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Cooling system based on recirculating water has a long history of application in various industrial sectors with its configurational/material simplicity, cost effectiveness and extreme cooling efficiency. While such systems are in use with many variations in system specifics such as the cooling media and component materials, the one with emerging interest is the configuration using microchannel Cu coldplate as a heat-exchanger because of its demand in high performance computing facilities. These facilities have to remove the intense amount of heat generated by a number of CPUs in parallel connection, forcing them to rely on the water based fluid and microchannel Cu coldplate attached to CPU as heat exchanger. One of the critical but less well address concern in such system is the danger of losing reliability by the process of corrosion. The concern exists especially because the Cu coldplate is likely to be subjected to a galvanic corrosion due to the brazing used during the assembly process. The brazing alloy is chemically different from pure Cu and may produce electrochemical potential sufficient to result in accelerated corrosion by the way of galvanic reaction. While such possibility has been noted at early stage of the system development, its risk to the system reliability and practical method of its suppression is not well known with lack of fundamental studies. Motivated by the growing need to better understand the corrosion mechanism active in such system, we have investigated the nature of corrosion in galvanic pair created by the braze. The braze is likely to suffer from galvanic corrosion due to multiple galvanic pairs. As shown in Fig.1-1, where microstructure of braze created by Cu-Ag-P alloy is shown, the braze consists of multiple phases. The inter-phase galvanic potential is expected. Also, the braze and surrounding Cu can form a galvanic pair. The existence of multiple galvanic pairs makes the study of corrosion to be extremely difficult and require new characterization strategies. For this, we have employed two new measurement techniques in order to isolate the galvanic corrosion mechanism from other background corrosion. Using these techniques we carried out extensive studies on braze/Cu corrosion with variation in coolant, filler materials and temperature. The first method was to measure the corrosion current under the configuration of ZRA (zero-resistance-analysis). The ZRA uses the Cu and the joint as the counter/working electrode as shown in Fig.1.2 and measures the galvanic potential and current across the joint. The second is to make a mini-galvanic cell consisting of a pure Cu plate and a Cu plate with the brazed part. Spaced by an O-ring, these two plates are forced to make a galvanic cell with a liquid filled the gap and an external short circuit. We were able to measure the resulting corrosion current, direction and the amount of which can be related to the galvanic corrosion created by the braze. This study presents key findings of ours evidencing that the galvanic corrosion can be better characterized by the use of techniques employed in our study. An example of such evidence can be found in Fig.1-3 where the Cu plate with the braze strip after microcell testing is shown. Note the corrosion happened along the interface between the strip and Cu plate. It is also found that the cell current corelates very well to the degree of corrosion as evidenced in Fig.1-4, enabling quantitative measurement of corrosion rate of the galvanic pair formed between Cu and the braze. A detailed description of such findings along with theoretical explanation will be provide in this paper. Figure 1

The mechanism behind corrosion of Cu and Cu alloys has been the subject of extensive studies within their applications in various engineering components. These studies explore the electrochemical mechanisms along with the contributing factors impacting the corrosion behaviors such as pH, temperature, and solution chemistry. Thanks to these studies, the corrosion behavior of Cu and Cu alloys is reasonably well understood to a degree to predict their reliability under conditions that have been a concern of related industries. However, it is noted that studies are very limited to the impact of a microscopic structure that forms a galvanic pair with Cu and Cu alloys. One of the most obvious but least studied examples of such a case is the corrosion located in the joint that is created by brazing material. The Cu or Cu alloy components with a brazed joint is commonly found in water handling devices and facilities (such as a heat exchanger). Since the filler material used for the brazing of Cu and Cu alloys contains a substantial number of elements other than Cu the joint changes chemically and microstructurally different. It then becomes a source of a galvanic couple. In this case, the most corrosion prone part is the area near or at the joint as the corrosion is accelerated by the galvanic potential. While such joints are abundantly placed in various engineering components (and therefore the concern for corrosion should be focused more on those parts), studies on the corrosion mechanism with the brazing joint are extremely limited. This is probably due to sufficient material redundancy in the joint and thus causing immunity to failure by corrosion. However, a recent development of an advanced cooling system, where heat from electronic devices such as a microcontroller is removed by the use of recirculating coolant, the redundancy is no longer valid and the corrosion at the brazed joint becomes a matter of critical and practical concern. Motivated by the growing need to better understand the corrosion mechanism active in advanced cooling system, we have investigated the nature of corrosion in Cu plate with a variation in filler materials (Cu-Ag eutectic alloy, Cu-Ag-P alloys), liquid type (water, water mixed with PG), and temperatures. One of the most challenging part of our investigation is the fact that there are multiple sources of galvanic potential due to the existence of multiple phases in the sample. For example, when a Cu-Ag-P alloy was chosen as a filler material, the joint contained three phases, Cu-rich, Ag-rich and Cu3P intermetallic phases. An example of such a microstructure is shown in Fig.1, where an SEM micrograph of the joint is displayed. In this case, the galvanic potential exists not only between the filler materials as a whole and the surrounding Cu, but also among these three phases. In order to overcome the difficulty of studying corrosion behaviors in such a structure, we adapted two different experimental approaches. The first is to measure the corrosion current under the configuration of ZRA (zero-resistance-analysis). The ZRA uses the Cu and the joint as the counter/working electrode as shown in Fig.2 and measures the galvanic potential and current across the joint. The second is to make a mini-galvanic cell consisting of a pure Cu plate and a Cu plate with the brazed part. Spaced by an O-ring, these two plates are forced to make a galvanic cell with a liquid filled the gap and an external short circuit. We were able to measure the resulting corrosion current, direction and the amount of which can be related to the galvanic corrosion created by the braze. This study presents some of our highlighting findings suggesting that the galvanic corrosion can be better characterized by the use of techniques employed in our study. An example of such evidence can be found in Fig.3 where the ZRA scan signal (voltage and current) in 1M KCl solution is shown with the filler area as the working and surrounding Cu as the counter electrode. Note the instability in the galvanic potential with time, which is attributable to the lack of sustainable passivation layer on top of Cu3P and is responsible for the complex corrosion response of the brazed part to the solution chemistry. A detailed description of such findings along with theoretical explanation will be provide in this paper. Figure 1

This paper concerns the new method of detecting the integration failures in porous low-k (PLK)/Cu interconnects using simple voltammetry-based techniques. In essence, the technique takes advantage of the fact that pores in PLK allow permeation of liquid, including electrolyte, into interconnect structures. The infiltration of electrolyte allows the formation of a micro-cell, consisting of two mating Cu interconnect electrodes and the electrolyte in PLK, where simple linear voltammetry can examine various integration reliability issues pertinent to PLK/Cu interconnects. Specifically, the technique is proven to be effective in detection of 1) failure in Ta barrier, 2) cracks in the capping layer, and 3) trapped impurity in pores in PLK. The working principle of the voltammetry technique and demonstration of its effectiveness is introduced in this paper.

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