Freescale Semiconductor
Application Note
AN1907
Rev. 1, 6/2006
Solder Reflow Attach Method for
High Power RF Devices in Plastic Packages
By: Wendi Stemmons, Jerry Mason, Rich Wetz, Tom Woods, Mahesh Shah and David Runton
INTRODUCTION
This application note describes a process to solder attach
the TO - 270 - 2 (Case 1265) as well as the TO - 270 WB - 4
(Case 1486) RF power plastic packages to a printed circuit
board and heatsink assembly. There are several issues that
are of concern that will be addressed here:
semiconductor packages. The technology and material used
(such as lead frame, die attach, wire bond and mold
compounds) have been used in many applications and are
known to provide robust semiconductor packages. These
plastic packages have been used for power devices in harsh
environments such as under - the - hood applications, without
any reliability degradation.
1. Establishing a good thermal path between the device
and heatsink by providing high quality solder joint
between the device heat spreader and the power
amplifier (PA) heatsink.
2. Obtaining a high quality solder joint between the device
leads and the pads on the printed circuit board (PCB).
3. Maintaining the package integrity so that the leads or
molded plastic are not overstressed.
DISCUSSION
A number of RF power devices are assembled in packages
with standard JEDEC designations such as TO - 270. RF
power packages, such as TO - 270 - 2 shown in Figure 1, were
developed using technology similar to that used in low
frequency power plastic packages. It is designed for an RF
power device utilizing either silicon (LDMOS) or GaAs
technology. The packaging technology is a conventional
over - molded plastic process, commonly used in most
© Freescale Semiconductor, Inc., 2006. All rights reserved.
RF Application Information
Freescale Semiconductor
Figure 1. Typical RF Power Plastic Device
Compatible for Solder Reflow Process
(Case 1265, TO - 270 - 2)
AN1907
1
PCB ASSEMBLY PROCESS
For better electrical and thermal performance, Freescale
highly recommends that the RF power device should be
soldered to a PCB and the heatsink, as shown in Figure 2. The
soldered interface at the heat spreader or the source contact
provides a better heat dissipation path from the device to the
heat spreader and then to the power amplifier (PA) heatsink,
resulting in a lower junction temperature. The reduction in
junction temperature typically is associated with an increased
Mean - Time - to - Failure (MTTF) for the semiconductor
devices. In addition, a soldered interface tends to provide
improved grounding for the RF power device and, thus,
improved electrical performance. For this kind of assembly
process, two types of special PCB technologies are available.
Figure 2. Reflow Pallet Assembly with Components and Soldering Fixture
In one type of PCB assembly, the conventional PCB is
attached to a full metal carrier or pallet that is the same size
or slightly larger than the PCB. This is known as an Integrated
Metal Carrier (IMC). The metal carrier is made from mostly
copper or aluminum material. The metal is plated to provide a
solderable surface. A copper pallet is typically plated with Ni
followed by Au. The Au thickness is fairly small and is
commonly known as Au flash. The aluminum material is
typically plated with some type of zinc, followed by a Ni and Au
top layer to prevent the Ni from oxidizing.
The second type of PCB assembly is a forged or machined
copper coin that is also plated with Ni and Au. The coin is
usually designed to be larger than the RF power device and
has two bolt holes on each side of the RF power device to bolt
the coin to the PA heatsink.
Both the coin and the pallet are attached to the under side
of the PCB using either a high temperature solder or a
conductive epoxy such as Ag- filled epoxy. If solder is used to
attach the coin or the pallet to the PCB, the solder selected
must have a higher melting temperature than the solder used
for the components on the PCB. In either case, the PCB
supplier will provide the PCB with either the IMC or coin
already attached to it. The typical process flow is shown in
Figure 3.
The assembly shown in Figure 2 was created using the
typical mounting process flow for a solder reflow process
described in Figure 3. The gate and drain leads of the device
are soldered to the pads on the top of the printed circuit board.
The heat spreader of the device (source contact for an
LDMOS device) is soldered to a machined cavity in the copper
plate through a hole in the PCB. The bottom of the PCB is tin
lead plated and attached to the copper plate.
The biggest challenge in assembling any device is to
overcome the accumulated tolerances in the stack - up
between the device, PCB and copper plate and to maintain
good thermal and electrical contact where necessary. If the
device leads are too high above the PCB surface, they may
not contact the solder paste, resulting in a weak or possibly
non - existent solder joint. If the device is placed too low, the
leads can be bent in an upward direction, resulting in possible
delamination in the device.
AN1907
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RF Application Information
Freescale Semiconductor
PROCURE PCB WITH COIN OR
PALLET ALREADY ATTACHED.
ALTERNATIVELY, PROCURE PCB,
PALLET OR COIN AND ATTACH TO PCB.
SIZE THE CAVITY DEPTH
OR PEDESTAL HEIGHT.
INCORPORATE IT IN THE
PALLET OR COIN DESIGN.
SCREEN PRINT SOLDER PASTE ON
THE PCB SOLDER PADS.
(SCREEN PRINT)
PLACE SOLDER PREFORM(S) IN THE
CAVITY THROUGH PCB SLOTS.
