Soldered electrical connections

Soldered joints present their own characteristic corrosion problems usually in the form of dissimilar metal attack often aided by inadequate flux removal after soldering. Such joints have always been a source of concern to the electrical industry. Lead-containing solders must be used with caution for some types of electrical connection since PbfOHjj.PbCOj may be found as a corrosion product and can interrupt current flow. Indium has been found to be a useful addition to Sn-Pb solders to improve their corrosion resistanceHowever, in view of the toxicity of lead and its alloys, the use of lead solders, particularly in contact with potable waters and foodstuff s, is likely to decline. 

The micro hole array is an arrangement similar to monoliths and particularly to gauzes employed for the same purposes, and hence is termed pgauze in the following. The pgauze strip is inserted in a structured ceramic frame that contains a recess for the strip. Embedded silver and metal solder rods serve for electrical connection via the ceramic material (Figure 3.23). 

Tin (Sn) Tin is a silvery white metal. Solder is an alloy of tin and lead and is used in electrical connections. Stannous fluoride (SnF2) was once a common ingredient in toothpaste, but has largely been replaced by sodium monofluo-rophosphate. 

To reduce expense, efforts are made to exploit integrated thin film technologies. For example, arrays have been produced via thin film deposition of the pyroelectric onto a sacrificial layer, e.g. a suitable metal or polysilicon, which is then selectively etched away. Thermal isolation of the pyroelectric element is achieved through engineering a gap between it and the ROIC silicon wafer. Yias in the supporting layer permit electrical connections to be made between the detector and the wafer via solder bonds. Imaging arrays have been produced in this way incorporating sputtered PST and sol-gel formed PZT films. 

Computers, microprocessors, and other microelectronic devices could not exist as we know them today without the technology of depositing thin metal or alloy films with fine lithographic patterns. For example, in a computer, the individual transistors that make up an integrated circuit must be electrically interconnected by a complex network of conducting lines and vias that are deposited above the semiconductor layers. Furthermore, the chips are joined to multi-chip packaging modules, a process in which many electrical connections are simultaneously established by solder balls. 

All wiring and electrical components used in any control system must be rated for the current and voltage carried. If they are not, there is a risk of overheating components and/or starting a fire on wire insulation, etc. All electrical connections whether mechanical or soldered need to be done well so that shorts do not occur between wires, terminals or soldered joints. 

Applications in which the electrical connections of the sensing element are made by soldering are rare, Ni can be used as contact material in such applications [6], To protect Ni against oxidation, it is often covered by a very thin layer of Au [6], Most applications make use of bond contacts of A1 or Au wires. Frequently used contact materials include Ni (A1 wire), Al, or Au. NiCr is a good adhesion layer for Ni. Therefore, Ni is a popular contact material for NiCr layers, which can then function simultaneously as both a sensing and an adhesion layer. However, Au is used for most automotive applications, because it is the most stable against corrosion. Ti and NiCr are suitable adhesion layers for Au. 

Sacrificial anodes for galvanic cathodic protection are commonly made from magnesium, which from Table 7-4 one can see is the least noble (most anodic) of all the metals listed. Zinc is also utilized, but it has a lower oxidation potential. Good electrical connections (by soldering or brazing) must be made between the sacrificial electrode and the structure to be protected. 

Adhesives may substitute for wire or solder in forming electrical connections for example, in connecting capacitors, resistors, or semiconductor devices to a printed-wiring board or to a thin-film or thick-film hybrid microcircuit. The best and most... 

Surface-mount technology replaces previous methods of inserting component leads into plated through-holes of PWBs, wave soldering from the back side to flow solder into the holes, and simultaneously forming both mechanical and electrical connections. SMT is highly automated and currently the most widely used production process for the assembly of single-layer, double-sided, and multilayer circuit boards. 

After mechanical attachment to a substrate, a leadframe, or to the inside of a package, bare die or chip devices are electrically connected by one of five methods wire bonding, flip-chip bonding, TAB, solder attachment, or attachment with electrically conductive adhesives. Fig. 1.6 shows some of these methods. 

Flip-chip devices have solder bumps, other metal bumps, or even conductive adhesive bumps on the face of the device for I/O connections. During assembly, the devices are flipped face down, then mated and bonded to corresponding solder or metal pads on the package or interconnect substrate. In the quest to eliminate tin-lead solders, electrically conductive epoxy adhesives are beginning to be used for the bumps. 

Chip is attached face down and wire bonded to the interposer. A thin elastomer, sandwiched between the chip and interposer, cushions the chip and the solder-ball interconnects, relieving stresses (see Fig. 1.13). The interposer generally consists of a metallized, flexible polyimide tape on which are formed electrical connections by photolithographic processes. As a final step, the exposed wire bonds and edges of the chip are molded with epoxy. 

For adhesive-attached components or chip devices that are electrically connected by wire, TAB, or ribbon, the leads must first be excised. In the case of solder-connected components, the solder is heated to its melt temperature and wicked off the part. 

The absorption of moisture in underfill adhesives induces a tensile hygrothermal stress on the solder-ball connections causing electrical opens in the connections and cracking in the adhesive. These tensile stresses offset the compressive stresses that underfill adhesives provide in improving the reliability of flip-chip and ball-grid-array devices. 

Silver-filled epoxies and other electrically conductive adhesives are widely used to electrically connect chip devices or packaged components to interconnect substrates or printed-circuit boards. Chip capacitors, resistors, transistors, diodes, and magnetic components may be attached with silver-filled epoxies whose volume resistivities range from 1 x 10 " to 3 x 10 " ohm-cm or with gold-filled epoxies whose volume resistivities are approximately 8 x 10 ohm-cm. Conductive adhesives are also finding use as replacements for solder balls in flip-chip devices. In all cases, to achieve reliable connections, initially low-contact resistances or volume resistivities must remain low on aging and on exposure to operational stress conditions, such as humidity, temperature, vibration, shock, and power. 

Up to now the electrical connection of devices and conducting tracks is made by solder processes, and therefore the adhesives have to withstand temperatures of about 260°C for a short time without remarkable losses in their performance. Up to now, conducting adhesives for electrical connections as an alternative have been nsed in special applications only. 

The potential difference established when two metals (alloys) are electrically connected in a conducting medium produces electron flow and causes the metal (alloy) with more negative potential to preferentially corrode. The more positive metal (alloy) becomes a cathode and is protected by the negative metal (alloy), which becomes an anode (Fig. 1.5). Because the driving force for corrosion is the potential difference between the metals (alloys), this form of corrosion is called galvanic corrosion. Steel fasteners in an aluminum foil, the solder on a copper pipe, and stainless steel in contact with... 

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