Laser Welding – Design Guideline for Hermetic Sealing
Precision components or sensors exposed to corrosive conditions, are required to have extreme performance characteristics, and need to be sealed hermetically into special packages. Epoxy gluing, resistance welding, soldering, electron-beam welding, and laser welding are several of the techniques available for sealing these packages. The disadvantage in the use of epoxies is that they do not create a hermetic seal as moisture migrates through the epoxy, destroying its hermetic seal.
Resistance seam welding is a reliable process that has been around for many years, but it has several disadvantages. For one, the process requires that the materials to be welded have high electrical resistance, and therefore cannot be used to weld materials such as aluminum or copper. Second, the electrodes must be “burned in” on several packages before production welding begins. Furthermore, resistance welding can be used only for a lap-weld joint.
Problems develop when solder-sealing is attempted on large packages. The process requires the whole assembly to be heated, which can compromise performance of some electronic components, and the use of low-melting-temperature alloys may result in the materials not being fully wetted.
Electron-beam welding has many of the same advantages as laser welding, but E-beam systems require vacuum chambers, X-ray shielding, and a fill of inert gas after welding, requiring a sealing operation by conventional means thereafter.
Reliability, minimal heat distortion, high processing speeds, a non-contact process, and the flexibility of CNC programming are just a few of the advantages that have resulted in the increased use of laser welding. However there are several considerations in package design that need to be addressed to enable the designer to maximize the unique functions, features and benefits of laser welding.
First, a short review of laser welding systems and the welding process. In its simplest form, a laser-welding system consists of a laser, beam delivery, and workstation. Lasers with wave lengths ranging between 808 nm to 1064 nm (Direct Diode and nd:YAG laser) are best suited for hermetic seam welding. Pulsing capabilities of nd:YAG lasers deliver high energy (joules/cm2) with short pulse duration to the workpiece which generate minimal heat input beyond the immediate weld joint area. These short wave length laser can also weld reflective materials, e.g.; aluminum, gold, silver, copper, beryllium copper, etc. The laser beam can be delivered to the workstation by standard hard optics or through a fiber optic delivery (FOBD) weld head. When standard optics are used, the laser is typically positioned on top of the workstation and a mirror angled at 45° directs the beam downward through the focusing lens to the work piece. A FOBD uses a flexible, optical cable to deliver the beam to the workstation allowing the laser to be remotely located away from the processing area. Time-sharing or energy-sharing fiber systems permit the output of one laser system to be used at several workstations. A closed-circuit television coaxial viewing system is an effective processing tool, as the camera is configured for a coaxial view directly down the beam-delivery path. Cross-hairs are electronically generated on the TV screen and centered where the focused laser beam strikes the work piece. The focused spot then can be automatically positioned to the weld joint by moving the part and/or moving the FOBD weld head. The CNC controller can also be programmed to include laser process parameters, e.g., shutter controls, laser power and pulse parameters, shield-gas delivery, etc.. Programs for each type of part to be welded can be stored in the CNC controller requiring little to no adjustments in the set-up.
These features facilitate the design and development of integrated automated welding systems. An enclosure such as a manual glove-box system or an automated enclose workstation that is sealed from the outside atmosphere while its interior is purged and filled with a purified, inert-gas mixture. Parts are loaded through a two-way isolated transfer chamber, where they are positioned for welding.
How a Laser Welds:
When the laser energy is absorbed by the material, heat is conducted into the material, creating a weld pool in a very localized area. With many metals no filler material is needed, but a tight fit-up of the parts is essential.
Figures l-a through l-h (located at end of document) show top views and cross-sections of welds in various hermetic weld sealed packages.
Typical weld penetrations can be adjusted within a range from 0.01″ to 0.06″, depending on the materials and configuration of the package joint. The heat input to the part is kept to a minimum because of the laser’s ability to deliver short duration pulse of high energy in a small (< 0.020") weld nugget. As a result, even high reflection materials such as aluminum, copper, and gold pulsed nd:YAG laser can produce consistent quality weld joints. Laser welding is typically performed in an inert atmosphere (nitrogen - argon), with less than 100 ppm of moisture and oxygen. A slow-flow cover-gas also should be supplied through the beam-delivery nozzle, or through a side jet, to keep debris from being deposited on the focusing lens. This keeps the weld free of contaminates. Additionally, the inert gas with 10% helium can be contained within the welded package, so that the helium can be used as a tracer gas for leak detection of the hermetic seal.
