Reliability of the adhesion

Cobalt stearate (or other cobalt salts) is sometimes used as rubber compounding ingredients to improve rubber-to-brass steel tire cord adhesion under certain circumstances. Commonly, a careful use of cobalt soap such as cobalt stearate may actually improve certain adhesion characteristics if it is used properly. Since rubber-substrate adhesion is a variable phenomenon, many technologists feel that the contribution of cobalt is to improve the reliability of the adhesion rather than the adhesion per se. Over the past three decades, this reliability of adhesion has been found to be of much importance in the manufacture of steel-wlre-reinforced tires and other rubber products. Thus the end result is a greater consistency of product quality, with fewer production rejects and subsequent failures in actual service. 

All the other test requirements deal with the performance and reliability of the adhesive joint, starting with the cure schedule that very often conditions the quality of the attachment. The bond strength requirements have been established to ensure that the assembly can be subjected to the subsequent manufacturing steps... 

There are at least three important requirements for the apphcation of conductive adhesives in the electrical industry. The first one is the electrical resistance. Because of the typical low electrical conductivity of the polymer materials, a high electrical conductivity or low resistivity is definitely the most important factor for the conductive adhesive. Second, the thermal properties of the adhesives should be carefully considered. The bonding process with the adhesives is optimized by their thermal properties, e.g., their Tg and curing behavior. Finally, thermo-mechanical properties of the adhesives would be discussed in this chapter. The thermo-mechanical properties determine the long-term reliability of the adhesive interconnection, so it is necessary to review how the thermo-mechanical properties can be measured and the results analyzed. 

AU the other test requirements deal with the performance and reliability of the adhesive joint, starting with the cure schedule that very often... 

PMDA-ODA on Si02. It is clear from Fig. 3 that the adhesion of PMDA-ODA to SiO, surface is significantly improved by the application of APS. This is not only seen initially but also after exposure to extended times at T H conditions, i.e. the reliability of the interface has been improved. Notice the spontaneous delamination (zero peel strength) of the PMDA-ODA film from non-APS treated silica surface after only 100 h in T H. It should be pointed out here that the 100 h exposure was the first point at which the samples were removed from the T H test chamber. It is possible that the delamination may have occurred much earlier than the 100 h reported here. Table 2 shows the locus of failure analysis results for the interfaces after initial peel and after exposure to T H for 100 (no APS only) and 700 h. 

Time and economics generally allow only short-term tests to verify the selection of the adhesive system relative to the environment. It is tempting to try to accelerate service life in the laboratory by increasing temperature or humidity, for example, and then to extrapolate the results to actual conditions. However, often too many interdependent variables and modes of potential adhesive degradation are in operation, and a reliable estimate of life using simple extrapolation techniques cannot be achieved. 

There are two categories of common tests for adhesives fundamental property tests and end-use tests. End-use tests, such as peel and shear, are those that try to simulate the type of loading and service conditions to which a joint will be subjected. These tests are relatively straightforward, but experience is required to establish the correct sample type and testing procedures, judge the reliability of the resulting data, and interpret the results and apply them to a practical application. 

Because the fracture toughness depends both on cure time and temperature, the arbitrary selection of time and temperature for accelerated tests may not be appropriate for reliable prediction of longterm service life of joints (J7). In order to reduce test variability and improve the durability prediction of adhesive joints, it would be necessary first to control the cure temperature and time required to produce a level of fracture toughness that does not change further (14). The study is thus an excellent example of a multidisciplinary approach combining chemistry, fracture mechanics, and wood science in the investigation of the adhesive bonding of wood. 

As mentioned above, application of the additivity law (eq. (1.5)) supposes a knowledge of the number of the components (or phases) with given microhardnesses and weight fractions. What is not explicitly given in this equation is the type and the extent of mutual dispersion of the components as well as the quality of the adhesion on the contact surface boundary between the components (phases). We wish to stress here that this has an influence on the reliability of the H values derived from the additivity law. 

All these factors fall into two classes those that affect the conditioning action, and those that affect the lifetime and reliability of the conditioning disk. The first class will be the major concern here the adhesion of the diamond to the mounting is the only effect from the second class that will be dealt with in this section. 

