Reliability study of cover plate for parallel seam welding

1 Introduction

　　At present, the parallel seam welding process is widely used in various electronic packages with airtightness requirements. Due to the advantages of low temperature rise of the package body during the sealing process, no use of solder, little impact on device performance, and high welding strength, it is widely used in the packaging of electronic components that are sensitive to temperature, such as integrated circuits, hybrid integrated circuits, surface-mounted quartz crystal oscillators, resonators, and surface acoustic wave filters (SAW). The airtightness of the package can reach: leakage rate L≤1×10-3Pa·cm3/S(He), which is a highly reliable capping method and can be used in packages with high airtightness requirements.

　　Although parallel seam welding is a highly reliable capping method, the cover plate for parallel seam welding, as an important component of parallel seam welding, is an important structural material for parallel seam welding, which has an important impact on the airtightness and airtightness yield rate of the package. In order to improve the reliability of parallel seam welding, in addition to the high quality of the package base, a high-quality cover plate is also required. A high-quality parallel seam welding cover plate must have the following characteristics: ① The thermal expansion coefficient is the same as that of the base welding ring and is similar to that of the porcelain body. ② The welding melting point temperature should be as low as possible. ③ Excellent corrosion resistance. ④ Small dimensional error. ⑤ Characteristics such as flatness, smoothness, small burrs, and less contamination. The following will explain several elements that a high-quality parallel seam welding cover plate must have.

　　2 Matching of thermal expansion coefficients

　　The thermal expansion coefficient of the parallel seam welded cover plate mainly depends on the cover plate base material itself. First of all, the choice of the cover plate base material should be determined based on the thermal expansion coefficient of the base welding ring. At present, the largest amount is the alumina ceramic base, followed by the Kovar (KOVAR alloy) base. The metal welding ring that matches the ceramic expansion coefficient is KOVAR alloy or 4J42 iron-nickel alloy. Therefore, the parallel seam welded cover plates produced by our company are all made of KOVAR alloy material, which matches the ceramic base. This material has a high capping yield (≥99.5%). After packaging, the device is subjected to -55℃～+150℃ and 50 cycles, and the gas leakage L≤9×10-8～7×10-10atm·cm3/s (equivalent to 9×10-3～7×10-5Pa·cm3/s(He)).

　　3. Requirements for welding melting point temperature

　　The melting point temperature of parallel seam welding is a key technical issue for protecting the performance of electronic components. We know that semiconductor devices and quartz crystal oscillators are sensitive to high temperatures. If the temperature rise of the package body is too high, the oscillation frequency of the quartz crystal will deviate from the design range, the error will increase, and even the device will fail. In order to protect the performance and yield rate of electronic components such as the packaged circuit and quartz crystal oscillator, efforts must be made to reduce the melting point temperature of welding to reduce the temperature rise generated during sealing. Sometimes the temperature rise is too high, which will cause the ceramic base and the welding ring to crack due to stress, and even cause the metallization of the welding ring to fall off, causing the entire device to be scrapped. Reducing the melting point temperature of welding is conducive to improving the yield and reliability of the sealing cap. The melting point of KOVAR material is 1460℃, and the melting point of iron-nickel alloy 4J42 is as high as 1440℃. If the melting point temperature of parallel seam welding is to be reduced, the problem must be solved on the cover plate coating. Parallel seam welding cover plates are commonly used with nickel plating, gold plating or nickel alloy plating processes. The melting point of pure nickel is 1453℃, and the temperature of pure gold is 1063℃. We began to use electroplated nickel, but found that it was impossible to lower the melting point welding temperature. Because electrolytic nickel is close to pure nickel, its melting point temperature is high, and users need to use a large current when welding, and the weldability is poor. Only alloy nickel can effectively lower the melting point temperature. Electroless nickel alloys include nickel-phosphorus alloy and nickel-boron alloy, and the phosphorus content reaches the lowest at 12.4Wt%. See Figures 1 and 2.

