HERMETIC SEALING USING LOW IMPEDANCE PROJECTION WELDING. By T.E.Salzer

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1 HERMETIC SEALING USING LOW IMPEDANCE PROJECTION WELDING By T.E.Salzer Over many years, various studies, articles, and inventions have been published relating the dynamics of resistance/projection welding parameters to the quality of the weld. Monitoring equipment is available to measure the important electrical and mechanical aspects involved in the formation of a weld, and from previously documented data, predict properties of the weld. Parameter measurements have been used in feedback loops to provide desired weld properties on a more consistent basis. This paper will describe a new class of resistance/projection welder and some of its technical features. Concerns with conventional welding equipment 1. Limitations of feedback. A concern with the use of feedback regulated welders is that projection welding of electronic packages generally requires 15-20kA of weld current for each linear inch of seal. Packages approaching 10.0 linear inch perimeters represent a significant quantity of current to regulate with a feedback system. In fact, no company currently offers projection welding sealing equipment that utilizes feedback regulation. 2. Lack of dynamic current regulation - excessive expulsion. This subject will be covered in greater detail, but our testing has demonstrated significant advantages of low impedance equipment over conventional welding equipment. 3. Low efficiency. This will be covered more extensively; however, with conventional welding equipment 25-90% of the energy utilized from the mains is wasted. 4. Limited choice of weld time. A final concern is that the use any welder requires a tradeoff between weld current and time. Most sealing, benefits from short welding time to minimize thermal shock to adjacent glass seals and components. Yet, if a machine is not capable of delivering the required weld current in a short time, there may be no other choice than to extend the weld time and increase the attendant thermal risk. Addressing the concerns We will show how the use of a low impedance welding machine mitigates these concerns. In addition, users of this equipment enjoy an increase in the percentage of good welds. Most find it preferable to experience a yield improvement, over the ability to identify and quarantine defective welds. Adopting low impedance welding technology does not negate the use of parameter measuring equipment to monitor the effectiveness of

2 the machine; however, experience has shown that both quality and yield demonstrate a significant increase. There are likely several explanations to account for the improvement. One reason we have studied is that the use of a low impedance welder shifts the maximum heating rate of the weld joint toward the initiation of the welding cycle. It is widely accepted that under adiabatic conditions, a fixed amount of energy is required to raise the temperature of a weld from ambient to weld temperature within a certain time. Conventional welders deliver lower power at the beginning of the weld cycle with the power dissipation increasing as the weld metal gets hot and the electrical resistance of the weld increases. Thus, toward the end of the weld cycle, the power dissipation is greatest and the rate of temperature rise is at a maximum. The result is rapid collapse of the weld projection which means the weld electrodes must accelerate rapidly to maintain adequate force on the melting projection. This is the time when molten metal particles may be expelled from the hot weld zone. Expulsion increases the risk of a defective weld. Expelled particles can also solidify inside the sealed part causing contamination and PIND failures. When making an identical weld with a low impedance machine, more power is dissipated at the beginning of the weld than as the weld approaches melting temperature. The reason is that with the low impedance process, the maximum power dissipation is controlled by the resistance of the weld, not by the impedance of the welding equipment. As the weld approaches the desired temperature, its resistance increases and both the weld current and power are commensurately and automatically reduced. Therefore, when using a low impedance welder, the heating rate is greatest at the beginning of the weld cycle and reduced as the weld proceeds. The result is more accurate control of the weld temperature profile and less rapid collapse of the weld projection. This means that the weld electrodes need not accelerate as rapidly as when using the higher impedance welder. Use of a low inertia, fast follow-up weld head is one approach to deal with the rapidly collapsing projections; however, test results indicate that low inertia in combination with controlled heating rate is more effective. It will become apparent that in low impedance welding, the changing resistance of the weld itself automatically regulates the magnitude of the weld current. The result is similar to what would be obtained using a feedback loop, but without the complexity or requirement for high power regulating circuitry. For those interested in specifics, the ratio of the resistivity of welded metals at their melting temperature to their resistivity at room temperature is about 10/1. Specific improvements resulting from a change to low impedance welding include a significant decrease in metal particle expulsion, an increase in weld strength, uniformity, and a major reduction in the amount of energy required to produce the weld. In conventional projection welding, a large percentage (typically 25-90%) of the energy taken from the mains is used to generate unnecessary magnetic fields. To the extent these fields are minimized, the impedance of the machine can be reduced and efficiency increased. This often-overlooked aspect is inherited from legacy welder designs that required a throat or gap into which, parts with certain dimensions could be inserted for welding. Thus, a weld in the center of a 4 x 8 sheet required at least a 2 throat. When sealing small parts, the need for a throat vanishes. The welder can actually surround the part, and only access for loading and unloading is required.

