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Mounting Methods for High Power RF Plastic Devices

AN-0004125

Rev. V1

Heat slug Attach – Solder-Down Method

The first consideration in mounting the high power

plastic package is the heat slug attach. There are

two ways this can be accomplished, the solder-down

method or bolt-down method. The best way to

mount the heat slug is the solder-down method.

When done correctly, it provides a consistent bond

between the package heat slug and the carrier of the

board ensuring good thermal contact and electrical

grounding for the life of the device.

The carrier is typically made of metal, usually copper

or aluminum, and has a solderable surface. A

copper carrier is usually plated with Ni followed by a

thin layer of Au to prevent the Ni from oxidizing. An

aluminum carrier is typically plated by a zincation

process, followed by Ni and a thin Au layer.

The solder can be introduced as either preform or

paste. Solder preform, which is a pre-cut solder foil

usually coated with flux, is the preferred material.

This typically produces less voiding than solder

paste, resulting in better thermal transfer from the

device to the carrier or heat sink. There are many Pb

-based and Pb-free solders available, along with

recommended solder reflow profiles, from the solder

material suppliers. No-clean flux is recommended as

this does not require an aqueous cleaning step after

solder reflow. The heat slug can be soldered to the

carrier as a separate process step or be done at the

same time as the other components and the leads of

the package are soldered to the PCB. Refer to Table

1 and Figure 7 for standard Sn-Pb and PB-free

reflow profiles. In general to minimize solder voiding

the soak time should be extended to drive off flux

and the peak temp should be below 240°C.

Heat Slug Attach – Bolt-Down Method

Even though soldering the heat slug provides the

best interface, there are still customers that prefer to

bolt down the part. With the bolt-down method, it is

even more important to use the correct hardware,

surface finish, cavity height, etc. because there is no

solder to compensate for variations between the

heat slug and carrier. For bolt-down mounting the

carrier is typically machined or die cast using an

aluminum alloy. The surface flatness of the carrier

should not exceed 0.4 mils / inch (0.4 micron / mm)

and the average surface roughness (Ra) of the

carrier should not exceed 32 micron-inches (0.8

microns). These values can typically be achieved by

conventional machining without needing to add

process steps such as lapping or polishing. Casting

can not usually achieve the appropriate flatness and

roughness without an additional machining step in

the area where the part is mounted to the carrier.

Using the bolt-down method it is important for the

center of the heat slug to be in direct contact with

the carrier, so that there is a good thermal path

directly beneath the semiconductor device. To

accomplish this, MACOM recommends using a

specially designed clamp that is placed on top of the

part during bolt-down, see Figure 2. The clamp has

a convex surface on the bottom that has been

demonstrated to apply the appropriate force to the

center of the package which is transferred to the

backside interface between the heat slug and the

carrier. The clamp is made of a polyamide glass-

fiber reinforced material that can handle the bolt-

down stress as well as maintain its properties during

the life of the device, even in different environmental

conditions. The clamp for the TO-272 style

packages can be purchased from Richardson RFPD

as part # TO270WB.