Maximizing the Effectiveness of Your SMD Assemblies

Application Note AN-0994
Maximizing the Effectiveness
of your SMD Assemblies
Table of Contents
Page
Method ....................................................................2
Thermal characteristics of SMDs..............................2
Adhesives ...............................................................4
Solder pastes..........................................................4
Reflow profiles ........................................................4
Rework ...................................................................6
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AN-0994
Version 6, May 2012
Page 1 of 6
calibration table to estimate Tj. The temperature rise due
to the heating pulse was calculated using this equation:
Introduction
This application note describes the measurement of
thermal resistance for International Rectifier’s surfacemounted devices (SMDs). It explains how packages
were mounted and the techniques used in heat sinking
and testing. It applies only to SMDs, not to throughhole devices such as TO-220, TO-247 and Fulpak.
ΔT = Temperature difference (C) between junction
temperature and reference temperature (TJ - TRef); TRef is the
ambient, package case or package lead temperature
Method
RTH = Thermal resistance (C/W) between junction and
reference point (package case or package lead)
ΔT = RTH X PD
where:
PD = Power dissipated (W)
Standard printed circuit boards were developed, to
which the devices under test were solder-mounted for
measuring thermal resistance. The boards measured
4.75 inches by 4.5 inches, and were made of FR-4
backed with 2 oz copper.
We can calculate the thermal resistance by inserting
measured values of temperature rise and power.
Measurements were taken from representative
samples of all the packages listed in Table 1.
Note: A specific thermal test board was used for DirectFET
because of this package’s unique connection design.
Thermal characteristics of SMDs
Three metal patterns were tested (Figure 1):
 one square inch of copper
 modified minimum: copper covered only the space
occupied by the device and lead pads
 minimum pattern: copper covered only the space
occupied by the lead pads.
Table 1 shows typical and maximum Rth (JA) and typical
Rth (JL) values of International Rectifier’s SMD packages
mounted using the metal patterns shown in Figure 1.
For Rth (JC) values, refer to the appropriate data sheet.
Modified
minimum
pattern
One square
inch pattern
Minimum
pattern
The device was placed in the middle of the pads shown.
Figure 1 Device placement used in testing
Notes
In accordance with industry practice, thermal resistance
was measured by first performing a reference
temperature estimate. A temperature sensitive electrical
parameter (TSEP) such as Vsd was measured and
compared with a calibration value to determine the
junction temperature (Tj). A heating pulse of known
power was then applied. This was followed by a second
TSEP measurement, which was compared with a
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Table 1 Typical and maximum thermal resistance
Package type Thermal resistance
One
Modified
Minimum
All
square inch minimum
Typ
Max Typ
Max Typ
Max Typ
Rth(JA) Rth(JA) Rth(JA) Rth(JA) Rth(JA) Rth(JA) Rth(JL)
D-Pak
20.2 26.3 42.0 54.6 59.5 77.3 2.0
D2-Pak
18.0 23.3 33.6 43.7 36.7 47.7 1.6
SO-8
33.5 50.0 66.3 86.2 70.6 91.8 10.6
SO-8 (dual)
54.5 62.5 73.1 95.1 94.7 123.1 28.7
SOT-223
27.2 60.0 49.0 63.7 66.1 86.0 4.9
TSOP6 (dual) 73.4 125.0 134.7 175.1 170.7 222.0 35.5
TSOP6 (single) 47.3 62.5 112.0 145.6 118.5 154.0 17.0
TSSOP8
60.9 83.0 106.4 138.3 117.0 152.1 35.5
u-3
169.2 230.0 237.1 308.2 263.6 342.6 139.3
u-6
47.1 75.0 112.5 146.3 124.9 162.4 14.7
u-8
39.9 70.0 102.4 133.2 126.1 163.9 17.0
DirectFET
A specific thermal test board was used because of
DirectFET’s unique connections. This table is split to
reflect this difference but, to improve readability, the
results are shown together on the following graphs.
Small can
32.1 60.0 49.2 64.0 68.1 88.5 NA
Medium can
32.3 60.0 55.6 72.3 62.2 80.9 NA
1. Three pieces of each package type were in the sample.
2. Rth (JL) (typical) and R th (JL) (one square inch) were
measured at the same time (Rth reference to drain lead).
3. Measurement conditions were as described in Method.
4. The board contributes greatly to the total thermal
resistance. If its material properties or dimensions vary
significantly from those used by International Rectifier,
actual thermal resistance may vary.
