very fine welds (spot sizes could range from approx. 10um up to 100um)

 reduced distortion and heat affected zone

 process reliability and stability (pulse to pulse stability for micro spot welding)

 high process speed (welding with scanner)

 welding of reflective materials

 welding dissimilar materials.

 low heat input: the weld energy is delivered only where it is needed and with exceptional control

 clean welds: in addition to the aesthetic benefits, clean welds result in products that are easier to sterilise or fit into other assemblies

 strong welds: lasers provide high strength with a minimum number of welds

 hermetic welds: unlike soldering or brazing, lasers can provide flawless hermetic welds essential for many micro-applications.

7.2.1 Beam quality

For the majority of microwelding applications, it is necessary to have a laser with good beam quality to produce a small spot size at the workpiece. A combination of good beam quality and small spot size offers a number of advantages during microwelding including:

The quality of a laser beam can be calculated by the beam parameter product (BPP),⁴ which is defined as the product of a laser beam’s divergence angle (half-angle) and the radius of the beam at its narrowest point (the beam waist):

[7.1]

where ω₀ and θ denote the beam waste radius and the laser beam’s divergence angle, respectively (Fig. 7.1) and the BPP quantifies how well the laser beam can be focused on to a small spot. A Gaussian beam has the lowest possible BPP, λ/π, where λ is the wavelength of the light.

The ratio of the BPP of an actual beam to that of an ideal Gaussian beam at the same wavelength is denoted M²

[7.2]

The beam quality M² parameter is a wavelength-independent measure of beam quality. Figure 7.2 shows beam quality data for commercial and laboratory lasers.⁴

For a number of years, low power (up to 300 W) lamp-pumped pulsed Nd:YAG lasers with an adequate beam quality (M² = 15–20) have been the laser of choice for a number of microwelding applications. At a wavelength of around 1 μm, the focusing optics are smaller and simpler to enable smaller spot sizes than equivalent CO₂ lasers. As these devices are getting smaller and smaller, it is very important to have laser sources that have good beam quality to produce a small spot size and produce a very stable output for a consistent process.

The need for more efficient, compact and high beam quality lasers for very fine microwelding has fuelled the rapid growth in developing fibre lasers. The following sections highlight the advantages and disadvantages for both pulsed lamp-pumped and fibre lasers for microwelding applications.

7.2.2 Laser structure

Figure 7.3 shows the basic components comprising a laser. The gain medium is capable of producing light when the excitation energy is applied to it. The front and rear mirrors define the resonator, which ensures the light reflects back and forth through the gain medium, thus gaining in intensity and reducing in angular spread. The fraction of the light which passes through the partially transmitting front mirror becomes the useful laser beam.

For a CO₂ laser, the gain medium is a mixture of gases (mainly helium, nitrogen and carbon dioxide) and the excitation energy comes from the electrical discharge through the gas. The laser beam wavelength is 10.6 μm. For a solid state laser, the gain medium is typically a rod of YAG (yttrium aluminium garnate) doped with about 1% Nd (neodymium) atoms, and the excitation energy is provided by the broadband light from a pulsed flashlamp of a continuous arc lamp. The laser beam wavelength is 1.064 μm.

For a fibre laser, the gain medium is an ytterbium-doped glass fibre, with the excitation energy being provided by laser diodes, operating around 950 nm, coupled by various schemes into the core of the doped fibre. The laser beam wavelength is typically in the range 1.07 μm to 1.09 μm and the physical dimensions of the gain medium for the fibre laser are very different from other laser types. A Nd:YAG rod might be 200 mm, a CO₂ discharge around 2 m, but the gain fibre in a fibre laser will be tens of metres long. The reflectors used in the fibre laser are physically very different from traditional lasers. Typically the mirror will be formed from a dielectric coating on substrate, which will be transmissive at the laser wavelength for the output coupler. For the fibre laser, Bragg gratings written into the core of a fibre are used. These fibre Bragg gratings (FBGs) consist of periodic refractive index variations. The longitudinal period of the grating determines the wavelength of the reflected light, and the magnitude of the variation controls the reflected percentage.

