AN1923 Motorola / Freescale Semiconductor, AN1923 Datasheet - Page 3

• Manufacturer Part Number: AN1923
• Description: AN1923 Application Note Mounting Method with Mechanical Fasteners for the MRF19090 and Similar Packages
• Manufacturer: Motorola / Freescale Semiconductor

necessary to ensure that the level of gold within the solder joint
does not exceed 4% by volume. One way to accomplish this
is to solder dip the leads into a solder pot of molten Sn/Pb
solder. An alternate way is to provide enough thickness of
solder paste so that the amount of Au in the solder joint is
within the desired limit set by the customer.
accomplishing this for the power life test assemblies, the PCB
was screen–printed with Sn/Pb/Ag solder paste using a
stainless steel stencil, 0.006 thick. The copper pallets were
plated with approximately 1,000 to 1,500 micro–inches of
electroless nickel. The pallets contained a recessed cavity in
which the components were placed. The PCB was placed on
a reflow boat. The solder reflow fixture shown in Figure 5 was
then fixed in place over the part using four #4–40 screws with
5 in.–lbs of torque (M3 screws with 0.8 N–m of torque). Finally,
the entire assembly was placed in a BTU convection reflow
furnace. The recommended pad size is shown in Figure 6.
held constant for a minimum of one minute to stabilize the
board temperature. A “spike” above the 183 C liquidus
temperature achieves best reflow characteristics. To achieve
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MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION
Prior to proceeding with assembly of the PCB, it is
In the reflow step, the board was preheated to 150 C and
Figure 6. Pad Size in Relation to the Lead
After
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 7 shows a typical reflow profile. After the
reflow operation, the fixtures were disconnected by removing
the screws. The fixture could then be reused. Thermal
compound was then evenly spread on the backside of the
component with a roller (0.0005 to 0.0010 thick). The PCB
and component were secured to a copper pallet using six
#4–40 socket head cap screws with 5 in.–lbs. of torque (M3
screws with 0.8 N–m of torque). The process flow is shown
in Figure 8.
screw mounted to the aluminum heatsink after evenly
spreading the backside of the copper pallet with 0.0005 to
0.001 of thermal compound. An actual board assembly used
for the power life test is shown in Figure 9. Care should be
taken to ensure that adequate electrical and thermal contact
is established between the pallet and the bottom of the device
flange.
The completed, reflowed board/pallet assemblies were
Figure 7. Typical Solder Reflow Profile
for Sn/Pb/Ag Solder
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