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Low-Impedance Resistance Welded Hermetic Seals

How the quality of longer perimeter resistance welded seals benefit from impedance reduction of the welding equipment By Thomas E. Salzer, president of Hermetric, Inc., Bedford, Mass. Reprinted with permission: The AWS Welding Journal Numerous applications exist for resistance spot welding, the most ubiquitous being in the automotive assembly field. When individual spot welds have insufficient strength, or don’t provide a leakproof seal, we may turn to alternate forms of welding including, but not limited to, resistance seam welding, laser beam welding, electron beam welding, various forms of arc welding, ultrasonic, etc. There are, however, applications when elongated spot welds can be accomplished with a single electrical discharge that performs the entire operation, saving the time required for alternate forms of seam welding technologies to traverse the entire weld perimeter. In addition, many of these applications involve close proximity of the molten weld metal to thermally sensitive locations such as glass-to-metal seals. Typical applications include semiconductors, hybrid circuits, photonic devices, medical devices, transducers, bulkhead seals, fuel rod seals, and diaphragm seals. Questions we need to ask when contemplating the use of elongated resistance welds are as follows: 1) The concern of expulsion. In many applications, weld expulsion results in contamination of the weld area that causes electrical and mechanical malfunctions that affect reliability, 2) the welding process must not excessively heat the sealed container or its contents, and 3) the effects of residual stress caused by the welding process. This article is a follow-up illustrating production applications for a low-impedance welding technology described in a previous article in the March 2004 issue of the Welding Journal. The welding system we describe here resolves these concerns in a unique and elegant manner and is described in detail by U.S. Patent No. 6756558. Early semiconductors were frequently hermetically sealed, using resistance welding, in dry gas-filled metal packages. There were good reasons for this, and much of the technology applies today. Many millions of semiconductors were packaged by resistance welding in packages, generically known as JETEC TO-XX packages. The technology was low cost, clean, high strength, required no noble metals or preforms, and delivered reliable results. As both device size, and I/O counts grew, and new packaging technologies evolved, it was learned that the original resistance welding technology that had served so well in the past was not ideal for welding the larger packages required for advanced systems. Larger packages required larger welding equipment. Although larger resistance welding equipment is available, users found that weld-related defects such as cracked glass-to-metal seals, leaks, and particle impact noise detection rejects caused by weld expulsion, could defeat attempts to meet required reliability specifications. Years of design, testing, and using commercial welding equipment have taught us that much of the problem is caused by the relationship between the resistance of the welding equipment and the weld itself. Specifically, it is because the resistance of the weld can be low relative to the resistance of the welding machine. The difficulties encountered in reliable sealing of larger packages has caused many manufacturers to turn to slower processes such as various forms of seam welding, including parallel seam and laser beam welding. Such seam technologies represent methods of impedance matching by making many small overlapping spot welds. These technologies work well, but can’t provide the speed and cost advantages of a weld that can seal the entire perimeter with a single discharge lasting a few milliseconds. In theory, both seam and laser welding could be accomplished with single-shot welding technology; however, single-discharge resistance welding remains the most cost effective and reliable method available.  The welding system we describe resolves all manufacturing requirements in a unique and elegant manner. The thought that the electrical and physical characteristics of resistance welding equipment could affect the quality of a weld is relatively recent. As recently as three decades ago, experts (Ref. 1) were attempting to quantify the characteristics of resistance welds while neglecting to specify the welding machine type or manufacturer. That all changed with the development of low-impedance welding equipment by Brown and Salem (Refs. 3, 4). Still, the performance of these “constant voltage” welding machines has been dependent on the use of voltage feedback to achieve low-impedance operation, and the maximum welding current has been limited to perhaps 20 kA and required the use of high current mains. Our goal was to design, construct, and utilize resistance welding equipment that displayed the low-impedance welding characteristics of constant voltage operation without the weld current limitations posed by constant voltage welding equipment as well as the requirement for high-current mains. Previous investigations had determined that the electrical resistance of a weld is inversely proportional to its perimeter. It had also been shown that low-expulsion welds can be