Wire Bonding

Wire Bonding

Cleanroom facility

Wire Bonding

There are two main wire bonding processes that are used for interconnecting bare-die IC Chip to PCB, Rigid-Flex, Flex or Ceramic substrates.

  • Gold Ball Wire Bonding (Thermosonic)
  • Aluminum Wedge Wire Bonding (Ultrasonic)

Gold Ball Wire Bonding Process (Thermosonic – 150°C)

In this wire bonding technique, the wire is fed through a hollow capillary and a Negative Electronic Flame-Off (EFO) system is used to melt a small portion of the molten gold wire forming a ball as the wire material solidifies. The ball is pressed onto the first bonding pad on the die (IC Chip) with sufficient force to cause deformation and interdiffusion of the wire and the underlying metallization, which ensures the contact between the two metal surfaces and form the first bond (ball bond) on the IC.  The capillary is then raised and moved to the second bond site creating a wire loop, deforming the wire against the bonding pad and thus creating a second wedge bond on the substrate.

  • Omnidirectional wire bonding process from 1st bond to 2nd bond
  • Substrate Temperature: 150°C during wire bonding
  • ENEPIG: Nickel 3-6um (118-236 uinches), Palladium 0.05-0.3um (2-12 uinches), Gold >0.1um (>4 uinches)
  • Small Wire Bonding Area: X-Axis=56mm, Y-Axis=80mm (2.2”x3.15”)
  • Gold wire diameter: 18um to 50um (0.7mil to 2.0mil)
  • Fine-pitch 45um inline, 30um staggered
  • Ball size is approximately 2 to 3 times wire diameter
  • Bond size should not exceed 75% of the IC pad size
  • Loop height: 100um (4mil). Typical 200um (8 mil)
  • Loop length: 300um to 5mm (12 mil–200 mil); max. 100X wire diameter 

Aluminum Wedge Wire Bonding Process (Ultrasonic – 25°C)

Wedge bonding was name based on the shape of its bonding tool.  In this wire bonding technique the wire is fed usually at a 45 degree angle from the horizontal bonding surface through a hole in the back of a bonding wedge tool. Normally, forward bonding is preferred (1st bond on the die (IC Chip) and the 2nd bond on the substrate or package).  The wedge tool descends on the IC bond pad, the wire is pinned against the pad surface and an Ultrasonic (U/S) first bond is performed. Next, the wedge tool rises and executes a motion to create a desired loop shape in the direction of the second bond, and then the wedge tool descends making the second bond on the substrate or package. Then the wire clamp is closed to break the wire while machine bonding force is maintained on the second bond (clamp tear).

Wedge Wire bonding process can be used with Aluminum wire, bonding at 25°C (room temperature) and with Gold wire, bonding at 150°C substrate temperature.

  • Unidirectional wire bonding process from 1st bond to 2nd bond
  • Substrate Temperature: 25°C (Room Temperature) during wire bonding
  • ENIG: Nickel 3-6um (118-236 uinches), Gold >0.05um (>2 uinches)
  • Large Wire Bonding Area: X=250mm, Y-Axis=200mm (10.0”x8.0”)
  • Aluminum wire diameter: 18um to 50um (0.7mil to 2.0mil)
  • Fine-pitch 45um inline, 30um staggered
  • Wedge bond (foot) is approximately 1.2 to 2.5 times wire diameter in width, and 1.5 to 3.0 times wire diameter in length
  • Bond size should not exceed 75% of the IC pad size
  • Loop height: 100um (4mil). Typical 200um (8 mil)
  • Loop length: 300um to 7.5mm (12 mil–300 mil); max. 150X wire diameter

Plasma Cleaning: Wire Bond surface preparation

To ensure bondability and reliability of the wire bonds, one of the critical conditions is that the bonding surface must be free of any organic contaminants. Plasma cleaning employs a radio frequency (RF) source to convert inert gas used into plasma. The high velocity of gas ions bombard the bonding surface and sputter off contaminants from the bonding surface by physically breaking apart the contamination molecules. In most cases, the gas ionized is Oxygen, Argon, and Nitrogen.

Metallurgical Systems (Au or Al Wire Bonding to Die Bond Pads)

  • Au Wire to Al Die Bond Pads. (Gold Ball or Gold Wedge)
  • Au Wire to Au Die Bond Pads. (Gold Ball or Gold Wedge)
  • Al Wire to Al Die Bond Pads. (Aluminum Wedge, Room Temperature)
  • Al Wire to Au Die Bond Pads. (Not recommended)