DISPENSE FLUX IF NECESSARY.
(PICK AND PLACE)
PLACE RF POWER DEVICE IN THE
PCB SLOT, WHILE POPULATING
THE PCB.
(PICK AND PLACE)
ADD THE FIXTURE TO KEEP THE RF
DEVICE IN PLACE WHILE SOLDERING.
(PICK AND PLACE)
REFLOW THE SOLDER IN A
CONVECTION REFLOW FURNACE.
(REFLOW)
REMOVE THE REFLOW FIXTURE AND
EXAMINE THE SOLDER JOINTS.
Figure 3. Process Flow for Board Assembly
Mechanical tolerances for this device are tightly controlled.
The manufacturing process results in a seating plane height
of 0.041 ± 0.001″ (1.04 ± 0.03 mm). The seating plane height
is defined as the distance from the bottom of the device lead
to the bottom of the package case. There is also a co- planarity
specification on these devices that indicates how level the
leads must be with respect to the flange. The co- planarity limit
is typically 0.041 ± 0.003″ (1.04 ± 0.07 mm). These tolerances
are much tighter than those for any of the metal ceramic
devices common in the industry for RF power application. It is
also important to note that the leads of the plastic packages
are made from 8 mil (0.20 mm) thick Cu- alloy rather than the
5 mil (0.13 mm) thick Fe- Ni alloy used in most metal ceramic
packages. The increased thickness makes the leads for
TO - 270 packages slightly stiffer than metal ceramic package
leads.
Typical tolerances of the PCB manufacturing process are
±10% of the PCB thickness. The tolerances of the machined
cavity in the copper plate can be kept to ±0.003″ (0.08 mm) or
better. The recess in the copper plate must be designed so
that the device leads are not assembled in a bent- up position.
We recommend using the square root of sum of squares
method to define the cavity depth rather than using the
worst - case tolerance stack - up analysis. Care should be
taken in the design of the heatsink so that the leads are not
bent to the point where this can contribute to delamination of
the plastic mold compound from the leads. Tests were
conducted to show that 0.015″ (0.38 mm) of lead tip deflection
during three solder reflow operations will not cause any
delamination of the mold compound from the lead frame.
In addition to the cavity depth, the next important
consideration is to ensure that the device is held in place with
the device heat spreader in contact with the solder preform in
the cavity and the leads are in contact with the solder paste on
the PCB solder pads. The PCB solder pads are designed to
be a minimum of 0.010″ (0.25 mm) larger than the
corresponding lead sizes as shown in Figure 4. In multi - lead
packages, where the lead spacing is very close, this distance
may have to be reduced to ensure that the device leads can
be soldered without getting shorted by solder bridging
between two pads.
AN1907
RF Application Information
Freescale Semiconductor
3
Device Lead
ÍÍÍÍÍÍÍÍÍ
ÍÍÍÍÍÍÍÍÍ
ÍÍÍÍÍÍÍÍÍ
ÍÍÍÍÍÍÍÍÍ
Solder Pad
3X
10 mils minimum
Figure 4. Pad Size and Spacing for Gate and
Drain Leads
For the solder reflow process, a fixture is usually needed to
(a) keep the device in place while running though the reflow
furnace, (b) prevent the device from lifting off due to buoyancy
forces when the solder melts and (c) keep the leads in contact
with the solder paste so it forms a good solder joint. The fixture
may also be required to apply force on the device to hold it in
place and to prevent it from lifting off. The amount of force
needed depends on the amount and type of solder used and
the soldering process.
RF power packages with the suffix “M” in the device part
number are provided with SnPb plating on all of the exposed
metal surfaces (source pad and gate and drain leads).
Packages with the suffix “N” in the device part number are
provided with matte- Sn finish on all exposed metal surfaces.
These devices are RoHS compliant.
RF power devices can be soldered using either
SnPb - based solder or most of the Pb - free solders in use. In
general, Freescale’s RF power devices are all being qualified
to meet the JEDEC J - STD - 20 requirements of MSL 3 at
260°C package peak temperature. Each device data sheet
identifies the package peak temperature and corresponding
MSL rating of the device. Devices with an MSL rating below
1 are normally shipped in a vacuum pack. The handling,
storage and use of such devices on the customer’s assembly
floor should strictly adhere to JEDEC J - STD - 33. This
standard defines the shelf life of the devices after they are
removed from their vacuum pack. It also defines the
conditions for drying such devices to reset the floor life after
moisture exposure. It should be noted that the drying is
typically specified at either 40°C, 90°C or 125°C. The baking
time for an MSL 3 rated part at 40°C is in months, which is not
very practical. In addition, the tape and the reel material in
which RF power devices are shipped cannot withstand
temperatures higher than 70°C. If such devices must be dried
to reset the floor life, they should be removed from the tape
and reel and dried in a tray that can handle a drying
temperature of 125°C.
