During wire-bonding of semiconductor devices, wherein electrical connections are made between bond pads of dice and/or substrates on which they are attached, it is common to utilize a wire clamp to feed a roll of bonding wire towards a bonding site. The clamp is opened to allow wire to feed through during threading of the wire through a capillary and thereafter closed to control the wire. The wire clamp may also be used to hold the wire in position during the making of a first bond and a second bond on the die and/or substrate. The clamp is further commonly used to enable looping of a length of bonding wire between electrical contact points on the die and/or substrate, and/or to pull wires from bonds after the bonds have been made.
The clamp typically comprises a movable arm or member, and a fixed arm or member. The movable arm is opened and closed by a solenoid or a linear motor, and is usually urged towards the fixed arm by a spring or the motor. The bonding wire is very fine, to the order of 1 mil or less. Thus the wire is easily broken if subjected to excessive force. It is important that a clamping force exerted by the wire clamp is sufficient to grasp the wire, but not too high so as to cause abnormal deformation or to break the wire. It is also important that a gap between the movable and fixed arms is sufficient for the wire to pass through, and yet not be so large as compared to the size of the wire when opened so that the clamping force cannot be easily or reliably controlled.
In view of the above, it is usually necessary to calibrate a wire clamp prior to using it. Prior art devices for calibrating wire clamps have been devised, but such prior art devices involve too much human intervention and have become less effective as the diameters of bonding wires decrease together with decreasing dimensions of semiconductor packages.
For example, in order to measure clamping force, a gram gauge (see FIG. 1) has been used in the prior art. The gram gauge has a deflectable lever, which measures a deflecting force exerted on the lever by a movable wire clamp member. The gram gauge functions in much the same way as a conventional weighing scale. However, a spring which biases the lever during deflection is not sufficiently sensitive where the clamping force is small and only deflects the deflectable lever minimally.
In order to measure a gap between clamping members, a thin gauge sheet of a known thickness may be inserted between the clamping members (see FIG. 2). If the gauge sheet cannot fit into the gap, it means that the distance between the clamp members is smaller than the thickness of the gauge sheet. If the gauge sheet can fit into the gap with space to spare, then the distance is much larger than the thickness of the gauge sheet. The distance between the clamp members should be adjusted so that the gauge sheet just fits into the gap. Thus, this method is based on trial-and-error, and the error margin gets larger as the distance between clamp members (for smaller diameters of wires) gets smaller. In the event, this method does not offer sufficient accuracy for thinner wires for smaller semiconductor packages.