Laser welding produces high-yield assembly and packaging

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Craig Marley and Mark Rodighiero

Reliable, permanent attachments are necessary elements at every stage of component assembly and packaging. Positional tolerances and alignments can be achieved by solder and epoxy, but laser welding yields higher production rates and provides an easier route to automation.

Every product in a fiberoptic network contains a chain of optical components, from lenses and crystals to optoelectronic devices such as laser diodes and photodetectors. All require precision alignments and reliable, permanent attachments. Additionally, one or more fiberoptic cables must be attached to the packages containing these components so the ends of the fibers are positioned in close proximity to other optical components in the chain. These precise alignments and attachments of optical components and fibers present technical challenges to the photonic-device manufacturing process.

Photonic-device manufacturing requires the highest levels of precision and miniaturization compared to electronic parts. The placement and attachment of miniature optical components within a photonic device often requires positional tolerances from 250 nm to as little as 50 nm in several axes. For this reason, manufacturers of photonic devices are now using pulsed Nd:YAG lasers to precisely attach components because equipment that incorporates laser welders meets precision manufacturing requirements.

Laser rods are diamond-cored from the YAG crystal and are optically polished to exact specifications for laser use. Pulsed YAG lasers are stimulated with a short pulse of energy from a flash lamp; they are more suited for spot-welding and seam-welding in thin metals up to 1 mm thick, and for welding dissimilar metals.

Pulsed Nd:YAG laser welding results in minimal heat-affected zones and is the only practical and reliable means to achieve optical alignment with submicron tolerances over the lifetime of the devices. Compared with solder and epoxy-adhesive attachment methods, laser welding offers significantly higher production rates, easier automation, excellent long-term stability of mechanical and optical performance, low contamination, strong clean joints, and low ecological impact. Pulsed Nd:YAG laser welding also permits peaking the power output via post-weld processing ("laser hammering") or mechanical bending.

Laser welding these components requires a pulsed Nd:YAG laser with 10 to 50 W of average power and 3 to 5 kW peak power. Flexible optical fibers deliver high-energy laser pulses through a focusing assembly to the part surface with a high degree of accuracy and repeatability under microprocessor control (see Fig. 1).

PRECISION CHALLENGES
The requirements for spot-size diameter, penetration depth, and even the shape of weld nugget, are much more stringent for photonic devices than for general-purpose laser welding. Another very critical concern in these welding applications is post-weld shift (PWS). PWS is the minute relative movement between the parts that are being welded as a result of the welding process. Because the tolerances for the mechanical connection are so tight, special attention must be paid to laser-weld process parameters, beam-delivery components, materials, and weld-joint design.

Laser-welding process parameters are the key to successful laser spot- and seam-welding of photonic devices. For spot-welding coaxial photonic devices, the laser is normally configured with two or three highly balanced energy-share outputs. Laser spot-welding of planar devices—for attaching fiber ferrules and clips within 14-pin butterfly packages, for example—requires two balanced energy-share outputs (see Fig. 2). The laser beams are focused to deliver equal energy at each spot simultaneously. Branch-to-branch energy balance, beam profile, and proper beam-distribution hardware are essential to minimize PWS.

Laser energy (joules) requirements for spot-welding applications will normally range from 1 to 5 joules with peak power of 0.8 to 1.5 kW. Focused beam diameters are typically 300 to 500 µm.

Pulse-to-pulse stability is a critical characteristic for successful laser welding and should be maintained at ±5%. This stability is particularly important for precision spot-welding using two- or three-way energy-share outputs.

Post-weld shift is introduced as a result of the asymmetrical cooling of the simultaneously applied spot-welds. When laser welded, the affected part will shift toward the source of the heat (the laser beam). Fillet weld joints are more tolerant to xy misalignment but are more susceptible to tilt shift. Butt welds are strong and produce lower tilt shift but are less tolerant to xy misalignment. Lap welds are good for structural joints with low PWS orthogonal-to-the-weld axis but require more laser energy and weld depth (heat).

Precision, low-noise, nanopositioning motion systems with highly versatile and responsive software are essential for positioning and aligning photonic components prior to laser welding. This equipment is also needed for adjusting PWS to the peak value by mechanical bending or "laser hammering." Depending on the device specifications, component alignment tolerances from 250 to as little as 50 nm in several planes may be required. Software algorithms are required to find first light and to achieve peak coupling efficiency, thus maximizing output power in the shortest possible time.