made when the source resistance of the welding machine is about equal to the resistance of the weld. Although we speak of the resistance of a welding machine and a weld, we know that both of these effects combine to restrict the flow of weld current, so we call the total effect by its proper name, impedance. The impedance of the welding machine is especially important because it can significantly affect the quality of the weld, and because it can be changed. Manufacturers of welding equipment have designed equipment with ten times the current rating, but they haven’t designed equipment with one-tenth the impedance. Reasons for this include the facts that 1) most resistance welding machines use output transformers with impedances of 125 to 500 µΩ, and 2) all weld equipment designs involve parasitic impedance losses that increase their electrical impedance. The design and construction of the welding system illustrated in Fig. 1 has been specifically engineered to minimize parasitic impedance by methods described in U.S. Patent No. 6756558, and is also useful for welding any form of continuous or multi-projection weld such as glass-to-metal bulkhead seals, diaphragm and burst discs, and projection welded hardware seals. It has more recently been used to hermetically seal more than 1,000,000 microelectronic components with no reported failures. The Importance of Impedance Control in Resistance Welding The concepts of constant voltage, power, and current are well understood in the field of welding smaller components using lower power welding equipment. One simply makes test welds using the three available weld modes. The mode that produces the best welds is selected and the process is optimized around that mode. When welding larger parts with welding machines that deliver many tens of thousands of amps of weld current, no mode switch is available. The source impedance has been built into the welding machine and is rarely specified. Depending on the parts being welded, this impedance can be relatively high compared to the impedance of the weld. Welding low-impedance components with a high-impedance (constant current) machine often causes expulsion because the weld current doesn’t know or care about all the minuscule variations in the surfaces between the welding electrodes. To reduce expulsion, the normal recommendation is to increase both the weld force and current. Increasing the weld force is required in order to smooth out the metal features that cause the expulsion. However, by reducing impedance of the welding machine, the voltage across the weld is reduced, which reduces the onset of expulsion, and the mechanical features that previously caused expulsion are heated more slowly and permitted to melt, instead of turning into sparks and vapor. It also means that the voltage drops between microscopic irregularities are smaller. This reduction of voltage across the weld provides a significant reduction in the onset of expulsion, which results in an increase in the width of weld lobes. Wider weld lobes desensitize process parameters in the desired direction (higher weld current with less expulsion), and represent the fundamental principle behind low-impedance welding and result in significantly improved weld quality with significantly less expulsion. It also permits a reduction in required weld force, which results in significantly longer electrode life, and minimizes the need for rapid follow-up mechanisms. Others have attempted to simulate low-impedance operation using “close coupled” or “impedance matched” technologies, and/or by use of multiple paralleled transformers. Our view is that minimizing overall welding machine impedance requires a holistic design approach that involves all parts of the welding machine from the transformer to the bus bars, to the welding electrodes. Even if one is able to shorten the bus bars by a factor of two, the advantage will be small if the transformer has excessive parasitic impedance losses. Measuring Impedance Mismatch We are frequently asked if there is a method to determine whether a lower-impedance welding technology will be advantageous in any particular situation. To answer this question, we recommend that the welding machine be equipped with a peak current monitor. We normally use a memory oscilloscope because it also captures details of the weld waveform. If a monitor is not part of the welding machine instrumentation package, it is easily added. We use a Rogowski coil and integrator; however, there are many good current measuring systems on the market. The first thing one needs to do is to measure and record the peak weld current while making a weld. Then, measure and record the peak current at the same settings with the welding electrodes short circuited. If the peak weld current is less than 5% lower than the short circuit current, the welding machine is behaving like a constant current machine, and is at increased risk of producing expulsion and defective welds. On the other hand, if the weld current is 30% or less than the short circuit current, the machine is acting like a low-impedance machine and should provide satisfactory results. An advantage of this method is that one need not count on the intentions or recommendations of the welding machine vendor to determine the suitability of a welding machine for a particular application. To date, well over 1,000,000 