3 Illustration 1 - Low Impedance Welder in Dry Box An example of an industry affected by failure to practice the principles of impedance control in resistance welding is semiconductor packaging. Early semiconductors were often hermetically sealed using projection welding. The weld perimeters were usually 1.0 inch or less. As the industry matured, larger packages were developed to hold larger semiconductors, crystals, hybrid circuits, etc. As package perimeters increased (currently approaching 10.0 inches), more weld current was required to weld them. The welder manufacturers responded by designing larger, more powerful welding machines. Users of these larger machines found that although they provided sufficient welding current, all process parameters became more critical. What equipment manufacturers failed to comprehend was that as package perimeters increased, the resistance of the welds decreased. While package size was increasing and the resistance of the welds decreasing, the output impedance of the larger welding machines was remaining constant, and sometimes increasing. Only since the 21st century development of low impedance welder technology, was it determined that large packages could be welded with the same process insensitive parameters that had been used to weld small packages some 50 years earlier. While electronic packaging technology has largely by-passed many of these semiconductor sealing issues, hermetic sealing with projection welding is still practiced today, and this technology still represents one of the most robust and cost effective hermetic packaging options as shown in the second illustration.

4 Illustration 2 Assorted Hermetic Welded Electronic and Electro-Optic Packages Diaphragms for burst discs, SAW devices, electro-optical devices, sensors, transducers, battery cell connections, microsensors and microsystems packaging, electrical connectors, medical devices and feedthrus are contemporary heirs to this advancement in hermetic welding technology. It should be noted that when cross wire welding high resistivity materials or spot welding thick metals, the resistance of the welds may be high enough that there is no advantage to using a low impedance welder. The advantages of a low impedance welder become more apparent when welding low resistance materials and combinations such as the ones previously listed. A procedure to evaluate whether a welding process can benefit from a low impedance welder is to measure the peak weld current while making a sample weld. Then discharge the welder at the same settings with the electrodes shorted. If the current is substantially the same under both conditions, the process could benefit from a lower impedance weld system. As the output impedance of the machine begins to approach the resistance of the weld, the whole process becomes more efficient and the amount of energy required to make the weld is reduced. It is common to find 75% or more of the electrical energy delivered from the power supply, used to generate a magnetic field instead of making a weld. Our final concern is related to weld time. Analysis and testing have shown there is a sweet spot in the weld current/time regime. We have designed the output characteristics of the Centaur welder to fall in the center of that sweet spot. Short circuit current of about

5 200,000 amperes is available from this surprisingly compact machine. Pulse rise and fall times have proven to be suitable for all applications encountered to date. Because welding equipment represents a long life capital investment, some potential customers opt to keep their old welding equipment instead of investing in modern low impedance technology. Although it may be true that ROI is difficult to justify from an energy standpoint alone, others have determined that the yield and quality increase obtained by properly matching equipment to application, represents money well spent. Demonstrations are available at our facility and comparisons against conventional equipment are encouraged. This introduction to low impedance welding is only the tip of the iceberg. For additional information, please call Hermetric, Inc. at