Adhesives screening test

There is a need for an accurate assessment of the mechanism of cathodic debonding of thick adherends. This would contribute to better control over the selection of formulations resistant to attack. One of the objectives of this work was to develop an accelerated screening test. The screening test was used to evaluate the relative performance profiles of a number of commercial adhesives which were recommended for marine applications. It is hoped that a screening test such as this one could be used to isolate measurable primer properties which can be related to the long term prospectus of the primer, and the variability of this prospectus under permutation of substrate type or additives. 

Initially, we tried to devise a screening test for adhesives and additives which imitated naturally occuring conditions. The environment we were Imitating was characterized by salt water... 

The preceding has been a description of our efforts to develop a screening test for adhesives which will be used on steel or monel substrates which are maintained at a cathodic potential, immersed in... 

The limits quoted above can be made more stringent if the materials are to be used in very critical areas. If required, the collector plate can be examined by an infrared spectrometer to determine the IR spectmm, and hence chemical nature, of the contaminants. The use of materials deemed acceptable by these limits does not necessarily ensure that contamination will not occur. The screening test is done at 125°C in order to accelerate the outgassing to ensure a measurable change is observed. Comparisons of various materials are thus only really valid at this temperature. The measurement of contamination is only comparative and also strictly valid for collectors at 25 C with similar surface adhesion properties. Also it is worth noting that this test says nothing about the species that is being evaporated. 

Performing a peel test after lamination should be considered as only a good screening test it is not sufficient for material selection. Plueddemann recommends a 7-day soak in room-temperature water as an additional test, with final peel tests demonstrating cohesive failure rather than adhesive failure (Reference... 

Peel tests as a measure of laminate adhesion are only partially useful. Most laminant systems exhibit either very low or very high adhesion after a 7-day soak in cold water. The soak test may not be a sufficient predictor for 20-yr service life however, it may be considered as a screening test for systems that should receive additional effort. 

Secondly, accelerated tests can be used to qualify adhesives for specific applications, and the data obtained can sometimes be extrapolated to long-term, real-time performance. Accelerated tests are also valuable as screen tests and as acceptance tests and are specified in material-procurement documents or in hardware-acceptance specifications. Adherence to material and process specifications and their quality-control provisions is an essential element in assuring reliability. Life prediction through modeling is yet another approach, but is outside the scope of this chapter. 

For hermetically sealed microcircuits several screen tests should be performed before delidding the package. A particle impact noise detection test (FIND [MIL-STD-883 Method 2020 or MEL STD-750D, Method 2052-2]) is recommended. This shake test combines a series of shocks and vibrations and, through an attached transponder, acoustically detects any loose particles (for example, chips of cured adhesive, solder, wire, ceramic, or loose plating) that have detached and can cause shorted or open circuits and damage or breakage of thin wires and connections. 

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. 

A much simpler version of this test, which can be used as a screening test, is to apply the adhesive tape to a freshly polished soft copper panel using a 100-grit aluminum oxide polishing wheel or 150-grit paper, then heat-age this at 100°C (212°F) for 72 h. After allowing to cool, the tape is removed and the panel inspected for any indication of staining. 

Most conductive adhesive failures are accelerated by elevated temperature and humidity. In a study of 12 commercially available isotropically conductive adhesives, joint resistance increased between 160 and 35,000% when exposed to 65°C and 85% relative humidity (65/85) [56]. However, some adhesive manufacturers claim resistance change of less than 10% after 1000 h at 60°C and 90% relative humidity [57] and less than 4% after 1000 h at 85/85 [58]. Anisotropically conductive adhesive joints are even more susceptible to early failures under accelerated test conditions due to process variations [16]. Reliability screening tests can be used effectively to iteratively optimize process parameters. 

Screen tests assure the reliability of the entire electronic product of which the adhesive joints are a part. 

In the case of radiation-curable (e.g., UV) adhesives, TLC represents a very useful screening test for the presence of several characteristic photo-initiators, e.g., benzophenone, 2,2 -diethoxyacetophenone, 1-benzoyl-cyclohexanol, and 4,4 -dimethylaminobenzophenone (Michler s ketone). 

For a screening test we chose to cure the adhesive material in film form and to test the material in simple "dogbone" tensile tests. The justification for using tensile tests, when structural designers inevitably seek to minimize the tensile stress in their applications of structural adhesives needs some explanation. 

Select and perform screening tests on candidate adhesives. 

Selecting appropriate screening tests is not always easy, but familiarity with the tests available and their use will simplify matters. Appendix 2 of this chapter is a complete listing of standards currently under the jurisdiction of ASTM Committee D-14 on Adhesives (plus a few adhesive standards from other ASTM committees) and should be helpful in looking for appropriate screening tests. 

Appendix 2 first lists ASTM s adhesive performance specifications, most of which are in the area of wood bonding. They are subdivided by substrate or application. Whenever a specification is available for the adhesive or bonding application being considered, it should certainly be used in the screening testing if at all possible. 

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