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Agilent Beam Lead Attachment Methods

Application Note

Figure 1. General setup for attaching beam lead devices

Introduction
This application note gives the first time user a general description of various attach-
ment methods for beam lead devices. The attached table summarizes the salient
features of all the methods including the advantages, disadvantages, and the associ-
ated cost of the equipment. Equipment selection as dictated by the type of substrate
being used is emphasized.

Attachment Methods
Beam lead devices can be attached by any of the conventional means, but
matching the method to the intended substrate, conductor metals and finish
is imperative to a reliable, cost effective process. The various methods should
be evaluated in light of this requirement. A general setup for attaching beam
leads is shown in Figure 1. Characteristics of the various methods are discussed
below.
Thermocompression bonding
Since nearly any thermocompression wire bonder can be modified for beam
leads, it is a common method. A typical setup for bonding beam leads is shown
in Figure 2. The wedge bonder tip contact area is 3 x 4 mils.

Because of the temperatures and pressure required, the thermocompression
method should be restricted to hard substrates which won't deform or allow the
conductor to move. Substrate deformation or conductor movement generally
results in the beam being torn or the device broken. Seldom is a reliable bond
achieved since the material is deforming under the pressure and does not allow
the necessary concentrated force at the bond area.

Figure 2. Typical setup for
thermocompression bonding of beam lead
devices

Reflow method

1. The deformation of the beam by the One alternative to the thermocompression bond on either soft or hard substrates
bonding tool causes the gold to move into is the reflow of either tin or solder. The conductor is prepared by plating the
the silicon chip. The movement of this base metal with 200 micro-inches of bright tin or solder coating. It is difficult to
metal results in the chip lifting away from control the amount of solder such that it does not alter the spacing required for
the substrate. This is commonly known as the beam lead, so tin is the preferred choice. In the reflow process, the wedge
"bugging". A certain amount of bugging is bonder tip is replaced with one which has a tip of approximately 10 x 10 mils
beneficial for providing a resilience to the and has enough mass to act as a soldering tip. (See Figure 3.) The temperature
chip and substrate for protection against of the electrode must be arrived at empirically since the materials being used
damaging stresses during thermal cycling
will dictate the temperature required to reflow the tin or solder.
in actual usage. However, excessive bug-
ging caused by a high deformation of the
beam leads could lead to unusual stresses One word of caution regarding the reflow method should be noted. It is probable
at the interface between the beam and the that the device will be under some mechanical stress when put down by this
chip. Failure due to peeling of the leads or method because there is no "bugging"1 taking place, therefore no strain relief
fracturing of the silicon could result. built in. When a soft substrate, which can be flexed or which has a large coef-
ficient of expansion is used, some strain relief must be provided by bowing the
part away from the mounting point before the second lead is attached.

2
Figure 3. Typical setup for reflow of low
temperature materials to attach beam
lead devices

Wobble bonding
Another thermocompression bonding method, which bonds all the leads in a
single operation, is shown in Figure 4. The wobble bonder allows the beam lead
to be picked up by a collet which has been designed to clear the body of the
part and have about 0.002" surface width to bond the leads. This collet is then
wobbled around the device with approximately a 1 degree angle with a force
sufficient to bond (~ 250 grams) to the circuit materials. This method has the
same problems with soft substrates as the basic thermocompression bonding
method. Because of the heat and pressure required, reliable bonds cannot be
made on any non-rigid substrate.

Figure 4. Thermocompression wobble
bonder for attaching beam lead devices

3
Epoxy attach
It is possible to use epoxy to attach beam leads to soft or hard substrates. With
the very pure materials available good results can be achieved. Equipment is
available to dispense very small dots of epoxy and then pick and place the beam
lead onto the epoxy. These setups are shown in Figures 5 and 6, respectively.
Solder paste can also be used in place of the epoxy. By using a good quality low
temperature solder (180

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