Technical Report on Ultrasonic Vibration Measurement of Wire Bonding Using a Laser Doppler Vibrometer 1

The vibrator has the characteristic of generating large vibrations at a specific frequency (resonant frequency), but the frequency can change depending on the conditions. Therefore, a feedback circuit is established in the oscillator driving the vibrator for frequency control. Furthermore, the vibrations propagated through the metal horn are amplified, resulting in an amplitude of several micrometers at the tool tip when unloaded. To perform stable bonding, it is necessary to stabilize the vibration amplitude at the tool tip. As a result, the need for direct measurement of the vibrations at the tool tip has been increasing year by year.

The cycle of wire bonding, using ball bonding as an example, is as follows.

The ball formed at the end of the wire at the tip of the capillary is pressed against the pad on the chip, which serves as the bonding surface, as the capillary descends. Next, the capillary moves over the lead while extending the wire and bonds in a similar manner. After that, the capillary rises, and during this process, the clamp closes, cutting the wire. The cut wire end is melted by high-pressure discharge from a torch, forming a new ball. The bonding to the IC pad at this time is referred to as the first bond, and the bonding to the lead is referred to as the second bond. The bonding parameters during wire bonding include the following, and they can be set individually for the first and second bonds.

Heater heating temperature (for ball bond only)
To facilitate wire bonding, the pad and lead are heated around 200°C.
Load
It is the pressing load when pressing the capillary against the pad lead. This is done by applying a load of about 10 to 100 g from the ultrasonic horn side.
Ultrasonic (Ultra Sonic) oscillation
assists in the plastic deformation of the wire and promotes fusion with the pad surface and lead surface.
Bonding time
is the time to apply load and ultrasound, and can vary from several tens of msec to several hundred msec. Among these bonding parameters, with the diversification of semiconductor materials, there has been a demand for lower heating temperatures in ①, and a demand for shortening the bonding time in ④ due to the improvement of manufacturing speed (tact time per cycle). Therefore, there have been cases where the functions that each of the parameters ① and ④ was responsible for must be supplemented by other parameters.

In such cases, the following measures are being taken.

Increase of US power or high-frequency oscillation of the frequency.
increase in load

However, since the quality of bonding (bondability) is established on a delicate balance of various bonding parameters, careless changes to these parameters can have a significant impact on product yield. Specifically, there may be an increase in failure incidents such as wire detachment or neck breakage, as well as destruction incidents like the loss of contact pads on the chip side due to excessive loading.

advantage: Can be addressed in a short term and at a relatively low cost.

shortcoming: Due to the lack of theoretical and numerical backing, it cannot be assumed that the new type of IC can be directly supported. Additionally, because the issues that arise during troubles are unclear, the countermeasures are limited, and once the yield deteriorates, it can lead to an endless cycle of problems.

advantage: To examine the tensile strength and joint strength of the wire itself, it can be easily tested.

shortcoming: It is a confirmation of results and does not lead to cause investigation. It will also be a destructive test.

advantage: Effective for confirming disconnection accidents and peeling conditions, etc.

shortcoming: While effective for confirming results, it is extremely limited in identifying causes. In particular, it cannot address problems arising from high frequencies such as US oscillations.

advantage: By installing it on each bonda, it is possible to achieve inline constant monitoring.

shortcoming: Unless the rate of change is small and there is a significant accident with a clear cause, differences are not noticeable, and the effects of the introduction are almost nonexistent.

The bonding process, as previously mentioned, is conducted between the tool or connector, referred to as a capillary, which is attached to the tip of a metal horn by screws, and the pads or lead frames. Therefore, it is easy to imagine that effective measurement points for verifying bondability would be the tool tip itself or directly measuring the pads or lead frames. However, measuring the tool, pads, or lead frames has traditionally been extremely difficult, and simply put, there were no applicable sensors available. For example, the tools used in ball bonding have a tapered tip called a capillary, with a fine diameter of about 1.5 mm and a length of about 11 mm, typically made of hard ceramic or ruby. Due to size constraints, it is impossible to attach contact-type vibration sensors. Furthermore, laser displacement meters, which are representative of non-contact sensors, are rare in being able to measure frequencies above 60 kHz, and due to the principles of triangulation, the measurement distance and resolution between the sensor and target are in a trade-off relationship, making measurement impossible. As a result, the application of laser Doppler methods for vibration measurement has become mainstream, yielding good results. Below, I will discuss the advantages and disadvantages of measurements using the laser Doppler method.

strengths

Since the resolution does not depend on the measurement distance, it is easy to quantify with high resolution, and because measurements are taken near bonding points, the rate of change in measurement data is large, making differences more likely to appear. In particular, if there are issues with oscillators or metal horns, having initial data allows for relatively easy identification of problems.

shortcomings

Due to the differences in the types of ICs being manufactured and the bonding methods and wires used, it is not possible to establish standards that would become industry norms, leading to a need for individual responses. As a result, creating standards takes time and lacks immediate effectiveness. Furthermore, constant monitoring is impossible, and some familiarity with measurements is required.

Although there are issues with measurements using the Laser Doppler Vibrometer, it cannot be considered 'universal' as a method for testing bondability. However, when other measurement methods are employed, they generally lead to situations where the cause is unknown or there is no effect, making it the 'best available measurement method' currently applicable.

For example, tools need to be replaced as consumables, but typically, the replacement of tools is done manually. Depending on the mounting position of the tool on the horn and the tightening torque of the fixing screws, the bonding state may change, so quantifying the vibration amplitude of the tool tip is important during maintenance evaluations.

Furthermore, if there is looseness in the fixing mechanism on the transport mechanism, it can cause the lead frame to move during bonding, leading to defects. A measurement system using a Laser Doppler Vibrometer as a sensor is optimal for these measurements.