AN1908 Freescale Semiconductor / Motorola, AN1908 Datasheet - Page 3

• Manufacturer Part Number: AN1908
• Description: Solder Mounting Method for the MRF19090S and Similar Packages
• Manufacturer: Freescale Semiconductor / Motorola

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. After 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 that was
plated with 0.0003 of tin lead (60–40). Prior to placing the
component in the recessed cavity, two 0.002 thick solder
preforms and two drops of no clean flux were set into the
recess. After placing the component, 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
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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
Device Lead
Freescale Semiconductor, Inc.
For More Information On This Product,
Solder Pad
10 mils minimum
Go to: www.freescale.com
3X
board temperature. A “spike” above the 183 C liquidus
temperature achieves best reflow characteristics. 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.
minutes. Figure 7 shows a typical reflow profile. After the
reflow operation, the fixture was disconnected by removing
the four screws. The fixture could then be reused. 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 PCB assembly step for
the process flow is shown in Figure 8. For small PCBs, it is
possible to solder the backside of the entire PCB to the copper
pallet instead of bolting it to the pallet.
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.
The completed, reflowed board/pallet assemblies were
250
200
150
100
50
0
0
Figure 7. Typical Solder Reflow Profile
for Sn/Pb/Ag Solder
100
Maximum time above 150 C is 5.5
Time (seconds)
200
AN1908
300
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