Assembly acceptability Adhesives

Specifications are a necessary part of a quality control program. A specification simply is a statement of the requirements that the adhesives or sealant must meet to be accepted for use. A specification is an agreement between supplier and user. Conformance to a specification does not mean that the adhesive or sealant will perform perfectly in service. It only means that the product conforms to the specification. Specifications not only account for the adhesive or sealant, but also define the adherends and the accessory processes for preparing the adherends and the joint assembly. 

Conductive adhesives are one of the feasible alternatives to lead for electronics assembly. Isotropically conductive adhesives are suitable for standard pitch (50- to 100-mil) surface-mounted components and numerous commercial materials are available (see commercial suppher Ksting, Section VI.E). Anisotropically conductive adhesives are more suited to flex to rigid connections, fine pitch components (15- to 20-mil pitch), and flip-chip assembly (4- to 12-mil pitch) [22]. Adhesives are not ready to replace solder throughout the electronics industry, however, due to questions that remain concerning the reliability of electrical interconnections. Their implementation is currently limited to low-cost applications using polyester substrates and specialty appHcations where solder cannot be used. Additionally, the lack of equipment for large-volume assembly with anisotropically conductive adhesives, which require the simultaneous appUcation of heat and pressure, impedes the acceptance of these promising materials. 

The mixing procedure, preparation conditions and cure schedule all have a profound effect on the mechanical properties of the adhesives and, consequently, will affect their durability [20, 73]. Because of this, expeditious on-site tests should be conducted to detect possible deficiencies. A number of test methods have been proposed by the CEN/TC193/SC1/WG11) in the pre-standard Adhesives for on-site assembling or restoration of timber structures — on-site acceptance testing . This... 

Standard Proposal CEN TC193/SC1AVG11 N20 Adhesives for on-site assembling or restoration of timber structures. On-site acceptance testing. Sampling and measurement of the adhesive s cure schedule. European Committee for Standardization (CEN), Brussels (2003). 

Many kinds of adhesives are utilized in the bonding of loudspeaker magnets, DC motor assemblies, and other types of magnet bonding. Some manufacturers still prefer to use mechanical clamps. The factors that contribute to the choice of a joining process include the cost of the adhesive, labor rates, speed of adhesive fixture, acceptability of odor level, and energy costs. Aerobic acrylic adhesives have been used primarily for their rapid fixture (as little as 10 seconds) resulting in faster assembly rates. 

Curing time is reduced by increasing the curing temperature. Resistance to weather, boiling water, and biodeterioration is satisfactory. Resistance to elevated temperatures is also satisfactory, but inferior to that of heat-cured phenolic and resorcinol adhesives. Excess acidity due to poor control of the acid catalyst content often leads to wood damage on exposure to warm humid air. The durability of joints at high and low temperatures for extended periods is usually acceptable. These adhesives are used for woodwork assemblies, where the service temperature does not exceed 40°C. Applications include furniture construction and, to a minor extent. 

Pressure devices should be designed to maintain constant pressure on the bond during the entire cure cycle. They must compensate for thickness reduction from adhesive flow-out or thermal expansion of assembly parts. Thus, screw-actuated devices like C-clamps and bolted fixtures are not acceptable when constant pressure is important. Spring pressure can often be used to supplement clamps and compensate for thickness variations. Deadweight loading may be applied in many instances however, this method is sometimes impractical, especially when heat cure is necessary. 

Acrylic adhesives are a versatile class of reactive adhesives that have in recent years gained wide acceptance in industry as bonding agents in the assembly of components. They are also to be found in the consumer market, where the industrially important benefits of rapid bonding, strength and ease of use can also be appreciated. 

As many PCB fabricators and assemblers transition to lead-free processing, they are faced with solder alloys that have an operating temperature up to 30°C (54°F) hotter than the traditional eutectic tin-lead solders. These increased soldering temperatures raise concerns about the ability of the solder mask (as well as all other materials) to resist embrittlement, discoloration, loss of adhesion, and cracking with repeated exposures to the higher temperatures. Some existing products may not be acceptable for lead-free processing. 

---