Adhesives bond reliability

However, even with these potential difficulties, adhesive bonding can be an easy and reliable method of fastening one type of plastic to itself, to another plastic, or to a nonplastic substrate. Pocius et al. provides an excellent treatise on the use of adhesives in joining plastics.51... 

Surface preparations must be carefully controlled for reliable production of adhesive-bonded parts. If a chemical surface treatment is required, the process must be monitored for proper sequence, bath temperature, solution concentration, and contaminants. If sand or grit blasting is employed, the abrasive must be changed regularly. An adequate supply of clean wiping cloths for solvent cleaning is also mandatory. Checks should be made to determine if cleaning cloths or solvent containers have become contaminated. 

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. 

The process steps to ensure high-quality, reliable adhesive bonds are ... 

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. 

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. 

Although important for structural adhesive bonds, fracture mechanics is not as critical for non-structural low load-bearing adhesives as used in most electronic modules. For the most part, passing minimum specification requirements for peel and tensile strengths both at ambient conditions and accelerated test conditions are sufficient. However, computer-simulated modeling and reliability analysis have been used for evaluating electrically conductive adhesives as used in electronics assembly. ... 

The reliability of electronic systems depends on extensive testing, inspection, and evaluation of all parts of the design and manufacturing process, including the adhesives used in the systems. Test methods for adhesives and adhesive-bonded assemblies may be generally classified as physical, electrical, environmental, thermal, mechanical or thermomechanical, and chemical (Table 7.1). This chapter summarizes most of the methods used to test and evaluate adhesives used in electronics. It is not intended to provide detailed instructions, but to lead the reader to the appropriate government or industry specifications, documents that are available from the various organizations listed in Table 7.2. 

Moisture absorption in uncured and cured adhesives is important for reliable adhesive bonding. Absorbed moisture in adhesives, particularly those used in surface-mounted parts, may result in voids in the bond hne due to moisture desorption or entrapment. Absorption of moisture after cure also affects reliability. Popcoming, the cracking of plastic parts during solder reflow, is due to the rapid vaporization and expulsion of moisture retained by the die-attach adhesive or the plastic encapsulant. 

For the future the evolution of adhesive bonding as a joining technique in automobile production points in two directions. On the one hand, well-known applications have to be optimized and improved to make them eost-elfeetive but nevertheless reliable and trustful processes enjoying increasing acceptanee for adhesive bonding. On the other hand, there will be new applications with different adhesive requirements, and adhesive suppliers must anticipate these changes and develop eompatible adhesive eompounds to satisfy the new requirements. 

These factors all favour materials such as composite resins, which exploit the growing technical knowledge of adhesion and the improved performance of bonding agents for their success. Current materials available for bonding are formulated with the aim of providing reliable adhesive bonds that are, in principle, capable of lasting for many... 

It has often been observed that the application of adhesives to metal fabrication, in common with many other technological innovations, was pioneered by the aircraft industry. It is ironic that this industry, in which safety and reliability command paramount attention, should lead the departure from traditional methods of joining. Today adhesives are used to bond critical parts in commercial and military aircraft and helicopters, spacecraft, rockets, missiles and the US Space Shuttle. The American Primary Adhesively Bonded Structure Technology (PABST) Programme, which ran from 1976-81, was an imaginative attempt to advance significantly the use of bonded... 

The author would like to thank the many researchers who responded to the questionnaire for the National Materials Advisory Board Workshop on "Reliability of Adhesive Bonds Under Severe Environments. Some of their names appear in the references. A summary of this paper, presented to the Workshop held on May 3-4, 1984 at the National Academy of Sciences, will become a part of the Committee Report to be published separately. 

The second step, after determining the quality of incoming materials, is adherend surface preparation. Surface preparation must be carefully controlled for reliable production of adhesive-bonded parts. 

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