Capillary Profiles Affecting Stitch Bond

Tip Diameter (T)

The selection of the tip diameter is very much based on the bond pad pitch of the device, which determine the maximum usable tip diameter without interfering with the adjacent wire during bonding. For non-fine pitch or standard application, the tip diameter can varies from 140µm to 710µm depending on the hole diameter. For fine pitch or ultra fine pitch applications, the tip diameter can be smaller than 80µm. As a guide, always select the largest tip diameter for larger stitch bond area and higher wire pull strength.

T(max) = 2BPP-WD-2(CLH-MBH)tan(BNA/2)

Face Angle (FA)

provides a certain level of thickness of the stitch bond with a proper combination of OR transition. Depending on the type of bonding applications, face angle of 4°, 8°, 11°, and 15° are commonly used. Generally, 8° FA is used for non-fine pitch and 11° FA for fine to ultra-fine pitch application.

Outer Radius (OR)

provides a proper heel curvature of the stitch bond to minimize heel crack. Different OR can affect the stitch bond strength and the heel shape. OR must complement face angle design for given tip diameter so as to provide adequate thickness and smooth transition of the stitch. Common OR varies from 8 to 75µm depending on the tip diameter. As a guide, the OR is reduced proportionally with the tip diameter.

Matte Finish – provides better stitch adhesion between Au wire and the substrate metallization due to its gripping effect during the ultrasonic wire bonding process. The matte finishing only appears in the capillary tip face (including the face angle and outer radius) while maintaining polished hole and chamfer area to ensure smooth exit of the wire during looping.

Polished Finish – has the advantages of lesser contamination load-up and consequently, extending its useful life span. SPT’s ultra-precision polishing techniques ensure uniformly smooth surface finish that is desirable for ultra-fine pitch capillary designed for 60um BPP and below.