Controlled Solder Self-alignment Sequence for an Optoelectroni

A new flip-chip solder technology has been developed for the precision self-alignment of an optoelectronic module without using mechanical stops. A main feature of the technology is the controlled self-alignment sequence: laser is aligned to a polymer waveguide in the lateral and vertical directions, then it slowly makes a contact with the waveguide in the axial direction. The elimination of the use of mechanical stops significantly simplifies the process. The axial distance between the laser and the waveguide is very small; it is determined by the contacting edges and a tilt angle of 12 . The lateral alignment accuracy is less than 2 μm. Introduction Solder self-alignment is important in the area of optoelectronics. In addition to providing electrical connections, solder is also one of the enabling technologies for passive precision alignments. Between 1990 and 1995, there were at least 60 papers and 8 U.S. patents reporting the different solder technologies developed for optoelectronic packaging. These studies were reviewed by Tan and Lee [1]. The technologies can be grouped into four categories: solder assembly with no precision selfalignments, and self-aligned solder assembly with no, one or two mechanical stops. Among these technologies, one approach related to this work is shown in Figure 1. It is a schematic illustrating an alignment concept in which mechanical stops are used to control both lateral and vertical movements of the chip [2]. The surface tension force of the misaligned molten solder joints push a mechanical stop on the laser against that on the substrate for the lateral alignment and pull the laser against the stops on the substrate for the height control. In principle, the lateral alignment and the height control could be accomplished automatically through a single solder reflow process. SUBSTRATE CHIP SPACERS