Feature: Interconnections Figure 2: Thermosonic bonding


rendering an excellent joining result at any height. The critical parameters that qualitatively influence the process result – such as force, temperature, ultrasonic welding power and process time – can be programmed individually. In general, higher setting of the parameters ensures a larger collapsed bump area and a higher strength of the joint. However, increasing the parameters also carries risks. For example, an electrical short-circuit can occur between adjacent bumps, or the chip can break.


The importance of collets The inverted pyramid (IP) collet is also of central importance. This collet is suitable for a wide range of applications. The use of an additional channel type collet (CH) is particularly suitable when access to the tool is restricted or it aligns with two sides of the chip. In this case, the chip is held by two side walls of the collet, with the wall length being shorter than that of the chip. Looking at the force distribution generated by the collet in


the chip, e.g., at the moment of chip placement, the operator can distinguish between an IP angle of 90° and a CH angle of 110°. The longitudinal and vertical forces have the same amplitude at an angle of 90°, but the force distribution in the mass is completely different. Thus, the longitudinal forces act on a thin superficial layer of the material, whereas the vertical forces act on the underside of the chip, with a theoretical force gradient in the direction of the symmetry axes. At an angle above 90°, the amplitude of the vertical force


component predominates over the longitudinal component. To reduce chip breakage, we recommend an angle of 110° or 120°,


36 May 2023 www.electronicsworld.co.uk


which should be applied in case the chip to be bonded exceeds 2mm length. If breakage occurs, an angle of over 90° will push the chip along the sides of the die instead of on its surface. Factors such as the cyclic movement of the collet during


ultrasonic bonding, the sensitive surface of the chip and heat transfer can be used to demonstrate the suitability of the collet for TSB. As soon as the ultrasonic bonding current is applied during the TSB process, the transducer vibrates. This causes the collet to move by micrometres in the x and y directions in a very short time, with the movement clocked in the kHz range. As the collet holds the chip at its edges, the chip follows the


vibration of the collet, which vibrates tens of thousands of times a second. At the same time, a certain bonding pressure is exerted by the bonding head, whereby the contact area between chip and collet is very small. As a result, very little heat is transferred onto the die collet. This is an extremely important aspect, since a lot of heat dissipating from the bonding temperature can accumulate between chip and substrate. The process makes it possible to dissipate the heat energy present in the pad bumps onto the substrate.


Ultrasonic bonding The next generation thermosonic bonding we offer allows ultrasonic power and collets to be used, which creates an excellent bond between chip and substrate. Working with low bonding temperatures and lower


pressure, thermosonic bonding has attracted great interest among customers. And, Tresky’s pick-and-place system offers excellent coplanarity and parallelism along the z-axis, ensuring precisely-bonded chips.


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