Solder-based interconnection

Prior to 1950, these industries were based on vacuum tube technology, and most electronic gear was assembled on metal chassis with mechanical attachment, soldering, and hand wiring. All the components of pretransistor electronic products—vacuum tubes, capacitors, inductors, and resistors— were manufactured by mechanical processes. A rapid evolution occurred after the invention of the transistor and the monolithic integrated circuit. Today s electronic equipment is filled with integrated circuits, interconnection boards, and other devices that are all manufactured by chemical processes. The medium used for the transmission of information and data over dis-... 

Currently, there is no unique solution to lead-free C4 material that is available at this time. Based on the above discussions, it is anticipated that the selection of lead-free solder for C4s will be dictated by the interconnect materials in the chip and by the selected packaging solution. Therefore, the lead-free C4 material selection is expected to be application-specific. 

In electronics production, two main principles of interconnection are used soldering using metal-based alloys and adhesive bonding with electrically conductive adhesives (Rahn 1993). 

To reduce the thermal stress for components as well as for thermoplastic base substrates, conductive adhesives and selective soldering can be used. In working with adhesives, good interconnections with a maximum temperature of only about lOCfC can be established. This temperature is necessary to harden the epoxy material. In the selective soldering technique, the heat is transferred only to places where interconnections have to be made between the component s termination and the pad on the substrate. This reduces the overall thermal stress situation for the whole 3D MID. 

Flex interposer-based CSPs. This CSP type uses a flexible circuit having solder balls or metal bumps as an interconnect interposer between the chip and the next circuit board level. The bare chip is attached facedown 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. 

ACA Bumped Flip Chips for High Frequency Applications. The high frequency behavior of ACA interconnections has attracted much attention in the past several years. Sihlbom and co-workers demonstrated that ACA-bonded flip chips can provide performance equivalent to solder flip chips in the frequency range of 45 MHz to 2 GHz on FR4 chip carriers and 1 to 21 GHz on a high frequency Telfon-based chip carrier (Fig. 5). The different particle sizes and materials in the conductive adhesives gave little difference in high frequency behavior of ACA joints (22,23). 

Yim and co-workers developed a microwave frequency model for ACF-based flip chip joints based on microwave network analysis and S-parameter measurements. By using this model, high frequency behavior of ACF flip chip interconnections with two filler particles, Ni and Au-coated polymer particles, was simulated. It was predicted that Au-coated polymer-particle-fllled ACF flip chip interconnections exhibited comparable transfer and loss characteristics to solder bumped flip chips up to about 13 GHz and thus they can be used for up to 13 GHz, but Ni-filled ACF joints can only be used for up to 8 GHz because the Ni particle has a higher inductance compared to the Au-coated particle. Polymeric resins with a low dielectric constant and conductive particles with low inductance are desirable for high resonance frequency applications (24). 

A high-temperature masking tape can be used to cover LGA lands on the PWB. This will protect it from flux residue, spattered solder, and so on. The tape should be capable of sustaining multiple reflow cycles. A polyimide tape with acryhc adhesive is recommended to minimize tape adhesive residue. Silicone-based adhesives have been found to leave residues that can interfere with electrical interconnect. Since silicone is largely insoluble in most common solvents, attempts to clean any residue will be futile. 

Thermal cycle experiments have been the current industry standard for assessing second-level interconnect reliability. These t5q)es of tests tend to produce solder joint and solder joint-pad interfacial fractures that are t5q)icaUy seen in field failures. The fact that thermal cycle tests produce the same physical failures allows for detailed acceleration transformations and finite element-based Ufe assessments to be made employing thermal cycle data as input. 

The cofirable metal systems based on W, Mo, and Mn are not able to be interconnected with traditional methods, that is they are not wettable with solder systems or wire- or ribbon-bondable. Therefore, these packages require postfire plating, electrolytic or electroless, with a Ni base layer followed by a thin Au layer to provide solder-wettable and wire-bondable terminations for interconnection. Electrolytic plating requires electrical connection to all pads requiring plating. The electroless systems plate all exposed metal surfaces. 

Base Metal Conductors. Copper conductors have gained some acceptance over the past decade because of their relatively low metal cost, low resistivity, good adhesion on AljOj substrates, excellent solder leach resistance, and low migration tendency. Advances in compatible thick film dielectric formulations have resulted in significant use of Cu in multilayer interconnect boards, primarily for military applications. Also, uses of Cu conductor materials have included power hybrid and microwave-related applications. Their applications in more complex systems and networks have been limited by the availability of state-of-the-art nitrogen-firable resistor systems. However, there are additional factors that complicate the widespread usage of this versatile material. 

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