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WIRE BONDING
When wire bonding at Acreo, a thermo-sonic gold wire bonding equipment is employed. The equipment has a heated work stage with a manual clamp holding the samples. Gold wires with a diameter ranging from 17.5 µm to 50 µm can be used. To insure intimate contact between pad and wire the chip/substrate is heated during bonding. Heating enhances the diffusion of material between the pad and wire.
Process
The following 4 steps gives a short explanation of how a wire bond is made..
- A “free air ball” is made with the EFO (Electrical Flame Off), which means that the wire is melted with a current discharge. The bond head moves to a pre-set location and height on a chip pad or substrate pad.
- The ball is placed on the pad, pressure is applied as well as ultra sonic (US) power under a pre-set time. The values of the different parameters to be used differ between different pad material and chip/substrate material.
- The first bond is made and the bond head moves to the place for the second bond, the stitch bond even called wedge bond. The same procedure as for the first bond is done. Pressure, US under a certain time.
- After the second bond is made the bond head moves in an upwards vertical direction stopping after a certain length of the wire has been fed out. Clamping the wire and making an upwards vertical motion of the bond head breaks the wire. The remaining wire is called the tail. The EFO uses the tail and makes a new “free air ball”.
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| 4 steps explaining how a wire bond is made (from http://education.netpack-europe.org/). |
FLIP CHIP ATTACHMENT
A flip chip interconnection is made by flipping a chip upside-down and attaching it to a substrate. The interconnection method is normally soldering, gluing (conductive polymer) or thermo-compression.
When soldering, the chips are pre-bumped with electroplated solder balls. The chip is flipped and the solder re-flowed in an oven.
Gluing can be made in two ways, either with a stud bump attached to the chip or with a screen printed conductive polymer bump. The chip is flipped and placed on the substrate, the glue is then cured at a certain temperature for a time period.
Thermo-compression interconnect demands a gold wire stud bumped chip and it is interconnected by heat and pressure. The heat is above 300˚C and the pressure varies with the amount of bumps on the chip.
Stud bumping
The procedure to make a stud bump is almost the same as when making a wire bond. A stud bump can be characterized as a smashed ball. A stud bump can be used when working with the interconnect method called flip chip.
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| Two stud bumps which can be used for flip chip interconnections. |
Process
The process is steps number 1-2 and 6, when making a wire bond. In short, a “free air ball” is made by the EFO and put on a pad located on a chip or a substrate. The bond head moves in an upwards vertical direction stopping after a certain length of the wire has been fed out. Clamping the wire and making an upwards vertical motion of the bond head breaks the wire. Alternatively, we can even perform electroplated studs.