In our experiment, we designed a PCB capable of powering
the device in DC mode. In addition, we also machined a
copper pallet with a cavity to accommodate the power
transistor. The copper pallets were plated with approximately
1,000 to 1,500 micro - inches (25 to 38 micrometers) of
electroless nickel. The pallets contain a recessed cavity that
is overplated with 0.0003″ to 0.0005″ (8 to 13 micrometers) of
tin plating to promote solder reflow. We used Sn plating
because the pallets were going to be soldered very quickly
after being received from the plating shop. For long - term
storage and use, we recommend using Au plating over
electroless Ni instead of Sn plating. In addition, we used a
special fixture to push the leads down at the tips so that the
lead tips were at a fixed distance above the top surface of the
PCB and the leads were in contact with the solder paste during
the reflow process. The cross-section of the assembly through
the fixture and the TO-270-2 device is shown in Figure 5. To
solder multiple components at one time, a simple fixture can be
designed to secure all of the components during the reflow
operation.
In the soldering process, the PCB was first attached to the
pallet. After that, the PCB was screen printed with Sn/Pb/Ag
solder paste using a 0.006″ (0.15 mm) - thick stainless steel
stencil. Prior to placing the device, two 0.002″ (0.05 mm)- thick
solder preforms and two drops of no clean flux were set into
the cavity. The device was then placed in the cavity through
the slot in the PCB. The solder reflow fixture was then attached
over the part. Finally, the entire assembly was placed in a
convection reflow furnace.
Clamp
Deformed
Copper Lead TO−270−2 Device
Solder Paste
Solder Preform
PCB
Figure 5. Concept of Fixture Holding the Leads Down on the PCB
AN1907
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RF Application Information
Freescale Semiconductor
In the reflow step, the board is preheated to 150_C and held
constant for a minimum of one minute to stabilize the board
temperature. A “spike” above the 183_C liquidus temperature
achieves the best reflow characteristics. In order to achieve
the appropriate temperature profile, the peak temperature and
belt speed of the reflow furnace are determined based on the
total mass of the assembly going through soldering.
Maximum time above the liquidus temperature is 90 seconds
with 30 to 60 seconds typical. Maximum time above 150_C
is 5.5 minutes. Figure 6 shows a typical reflow profile used in
the reflow of SnPb eutectic or similar solder. Similarly, Figure
7 shows the typical reflow profile for Pb- free solder. We want
to emphasize that these profiles are shown here only as an
example. The solder supplier should specify the required
profile. After the reflow operation, the fixture is removed. The
fixture can then be reused. An actual board assembly is shown
in Figure 8.
250
Temperature ( C)
200
° 150
100
50
0
0
100
200
300
Time (seconds)
Figure 6. Typical Solder Reflow Profile for Sn63 or
Similar Solder
Figure 7. Typical Solder Reflow Profile for Pb - free (SnAgCu) Solder
AN1907
RF Application Information
Freescale Semiconductor
5
Figure 8. Complete PCB Assembly with TO - 270 - 2 (Case 1265) Package on a Cu Pallet
RESULTS
As mentioned earlier, the lead tips can be pushed down by
0.015″ (0.38 mm) maximum to provide a good solder joint. An
evaluation was performed to determine whether bending the
lead and then reflowing the components caused any
delamination on the lead to plastic interface. The leads to
plastic interface of several TO - 270 packages were examined
using acoustic microscopy. The leads were then pushed down
at the tips by 0.015″ (0.38 mm) using a fixture similar to the one
used for the soldering operation. The assembly was then
exposed to the standard reflow temperature profile three
times. Figures 9 and 10 show sonoscan images of the
interface on two typical parts before the reflow exposure.
Figures 11 and 12 show sonoscan images of the interface on
the same parts after three reflow exposures. There is no
evidence of delamination in the parts caused by combined
stresses of the soldering temperature exposure (a maximum
of three times) and lead deflection of 0.015″ (0.38 mm).
The device MRF9045NR1 in the TO - 270 - 2 package
has a typical junction to case resistance (θ JC ) of 0.85_C/W.
For the installation described here, the device is soldered
down in the cavity of a copper pallet. The difference
between the maximum temperature in the solder joint at
the source contact and the maximum temperature in the
die will be equal to 0.8 times the dissipated power (in
watts). Once the ambient temperature in the base station
is known, the system design can be evaluated to determine
the temperature at the solder joint at the source contact of
the device. The temperature rise, calculated above based
on the power dissipation, can be added to determine the
junction temperature.
AN1907
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RF Application Information
Freescale Semiconductor
Figure 9. Lead to Plastic Interface of Part A Prior
to Lead Bending and Reflow as Viewed Using an
Acoustic Microscope
Figure 10. Lead to Plastic Interface of Part B Prior
to Lead Bending and Reflow as Viewed Using an
Acoustic Microscope
Figure 11. Lead to Plastic Interface of Part A After
Bending and Reflow (Three Times) as Viewed
Using an Acoustic Microscope
Figure 12. Lead to Plastic Interface of Part B After
Bending and Reflow (Three Times) as Viewed
Using an Acoustic Microscope
AN1907
RF Application Information
Freescale Semiconductor
7
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AN1907
AN1907
8Rev. 1, 6/2006
RF Application Information
Freescale Semiconductor