.026
0.075
0.016
0.035
Figure 22. PCB Pad Layout
( dimensions in inches) .
SMT Assembly
Reliable assembly of surface
mount components is a complex
process that involves many
material, process, and equipment
factors, including: method of
heating (e.g., IR or vapor phase
reflow, wave soldering, etc.)
circuit board material, conductor
thickness and pattern, type of
solder alloy, and the thermal
conductivity and thermal mass of
components. Components with a
low mass, such as the SOT-363
package, will reach solder reflow
temperatures faster than those
with a greater mass.
HP’s SOT-363 diodes have been
qualified to the time-temperature
profile shown in Figure 23. This
profile is representative of an IR
reflow type of surface mount
assembly process.
After ramping up from room
temperature, the circuit board
with components attached to it
(held in place with solder paste)
passes through one or more
preheat zones. The preheat zones
increase the temperature of the
board and components to prevent
thermal shock and begin evaporat-
ing solvents from the solder paste.
The reflow zone briefly elevates
the temperature sufficiently to
produce a reflow of the solder.
The rates of change of tempera-
ture for the ramp-up and cool-
down zones are chosen to be low
enough to not cause deformation
of the board or damage to compo-
nents due to thermal shock. The
maximum temperature in the
reflow zone (T
MAX
) should not
exceed 235
°
C.
These parameters are typical for a
surface mount assembly process
for HP SOT-363 diodes. As a
general guideline, the circuit board
and components should be exposed
only to the minimum temperatures
and times necessary to achieve a
uniform reflow of solder.
[6]
Hewlett-Packard Application Note 956-4,
Schottky Diode Voltage Doubler.