RU2734854C1 - Method for multi-crystal modules thermo-sound micro-welding - Google Patents

Abstract

FIELD: physics.SUBSTANCE: invention can be used for connection of thermic-sound micro-welding of multichip semiconductor microcircuits modules with high assembly density. Welding is performed by capillary electrode with wire melted by supply of reverse polarity current pulses. Ball is formed at the first connection point. Wire is cut off and the ball is formed at the second connection point, and then the jumper is pulled to the first connection point. Ball of the first connection point is flattened to form a wedge and the wire is torn off. Capillary electrode with a conical tip is used, the end diameter of which and cone angle are selected from the condition of receiving the height of the jumper, which excludes contact of the adjacent jumper by the electrode. Surface of contact platform is prepared by purification with alcohol and subsequent purification in plasma for 700–800 s in medium containing 20 % Ar and 80 % O.EFFECT: technical result is high density of installation and high strength of connection.1 cl, 3 dwg, 1 tbl

Description

The invention relates to the technology of thermo-sonic microwelding, namely to the connection by thermo-sonic microwelding of multichip modules of semiconductor microcircuits.

From the prior art, in particular from (see article V. Lanin et al. "Ultrasonic equipment for welding microconductors" magazine "Components and Technologies" No. 8, 2009, pp. 124-128), it is known to use thermo-sonic microwelding for joining microconductors. The most common type of such connection is the ball-wedge and wedge-wedge connection.

The disadvantage of this method is the low bond strength.

A number of methods are also known in the art in which the above joining techniques are applied and improved. In the methods known from (see US 5967401, 19.10.1999; US 2004/0152292, 05.08.2004; US 2009/0008761, 08.01.2004; US 2010/0206940 A, 19.08.2010; US 2004/0104477, 03.06. 2006), US 2010/0206940 A, 19.08.2010; US 2005/0072833, 07.04.2005, US 2004/0104477, 03.06.2006) the strength of the connection is achieved by changing the shape by bending the wire, which forms a hardening zone, however, such methods are very complicated; in the methods known from (US 2005/0191839, 01.09.2005; US 2010/0314754, 16.12.2010), the strength of the connection is achieved by strengthening it with multiple connections, which also leads to a complication of the method and an increase in the consumable material.

The closest analogue of the claimed method is the technology of thermosonic microwelding by the "ball-wedge-ball" method (see the article "Technology of thermosonic microwelding" ball-wedge-ball "and control of microwelded joints", M. Shmakov et al., "Technologies in the electronic industry ", No. 7 2007, pp. 70-72), including the following operations:

- preparatory,

- turning on the installation,

- checking parameters,

- refueling of gold wire,

- creation of test jumpers,

- weldability control,

- creation of jumpers, at the first connection point of which a ball is formed using a special tool, then the bridge is pulled out and a wedge is formed at the second connection point, which is additionally reinforced with a ball, and

- quality control of welded joints.

The disadvantage of the above-mentioned closest analogue is the insufficient strength of the connection for separation.

The technical result of the claimed invention is to increase the assembly density and increase the strength of the connection.

The claimed technical result is achieved by creating a method for thermo-sonic microwelding of multichip modules, including preparing the surface of the contact pad, turning on the installation and checking the parameters, monitoring and threading the wire into the installation, creating test jumpers, monitoring the quality of welded joints, while to create jumpers, a ball is formed using a capillary tool at the first connection point, then the wire is cut to form a wedge, a ball is formed in the second connection current, the jumper is pulled and the wire is cut off to form a wedge at the first connection point.

The claimed invention is illustrated by the following drawings:

fig. 1 - the process of ball formation a) the shape of the original ball before welding; b) the profile of the deformed ball after attachment; c) view of the working end of the capillary with wire at the connection position.

fig. 2 - capillary instrument;

fig. 3 - welded joint made on Lange bridges: a) top view, b) side view of the "ball-wedge" connection, c) side view of the joint.

FIG. 1:

L is the capillary length;

OD is the outer diameter of the capillary;

FA - capillary end angle;

OR is the outer radius.

The method of thermo-sonic microwelding of multicrystal modules begins with the formation of a ball formed as a result of wire melting by a spark discharge obtained by a pulse of negative or positive polarity. In the first case, the plasma sheath surrounds only the lower half of the ball, which makes it possible to exclude damage to the capillary and minimize the annealing of the "wire - ball" transition; in the second, the pulse creates a plasma sheath around the entire ball, which leads to faster wear of the capillary and worsens the "transition" in the zone wire ball. The temperature of the plasma formed around the wire reaches the melting point of the wire material and when its end melts, the surface tension forces form a ball, the shape of which is one of the parameters affecting the quality of the welded joint. Hence it follows that the shape of the ball depends on the selected equipment and the shape of the capillary, which must have high wear resistance, have insignificant acoustic losses, low tendency to adhesion with the material to be welded, and high strength.

To obtain a high-quality welded joint, a stable diameter and symmetry of the ball are required, and at a high mounting density (with a distance between the CP (contact pads) ≤60 μm), to prevent the capillary end from touching adjacent joints and bridges, it is necessary to select capillaries with a thinned tip. When selecting this capillary, the end diameter T is taken into account, which is determined by the pitch between the centers of the gearbox (FPP parameter) and the value of the cone angle, which determines the height of the bulkhead at which the capillary will not touch the adjacent bulkhead. The minimum FPP step can be calculated using the formula (1):

where, Q - capillary end angle, h - bulkhead height, T - working end diameter

The optimum ball diameter is considered to be 2-2.5 wire diameters. With a wire diameter of 25 microns, the diameter of the ball will be 50-62.5 microns, which is not suitable for gearboxes with dimensions of 60x60 microns or less, and a decrease in the diameter of the wire will lead to an increase in the likelihood of its breakage during installation and, as a consequence, an increase in the percentage of rejects. In this regard, the diameter of the ball was calculated to be 2.0-2.3 wire diameters, which will provide the required strength of the welded joint and will not exceed the dimensions of the KP.

