Technical Report: Ultrasonic Vibration Measurement of Wire Laser Doppler Vibrometer 1

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.

The wire bonding cycle, using a ball bonder as an example, is as follows:

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.

Heater heating temperature (ball bonder only)
To facilitate wire fusion, heat the pads and leads to around 200°C.
load
This is the pressure applied when pressing the capillary against the pad lead. This is done by applying a load of approximately 10 to 100 g from the ultrasonic horn side.
Ultrasonic oscillation
It assists in the plastic deformation of the wire and promotes fusion with the pad surface and lead surface.
Bonding time
This refers to the time for applying load and ultrasound, and can be varied from tens of milliseconds to hundreds of milliseconds. Among these bonding parameters, with the diversification of semiconductor materials, there is a demand for lower heating temperatures (①) and a demand for shorter bonding times (④) to improve manufacturing speed (cycle time). As a result, there are cases where the functions previously performed by parameters ① and ④ must be compensated for by other parameters.

In such cases, the following measures are taken:

Increased US power or higher oscillation frequency
Increased load

However, since the quality of bonding (bondability) depends on a delicate balance of each bonding parameter, careless parameter changes can have a significant impact on product yield. Specifically, this can lead to an increase in wire breakage accidents due to wire delamination or neck breakage, or damage such as contact pad failure on the tip side due to excessive load.

advantage: It can be handled quickly and at a relatively low cost.

Disadvantages: Because there is no theoretical or numerical backing, it is not guaranteed that the system can directly handle new IC varieties. Furthermore, since the cause of problems when they occur is unclear, countermeasures are limited, and once the yield deteriorates, it becomes an endless cycle.

advantage: It can be easily tested to check the tensile strength and joint strength of the wire itself.

Disadvantages: This is merely a confirmation of the results and does not determine the cause. Furthermore, it involves destructive testing.

advantage: Effective for checking for wire breaks and delamination.

Disadvantages: While effective for confirming results, it is extremely limited in its ability to determine the cause. In particular, it cannot address problems caused by high frequencies, such as US oscillations.

advantage: By installing it on each bonder, continuous inline monitoring is possible.

Disadvantages: The rate of change is small, and unless there is a major accident with a clear cause, the difference will not be noticeable, and the effect of implementation will be minimal.

As mentioned earlier, the bonding process takes place between a wire and a connecting crimper called a tool or capillary, which is attached to the tip of a metal horn by screws, and the pad or lead frame. Therefore, it is easy to imagine that directly measuring the tool tip, pad, or lead frame, which are the points of application, would be effective as measurement points for verifying bondability. However, measuring the tool, pad, or lead frame has traditionally been extremely difficult, and to put it simply, there were no applicable sensors. For example, the tool used in ball bonders is a hollow needle called a capillary, with a tapered tip, a fine diameter of approximately 1.5 mm, and a length of approximately 11 mm, and is generally made of hard ceramic or ruby. Therefore, due to size constraints, it is impossible to attach contact-type vibration sensors, etc. Also, among non-contact sensors, triangulation-type laser displacement meters that can measure frequencies above 60 kHz are rare, and due to the principle of triangulation, there is a trade-off between the measurement distance between the sensor and the target and the resolution, making measurement impossible. Therefore, the laser Doppler method is currently the mainstream approach for vibration measurement, and good results are being obtained. Below, we will discuss the advantages and disadvantages of laser Doppler measurement.

Strong Points

Because the resolution does not depend on the measurement distance, it is easy to quantify with high resolution, and since the measurement is performed near the bonding point, the rate of change in the measurement data is large, making differences easily apparent. In particular, when there is a problem with the oscillator or metal horn, it can be identified relatively easily if initial data is available.

Cons

Because the characteristics vary depending on the type of IC manufactured and the bonder and wire used, it is not possible to create an industry standard, and each case must be handled individually. Therefore, it takes time to create standards, and the immediate effects are limited. Furthermore, continuous monitoring is impossible, and some familiarity with the measurement process is required.

While Laser Doppler Vibrometer measurements also have their drawbacks, and therefore cannot be considered a "universal" method for testing bondability, it is the "best measurement method currently available" in situations where other measurement methods generally yield results of "unknown cause" or "no effect," leaving no room for debate.

For example, tools are consumables that need to be replaced, but tool replacement is usually done manually. Depending on the mounting position of the tool on the horn and the tightening torque of the fixing screws, the bonding condition may change, so quantifying the vibration amplitude of the tool tip is important when evaluating during maintenance.

Furthermore, if there is any looseness in the fixing of the retaining plate or other components on the transport mechanism, the lead frame may move during bonding, causing malfunctions. A measurement system using Laser Doppler Vibrometer as a sensor is ideal for these measurements.