Material Considerations:
The metallurgy of laser welding is not much different than other welding techniques, but there are two special design attributes that must be kept mind. The first attribute is that laser welding is usually an autogenous process, which means that unlike GMA welding, no metal is added during the process (Except highly reflective metals as discussed above). The second special attribute of laser welding is the relatively rapid cooling rate of the solidifying metal, which places some special constraints on a few metal choices.
Aluminum is usually the first choice when a designer requires a lightweight, corrosion-resistant, heat-dissipating, robust, and economical package. Aerospace packages for microwave circuits, sensor mounts, or small-ordinance initiators are the most common examples of aluminum components that can be laser welded. Laser welding with penetrations up to 1.5 mm are common in aluminum alloys. The high reflectivity and conductivity of aluminum requires higher-peak-power pulses than are needed for ferrous alloys, but standard ND:YAG lasers easily produce these powerful pulses.
Type 6061-T6 is the material of choice because of economics, rigidity, and ease of machining.
However, the material cannot be successfully laser welded to itself, because the partially solidified melt zone cannot withstand the stress of shrinkage upon solidifying, and cracks are formed (termed "solidification-cracking" or "hot-cracking"). The solution to this problem is to improve the ductility of the weld metal by using aluminum with high silicon-content, such as alloy 4047 (Al-12 percent Si). This alloy is very ductile as a solid and difficult to machine into complex shapes. Therefore, 6061 is usually employed as the package component with intricate features, and 4047 is used as a simple lid that is relatively thin (typically less than 1mm). A 4047 perform filler insert can be sandwiched between 6061 components to produce excellent welds (see figure 2b).
Alloy 2219 and many other popular aluminum alloys are also wieldable using 4047 filler metal. The only aluminum alloys that can be welded with low heat input and without the use of 4047 filler are the 1000 and 1100 alloys. These commercially-pure aluminum alloys have the metallurgical characteristics to avoid hot cracking, but their poor mechanical and machining properties usually prohibit use in most applications.
Kovar (Fe-29 percent Ni, 17 percent Co) is typically chosen as a package material because its thermal-expansion coefficient matches that of other package constituents such as glass-to-metal seals. Plated Kovar offers good corrosion resistance and can be machined and drawn relatively easily. Kovar is denser and heavier than aluminum, but it presents few metallurgical problems compared to those of aluminum. In addition, it provides the benefit of a low coefficient of thermal expansion. Kovar can be welded to itself with ease with up to 2 mm penetration. It is important to consider plating options when specifying Kovar package components.
Stainless Steel provides excellent corrosion resistance and good metallurgical characteristics useful for a hermetic package. Stainless steel is slightly more difficult to machine, and is heavier and more expensive than aluminum. Some aerospace packages employ stainless steel, but the majority of uses seem to be in the military, the medical field, or in automotive airbag systems. The austenitic stainless steels (AISI-300-series alloys) have high nickel contents that are beneficial for laser welding. Types AISI-301, 304, 304L, 316, and 318 are the most popular choices for electronics packaging, with 304L and 316 the leading candidates. Although these grades of stainless steel generally produce hermetic laser welds, specific metallurgical compositions of alloys such as 304L are susceptible to cracking. This can be avoided by specifying the specific composition of the lot and verifying with weld tests. Because of their high sulfur and high phosphorous content, free machining stainless steels, such as AISI-303, should be avoided. These elements segregate to the weld center line, causing a brittle zone that cracks under the stress of solidification (hot cracking) Type 303 can sometimes be welded to another 300-series alloy, but different lots of 303 can have inconsistent welding characteristics. The ferritic stainless steels (400 series alloys) are generally not good candidates for laser welding, because the high cooling rates of laser welding cause martensitic formation in the weld zone. This brittle martensite can crack under solidification-shrinkage stress or in service. Pre-heating can reduce martensite formation in 400 series alloys. Some 400-series alloy can be welded to 300-series alloys with good results, but again, results can vary from batch-to-batch, or with variations in heat input. Stainless steel can be welded to 2mm penetration.
Fe-Ni alloys, such as Alloy 42 a Mu-Metal, are usually chosen for their electrical or electromagnetic characteristics. Alloy 42 has good electrical conductivity and is sometimes used as replacement for brass.
Mu-Metal has the correct magnetic properties for gyroscope guidance and similar components.
Invar is used in fiber communications assemblies or any other package that requires a near-zero coefficient of thermal expansion. All of these materials weld well and have laser welding characteristics that are similar to those of Kovar.
Titanium is chosen for its biocompatibility in pacemaker and pacemaker battery packages.
Commercially-pure titanium and Ti 6Al4V weld extremely well, but nitrogen cannot be used as cover gas because of the formation of titanium nitride. Argon or helium must be used to prevent oxidization.
Zircalloy is another excellent material to laser welding. Nuclear applications are the main use of zircalloy. Both titanium and zircalloy have similar welding characteristics and penetration to 2mm is easily achieved.
Copper alloys are used in hermetic packages where electrical and thermal conductivity are important or where non-magnetism is a consideration. Pure copper has good metallurgy for welding, but it is highly reflective and has high thermal conductivity, therefore making it difficult to achieve weld penetrations greater than 0.5 mm. The reflectivity of pure copper can be overcome by plating it with electroless nickel before welding, or with the use of a laser system that has pulse forming capabilities. Beryllium copper (BeCu) has better weldability and can produce very good welds to pure copper.
Copper-tungsten is very heavy, but has very good heat conductivity, as well as a thermal expansion close to that of many electrical components. Copper-tungsten and copper-nickel alloys weld well. Brass alloys are not good candidates because of their zinc content. Zinc vaporizes near the melting temperature of other metals, and vapor expansion tends to expel metal out of the weld pool. The little molten metal remaining solidifies, trapping the gas pockets in the joint creating undercutting and porosity.
Silver and gold are weldable but with penetration limited to less than 0.5 mm because of high reflectivity and thermal conductivity.
Platinum, however welds well up to about 2mm penetration depending upon the alloy. These metals are used for special applications in aerospace, electronics, and military ordnance, where corrosion resistance and electrical conductivity are paramount.
In summary, most metals used for hermetic sealing electronic packaging can be welded with a ND: YAG laser. Low heat input is a key feature for heat-sensitive components that require weld penetrations typically less than 2mm. Within each metal group, there are alloys that have better weldability than others and, in some cases, there are compositions of alloys that cannot be welded. In general, care in the selection and control of materials provides high quality, crack-free, hermetic weld joints.
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