Showcase of wire bonding research

Background

Microelectronic wire bonding is a process used to interconnect integrated circuits (ICs) on microchips to substrates or boards. Wire bonds provide electrical signals and power for the IC. The most common process to attach the wires to the metallized surfaces is using ultrasound as a bonding agent. Minimizing cost and size while maximizing quality are the goals. By improving microjoining of the internal wires in devices, the size of devices can go down and costs can be reduced.

Old technology evolving into new technology

Research areas - New materials

Cu bonding wires

The development of new bonding materials is leading to smaller, more economical, and more reliable solutions. Currently, bare gold wires are mainly used for microelectronic wire bonding. Bare gold is very clean, deforms under low forces, is reliable in many situations, but is also expensive. In contrast, bare Cu wire is inexpensive, but harder, and forms an oxide.

Insulated bonding wires

Insulated bonding wires have a thin layer of insulation on them which allows the wire connections and loops to be much closer than usual to the point where they actually touch each other. So, the IC interconnections (bond wires) can be designed extremely dense if insulated wires are used, leading to a miniaturization of the product.

Ni based metallizations

Ni based metallization provides improved mechanical protection of the microchip from the vertical and ultrasonic forces during the bonding process.


Research with all these new materials is underway, addressing issues and roadblocks that currently slow down the introduction of these materials together with their expected benefits.

Research areas - Low-stress bonding

The focus is on enabling low-stress ball bonding using Cu and insulated wires. The goal is to find ways to reduce the mechanical forces and stresses required to form acceptable bonds. Some objectives of the research are: (i) to investigate the effect of various input parameters on the physical mechanisms underlying the process process, and (ii) to analyze the failure criteria for underpad damage and correlate it with the process mechanism.

Box plots of maxima of ultrasonic forces

Comparison of maximum ultrasonic force measured by the microsensor for five different processes [Shah et al, Proc. of 58th ECTC, 2008 (PDF)]

Mechanisms of bonding and failure

The focus is on understanding the bonding and failure mechanisms of different wire bond processes such as Au/Al process, Cu/Al process, Al/Al process. Specific topics of interest include: (i) long-term reliability tests (ii) in situ process monitoring using microsensors and (iii) theoretical modeling and finite element simulation.

Copper ball bond after aging at 250°C for four hours

Cu ball bond after aging at 250C for 4 hours [Hang et al, Microelectronics Reliability, 2008 (PDF)]

Gold ball bond after aging at 200°C for four hours

Au ball bond after aging at 200C for 4 hours [Hang et al, Microelectronics Reliability, 2008 (PDF)]

Facilities

Wire bonding

Mechanical testing

  • DAGE 4000 Tester for wire bond shear and pull testing

Gold wire pull-test

Non-destructive gold wire pull-test

Footprint of a sheared Copper ball bond

Footprint of a sheared copper ball bond

  • Instron 5548 Micro Tester for tensile testing of bonding wire
  • Hardness testing using microhardness tester and nano-indentor

A cross-section of a copper ball bond with Vicker's microhardness indentation mark

Cross-section of a Cu ball bond with Vickers microhardness indentation mark [Pequegnat et al, J. of Mat.: Mat. in Elec., 2009 (PDF)]

A cross-section of a gold free air ball with Vicker's nanoharness indentation marks

Cross-section of a Au free air ball with Vickers nanohardness indentation marks

Modeling and simulation

  • State of the art computing facilities with access to commercial simulation software such as COMSOL, ABAQUS, ANSYS

FE plane stress-strain model of ball bonding using COMSOL 3.3

FE plane stress-strain model of ball bonding using COMSOL 3.3 [Shah et al, Sensors and Actuators A148(2), 2008 (PDF)]

Microsensors

  • Low-stress bonding and chip damage studies
  • Long-term reliability studies
  • Bonding and failure mechanism studies

Microsensor test chip

Microsensor test chip
[Shah et al, Microelectronic Eng., 2008 (PDF)]

Real-time in situ force signal

Real-time in situ force signal [Shah et al, Sensors and Actuators A148(2), 2008 (PDF)]

Sample preparation

  • Mechanical polishing
  • Ion milling: low angle ion milling and polishing system Model 1010, Fischione Instruments
  • Chemical etching: for underpad silicon damage analysis

Microscopy

  • Optical microscopes
  • Scanning electron microscopes (SEM)
  • Environmental scanning electron microscope