There are three types of laser weld joints – butt, lap, and fillet – each with its associated joint configuration. Figures 2a through 2e below show the geometry of each of these joint types.
A butt weld design is typically used for larger housings. Tight fit-up tolerances between the lid and base are important. Usually no special tooling is required to hold the lid in position. Since this laser welding process does not usually require any filler material, the fit-up of the lid to the package is critical in order to ensure a hermetic seal. If the gap is too large, the materials may not flow together to make the weld joint.
Figure 2a shows a butt weld with the dimensions necessary for the cover to drop into the housing.
The cover and housing dimensions have tolerances that will ensure the lid drops easily into the housing and the gap is kept to a minimum. The lip on which the cover rests has a minimum width of 0.03″ and the edge around the lip is typically 0.06″.
As shown in figure 2-b below, a filler material is used to weld high reflective metals (aluminum, copper, Also see Plating Considerations below). With a drop in cover, the lid is reduced in size to provide for the filler. The filler thickness can range from 0.005″ – 0.015″.
With a lap weld, the lid tolerance is less important +/- 0.007″. Fit-up is determined by the flatness of the lid which should be +/- .002 – 0.004″. Figure 2-d below shows the geometry of this package.
The laser fillet weld, shown in figures 2-e and 2-f below, is commonly used for hermetic laser weld sealing. The lid should cover the housing by twice the thickness of the lid and leave at least the same amount of housing exposed from the edge of the lid to the edge of the housing.
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.
Many hermetic packages employ plating to improve corrosion resistance, solderability, or for better absorption of laser energy. Nickel and gold are the most common plating choices. Nickel can be used alone, and it is almost always used as an under-plating for gold or tin. These metals are plated on top of the nickel, because they do not plate well directly to other base metals. Nickel can be plated by an electrolytic or electroless process. Ferrous alloys such as Kovar and stainless steel must be plated with electrolytic nickel. Electroless nickel plating contains phosphorous, which produces inter-metallics within the weld that cause porosity and cracking. Gold-plated Kovar packages weld well, as long as there is not an under-plating of electroless nickel. Gold-plated mild steel produces weld voids due to inter-metallic segregation in the weld. Aluminum and copper can benefit from nickel plating, because nickel absorbs the nd:YAG laser-energy much better than the base metals. This technique is not often used with aluminum but is an excellent aid in the welding of copper and its alloys. Tin and zinc have low melting and vaporization points, which result in porous welds. Tin and zinc plating should be absent from the weld joint.
Even when plating beneficial to the laser welding process are chosen there are other plating additives, such as organic brighteners, that can cause problems. Organic brighteners are gaseous when metals are liquid and, as discussed before, they create voids in the weld. Matte finishes with no organic brighteners work best for improving laser welding performance. If a plating is used that are not compatible with laser welding, they must not be present in the weld joint. Masking the weld area before plating, or machine removal of the plating in the weld zone is a common practice. In all cases, it is important to specify the correct plating, and perform weld tests before finalizing the weld joint design.
The Welding Process:
The hermetic weld sealing process begins by placing the parts in a vacuum bake-out oven to remove moisture. The parts are typically baked for 24 hours at 125 °C. Since this is a long process, more than one oven may be needed to fully utilize the laser’s welding capabilities. When the baking cycle has been completed the chamber is filled with an inert atmosphere. The parts can now be transferred to the glove-box or automated enclosure for further assembly and for laser welding.
The component parts need to mounted into a fixture designed to hold them securely for the accurate positioning during the programmed weld cycle. The fixture can be a tray matrix designed to position a number of component parts. Complex parts may need to be tack-welded before seam welding can be performed. The welding cycle can last from a few seconds to a few minutes, depending on the length of the weld and the number of parts being welded. After welding the parts are removed from the enclosure (glove-box). If rework is required, the weld joint can be machine out and the lid removed, and a new lid can be welded in place. In some cases an over size lid may be required.
The laser welding process allows designers to expand upon the types of joints, material, and plating options for their products. These options, along with the consistent quality, speed, reliability, flexibility, reduced energy consumption, and other advantages, are the reasons the use of laser welding is becoming the leading technology for hermetic sealing.