For that reason, the properties of stainless steel joints bonded with epoxy systems are of special interest. According to the literature, the aging behavior of such joints is critical [1, 2]. Mechanical tests reveal that the combined attack of water and temperature causes a strong deterioration in their performance [3]. It is necessary to understand the processes that are going on during aging in order to improve the reliability of structural adhesive bonding. 

Adhesion at the interface of the polymer-solid is a determining factor for the physical and mechanical properties of polymer composites. Many properties of particulate-filled composites are determined by the adhesion level at the filler-matrix interface. The problems involved in adhesion are very complicated and adhesion cannot be described by any single theory [1]. Many properties of particulate-filled composites are determined by the adhesion level at the filler-matrix interface [2]. However, no theory allows calculation of the adhesion strength of joints from the data on the nature of substrate and polymer or reliable calculation of the energy of the adhesive interaction. The reason for it is the number of factors that influence adhesion [3-9]. 

The prime function of adhesives is to mechanically attach or bond devices, components, heat sinks, wire, connectors, and other parts onto a circuit board or an interconnect substrate. Adhesives are also used as pastes or films to attach lids in sealing cavity packages and as dielectric films in fabricating multilayer interconnect substrates. The most important consideration in obtaining a reliable adhesive bond is the ability of the adhesive to flow and wet the surfaces. For a reliable bond, strong adhesion to both surfaces and strong cohesion within the adhesive are necessary. 

After the solder has been reflowed and the adhesive has been cured, the adhesive has served its primary function of holding the components in position during solder reflow. However, since the adhesive remains permanently on the board, its cured properties must be such that it will not reduce the reliability of the assembly or cause subsequent failures. 

The reliability of an adhesive and its impact on the performance of an electronic assembly should be considered in the initial selection of the adhesive and the design of the system. The function that the adhesive must perform for a specific application, the environment it is expected to encounter, and its duration are all important. Various approaches may be used to predict and assure reliability. Key among these approaches is a basic understanding of possible failure modes and mechanisms. Most failure modes attributed to adhesives are now well understood and documented so that they can be avoided in the initial selection and qualification of the adhesive and in its processing. 

A reliability concern with semiconductor devices, such as chip transistors in which one of the connections is made through the backside metallization by attaching with either eutectic alloy or conductive epoxy, is the loss of backside ohmic contact. Loss of ohmic contact may be due either to mechanical/physical or chemical mechanisms. Mechanical failures occur from partial or complete delamination of the adhesive interface. The smooth surface of the die metallization contributes to the problem whereas roughening the surface improves results. Loss of contact is evident from increases in resistance during initial electrical testing, but may better be detected... 

It should be noted that in using MIL-HBK-217, the rehability predictions are based on empirical data for different component types and do not necessarily take into account the rehabihty of adhesive attached and interconnected components. The effect of the adhesive and its various possible failure modes and mechanisms on the reliability of devices under both operating conditions and long-term accelerated conditions should be considered a part of the equation. 

For Class K (space-grade), the additional tests of nondestructive wire bond pull, FIND, and radiography are imposed. Screen tests assure the reliability of the entire electronic product of which the adhesive joints are a part. 

Reliability of the electrical properties of silicone-based isotropic adhesives has been the major difficulty to overcome and has essentially prevented commercialization. Another problem associated with silicones is that the addition polymerization reaction of silicones must be carefully controlled to prevent cure inhibition from various common chemical contaminants such as amines and sulfides. Other concerns include low-molecular-weight silicone polymer migration onto wirebond pads and very high GTE. There has been some activity in the development of hybrid resins that contain silicone blocks as comonomer with epoxies such that the epoxy processing can be maintained with the added stress reduction property of the silicones [52]. 

The reliability of conductive adhesive electrical interconnections depends on the individual formulation and process employed [55]. In addition, the test vehicle configuration will strongly influence results. No comprehensive studies have been published, however, and no attempts to correlate chemical composition or a specific process variable to reliability performance have been reported. 

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