7.2.3 Beam delivery

From the laser microwelding point of view, the laser by itself is very much ineffectual. The laser must be used in conjunction with several different items of optical and mechanical equipment, which need to be integrated into a functional unit, in order to be able to weld materials. It comprises three key elements:

7.2.4 Beam profiles

Apart from different temporal pulse shapes of both lasers, another factor to consider when choosing a laser for microwelding is the beam profile of the lasers. The beam profile of the laser can affect the welding performance, i.e. weld penetration depth, weld profile and weld quality. A top hat beam profile as for lamp-pumped pulsed Nd:YAG lasers (Fig. 7.14), for example, is more forgiving since the power and energy are evenly distributed across the weld zone. Top hat energy distribution is achieved when the laser beam is delivered to the workpiece via a fibre optic cable. Increasing the pulse energy increases the weld depth (Fig. 7.15). Vaporisation does not occur if the power/energy density remains below the vaporisation point.

The beam profile from a single-mode fibre laser is Gaussian in profile (Fig. 7.16). This produces very deep and narrow welds (Fig. 7.17). For overlap joint welds, this weld shape is not so good in terms of mechanical properties because the interface is very small compared to the top hat beam profile. However, it is possible to have a low power fibre laser 100–500 W with a flat top beam profile (Fig. 7.18). This leads to a lower beam quality by introducing extra fibre modes and the delivery fibre is of large core diameter, i.e. 100 μm compared to 10–20 μm diameter fibre for the Gaussian beam profile. Both of these beam profiles can provide advantages during microwelding.

7.2.5 Nd:YAG versus fibre

Considering the differences in beam quality and pulsed performance between the two types of laser, there are different operating regimes for the two types of laser. The lamp-pumped YAG laser is characterised by long high-energy pulses but poorer beam quality, and the fibre laser is associated with high repetition rate, on-off type modulation, single-mode beam quality but low pulse energy. From the perspective of microwelding applications, both of these regimes have their advantages and a few of the application examples are highlighted in Section 7.5.

7.3.1 Welding modes

The heat transfer and molten metal flow experienced during laser microwelding can significantly influence the weld pool geometry, solidification structure,^{21, 22} and defect formation.²³ Laser beam microwelding is a process that melts and joins metals by heating with a focused laser beam. Figure 7.19 shows the schematic setup for laser welding. The focused laser beam is made to melt the workpiece or joint at given laser power and welding speed. A shielding gas, usually an inert gas, is supplied either coaxially or by a pipe to protect the molten weld pool from oxidation. The main features of laser microwelding, which make it an attractive alternative compared to other fusion microwelding processes (i.e. resistance microwelding or arc microwelding), are:

Laser microwelding of metals is classified into two welding modes i.e. thermal conduction and keyhole or sometimes known as deep penetration welding.^{24, 25} Both the laser beam power and the focused spot size with respect to the workpiece will govern the welding modes.

Conduction limited welding (Fig. 7.20) occurs when the laser beam is out of focus and power density is insufficient to cause boiling at the given welding speed. In this welding mode, overlapping spots from a pulsed laser or the beam from a continuous laser is absorbed by the surface of the material and the volume below the surface is heated by thermal conduction producing a semi-circular cross section. The minimum laser power needed to melt the material’s surface can be evaluated by a simple equation.²⁶ When a stationary Gaussian beam interacts with material surface, the temperature at the centre of the heat source T(t) is given by:²⁷

[7.6]

where d is focused laser spot diameter, W is the laser power, A is the absorptivity of the surface, K is the thermal conductivity and α is the thermal diffusivity. Thus the minimum laser power needed to melt a metal of melting temperature T_m is given by:

[7.7]

One of the most important parameters influencing the effects of laser–material interactions is the absorptivity of the material for laser radiation and the absorptivity of the material is greatly influenced by the wavelengths and temperature.²⁸ When the laser beam strikes the target material, part of the energy is reflected, the rest is absorbed, and the absorbed energy heats up the target material. The absorptivity is strongly dependent on the material being irradiated and on the laser wavelength. Figure 7.21 shows the absorptivity of some common metals which are routinely used in a range of microwelding applications with different laser wavelengths.²⁸ Figure 7.22²⁶ shows the minimum laser power needed with a given focused spot size to melt common microwelding metals with Nd:YAG wavelength (1.06 μm). It can be seen that silver is the most difficult material for conduction mode welding, requiring almost 900 W to melt. Aluminium-based alloys, steels and nickel are the easiest to weld. Copper is also difficult for conduction mode laser welding because it requires approximately 300 W to melt. Sometimes copper-based alloys are nickel plated to enhance laser absorption and reduce the laser power needed to weld this material.

