HP A005 User Manual

Transistor Chip Use

Application Note A005

Part I. Assembly Considerations

1.0 Chip Packaging for Shipment

1.1 General

Hewlett-Packard transistor chips are shipped in chip carriers with a
clear or black elastomer as a carrier medium. There are up to 100 chips
in each pack. One of the corners of the pack is beveled to provide
orientation for chip selection.

1.2 Opening/inspection

Chip carriers should be opened only at a clean, well-lighted station
without fast-moving air. A white working surface is recommended for
best visibility. The chips are kept in place by the surface tension of the
elastomer on which they are placed. Once the clamps have been
removed, the pack should be set down on the surface and the lid
removed. Then, the paper can slowly (to avoid static-electricity
generation) be lifted off the lower half. Occasionally a chip may work
loose from the surface of the elastomer and turn upside down. If this
happens, the chip should be carefully picked up with tweezers, turned
over, and placed back on the elastomer. At this point the die can be
counted or visually inspected. The reverse of the procedure outlined
above should be used to close the waffle pack prior to storage or die
attach operations.

1.3 Long Term Storage

Die that will be stored for long periods of time (greater than 1 – 2
weeks) should be kept in a dry nitrogen atmosphere for optimum
performance and reliability. The gold-based metal systems of all
Hewlett-Packard transistors are very resistant to deterioration, but the
best practice is to store them in an inert atmosphere.

2.0 Handling Chips with Tweezers

Unfortunately, both silicon and GaAs materials are quite brittle (GaAs
is more so) and the sharp edges of the transistor chips are easily dam­aged if too much pressure is applied with tweezers. A good precaution
is to use only the sharpest possible tweezers with good point alignment
to handle transistor chips. New operators should practice with bonding
samples, which HP generally can supply free of charge.

3.0 Die Attach Procedure

Different die attach procedures are used for silicon chips and GaAs
devices. To die attach a silicon chip, the chip and mounting surface are
heated sufficiently for the gold of the mounting surface to mix with the
gold backside of the chip and melt into the silicon of the chip, forming
a gold-silicon eutectic bond. This technique is suitable because silicon
can tolerate the relatively high temperatures needed for eutectic
formation without endangering the silicon device.

GaAs devices are more temperature sensitive, and use instead a lower
temperature gold alloy “solderdown” technique. In both cases the die­attach station should have the actual die attach area flooded with an
inert (forming gas or nitrogen) atmosphere.

3.1 Silicon Bipolar Transistor Die Attach Procedure

3.1.1 The heater block temperature should be 410°C ±10°.

3.1.2 Place the circuit or package into which the transistor or capacitor
will be attached on the heater block. Allow sufficient time to heat
thoroughly (5 to 15 seconds depending on thermal mass).

2

3.1.3 Pick up a chip with tweezers (EREM type 5 SA recommended)
and orient it properly prior to placement on the heated surface.

3.1.4 Place the chip on the heated surface and scrub with a back-and­forth motion, being careful not to scratch the top surface of the chip.
Continue this until wetting occurs; this should take place within three
to four scrubs.

3.1.5 If wetting does not occur check that the heater block
temperature is correct and that the inert atmosphere blanket is present.

3.1.6 When wetting occurs, perform a circular scrub for one stroke to
insure wetting of the chip perimeter. Carefully remove the tweezers
from the die.

3.1.7 Remove the circuit or package from the heated surface and allow
it to cool in air.

3.2 GaAs FET Attach Procedure

3.2.1 The heater block temperature should be 300°C ±10°.

3.2.2 Place the circuit or package into which the transistor will be
attached on the heater block. Allow sufficient time to heat thoroughly
(5 to 15 seconds depending on thermal mass).

3.2.3 Pick up a Au-Sn (gold-tin) preform and place it on the circuit/
package in the die attach location. Use a sufficient quantity to insure
good wetting and to produce a fillet around the chip.