AN-0994
Version 7, August 2012
Application Note
Page 2 of 6
Using the Max Rth (JA) values from Table 1, Figures 2 to
4 plot power dissipation against ambient temperature
for each metal pattern.
Larger packages with exposed heat sinks (such as
D2-Pak, D-Pak and SOT-223) usually have the
highest power dissipation capabilities.
Increasing the area of the metal pattern reduces the
thermal resistance. The measurements taken using
the three different pattern areas reflect this, with the
one square inch pattern giving the lowest resistance.
Figure 3 Ambient power dissipation – modified minimum
Figure 2 Ambient power dissipation – one square inch
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Figure 4 Ambient power dissipation – minimum
AN-0994
Version 7, August 2012
Application Note
Page 3 of 6
Adhesives
vendor and board fabricator for recommendations
specific to their application.
As the connections on an SMD do not pass through
the board on which it is mounted, such devices rely
solely on the strength of the solder joint for mechanical
as well as electrical connection.
Reflow profiles
Many assemblies require devices to be mounted on
both sides of the board. The reflow process is typically
done only once, with both sides of the board pasted at
the same time. Devices are placed on one side of the
board, using an adhesive to hold them in place, and
then the board is turned through 180 degrees for
devices to be placed on the other side. The board is
subjected to a thermal process to melt the solder
paste. When the devices have been soldered to the
board, the adhesive serves no further purpose.
The adhesive used must secure the devices to
prevent movement during handling and soldering.
However, it must be possible to break the bond with
minimal disturbance to the populated board so that
incorrectly positioned devices can be moved before
soldering. Adhesion must be maintained during
preheating and the adhesive should not interfere with
solder flow during the reflow or wave soldering process.
Typical adhesives of this type are made from nonactivated resins (R), which can be used in a forming
gas atmosphere to reduce oxides. Some are mildly
activated resin (RMA), which can be used in normal
factory environments. The activators in these resins
dissolve small amounts of oxidation from solderable
surfaces and solder particles in the paste.
Solder pastes
A wide range of solder pastes is available for surface
mounting applications. Typically, they are composed of
a homogeneous mixture of pre-alloyed solder powder
with a specific grain size. Fluxes are included, as they
are essential to the surface mounting process.
In today’s densely populated assemblies, the leads on
SMDs are significantly closer together. Spacing of less
than 0.4mm is common and it can cause problems
such as solder bridging, insufficient solder on leads
and inaccurate device placement. The stencil’s
thickness, dimensions and registration accuracy and
the solder paste’s composition and particle size are all
critical in soldering these assemblies successfully.
Given these complexities, advising on the choice of
solder paste is beyond the scope of this application
note. Customers should contact their solder paste
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A major problem associated with mounting SMDs,
especially those with mismatched internal expansion
coefficients, is the thermal shock of the soldering
process. The advent of lead-free assemblies has
driven the requirement to develop and implement new
handling techniques for SMDs. Lead-free solders
typically have higher melting points than traditional
lead-based solders. Where lead-based assemblies
could be mounted at peak reflow temperatures in the
range 220°C to 245°C, lead-free assemblies require
temperatures in the range 245°C to 260°C. Higher
peak temperatures require careful control of the reflow
environment to prevent over-temperature conditions
that can severely degrade the reliability of SMDs.
Caution must be taken when choosing a reflow profile
to optimize the thermal stresses that are applied to the
assemblies, whether lead-based or lead-free.
Conversely, under-temperature conditions can result
in failed mechanical attachment.
A carefully controlled pre-heat and post-cooling
sequence is necessary. Properly controlling the preheat cycle removes any volatile components of the
solder paste, such as alcohol or water, by evaporation
before the solder fusing cycle starts. This reduces the
chances of forming voids or solder balls. Table 2
shows critical parameters of the reflow profile.
Table 2 Critical parameters by solder paste and device volume
Profile feature
Lead-free3 Lead-based
All sizes Large
Small
body1
body2
Average ramp-up rate,
3
3
3
TL to Tp (°C/sec max)
Preheat
150
100
100
Temp min, Tsmin (°C)
200
150
150
Temp max, Tsmax (°C)
60–180
60–120
60–120
Time, Tsmin to Tsmax (sec)
Max ramp-up rate, Tsmax to TL 3
—
—
(°C/sec)
Time maintained above:
217
183
183
Temp, TL (°C)
60–150
60–150
60–150
Time, tL (sec)
260 +0/-54 225 +0/-5 240 +0/-5
Peak temperature, Tp (°C)
10–30
10–30
Time within 5°C of actual peak 10–30
temperature, tp (sec)
6
6
Max ramp-down rate (°C/sec) 6
8
6
6
Max time 25°C to peak
temperature (min)
AN-0994
Version 7, August 2012
Application Note
Page 4 of 6
Notes:
1. Large Body: TO-220, D2pak and larger (package
3
thickness ≥2.5 mm or package volume ≥ 350 mm )
2. Small Body: Dpak, Ipak and smaller (package thickness
3
< 2.5 mm or package volume < 350 mm )
3. Lead-free devices have the suffix PbF in the part
number. To check if a device is lead-free, contact the
sales representative or the factory.