For a capillary that allows you to obtain the required ball diameter, you can use the following relationship (2) [5]:

where H is the diameter of the capillary hole (for free sliding of the wire, it should be 1.2-1.4 times the diameter of the wire); WD - wire diameter; CD is the diameter of the inner chamfer and CA is the angle of the inner chamfer (determine the shape of the deformed ball, the larger the angle CA, the larger the diameter of the deformed ball); MBD is the diameter of the deformed ball; MBH is the deformed ball height.

As a non-exclusive example, we can cite the capillary type 41xxx-A210-R34 selected taking into account relation (2) with the parameters H = 36 μm, CD = 48 μm, CA = 90 ° and the selected ball parameters H = 25 μm, MBD = 55 μm , MBH = 25 μm. With these parameters, we get FAB = 53.22. The ratio of the obtained ball diameter to the wire diameter is 53.22 / 25 = 2.1, which falls within the calculated range. From formula (1), the parameters of the capillary Q = 20 °, T = 127 μm and h = 150 μm (regulated by the requirements for space instrumentation products), we obtain the minimum step of the welded joint FPP = 102.4 μm, which is permissible when using the capillary shooting gallery 41xxx-A210 -R34, which allows welding with a distance between the KP of at least 42.4 microns and allows you to solve the problem.

The type of capillary is also due to the use of the Iconn automatic thermo-sonic microwelding machine, characterized by a high positioning accuracy of the tool of ± 2 microns, which minimizes the likelihood of the welded joint going beyond the KP and damage to the crystal, and a high welding speed, which ensures high productivity, repeatability and stability. process.

Based on the analysis performed and the welding method selected based on its results, the type of welded joint and the type of capillary, a new technological process of micro-welding was developed and introduced into production.

To develop the technology of microwelded joints, substrates made of alumina ceramics (VK-100), crystals and metal-ceramic bodies with a gold coating KP of at least 3 microns were used. The selection of welding modes was carried out for gold wire with a diameter of 25 microns.

The following sequence of technological operations has been developed:

1. Preparation of the CP surface: cleaning with alcohol, cleaning in plasma for 700 ... 800 s in an Ar (20%) / O ₂ (80%) medium (the cleaning mode in plasma was selected with control of the CP purity by the contact angle).

Note: Plasma cleaning is performed only in permissible cases.

2. Switching on the installation and checking the parameters.

3. Checking the wire and threading the wire into the installation.

4. Creation of test jumpers (the operation is carried out before starting work on 2-3 technological samples from each batch).

5. Control for weldability.

6. Making jumpers (SSB Bump 1 - first ball, SSB Bond1 - second ball (first jumper connection), Bond 2 - wedge (second jumper connection) welded into SSB Bump 1).

7. Quality control of welded joints.

The technological parameters of the TZS were selected experimentally in the following sequence: selection of the optimal current strength for the formation of the required ball shape, the minimum pressure at which the setting of materials is achieved, the duration of microwelding and the amplitude of oscillations of the tool (generator power) by gradually increasing them until a welded joint of the required strength is obtained. The selected parameters are presented in Table 1. The heating of the working table is chosen equal to 150 ° C.

Table 1. Parameters of the microwelded joint.

Weldable surfaces Welding parameters SSB Bump 1 (single ball) SSB Bond1 (ball) Bond 2 (wedge) USG,
mA Time,
ms USG,
mA Time,
ms USG,
mA Time,
ms Lange Bridges (Au) 95 ± 5 42 ± 5 95 ± 5 42 ± 5 9 ± 2 5 ± 2 Crystal / body
(Al / Au) 95 ± 5 40 ± 5 60 ± 5 15 ± 5 9 ± 2 5 ± 2

Based on the results of testing the technological process, welded joints of the required quality and high strength (at least 13 grams) were obtained. The appearance of the resulting conductors is shown in Fig. 3.

The claimed invention will increase the tensile strength of the connection when welding a wire of small diameter 25 μm on a small contact pad 60 × 60 μm with a high packing density and ensuring high accuracy of positioning the wire relative to the center of the contact pad.

Claims (2)

1. A method for thermo-sonic microwelding of multichip modules with a high packing density, including preparing the surface of the contact area, welding using a capillary electrode with a wire melted by applying current pulses, while forming a ball at the first and second points of the welded joint and obtaining a jumper between them, characterized in that current pulses of reverse polarity are applied to said electrode, while a ball is formed at the first connection point, then the wire is cut off and a ball is formed at the second connection point, then the jumper is pulled to the first connection point, the ball of the first connection point is flattened to form a wedge and the wire is cut off, and a capillary electrode with a conical tip is used, the diameter of the end of which and the angle of the cone are selected so that the height of the bridge is obtained, excluding the electrode touching the adjacent bridge. 2. The method according to claim 1, characterized in that the preparation of the surface of the contact area is carried out by cleaning with alcohol and subsequent cleaning in plasma for 700-800 s in an environment containing 20% Ar and 80% O ₂ .

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