In keyhole or deep penetration welding (Fig. 7.23), the metal surface is heated by a focused laser beam (increased power density) to a temperature where the surface evaporation creates a recoil force sufficient to depress the molten surface into a cavity or a small keyhole. The keyhole grows in depth because of increased coupling of radiation into the workpiece, through multiple reflections of the laser beam off the keyhole walls, and material vaporisation.^{25, 29, 30} The balance between the hydrostatic forces of the liquid metal surrounding the keyhole governs its existence and the pressure of vaporised and ionised material or plasma within it.

Figure 7.24 shows the transition in the weld bead shape and the weld penetration by changing the power density at the workpiece. The welds were made in stainless steel with a 400 W single-mode fibre laser.³¹ The micrographs show that as the power density increases, the welding mode changes from conduction to keyhole. During these tests the welding speed was kept constant at 9 m/min.

7.3.2 Welding joints

When considering laser microwelding, the design of the joint and possibly the fabrication method of the whole product should be assessed in order to gain the maximum advantage from the process. Laser microwelding requires high tolerances in gap control and joint positioning (Fig. 7.25). For macrowelding, filler material can be used to widen the tolerance field, but in practice this is not very common because of the associated cost related with the process and also reduction in the weld speed. Laser microwelding is usually performed without any filler metals so gaps must be filled with metal from the adjacent area and bridging a gap requires extra laser energy. Generally, a gap no more than 10% of the thickness of the thinnest component is allowed. This can be relaxed for thicker materials greater than 1 or 2 mm but might need to be reduced to 5% or lower for materials less than 0.2 mm. Gap problems show themselves as very concave weld beads or failure to bridge the gap between parts. It is also much more difficult to start a laser weld in a large-gap area compared to seam welds through a short section of high-gap area where bridging can sometimes be maintained. In addition to the suitable preparation and alignment of the joint faces, they should be free from contaminants such as grease, paint, dirt and oxide scales. Residue from chemical degreasing and cleaning agents should be carefully removed, since weld spatter and porosity (see below) can result if these substances are present on the workpiece.

7.3.3 Materials

The metallurgy of laser welding is not much different from other conventional welding techniques, but there are two very important features which make laser microwelding very challenging and the engineer/designer must be aware of these before designing the component and deciding on the material type. The first point to note is that laser welding is almost always an autogenous process, which means that no metal is added during the welding process. This was discussed previously in the consideration of fit-up tolerances. The second special feature of laser welding is the relatively rapid cooling rate of the solidifying metal, which places some special constraints on a few metal choices.

When microwelding microwave packages, a designer requires a lightweight, corrosion-resistant, heat-dissipating, robust, and cheap material. Aluminium is usually the first choice. Aerospace packages for microwave circuits, sensor mounts, or small-ordinance initiators are the most common examples of aluminium components that need to be laser welded. Laser welding with penetrations up to 1.5 mm are common in these applications and 6061-T6 aluminium alloy (Al-Mg-Si) 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, known as solidification cracking or hot cracking. Hot cracking is the cause of almost all cracking in aluminium weldments. Hot cracking is a high-temperature cracking mechanism and is mainly a function of how metal alloy systems solidify. This cracking mechanism is also known as hot shortness, hot Assuring, solidification cracking and liquation cracking.

Figure 7.26³² shows the aluminium crack sensitivity curves and is a helpful tool in understanding why certain aluminium welds crack. The crack sensitivity curves show that with the addition of small amounts of alloying elements, the crack sensitivity becomes more severe, reaches a maximum, and then falls off to relatively low levels. After studying the crack sensitivity curves, it is easy to recognise that most of the aluminium-based alloys are considered unweldable autogenously (without filler alloy addition). The solution to this problem is to improve the ductility of the weld metal by using aluminium with high silicon content, such as alloy 4047 (Al-12% 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 1 mm). The only aluminium 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 aluminium 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% Ni, 17% Co) is another material which is often chosen for microwelding 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 aluminium, but it presents few metallurgical problems compared to those of aluminium. In addition, it provides the benefit of a low coefficient of thermal expansion. Usually welded to itself, Kovar welds with ease up to 2 mm penetration. It is important to consider plating constituents when specifying Kovar package components.

