Controlling the Bond Process
BY FARHAD FARASSAT
Developments in wire bonding technology have been aimed toward requirements for increased speed and finer pitch, as well as controlling the bond process to ensure consistent quality through monitoring and documenting reliability.
Bond Process Control
Ultrasonic bonding of aluminum wire is a friction weld process in which two metals are pressed together at room temperature and rubbed together ultrasonically at the same time. Essentially, there are two steps to the process.
Step 1: Touchdown and Pre-deformation. A bonding wedge is used to flatten a bond wire onto the bond surface area. Depending on programmed parameters and dynamics of the bonder, the mere act of bringing the wire into contact with the bond surface results in a squashed or pre-deformed wire. Pre-deformation plays an important role in determining the quality of the subsequent welding process. The lattice structure of the bond wire and the bond surface are changed significantly if pre-deformation is too high, and the quality of subsequent bonds suffer accordingly.
Step 2: Ultrasonic Stage and Welding. By applying an ultrasonic frequency to the transducer, the wedge that is connected to the transducer vibrates along the wire. The amplitude of the vibrations, 1 to 5 µm, is small compared to the 25- to 50-µm diameter of the fine wires typically used. On standard machines, a frequency of 60 to 100 kHz is used. Initially, the wedge and wire move together and create friction at a constant pressure on the interface between the wire and bonding surface. After a short time, the wedge begins to deform, heat up and welding takes place. Both effects are crucial for the quality of the weld. Precise dynamic analysis of bond surface temperature and wire deformation shows that the procedure can be split into cleaning, mixing and diffusion phases (Figure 1).
Figure 1. Dynamic analysis of the bond surface temperature and wire deformation shows that the welding procedure can be split into three phases.
In the cleaning phase, which usually lasts 4 to 10 ms, hardly any deformation occurs and the temperature of the bond surface rises slowly. The ultrasonic energy applied is used mainly for surface cleaning (the removal of surface oxides and contamination layers) and heating. Throughout this phase, the wedge rubs the wire across the bond surface.
During the second mixing phase, the temperature rises sharply and the wire deforms accordingly. Ultrasonic power is used to level out the metal surfaces to cause a distinct rise in the bond surface temperature. The metals are brought together until they are merely one atomic lattice apart over as much as the interface area as possible. The elevated temperature enhances diffusion of atomic lattice dislocations and relaxes the weld area. A partial weld ensues and the wire sticks to the bond pad. The wedge now rubs onto the essentially immobile wire, thereby generating a further increase in temperature.
The third phase involves diffusion. No significant deformation or increase in temperature occurs in this phase. The heat generated by the friction of the bond wedge on the wire's surface causes the bond area temperature to rise, which increases the relaxation of the weld area. This tempering process stabilizes the bond by curing the diffusion-rich interface area and prevents it from becoming brittle.
Every bond goes through all of these phases, although the length of each phase varies for reasons such as nonhomogeneous wire composition or surface properties and variable contamination levels. In an ideal situation, as soon as the deformation curve levels off, the energy being supplied is reduced and removed shortly afterward. On most machines, however, high maximum energy level is programmed to ensure that all bonds are made. This method carries with it the risk of a large number of bonds receiving too much energy and, thus, results in significant quality loss (Figure 2).
Figure 2. Two typical deformation curves as they would occur on a machine without bond process control.
On conventional bonders, the same ultrasonic energy is applied for all three phases. But in reality, each phase requires application of a different level of energy. The surface cleaning phase requires more power than the others. The diffusion phase requires only enough power to heat the bond surface.
Bonding at a constant power and time is common practice, but not the best method. If it were possible to monitor the three bond process phases automatically during bonding, and to regulate power accordingly, bond quality could be improved significantly.
Figure 3. Bonder concept.
One company* has developed a technology** for control in aluminum fine wire bonders. The key component of the method is a high-resolution proximity sensor mounted on the transducer bearing, which measures the downward movement of the wedge after successful touchdown — registering ensuing deformation with accuracy (Figure 3). The sensor signal is digitized by an A/D converter and then processed by the controller. This, in turn, regulates the power applied, depending on predefined reference values. At the same time, the measured data and the bond curves can be displayed on a standard PC and stored away for processing.
The new technology generates a high yield of acceptable bonds. Bond development can be followed on a monitor but, unless the bonder has been programmed to stop and request operator intervention in the event of a failure, the only way to check for bad bonds is to actually look for them on the monitor. It is not always necessary to interrupt the bonding sequence, however, because the operator cannot repair the faulty bond immediately and can only mark it to check or repair later. For such applications, the bond process-monitoring mode displays bond data for storage and off-line processing.
One of the central issues in wire bonding technology is the difficulty in monitoring and influencing bond quality during the bond process. Control is usually restricted to optical inspection and destructive pull and shear tests on a sampling basis. In today's age of zero defect manufacturing, this blindfold-type of production is insufficient.
**Bond Process Control Unit.
FARHAD FARASSAT, owner and president, may be contacted at F&K Delvotec GmbH, Daimlerstraße 5-7, 85521 Ottobrunn, Germany; 49 (89) 62 9950; e-mail: email@example.com.