Laser welding with pulsed Nd:YAG lasers necessitates the use of a Class I eyesafe enclosure/workcell. The workcell must have a vibration-free baseplate and be ergonomic. Class I workcells must accommodate the computerized numerical-control motion hardware, nanopositioning stages, computer, controls, tooling, and beam-delivery optics and mounts. Class I eyesafe enclosures protect against exposure to the high peak power of a pulsed Nd:YAG laser, which can result in eye damage or serious skin burns.

BEST MATERIALS
Materials best suited for laser welding photonic devices include 304/304L stainless steels, nickel alloys, Kovar, Inconel, and Alloy 42. Package designers should avoid using 303, 316, and 400 series stainless steels because of weld splatter from trace elements found in these alloys.

Aluminum and carbon steels should also be avoided. Aluminum requires three to five times more heat input, and steels with greater than 0.08% carbon or "free-machining" steels with high sulfur-alloy content exhibit micro-cracks when laser welded.

In butterfly packages, some materials are selected for good thermal transfer and mechanical stability, and may not be optimum for laser welding. For example, sometimes nickel to nickel-plated molybdenum or copper tungsten materials are used. Nickel plating can present problems: nickel-plating baths that do not use electrolysis often contain trace contamination of hydrocarbons, mill oil, phosphorus, and mill scale. Under the intense power density of the laser-welding process, these trace elements typically explode, creating splatter and voids. If nickel plating is required, electrolytic nickel plating should be used with a tight tolerance band on plating thickness.

Gold plating can also present inconsistent laser-weld joints. Gold is very reflective at the 1064-nm Nd:YAG laser wavelength. Laser welding through gold plating requires more energy and heat. Gold plating 250 micro-inch thick may require a different laser weld schedule than a 50 micro-inch layer. A narrower tolerance band for gold-plating thickness is required for consistent results. Finally, gold plating can introduce latent cracking in seam welds used to hermetically seal Kovar photonics packages.

Each of these material combinations presents unique welding-process control issues. The device manufacturer should team up with a laser provider who is well versed in process development to make sure the proper beam-delivery system and weld parameters are selected for a robust welding process.

DESIGN DETAILS
Joint, package, and lid designs should be carefully considered to ensure they are "laser friendly." Poor designs may result in excessive PWS and weak joints. Improper package design may increase weld-cycle time. Butt, lap, and fillet weld joints are all possible options when good manufacturing and part-tolerance rules are applied. Part fit-up also plays an important role: poor fit-up may result in a broken or weak joint and PWS that cannot be corrected.

Laser hermetic seam-weldings of butterfly-package lids, microelectromechanical systems (MEMS) components, and TO caps are also being implemented as a means to reduce thermal and mechanical stresses that may result from more traditional resistance-welding methods. Laser seam-welding requires a 100- to 300-W pulsed Nd:YAG laser for increased welding speed at higher repetition rates. Pulsed Nd:YAG solid-state lasers are easy to automate and provide the flexibility needed for seam-welding a broad range of photonic and MEMS packages.

Hermetic laser seam-sealing may require a glove box enclosure with a bake-out oven to remove any traces of moisture. This oven enclosure is attached to the glove box, which provides an inert environment to prevent oxidation of the weld zone during the seam-welding process. Argon or nitrogen with a small percentage of helium as a trace gas is typically used in laser welding glove box systems. Laser-beam delivery optics and precision computerized numerical control motion equipment are integrated inside the glove box enclosure and are controlled via a PC and Windows-based software (see Fig. 3).

Manufacturers of next-generation photonic devices will utilize laser spot- and seam-welding systems for precise alignment and attachment of individual component parts and package-lid sealing. High levels of device performance, manufacturing yields, and productivity will be achieved when the photonic design elements, materials, and tolerances are optimized for advanced laser-welding processes.

Craig Marley is the director of business development at the photonic systems division, and Mark Rodighiero is vice president of the laser and systems division at Unitek Miyachi, 1820 S. Myrtle Ave., Monrovia, CA 91016. Craig Marley can be contacted at 909-245-3944 or at craig@unitekmiyachi.com.

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