Stainless steel with its excellent corrosion resistance and good metallurgical characteristics is used in a number of microwelding applications. Stainless steel is slightly more difficult to machine, and is heavier and more expensive than aluminium. 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 various types of stainless steel are identified, and guidance given on welding processes and techniques that can be employed in fabricating stainless steel components without impairing the corrosion, oxidation and mechanical properties of the material or introducing defects into the weld. The unique properties of stainless steels are derived from the addition of alloying elements, principally chromium and nickel, to steel. Typically, more than 12 or 13% chromium is required to produce a stainless iron.

7.3.4 Microwelding of dissimilar materials

Many applications in various industrial sectors, i.e. electronics, medical, consumer goods, automotive applications, etc., require joining of dissimilar metals. The trends of joining dissimilar metals of components are creating big challenges for welding methods. The weldability of dissimilar metals depends on many different factors. The physical properties have a big influence on the amount of coupled-in energy and the heat transfer.³⁴ In principle, a laser can also weld any material that can be joined by conventional processes. Illustrated in Table 7.3 is the weldability of metal pairs. In the welding of dissimilar metals, good solid solubility is essential for sound joint properties. This is achieved only with metals having compatible melting temperature ranges. If the melting temperature of one material is near the vaporisation temperature of the other, poor weldability is obtained and often involves the formation of brittle intermetallics. When joining dissimilar materials, there may be certain advantages in using laser welding even though brittle intermetallics may tend to form. Since the weld itself is narrow, the volume of intermetallics may also be reduced to acceptable limits. Again, it may be possible to offset the beam in one direction or the other, thus allowing some control over composition of the resulting alloy. Although it may be possible to produce sound joints by these methods on a laboratory scale, it is more difficult to achieve similar control under production conditions. Mixing the molten metal in a laser weld seldom produces a chemically homogeneous fused zone between the two dissimilar materials. Although the average chemical composition of the weld may be acceptable, local heterogeneity can be responsible for the presence of brittle zones. It will also be apparent that minor variations in the beam position can significantly influence the relative proportions of the two main constituents in the weld zone.

To date most microwelding of dissimilar metals has been carried out with pulsed lamp Nd:YAG lasers.^{35, 36} Lamp-pumped lasers are capable of producing long, multi-ms pulses with peak powers many times the rated average power of the laser, provided that the duty cycle is sufficiently low. This ability stems from the flash-lamp itself, which is often more constrained by the maximum average thermal load than the peak power output. High peak powers of pulsed lamp pumped Nd:YAG lasers coupled with pulse shaping make these lasers ideal for welding dissimilar materials. The shaping of pulses is of great importance since the temperature has to be controlled in the region where the two molten phases are mixed. Weld depth that is too deep – which can lead to defective joints – and also insufficient weld depths can be avoided by adjusting the high starting power and the correct decreasing power to the joint geometry and the material properties of the pulse shape. Figure 7.28 shows examples of dissimilar material welds made with a lamp pumped Nd:YAG laser. These welds were made with a ramp down pulse shape (Fig. 7.29).

Compared to conventional lamp pumped Nd:YAG lasers, fibre lasers offer a number of advantages when microwelding dissimilar materials. The big advantage of these new laser sources is the beam quality or focusability. Focusability is understood as the ability to achieve a small focus diameter with a given optical element.³⁷ The small focus diameter of the fibre lasers offers a number of advantages during laser microwelding, i.e.

7.3.5 Plating and coatings

The majority of the materials used in microwelding applications are either plated or coated to improve corrosion resistance, solderability, or for better absorption of the laser energy. Nickel and gold are the most common plating choices. Nickel is used alone, and it is almost always used as 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 intermetallics within the weld that cause porosity and cracking. Gold-plated Kovar packages weld well, as long as there is not under-plating of electroless nickel. Gold-plated mild steel produces weld voids due to intermetallic segregation in the weld. Aluminium and copper can benefit from nickel plating, because the plating has better laser-energy absorbability than the base metals. This technique is not often used with aluminium but is an excellent aid in the welding of copper and its alloys.