parts have been hermetically welded using the Centaur system, with no reported failures. Every resistance welded component has an electrical resistance/impedance. The amount of the impedance is a function of both the material and the geometry of the parts being welded. For example, auto-body spot welds have been shown to have an electrical impedance of 100 to 200 µΩ (Refs. 1, 2). Traditional resistance welding equipment also has impedance roughly equivalent to these values, making this welding equipment a good impedance match for automotive-type welds. On the other hand, the impedance of a long perimeter seal can be less than an order of magnitude lower than a conventional automotive spot weld, and therefore requires the use of lower-impedance welding equipment to achieve satisfactory results. Attempting to weld large perimeter packages using an automotive-type welding machine is likely to result in unwanted expulsion and leak failures that degrade the performance and reliability of the welds. Armed with this information, one can clearly deduce why the term, “low-impedance welding machine” means different things to different people. Little, if any, information suggesting a relationship between welding machine impedance and weld length has ever been published. Perhaps this explains the reluctance of manufacturers to invest in low-impedance equipment designs and to not disclose the impedance values of their machines. We only learned of the relationship between package size and welding machine impedance after extensive evaluation and testing. To make things even more complicated, resistance welding machine manufacturers don’t normally provide data on the impedance of their equipment in their specifications, so even if users knew they require a machine with an impedance of 50 µΩ, they won’t find that information on any data sheet. Of course there are reasons for this. However, those reasons are not part of this article and should be addressed to the welding machine manufacturers. As the impedance of a resistance welding machine is reduced, the rate of welding temperature rise (dT/dt) is reduced. This results in reduction of the static weld force required to make good welds, as well as minimizes the requirement for fast follow-up devices required to maintain weld force on rapidly melting welds. In addition, the voltage drop across the weld is reduced, minimizing the onset of the expulsion event. The overall effect is one of significantly extending electrode life as well as producing higher quality welds and wider weld lobes. Another benefit of lowering the impedance of a resistance welding machine is that the lower rate of temperature rise can eliminate the need for “dual pulse” or “up-slope” because the weld current amplitude is regulated by the voltage across the weld. Applications for Low-Impedance Hermetic Seals Applications for low-impedance welded seals include, but are not limited to, semiconductors, oscillators, hybrid circuits, electro-optics, crystals, transducers, bulkhead feedthrough seals, and projection welded hardware seals. Advantages of low-impedance resistance welded sealing technology are
  • High-speed sealing. Seals entire perimeter in a few milliseconds.
  • Low-impedance operation provides large reduction of expulsion, increased efficiency, and increased weld current.
  • Truly hermetic seals.
  • No start-up or warm-up time required, as in oven or furnace seals.
  • Low-cost process. No plating, preforms, or precious metals required.
  • High strength (used in deep well applications from vacuum to more than 50 ksi).
  • Fast welding pulses (0.005–0.025 s) eliminate thermal shock to adjacent glass seals and components.
  • Lower required weld force provides extended electrode life, eliminates the need for fast follow-up devices, and provides larger area weld lobes.
  • Stainless steel lids eliminate corrosion concerns and require no plating.
  • Small footprint and low machine installation cost.
  • High efficiency means minimal cooling requirement.
  • Operates from 110 V through 15-A outlet.
Conclusion It may be difficult to believe that the difference between constant voltage and constant current resistance welding impedance levels could be less than 400 µΩ, but it’s true. If you are welding smaller parts such as TO-5, TO-18, and TO-56 and are satisfied with the operation of your welding process, there is probably little advantage to changing it. However, if you are attempting to weld larger (lower impedance) parts as described in this article, and wish to reduce the number of defects, you will likely experience significant benefits from changing to a lower-impedance welding system. References
  1. Gedeon, S. A., Sorenson, C. D., Ulrich, K. T., and Eagar, T. W. 1987. Measurement of dynamic electrical and mechanical properties of resistance spot welds. Welding Journal 66(12): 387-s.
  2. Salzer, T. E. 2004. Optimizingprojection welding for hermetic sealing. Welding Journal 42(3): 42–46.
  3. Brown, L. J., and Lin, J. 2005. Power supply designed for small scale resistance spot welding. Welding Journal: 32(7): 32–37.
  4. Salem, M. Control and power supply for resistance spot welding (RSW) ir.lib.uwo.ca/cgi/viewcontent.cgi?article=1129&context=etd.
  5. Zhang and Senakra. 2006. Resistance welding fundamentals and applications. CRC pp 347-351.

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