Arthur S. Francomacaro

Wirebonding at higher ultrasonic frequencies: reliability and process implications

Abstract Higher-frequency ultrasonics have been utilized to improve the bondability of difficult substrates, i.e., substrates that would not bond or that bonded poorly using conventional ultrasonics (nominally at 60 kHz). A systematic study of the influence of higher-frequency ultrasonics on bond strength and the bondability of various substrates is reported. The studies were carried out using two essentially identical thermosonic ball bonding machines, one bonding at nominally 60 kHz and the other at 100 kHz. The only differences between the bonding machines were the ultrasonic generators’ operating frequency and the transducer horns. Key to the study was the ability to make the bonding experiments as controlled, repeatable, and independent of all variables (except frequency) as possible. Control techniques included setting the electronic flame-off to produce consistently sized free-air balls; monitoring the ultrasonic voltage and current waveforms; and picking force, dwell, energy, and substrate heat settings that would allow strong bonds to be formed at both frequencies. Wirebonds (ball bonds) in this study were evaluated primarily by the ball bond shear test. Statistical methods were used to determine whether the differences in the means and variances between comparable samples sets (one bonded at 60 kHz and the other bonded at 100 kHz) were significant. Results of our studies indicate that significant differences exist between bonding at nominally 60 kHz and bonding at 100 kHz. In particular, we describe effects associated with (1) the ball shear strength before and after thermal aging (temperatures up to 200 °C) for both 60- and 100-kHz bonds, (2) the influence of substrate-metallizations combinations on the geometry and strength of the bonds at the different frequencies, and (3) the sensitivity and control of the overall bonding processes.

High-frequency wirebonding: process and reliability implications

Most of the current wirebonding machines operate at a nominal frequency of 60 kHz. The choice of 60 kHz was made several decades ago based on appropriate transducer dimensions for the product sizes of the era and stability during the bonding load. A wide range of frequencies (25 to over 300 kHz) has been used to ultrasonically attach wires. Todays interest in higher-frequency wirebonding stems from reports that using higher ultrasonic frequencies produces better welding at lower temperatures in shorter bonding times (dwell times). It has also been indicated that higher-frequency wirebonding improves bonding to pads placed over soft polymers such as Teflon/spl reg/ and unreinforced polyimide. Despite these reports, few, if any, systematic side-by-side studies using controlled conditions have been performed. The current work continues our systematic efforts to evaluate the effects of using higher bonding frequency on bond quality and reliability. Using two identical bonding machines (except for ultrasonic frequency), we have investigated the bonding process on a variety of metallizations and substrates. In this study, statistical methods were used to determine whether the differences in the means and variances between comparable samples sets (one bonded at 60 kHz and the other bonded at 100 kHz) were significant. Results of our studies indicate that significant differences exist between bonding at nominally 60 kHz and bonding at 100 kHz. In particular, we describe effects associated with: (1) the ball shear strength before and after thermal aging (150/spl deg/C for 120 hours) for both 60 kHz and 100 kHz bonds, (2) the influence of substrate-metallizations combinations on the geometry and strength of the bonds at the different frequencies, and (3) the sensitivity and control of the overall bonding processes.

The development of poled polyimide dielectric layers for simultaneous testing and light guiding applications in MCM-Ds

Abstract Noninvasive techniques for measuring electric field strengths in multichip module (MCM) substrates can be extremely important in determining ultimate module performance. Certain polymers such as polyimide exhibit an electro-optic response, after appropriate doping and poling, that permits direct measurement of the internal fields with a laser probe. We have built MCM circuit structures using electro-optic polyimides for the dielectric layer. We report thermal, electrical, optical, and electro-optic properties for these dielectric layers. The optical properties of doped and poled polyimides make them attractive for building optical waveguides. We report results for poled polyimide use in both multiple layer structures and optical waveguide formation, and recommend processing guidelines. Further development of doped and poled polyimides may permit optical and electrical interlayers on the same thin-film MCM-D structure.

Poled polymers for MCMs with integrated dielectric and optical layers

Low dielectric constants and ease of processing make polymers attractive materials for building both optical waveguides and dielectric layers. The change in the index of refraction of certain polymers such as polyimide, after appropriate doping and poling, makes it possible to build an optical waveguide using established polymer processing techniques. Our work uses these results to demonstrate optical waveguide fabrication for applications requiring optical and electrical interlayers on the same thin-film multichip module structure (MCM-D).

MCM structures with poled dielectrics to improve testability

To improve multichip module (MCM) testability, we have used a new, recently-demonstrated technique to detect on-substrate electric field strength. This technique employs a noninvasive, laser-based instrument to probe MCM structures fabricated with poled polyimide interlayer dielectrics and thin film metallizations on silicon carriers. Circuit elements characteristic of MCMs were probed to detect electric field strength. The electrical, mechanical, and optical properties of these electro-optic dielectric layers were determined to investigate the effect of the poling and processing operations on the efficacy of the polyimide as both a dielectric layer and an electro-optic material suitable for laser probing.