Tin and zinc have low melting and vaporisation points, which result in porous welds. These platings should be absent from the weld joint. Even when platings beneficial to laser welding are chosen, there are other plating additives, such as organic brighteners, which can cause problems. Organic brighteners are gaseous when metals are liquid and, as discussed above, they create voids in the weld. Matte finishes with no organic brighteners work best for improving laser welding performance. Platings that only exist on top and not in the joint are often vaporised by the impinging laser beam and will not affect the weld quality.

7.4.1 Weld defects

Welding defects such as porosity, lack of fusion, spatter, burn through, oxidation and different forms of cracking can occur during laser microwelding either due to material-related problems as outlined in Section 7.3.3 or during the welding process.

7.4.2 Evaluation of welds

Once the microweld has been made, there are a number of different methods available (visual, metallographic sections, mechanical testing and leakage testing) to evaluate the quality of the welds, i.e. cosmetic appearance, porosity, cracking, hermeticity and strength.

The simplest and most widely used method is visual observation of the surface of the weld. This method is very useful for determining if the microwelds are free from spatter and surface oxidation, and for in-depth analysis (i.e. cracking, porosity weld penetration, etc.), metallographic methods are required. This method is very slow but very necessary to evaluate the microwelds, and extreme care should be taken when sectioning very small components. The metallurgical laboratory should be well equipped with the suitable equipment to cut and polish very small parts.

Leak testing is another method which is very common for checking hermeticity of the microwelds. Lithium ion batteries and microwave packages are two of the applications which are tested for leaks. Figure 7.36 shows a typical leak detection jig used for checking welded batteries. Leak tests are performed on the welded samples to check that battery case (can and cap) and electrolyte inlet (ball and cap) are hermetically sealed. The samples are tested with nitrogen gas at 15 kgf/cm² for 5 minutes in water. The schematic of the leak test is shown in Fig. 7.37. The welds are good, i.e. hermetically sealed, if no bubbles are observed along the length of the weld and around the ball and cap.

For tensile strength, small-scale tensile testing machines are used to evaluate the strength of the microwelds. During the pull test, the welded joint is often exposed to a complex stress state, resulting most often in tensile shear of the weld zone.⁴² During laser microwelding it is important to consider the effect of the weld process on the raw parent material. The strength of the overlap weld joint is governed by power density and the spot size of the laser beam.

The mechanical properties of these joints are governed to an extent by the weld width at the interface. A weld width equal to or greater than the thinnest sheet thickness is a suitable target. Table 7.4 shows the effect of spot weld diameter and the nugget size on the shear strength of the spot welds. These spot welds were made with single-mode 200 W fibre laser in thin foils of stainless steel. At low power the weld had very little strength due to very small nugget size at the interface. As the power increases, the weld diameter also increases, which produces spots welds with good strength due to bigger nugget size.

7.5.1 Examples of laser microwelding applications

Lithium ion batteries for portable applications such as mobile phones and laptops are made in very high volumes and as such need very high throughput on production lines to make manufacture cost effective. Lithium cells are poisoned by water vapour, so a hermetic seal of the cell is critical for long life. Laser welding a lid to the can is fast, reliable, low cost, and easy to automate. Pulsed YAG lasers produce the necessary pulse energy and peak power to produce hermetically sealed welds with the required penetration for the mechanical strength. Galvo-mirrors or translating optics can be used to match the speed of the cells if a weld-on-the-fly application is needed. Figure 7.38 shows an example of a lithium ion battery welded with a JK300HP (300 W average power).

Disk drive flexures (Fig. 7.39) hold the ferrite reader that floats over the spinning magnetic media in a hard drive. The high speeds of positioning and the ultra-close proximity of the ferrite reader mean that this assembly must be accurately assembled from three or four pieces of stainless steel with thicknesses ranging from 20 μm to more than 200 μm. Their design must accommodate resonances, stiffness, and overall component accuracy in three planes. Assembly of these flexures must be highly automated for low-cost, repeatable manufacture.

