Optimization of Ultrasonic and Thermosonic Wire-Bonding Parameters on Au/Ni Plated PCB Substrate

 Abstract

The wire-bonding process is most widely used in the laser diode packaging industry to provide electrical connections between laser diode and lead frames of the package. The ultrasonic and thermosonic wedge bonding with gold wire having diameter, 25m, was optimized at different temperatures. Especially designed Au/Ni plated printed circuit board (PCB) substrate was used to optimize the process parameters viz. bonding force, temperature, ultrasonic power, and bonding time. The wedge bond foot-prints of the formed bond-loop were observed under optical microscope. The optimized power for ultrasonic bonding had a range between 0.6 - 0.7 watt and in the case of thermosonic bonding it was found to be 0.5 - 0.6 watt at 50°C.

Keywords: Laser diode, Wire-bonding, Thermosonic bonding, Gold wire, Au/Ni.

Introduction

The leading factor in the laser diode (LD) packaging is the die-bonding and wire-bonding. The wire-bonding is the most essential process which provides external electrical connection on p or n-type metallization of the LD chip/bar. Thin metal wire was connected between the contact-stripe on the LD chip and the corresponding contact-lead on the package.

Wire-bonding is a very delicate procedure which requires expertise to utilise wire-bonder machine. The wire-bonder machine bonds one end of the wire to the metalized substrate using an ultrasonic pulse, creates a loop with the wire, makes a second bond, and finally the process ends with the wire cutting at that end, as shown in Fig. 1. Normally, the wire-bonding techniques can be categorized into three major processes: thermo-compression bonding (T/C), ultrasonic bonding (U/S), and thermosonic bonding (T/S), depending on the bonding parameters i.e. ultrasonic power and/or heat. The most commonly employed technique to achieve wire-bonding is the thermosonic (T/S) bonding [1], performed at elevated temperature by ultrasonic power.

Basically, there are two types of wire-bonding techniques (1) ball bonding and (2) wedge bonding. In ball bonding small metal ball is formed by means of ‘electric flame off’ (EFO) at the end of the wire. On the other hand, in wedge bonding process the wire is pressed horizontally on a substrate and attached to it. We have employed wedge bonding technique to bond gold wire on the substrate instead of ball bonding due to its simplicity of implementation and no EFO is required. The quality wire-bond can be achieved by means of optimizing various bonding parameters viz. heat, pressure, ultrasonic energy and bond-time. These parameters are critical for consistent wire-bonding and hence ensure the reliability of the bonded device.

The first ever gold wire-bonding was optimized by means of thermocompression and reported by Bell Laboratories in 1957 [2]. Since then, the technique has been extremely improved. Much recently, copper (Cu) and aluminium (Al) are also used for wire-bonding in packaging industries due to its availability and low cost [3, 4]. However, the problem with Cu and Al is its hardness. Also, the coefficient of thermal expansion (CTE) mismatches with the laser diode material. Furthermore pure Al is too soft to form a very fine wire. Hence, it is often alloyed with silicon (Si) or magnesium (Mg). Nevertheless, at high temperature the Al wire degrades and may causes fatigue failure [3]. In case of Cu, the wire-bonding need to be performed under inert ambient conditions to avoid the oxidation of Cu [3]. Also, the mismatch of CTE cause damage to the laser diode while die/wire-bonding process [5]. The present article discusses optimization of wire-bonding process for laser diodes using gold wire on Au/Ni plated printed circuit board (PCB).

Experimental:

The 1 mil (25.4 µm - diameter) gold wire was used to optimize the bonding process. The wire-bonding was achieved using manual wire-bonder (K & S make universal bonder) shown in Fig. 2. The wire-bonder mainly consists of ultrasonic power supply, transducer, bonding head, bonding tool, heating stage, temperature controller, and microscope.

The complete bonding process was optimized on a specially fabricated and organically cleaned gold plated PCB. A good wire-bonding can only be attained by optimizing the parameters viz. bonding force, temperature, ultrasonic power, and finally the bonding time in a sequential manner. We have bonded Au wire at three various temperatures i.e. room temperature (RT), 50C and 100C. The bonding force, ultrasonic power and bonding time employed to achieve wire bond was varied between 8-12 gm, 0.3-1.0 watt, and 1-10 ms, respectively.

Results and Discussions:

The gold wire was bonded on Au/Ni plated PCB. We have optimized the bonding parameters, viz. bonding temperature, bonding force, ultrasonic power, and bonding time, together in a sequential manner by varying one parameter and keeping other constant. The wire-bonding has been optimized by observing the footprints of the formed bond-loop under microscope [5]. Table 1 shows the experimental and optimized wire-bonding parameters.

Starting with RT we went up to 100C to optimize wedge wire-bonding. The temperature will influence the bonding material and device if it is too high, or it will lead to undesirable bonding quality if too low [6]. Also, at 100C the PCB started getting burned. Hence, the process was optimized at RT and 50C. As we already have discussed that the bonding process optimized at RT is known as ultrasonic bonding and the thermosonic bonding is the process which is optimized at elevated temperature. In the case of bonding force, there was no bonding took place below 8 gm, even at elevated ultrasonic power and appreciable bonding time. While, exceeding 12 gm bonding force the gold wire was broken at the bonding neck and no wire-bond could be achieved. The good bond can only be achieved at 11 and 12 gm bond force in combination of other parameters. We have applied ultrasonic power ranges from 0.3 to 1.0 watt for wire-bonding and hence achieved the optimized value ranges between 0.5 - 0.7 watt. Here, lower or higher ultrasonic power, with respect to optimized value, applied in bonding was result in no bonding or wire deformation, respectively. The optimized power for ultrasonic bonding was ranging from 0.6 - 0.7 watt and in case of thermosonic bonding it was 0.5 - 0.6 watt at 50°C. Here, applied temperature for thermosonic bonding lowers the requirement of ultrasonic power and time for bonding.

Finally we have optimized bonding time. The lower bonding time (< 4 ms) results in no boding because it will not provide sufficient time to attach wire to the bond-pad. While at the higher bond time (> 6 ms) gold wire deformed and eventually gets broken from the bonding neck.

Here, the whole range of optimized parameters is known as the process window. If the value of bonding parameters set below or above this process window, it will result in either no bonding or wire breakage and will further damage to the substrate. Figure 3(a) shows the optimized bond footprint while Fig. 3(b) shows the broken bond neck due to excess ultrasonic power applied during wire-bonding process.

Conclusion:

We have successfully optimized ultrasonic and thermosonic wedge bonding process for gold wire at room temperature and at 50C, respectively. Moderated ultrasonic power and bonding time was optimized by observing the wire-bond footprint under the microscope. If the value of bonding parameters set below or above the optimized value, it will result in either no bonding or wire breakage and further will cause damage to the substrate.

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