As the semiconductor industry braced itself for the transition from gold to copper, the copper wire conversion is relatively a tougher process to define as compared to gold wire bonding with challenges both on the ball bond and stitch bond. The problem the customer may face varies, depending on the bond pad metallization structure and the substrate or lead frame surface condition.
One of the basic principles to achieve an optimized wire bonding process is through proper capillary design selection. The synergy of different process variables coming from the wire (e.g. gold and copper), substrate or lead frame based metallization, bond pad metallization, and wire bonder are influential to the final geometrical design of the capillary.
The proper selection of the copper wire (bare or coated) and capillary type are critical to resolve wire bonding issues like excessive aluminium splashed out, short tail, or fish tail which are inherent problems related to copper wire bonding. A proper design of experiment (DOE) needs to be conducted, not only involving the base settings but also considering auxiliary parameters such as scrub function and force/power profiling. Some high-end wire bonder platforms may have the feature to utilize segmented parameter profiling for specific bonding location of interest. The capillary selection process for gold and copper starts with the determination of the following information defined by customers’ device and package design configuration:
|Abbreviation used for Capillary & Wire Bonding:|
|Hole Diameter||H||Wire Diameter||WD|
|Chamfer Diameter||CD||Bond Pad Pitch||BPP|
|Chamfer Angle||CA||Bond Pad Opening||BPO|
|Tip Diameter||T||Loop Height||LH|
|Face Angle||FA||Free Air Ball||FAB|
|Outside Radius||OR||Mashed Ball Diameter||MBD|
|Main Taper Angle||MTA||Ball Height||MBH|
|Bottleneck Angle||BNA||Ball Contact Diameter||BCD|
|Bottleneck Height||BNH||Electric Flame Off||EFO|
Bond Pad Pitch (BPP) - is defined as the center distance between two adjacent bond pads. Specifically for ultra- fine pitch application, the BPP dictates the design of tip diameter (T), bottleneck angle (BNA) and chamfer angle (CA).
Bond Pad Opening (BPO) - is defined as the unpassivated area of the bond pad where the actual ball bonds are ultrasonically welded.
Critical Loop Heights (CLH) - is defined as the height of the loop that is in-line with the centerline of the capillary when viewed from the side or parallel to the adjacent wire. Once the wire passed the centerline, the capillary has already cleared and no adjacent loop disturbance is observed.
Cross Section diagram of Capillary Tip Profile