George A. Riley
A clever modification of the gold stud bump, developed at various sites throughout the US and Europe, reshapes the short, fat gold stud bump into a tall, slender, Micro-Post. The result preserves or enhances all of the usual advantages of gold stud bumps, while adding several new ones.
Like stud bumps, the patented Micro-Posts are formed in a conventional thermosonic ball bonder, with standard stud bump gold wire. The bumps require no special Under- Bump Metal (UBM), and can be placed directly on the wire-bondable pads of singulated die or on wafers. However, instead of squashing the stud bump down in the conventional manner, the bonder reshapes it into a post, with a height as much as eight times greater than its mean diameter. Figure 1 show a conceptual diagram of forming a Micro-Post in the capillary, resulting in a large attachment area, a compliant shaft, and a tapered end.
Fig. 1. Micro-post formation, conceptual diagram.
The greater height-to-diameter ratio of the Micro-Post gives a more mechanically compliant connection, able to compensate for larger differences in thermal expansion coefficients (CTE), and to operate over a wider temperature range without underfill. The slender post is easily compressed during assembly by as much as 30% of its height, to co-planarize or to readily adapt to surface features or irregularities on the substrate, without requiring excessive force. The smaller post diameter opens the tolerance window for bump alignment onto the substrate pad, allowing faster assembly with less costly equipment.
Micro-Post die may be assembled by a wide variety of methods. Thermosonic or room- temperature ultrasonic bonding are possible. Thermocompressive assembly has been made to delicate GaAs devices, and to InGaAs photo diodes. Furnace assembly onto tin-plated ceramic substrate pads makes gold-tin eutectic contacts. Gold intermetallic contacts result from furnace assembly to gold on nickel ceramic substrate pads. Conductive or non-conductive adhesive assemblies are also suitable.
Micro-Posts are also now being used as Micro-vias, in low cost end connectors for flexibles, to form local areas of ultra-fine interconnect and in Chip-In-Board assemblies. Figure 2 shows a portion of a Micro-Post array.
Figure 2. Micro-Posts; Diameter 65 microns, height 200 microns.
For more information
For information about Micro-Post sampling or licensing, contact:
Binders Business Park
Cryers Hill, High Wycombe
Bucks. HP15 6LJ England
TEL: +44 1494 714010
FAX: +44 1494 712400