2005 Jun 03 7
Philips Semiconductors Product specification
CDMA800 power module BGF802-20
MOUNTING RECOMMENDATIONS
General
LDMOST base station modules are manufactured with the
dies directly mounted onto a copper flange. The matching
and bias circuit components are mounted on a
printed-circuit board (PCB), which is also soldered onto
the copper flange. The dies and the PCB are encapsulated
in a plastic cap, and pins extending from the module
provide a means of electrical connection. This construction
allows the module to withstand a limited amount of flexing,
although bending of the module is to be avoided as much
as possible. Mechanical stress can occur if the bottom
surface of the module and the surface of the amplifier
casing (external heatsink) are not mutually flat. This,
therefore, should be a consideration when mounting the
module in the amplifier. Another cause of mechanical
stress can arise from thermal mismatch after soldering of
the pins. Precautions should be taken during soldering,
and efforts made to ensure a good thermal contact
between the flange and the external heatsink.
External heatsink (amplifier casing)
The module should always be mounted on a heatsink with
a low thermal resistance to keep the module temperature
as low as possible. The mounting area of the heatsink
should be flat and free from burrs and loose particles. We
recommend a flatness for the mounting area of between
50 µm concave and 50 µm convex. The 50 µm concave
value is to ensure optimal thermal behaviour, while the
50 µm convex value is intended to limit mechanical stress
due to bending.
In order to ensure optimal thermal behaviour, the use of
thermal compound is recommended when mounting the
module onto the amplifier external heatsink.
The following recommended thermal compounds have a
thermal conductivity of >0.5 W/mK:
• WPS II (silicone-free) from Austerlitz-Electronics
• Comp. Trans. from KF
• 340 from Dow Corning
• Trans-Heat from E. Friis-Mikkelsen.
The use of thermal pads instead of thermal compound is
not recommended as the pads may not maintain a uniform
flatness over a period of time.
Mounting
PREPARATION
Ensure that the surface finishes are free from burrs, dirt
and grease.
CAUTION
During the following procedures ESD precautions should
be taken to protect the device from electrostatic damage.
PROCEDURE
1. Apply a thin, evenly spread layer of thermal compound
to the module flange bottom surface. Excessive use of
thermal compound may result in increased thermal
resistance and possible bending of the of the flange.
Too little thermal compound will result in an increase
in thermal resistance.
2. Take care that there is some space between the cap
and the PCB. Bring the module into contact with the
external heatsink casing, ensuring that there is
sufficient space for excessive thermal compound to
escape.
3. Carefully align the module with the heatsink casing
mounting holes, and secure with two 3 mm bolts and
two flat washers. Initially tighten the bolts to “finger
tight” (approximately 0.05 Nm). Using a torque
wrench, tighten each bolt in alternating steps to a final
torque of 0.4 Nm.
4. After the module is secured to the casing, the module
leads may be soldered to the PCB. The leads are for
electrical connection only, and should not be used to
support the module at any time in the assembly
process.
A soldering iron may be used up to a temperature of
250 °C for a maximum of 10 seconds. Avoid contact
between the soldering iron and the plastic cap.
Electrical connections
The main ground path of all modules is via the flange. It is
therefore important that the flange is well grounded and
that return paths are kept as short as possible. An
incorrectly grounded flange can result in a loss of output
power or in oscillation.
The RF input and output of the module are designed for
50 Ω connections.
Incoming inspection
When incoming inspection is performed, use a properly
designed test fixture to avoid excessive mechanical stress
and to ensure optimal RF performance. Philips can deliver
dedicated test fixtures on request.
