Technical Report: Ultrasonic Vibration Measurement of Wire Laser Doppler Vibrometer Part 2)

A vibrator has the characteristic of generating large vibrations at a specific frequency (resonant frequency), but since the frequency can change depending on the conditions, a feedback circuit is installed in the oscillator that drives the vibrator to control the frequency. In addition, vibrations propagated by the metal horn are amplified, so the amplitude at the tool tip is about a few micrometers under no load. To perform stable bonding, it is necessary to stabilize the vibration amplitude at the tool tip. For this reason, the need to directly measure the vibration at the tool tip is increasing year by year.

Wire bonders have a function that vibrates the tool by inputting a test drive signal. In wire bonders that have been used for a long time, fatigue accumulates in the transducer and horn, and they may start to vibrate differently than when they were first used. Therefore, when replacing with a new transducer or horn, recording the vibration value at the tip of the horn during the initial use allows for quantitative maintenance of the transducer or horn based on subsequent changes in the vibration value at the tip of the horn. In particular, measuring whether the actual vibration amplitude of the horn tip or tool increases or decreases proportionally to increases or decreases in the strength of the drive signal provides useful data for subsequent maintenance. Furthermore, when considering changing the transducer, horn, or US oscillator from the bonder manufacturer's original parts to specialized manufacturer parts during the introduction or maintenance of a wire bonder, this can be a powerful verification method for comparing the performance of each piece of equipment.

As mentioned earlier, conventional methods for estimating tool vibration during bonding only involved measuring changes in the resistance (impedance) of an ultrasonic transducer and echoes. However, using a laser Doppler vibrometer makes it possible to verify bondability from the actual tool vibration during bonding. However, because it is difficult to direct the laser beam in sync with the bonding process, measurements cannot be taken when the wire bonder is continuously performing bonding operations (auto-bonding). By using the function that allows for a single, isolated bond (manual bonding), it is possible to measure the tool's behavior at the moment of bonding.

When the tool is actually pressed against the chip or lead frame and the vibration is measured, amplitude modulation due to differences in contact pressure and US can be observed, as can frequency modulation (sideband frequencies alongside the fundamental frequency) due to the resonant frequency of the lead frame. Furthermore, since the tool is a replaceable part, replacement work is required, and depending on the mounting position of the tool to the horn and the tightening torque of the fixing screws, the amplitude of the tool may change during bonding. It has also been found that the vibration amplitude changes depending on the material and shape of the tool.

In any case, the laser Doppler method is unique in that it can obtain measurement data with a large rate of change. However, the creation of "bondability evaluation criteria" associated with data acquisition must be set individually for each bonding machine model and business location, so it is necessary to acquire and accumulate data regularly or when events occur.

As tools are structures, they naturally possess vibration modes due to their resonant frequencies. In particular, when a tool vibrates at frequencies above 60 kHz or 100 kHz, measuring data at multiple points along its length makes it possible to measure how subtle differences in the tool's material and shape affect its vibration modes. While it is generally known that subtle changes in tool material and shape alter bondability, it is also understood that bondability is influenced by the tool's resonant frequency or vibration modes, along with the finish of its surface and interior. Therefore, Laser Doppler Vibrometer can provide valuable information when developing and verifying tools with new shapes and materials.

As the integration density per chip increases, lead frames are also becoming more pin-heavy or finer-pitch. In wire bonding, as mentioned above, vibration is excited by ultrasound, so as the pins on the lead frame become narrower, there is an increasing number of cases where vibration of one pin during bonding adversely affects other pins due to resonance phenomena, etc. This is attracting attention as a cause of defects such as wire breakage and fracture. Two methods can be considered to suppress vibration. One is to reduce the amount of vibration of the excitation source or to vibrate at a frequency that is outside the resonant frequency of the lead frame, and the other is to dampen vibration by changing the structure of the lead frame or using vibration-damping materials to prevent vibration propagation.

However, in the case of wire bonders, vibration during bonding is unavoidable, making it virtually impossible to reduce the amount of vibration or change its frequency. Therefore, as a method to suppress vibration, it is necessary to consider modifying the lead frame design to reduce vibration propagation. When considering structural changes or selecting vibration-damping materials, the optimal method may vary depending on the material and form of the lead frame in question, and it is essential to conduct vibration tests while making these considerations.

The ball at the end of the wire, formed at the tip of the capillary, is pressed against a bonding surface on the tip called a pad as the capillary descends, and a bond is formed. Next, the capillary moves along the lead while feeding out the wire, and a bond is formed in the same way. After that, the capillary rises, and the clamp closes, cutting the wire. High-voltage discharge from the torch melts the cut wire end, and a new ball is formed. This bonding to the IC pad is called the first bond, and bonding to the lead is called the second bond. The bonding parameters for wire bonding are as follows, and the first and second bonds can be set individually.

Regarding ultrasonic bonding in the wire bonding process, academic analysis and verification have been largely neglected to date, with a few exceptions. In actual manufacturing settings, the dominant approach to bonding is still based on past empirical rules, and in many cases, problems are resolved through these rules alone. However, it is clear that future wire bonding will involve an increasing number of uncertainties, such as the use of new materials in ICs, multi-pin and narrow-pitch designs, and higher speeds and frequencies on the wire bonder side. Therefore, academic analysis and verification of ultrasonic bonding will be required not only for wire bonder manufacturers but also for device manufacturers producing various devices. As a first step towards "intelligence in the wire bonding process," we recommend measurement using Laser Doppler Vibrometer.