4. The recommended peak reflow temperature for some
large-body packages (such as PLCC-44 / MQFP64)
is 250°C +0/-5°C. To check the peak temperature for a
device, contact the sales representative.
This thermal conditioning can be applied in several
ways, including infrared/convection reflow, vapor
phase reflow and wave soldering. Figures 5 and 6
show recommended profiles for infrared/convection
reflow and wave soldering. Table 3 and Table 4 give
recommendations for specific SMDs.
temperature
ramp up
tp
TP
ramp
down
TL
tL
TSmax
TSmin
tspreheat
t25ºC to peak
time
Figure 5 Infrared / convection reflow profile
temperature
10±1 sec
*Tpeak
Table 3 Infrared/convection reflow recommendations
Package
Reflow temp (°C)
Name
Size
Lead-free
Lead-based
D2PAK
L
250
225
D-61-8
L
250
225
D-PAK
S
260
245
Micro-3/SOT 23 S
260
245
Micro-6/SOT 6 S
260
245
Micro-8
S
260
245
MLP-20 4x4
S
260
245
MLP-28 5x5
S
260
245
MLP-48 7x7
S
260
245
MLP-6 3x3
S
260
245
MQFP64
L
250
225
PDIP-14
L
250
225
PDIP-16
L
250
225
PDIP-20
L
250
225
PDIP-28
L
250
225
PDIP-8
L
250
225
PLCC44
L
250
225
SMA
S
260
245
SMB
S
260
245
SMC
S
260
245
SOICN-14
S
260
245
SOICN-16
S
260
245
SOICN-8
S
260
245
SOICW-16
S
260
245
SOICW-20
S
260
245
SOICW-28
S
260
245
SOT223
S
260
245
TO-220
L
250
225
TO-247
L
250
225
TO-262
L
250
225
TSSOP20
S
260
245
TSSOP24
S
260
245
TSSOP8
S
260
245
Thickness > 2.5 mm or volume > 350 mm2, reflow 250+0/-5°C (Pbfree), 225+0/-5°C (SnPb eutectic)
Thickness < 2.5 mm and volume < 350 mm2, reflow 260+0/-5°C
(Pb-free), 240+0/-5°C (SnPb eutectic)
Disclaimer:
1st wave
150−200ºC / sec
2nd wave
5ºC / sec
If you attach packages using
wave solder immersion, you
may need to make special
evaluations because of
Tpeak 1st wave
≤ 10ºC of Tpeak
differences in the process
2ºC / sec
140ºC
25ºC
1.5 sec
time
80 sec
100 sec
Figure 6 Wave soldering profile
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200 sec
Table 4 Wave solder recommendations
Package
Reflow temp (°C)
D2PAK
225
D2PAK
260
DPAK
260
D-PAK
235
SOICN-14L
240
SOICN-14L
260
SOICN-16L
260
SOICN-28L
240
SOICN-8L
240
SOICN-8L
260
SOICW-16L
240
SOICW-16L
260
* Contact manufacturer for more information
AN-0994
Version 7, August 2012
Remarks
Not lead-free
Lead-free
Lead-free*
Not lead-free*
Not lead-free
Lead-free
Lead-free
Not lead-free
Not lead-free
Lead-free
Not lead-free
Lead-free
Application Note
Page 5 of 6
Rework
When replacing an SMD that has been soldered onto
a substrate, the main problem is applying enough heat
to melt all the connections on the device that is being
rep laced without overheating adjacent devices. This
is achieved by using a soldering iron with a specially
shaped tip and, because of the diversity of SMD
package styles, a corresponding variety of tips is
required. The tip must have a gripping function so that,
when the solder is reflowed, the device can be
extracted from the board.
When the replacement SMD is mounted, the tool must
perform the reverse procedure. Flux must be applied
to the new device before localized reflow.
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Version 7, August 2012
Application Note
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