Automating Optoelectronics Soldering

The unprecedented growth in the telecommunications sector in the mid to late 1990’s saw a high demand in the area of optical packages and optical networks. This sudden growth in demand for optical technology in telecom and data communication applications could not be supported by the existing manufacturing infrastructure; so, to meet the growing volume needs of the industry – the electronics manufacturing sector adopted what was still a laboratory method of assembling technique to high volume manufacturing. Since most of the assembly of the optical component and attaching the component to the board involved tedious manual labor, the process often resulted in a high cycle time and low yields. This drove the need for automated or semi-automated assembly processes that would address these issues and result in a lower cost solution to the contract electronic manufacturer. This paper discusses one of the National Electronics Manufacturing Initiative’s (NEMI) projects addressing this issue. The main purpose of this research project is to identify low-cost, high-yield, data-driven processes such as laser selective soldering and infra-red (IR) soldering to attach non-reflowable optoelectronic packages to circuit boards. Packages whose form factors are currently not standardized will be assembled, placed, affixed to board and connected using automated techniques in order to reduce costs significantly in terms of cycle time, rework, and unreworkable scrap. With the current and foreseeable lack of standard packages formats, the use of data-driven selective soldering techniques hold the promise of maintaining flexibility while replacing the manual assembly techniques that incur high costs and high yields. This paper will discuss the need for automation – the factors supporting and challenging the needs for the same are also addressed. One of the actions undertaken to quantify and justify the need for an automated process is the creation of a Cost of Ownership (COO) model based on the industry standard SEMI E35. Results from the cost model are discussed in this paper. INTRODUCTION The current theme in the OE assembly industry is to drive down costs and many organizations are exploring different ways and means to achieve these goals. The recent downturn in the industry has raised concerns regarding the comeback in the OE sector. Although, this phase is temporary, it is a fact that there will be a constant demand for optical technology in the telecom and datacom sectors. The major driving forces behind this trend are higher speed, higher bandwidth and better cost-to-performance solution. Keeping in mind the current needs to reduce costs and the future demands of high volume – it is just common sense to explore different automated/semi-automated manufacturing techniques. Although changing component trends, lack of standards and standard operating procedures may hinder fully automated solutions; the option of semi-automated solutions is a distinct possibility. Soldering Techniques for Optoelectronics Soldering is the typical method of preference to join and connect many components of hermetically sealed OE packages. Most solders tend to require a reducing atmosphere and surface preparation, or a flux to aid adhesion but a flux is not acceptable within optical systems where trace amounts of organic on the optical train can absorb the infra-red (IR) laser radiation. Various adhesives also are used in assembling the optical train; it is essential that these do not outgas, causing contamination. Hermetic sealing is used to exclude moisture, which also can degrade optical components. Although hermetic packages will continue to be used for most long-haul telecommunications, one can expect non-hermetic packaged, SMT-assembled devices and low-cost liquid crystal polymer (LCP) packages finding increased use in OE boards. Hermetic packages typically are supplied in a "butterfly" format, with a typical package having 14 low-frequency leads. These packages are hand placed, often bolted to the board on thermal grease, and the leads hand soldered; yields resulting are thus poor. Any process heating of the package has to consider not only the fiber, but also many of the other components used in construction. Many packages use lowmelting solder or a hierarchy of solders to affix components. Some of the most common issues in level two OE attach are listed below: Manual soldering is high cost and unreliable Solder as a joining technique is preferable to adhesive bonding or laser welding in many cases because of the high electrical conductivity, high thermal conductivity, reliability and reworkability of solder. Because OE components and modules often require manual mechanical attach prior to or after solder attach, manual soldering is usually performed by the same operator, and the cost is incremental. To completely remove the operator step would also require automating the mechanical attach process, which may not be feasible because of the diversity of package & attach styles, card thickness, torque requirements, etc. Although some devices such as SFF transceiver packages can be reflowed, most OE packages are unlikely to withstand 80C (i.e. can not be mass reflowed) because of the fiber jacket temperature limit unless the “pigtail paradigm” changes and fiber pigtails are no longer attached. Packages may also contain lowtemperature solders and precision aligned components whose alignment may be damaged by thermal expansion. Diverse package formats, including through-hole and SMT, may require both bottom-side (or assembly invert) and top-side soldering capability. Alternative joining techniques such as selective laser soldering and IR soldering are adaptable to diverse package formats but their performance is not well characterized. Laser Soldering The key benefit of laser soldering is the ability to apply the right amount of heat, only where needed, and without undue thermal stress on surrounding materials. Once programmed, the laser soldering system can provide repeatable results, run after run. Process control parameter flexibility is an important factor when evaluating an appropriate laser system in a particular application area. With the large variation in OE modules and components, it is difficult to predict future pad and connection geometries. A noncontact laser soldering system that maximizes flexibility will allow precise positioning to handle future needs. To obtain good soldering results without damaging the surrounding devices or materials, it is necessary to be able to manipulate the laser energy being applied. Focused Light Beam (Soft Beam) Soldering Here the light from a xenon-arc lamp is converged on a second focus, and the converged light is passed through an optical fiber, which is connected, to a converging lens, which focuses the light beam to a spot diameter of approximately 1 mm. The fiber optic guided light beam from a xenon lamp is used as a heating source for microsoldering applications. OE ASSEMBLY OPPORTUNITIES Level 1 Packaging Optoelectronic device assembly in packages is complex. Devices may be active (e.g., lasers) or passive (e.g., filters). Thermal transfer is achieved through solder or conductive adhesive die attach for active devices. For passive components, it is achieved through solder, welding or adhesives. Interconnection typically is by wire or ribbon bonding. A typical package may consist of a laser, laser submount, ancillary devices and an optical bench (substrate). The bench may contain other elements such as lenses, modulators, filters, etc., depending on the package type. Devices may be soldered using fluxless solder, such as Au/Sn, or mounted using adhesives. Some devices are soldered and then cleaned using intensive cleaning processes. Given the technical requirements of OE packaging, the focus has been on meeting performance requirements and not manufacturability, leading to a predominance of goldplated Kovar hermetic packages with numerous electrical outputs and a fiber "pigtail." The optical pigtail fiber is stripped of its protective coating and glued or soldered into a metal, plastic or ceramic ferrule. The end of the fiber may be ground cylindrically to compensate for the elliptical pattern of the laser diode output configuration. The fiber is introduced through a port in the package, precisely aligned, and then welded, soldered or glued into place using a heated positioner or laser welding system. The active fiber core in a 125 μm area may be only 6 to 9 μm in diameter and, even with the most accurate positioning, only 75 percent of the light can be captured. Many opportunities for selective laser soldering exist in component attachment to the optical bench, as well as optical fiber ferrule attachment. Level 2 Board Assembly – The vast majority of assembly operations for an OE to printed circuit board (PCB) assembly consist of standard surface mount technology (SMT) processes, with two major exceptions. The first exception is the attachment of the OE module to the board. Due to its thermal sensitivity, the OE module will typically have to be hand inserted and either hand, wave or selective soldered. The second exception is the handling of fiber. In many OE PWBs the fiber is attached to the OSA in the module. This attached fiber is called a pigtail. The fiber must be handled with care. It cannot be bent around a radius less than about 75 mm and the fiber end must be protected with a cap. Efforts are being made by assemblers to develop techniques to carefully handle the fiber during assembly. However, the assembly of OE boards is not a big stretch from standard PCB assembly. Typically the vast majority of the assembly is handled in a straightforward SMT approach, such as shown in Figure 1. The OE components will typically be hand inserted and wave soldered, or hand soldered due to their thermal sensitivity near the end of the