　　As can be seen from Figure 1, the lowest melting point of nickel-phosphorus alloy is 880℃, which is lower than the lowest temperature of nickel-boron at 1093℃. Obviously, it is better to use nickel-phosphorus alloy. However, the phosphorus content should not be too high. If it is too high, the glass phase will increase, the hardness will be too high, and micro cracks will be easily generated during parallel seam welding. The process formula with moderate phosphorus content has achieved satisfactory results in actual production and user use. There is no obvious difference in the use effect of nickel-plated cover plate after high temperature (780℃) annealing in hydrogen furnace and without annealing.

　　Another consideration for choosing chemical nickel-phosphorus plating is that the body resistance and contact resistance of the plating are relatively large. The electrical conductivity of the chemical nickel-phosphorus plating is generally (1.47~2.22)×106n-1·m-1, and the electrical conductivity of the electrolytic plating is 13.7×106n-1·m-1. Obviously, the resistance of the chemical nickel plating layer is greater than that of the electrolytic nickel plating layer. Because parallel seam welding is essentially resistance welding, its resistance is concentrated at the contact point between the electrode and the cover plate during the welding process, that is, the plating part should have a larger resistance. The contact resistance of the plating with a large body resistance is large. In this way, when the pulse current passes through, the heat generated is concentrated at the contact point of the electrode, so that the cover plate and the welding frame at the contact point melt and combine together. Therefore, it is beneficial to have a larger resistance.

　　4 Corrosion resistance

　　As a high-reliability parallel seam welded cover plate, corrosion resistance is very important. Especially when the device works under humid and marine climate conditions. If the corrosion resistance is poor, the device will quickly corrode and perforate and fail. Therefore, people have done a lot of work on the nickel plating process formula and process optimization of the cover plate. It is found that the corrosion resistance of chemical nickel plating is better than that of electrolytic nickel plating under the same conditions of coating thickness, acidic chemical nickel plating is better than alkaline chemical nickel plating, thicker coating is better than thinner coating, only nickel plating is better than nickel plating and then gold plating, and the base material with high finish is better than that with low finish. Many foreign military cover plates with corrosion resistance requirements are only nickel-plated but not gold-plated. In the preliminary test of a crystal oscillator manufacturer, our company's nickel-plated cover plate has passed the corrosion resistance test of 40°C, 90% to 95% relative humidity, and 500 hours, and the results are comparable to similar Japanese products.

　　5 Determination and error of parallel seam welded cover plate size

　　The parallel seam welding process is generally an important process at the end of product production, and its yield rate has a great impact on the cost. Especially in today's large-scale industrial production of electronic components, very stringent requirements are placed on the yield and reliability of the products, and the yield rate must reach more than 99.8%, so higher requirements are placed on the dimensional accuracy of the cover. As a parallel seam welding for IC capping, the cover size should be 0.10-0.20mm smaller than the welding ring size. As a flat seam welding cover for the shell of a quartz crystal oscillator, the size should also be 0.05-0.2mm smaller than the welding ring size. However, after the size is determined, the tolerance of the same batch of products should be within ±0.03mm, otherwise the yield rate in the parallel seam welding process will fluctuate greatly, and even make production unable to proceed normally. 6 Flatness, burrs and surface quality of parallel seam welding cover The parallel seam welding cover requires high flatness, and its material requires flatness and should not be bent. The flatness of the finished cover plate should be less than 0.005mm/mm, and the burr should also be less than 0.005mm, otherwise it will affect the air tightness of the cap, the air tightness yield rate, and the strength of the cap. Surface finish, dust, etc. also have a great impact on the quality of device products. The dust particle size is required to be as small as possible and the number is as small as possible. Especially for high-density packaged integrated circuits and surface acoustic waves (SAW), the dust particle size should be less than 1μ, and the number of particles should not exceed 4. Therefore, the production, coating, inspection and packaging of the cover plate should be carried out in a clean workshop.

　　7 Summary

　　The above describes five issues related to the reliability of the cover package for parallel seam welding. Each of them has a great impact on the quality and reliability of the package, and none of them can be missing. We have done some research and exploration in this regard, and we hope that experts and readers will criticize and correct us if there are any deficiencies. In our work, we have received guidance and help from Professor Jia Songliang of Tsinghua University, Senior Engineer Xu Weiyuan of the Institute of Electronics of the Chinese Academy of Sciences, and Director Ding Rongzheng of the 58th Institute of China Electronics Technology, and we would like to express our deep gratitude!