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Mounting & Bonding Techniques for Millimeterwave GaAs MMICs

The die should be attached directly to the ground plane eutectically or with

conductive epoxy (see HMC general Handling, Mounting, Bonding Note).

50 Ohm Microstrip transmission lines on 0.127mm (5 mil) thick alumina

thin film substrates are recommended for bringing RF to and from the chip

(Figure 1). If 0.254mm (10 mil) thick alumina thin film substrates must be

used, the die should be raised 0.150mm (6 mils) so that the surface of

the die is coplanar with the surface of the substrate. One way to accom-

plish this is to attach the 0.102mm (4 mil) thick die to a 0.150mm (6 mil)

thick molybdenum heat spreader (moly-tab) which is then attached to the

ground plane (Figure 2).

Microstrip substrates should brought as close to the die as possible in

order to minimize bond wire length. Typical die-to-substrate spacing is

0.076mm to 0.152 mm (3 to 6 mils).

Handling Precautions

Follow these precautions to avoid permanent damage.

Storage: All bare die are placed in either Waffle or Gel based ESD protec-

tive containers, and then sealed in an ESD protective bag for shipment.

Once the sealed ESD protective bag has been opened, all die should be

stored in a dry nitrogen environment.

Cleanliness: Handle the chips in a clean environment. DO NOT attempt

to clean the chip using liquid cleaning systems.

Static Sensitivity:

Follow ESD precautions to protect against ESD

strikes.

Transients: Suppress instrument and bias supply transients while bias

is applied. Use shielded signal and bias cables to minimize inductive

pick-up.

General Handling: Handle the chip along the edges with a vacuum collet or with a sharp pair of bent tweezers. The

surface of the chip has fragile air bridges and should not be touched with vacuum collet, tweezers, or fingers.

Mounting

The chip is back-metallized and can be die mounted with AuSn eutectic preforms or with electrically conductive epoxy.

The mounting surface should be clean and flat.

Eutectic Die Attach: A 80/20 gold tin preform is recommended with a work surface temperature of 255 °C and a tool

temperature of 265 °C. When hot 90/10 nitrogen/hydrogen gas is applied, tool tip temperature should be 290 °C. DO

NOT expose the chip to a temperature greater than 320 °C for more than 20 seconds. No more than 3 seconds of

scrubbing should be required for attachment.

Epoxy Die Attach: Apply a minimum amount of epoxy to the mounting surface so that a thin epoxy fillet is observed

around the perimeter of the chip once it is placed into position. Cure epoxy per the manufacturer’s schedule.

Wire Bonding

Ball or wedge bond with 0.025mm (1 mil) diameter pure gold wire. Thermosonic wirebonding with a nominal stage

temperature of 150 °C and a ball bonding force of 40 to 50 grams or wedge bonding force of 18 to 22 grams is recom-

mended. Use the minimum level of ultrasonic energy to achieve reliable wirebonds. Wirebonds should be started on

the chip and terminated on the package or substrate. All bonds should be as short as possible <0.31mm (12 mils).

0.102mm (0.004”) Thick GaAs MMIC

Wire Bond

RF Ground Plane

0.127mm (0.005”) Thick Alumina

Thin Film Substrate

0.076mm

(0.003”)

Figure 1.

0.102mm (0.004”) Thick GaAs MMIC

Wire Bond

RF Ground Plane

0.254mm (0.010”) Thick Alumina

Thin Film Substrate

0.076mm

(0.003”)

Figure 2.

0.150mm (0.005”) Thick

Moly Tab

HMC425

0.5 dB LSB GaAs MMIC 6-BIT DIGITAL

POSITIVE CONTROL ATTENUATOR, 2.4 - 8.0 GHz

v02.0907