Lasers have been the only joining solution available for this application since the early 1980s. Their ability to produce discreet energy levels and pulse shapes for each weld location and relative thickness means that each weld is tailored to its target and the welding rate can be over 150 welds/second. Most systems today use galvo-mirrors that direct the laser beam through a flat-field lens onto the part. Ultra-precise fixtures hold the parts together for laser welding and also feed inert gas to the weld area for clean, smooth, oxide-free welds. Any brittle oxide or melt spatter on the surface can flake off and contaminate the drive rendering it useless. For this application both low power Nd:YAG and fibre lasers can be used to produce welds. Figure 7.40 shows spot welds made with both lasers.

Medical devices – pacemakers, defibrillator and drug-pump – welding implanted medical devices that incorporate electronics and sensors, require process control and end-product quality that is second to none. These devices must not only function flawlessly, they must have an aesthetic quality that helps with sterilisation and does not irritate the tissue. The welds on these implantable devices must be a hermetic seal very close to heat-sensitive components and since the value of the component before welding is very high, the yield from the welding process must also be high. Pacermakers and defibrillators are electronic devices that are encased in a titanium package that holds the circuitry and battery. Most of the laser welding employed with these units is in the outer case weld. Drug pumps are more complex since at their core they are a mechanical pump and reservoir system along with control electronics and batteries. Laser welding of the different mechanical sub-assemblies and reservoirs means that dozens of laser welds can be part of these systems.

Low-power pulsed YAG lasers (300 W) are the best choice for medical package assembly and sealing. Their high peak powers and low heat input can handle all the different alloys and create a very tolerant process while maintaining deep penetrations. Fibre optic beam delivery is most common for these parts because of the consistent focus spot size and top-hat energy distribution which helps with fillet and butt joints to fill any gaps and get good weld nugget size for maximum yield. It may be also possible to weld some of these medical devices with CW fibre lasers (200 W), but care is needed to optimise the laser parameters because pure CW power may cause distortion of the sensitive parts of the component. Figure 7.41 shows a titanium hearth pacemaker welded with a JK300HP (300 W). The weld penetration is approx. 200–250 μm.

Radioactive isotopes (Fig. 7.42) used for cancer therapy have a rather short half-life and must be produced to order and shipped immediately for implanting into the patient. The capsules or seeds containing radioactive elements are implanted into the body inside or near the cancer itself to produce a much localised radiation treatment. Other radioactive isotopes are used in heart procedures where the radioactivity helps reduce scarring around stents that begin to close the vessel sometime after the stent is inserted. In this case the radioactive material is laser sealed into a capsule and the capsule attached or laser welded to a lead for insertion into the body. It takes several minutes to locally radiate the artery around the stent.

For implantable medical applications, the alloy of choice is titanium which is very easy to weld due to its low thermal conductivity. Its reactivity with oxygen and nitrogen is the only process drawback. Weld shielding with argon is critical for bright silver welds. Pulsed YAG lasers are the best choice for capsule and seed assembly and sealing. Their high peak powers and low heat input create a very tolerant process while maintaining consistent shallow weld penetrations (200 μm).

In microwave packages welding (Fig. 7.43), the microwave circuitry is different from many other electronic devices in that the package around the active elements is also part of the microwave circuit. Since most microwave circuits involve expensive radar or communication systems, they require a very high reliability hermetic seal and a high yield. Therefore, their packaging must be robust, hermetically sealed, and capable of being opened and reworked when needed. Laser welding meets these requirements. A laser weld produces a hermetic seal so that no contaminants can reach the circuits and the weld also traps a very high quality inert atmosphere in the package. Laser welding has very low heat input and can be placed very close to polymer seals, glass-to-metal seals, soldered components and electronic circuits.

For aerospace applications, the alloy of choice for these packages is aluminium. It has the electrical and thermal properties, is easy to machine, and very light weight. 6061-T6 alloy is used for the base of the package and then a 4047 alloy lid is laser welded onto the base with a butt, lap or fillet joint. The 12% silicon-aluminium alloy 4047 can be fusion welded to the 6061 without cracking, creating a strong hermetic seal. For rework, an end mill will machine through the laser weld back to the same level as the original base. Once repaired, another lid can be located on the package and laser welded again. The number of repairs is virtually unlimited. In some instances, a laser weld is required through the lid into dividing labyrinth walls in the base to isolate certain parts of the circuit from others. Some microwave packages can use stainless steel or Kovar components.

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