Gregory B. Hotchkiss

Basic properties of the spheral solar cell

Spheral solar technology consists of silicon spheres embedded in aluminum foil. Sphere processing consists of obtaining metallurgical-grade silicon feedstock in the form of irregular shaped particles, melting them into spheres, upgrading them in purity, and diffusion. Cell processing consists of bonding the spheres to a perforated foil, exposing the core, and applying an insulator and a backfoil to complete the cell. Flexible 10 cm/sup 2/ solar cells of 10 cm/sup 2/ are repeatably produced with average and peak efficiencies of 10% and 11%, respectively.<<ETX>>

Recent progress in the design and fabrication of Spheral Solar modules

Texas Instruments has begun design and fabrication of photovoltaic modules using its unique Spheral Solar technology. The solar cell structure, especially the aluminum foil conductors, presents challenges in several areas of product design. This paper presents the design of the cell interconnection and junction box, both of which are affected by the aluminum cell structure and are also key to product reliability. Environmental testing of the cells and modules is discussed, and electrical performance data are presented.<<ETX>>

Tacky Dots/sup TM/ transfer of solder spheres for flip chip and electronic package applications

The use of preformed solder spheres for bumping flip chip wafers has not gained wide acceptance within the semiconductor industry. Due in part to equipment shortcomings, solder sphere transfer until now was commonly limited to spheres 300 /spl mu/m or larger, much too large for the typical flip chip applications of 150 /spl mu/m or less. To address this need, Texas Instruments and DuPontB have jointly developed a process for transferring 127 /spl mu/m diameter solder spheres to wafers. The process, called Tacky Dots/sup TM/, forms are array of sticky or tacky dots in a photoimageable adhesive coating. Solder spheres sprinkled on the adhesive coating are then captured and retained by the tacky dots until the spheres are aligned and reflowed to the wafer. This paper describes the equipment and processes developed for bumping wafers using Tacky Dots/sup TM/. The compliant polyimide sheet used in Tacky Dots/sup TM/ required a new and unique equipment design that aligns the solder spheres to the wafer and then reflows the solder without moving the wafer. Post reflow analysis of the bumped dies before and after environmental testing is reviewed. Tests conducted with a leadless chip carrier package design are also reviewed to demonstrate the capability of Tacky Dots/sup TM/ at transferring spheres to electronic packages and substrates other than wafers.

Aluminum decal for transferring solder spheres during electronic package assembly

Ball grid array (BGA) and flip chip package designs use solder spheres or bumps for electrically connecting the integrated circuit to another electronic component such as a printed circuit board. This paper introduces a novel method for transferring solder spheres to BGA substrates or flip chip wafers that uses a decal made from aluminum foil. This decal design differs from most other decals in that preformed solder spheres, and not plated or evaporated solder bumps, are utilized. Aluminum decals offer a solder source that can be applied at all stages of assembly including initial sphere attach. For BGA applications, aluminum decals should reduce equipment costs and save valuable floor space by decreasing the complexity and physical size of the sphere loading equipment. For flip chip applications, expensive plating equipment and sometimes difficult to control chemical processes are avoided. For a BGA rework application, the aluminum decal can be viewed as the solder sphere equivalent to the solder grid preforms commonly used for reworking BGA packages. The paper focuses on the design, fabrication, and experimental testing of an aluminum decal for a 256 I/O BGA organic substrate. Benefits and disadvantages of such an approach will be discussed. Reflow of the solder spheres to the BGA substrate and the resultant shear data are reviewed.