-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueComing to Terms With AI
In this issue, we examine the profound effect artificial intelligence and machine learning are having on manufacturing and business processes. We follow technology, innovation, and money as automation becomes the new key indicator of growth in our industry.
Box Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Soldering Techniques for OptoElectronics
December 31, 1969 |Estimated reading time: 3 minutes
A review of available joining techniques and materials within the optoelectronic package and in board assembly, focusing on laser soldering to illustrate some of the outstanding issues.
By Alan Rae
Soldering is the typical method of preference to join and connect many components of hermetically sealed optoelectronic 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 infrared (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, expect non-hermetic packaged, SMT-assembled devices with edge-emitting lasers or vertical cavity surface-emitting laser (VCSEL) arrays firing directly into embedded optical waveguides embedded in 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 are 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 low-melting solder or a hierarchy of solders to affix components.
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 optoelectronic 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.
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. Wire bonding typically is used for internal electrical connection.
Given the technical requirements of optoelectronics packaging, the focus has been on meeting performance requirements and not manufacturability, leading to a predominance of gold-plated 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. Currently, attachment of the completed optoelectronic module to the substrate is achieved by hand soldering. Laser selective soldering has shown its merits in other packaging and assembly areas through its ability to deliver precisely programmed and directed heat to where it is needed — exactly the conditions required for a heat-sensitive optoelectronics package.
Alan Rae, vice president of technology, Cookson Electronics, may be contacted at 25 Forbes Blvd., Suite 3, Foxborough, MA 02035; (508) 698-7238; Fax: (508) 698-7201; E-mail: arae@cooksonelectronics.com.