Bondline thickness

The two components cure without evolution of any by-products, little or no volume change occurs and the system is ideally suited for use in thick and variable thickness bondlines. Solventless and 100% solid systems are possible which cure at room temperature within a few minutes. Viscosity, curing rate and cure temperature requirements can be tailored to meet the user s needs. Application of the system is generally made by two-part metering and mixing equipment. 

T0 assure optimum conductance, the adhesive must be applied as thinly and uniformly as possible. To control the thicknesses bondlines, thermally conductive paste adhesives have been formulated with collapsible spacers. The spacers are reported to control bondline thickness to 1.2 mils. These adhesives were developed for stacked die packages, but may also be used to attach ICs and other devices to substrates in plastic EGAs, CSPs, and array packages based on flexible tape or plastic laminates.Bondline thicknesses and uniformity may also be achieved by using film or preform tape adhesives and controlling the applied pressure and heat during cure. 

The working characteristics of the adhesive relevant to the application conditions must be determined. For instance viscosity is often temperature, shear-rate and time-dependent, and this will influence the choice of dispensing equipment, the method of application, the usable life and the open time. The viscosity should therefore be regulated bearing in mind the adherend rugosity and surface pretreatment, the method and location of application, and the cure temperature and duration of application. A thixotropic material may be required for application to vertical or soffit surfaces. Generally, relatively thick bondlines are encountered so that the adhesive should be able to cure in thick and/or uneven layers. It should also be remembered that for about every 8 C change in... 

Another significant benefit of adhesives is their ability to join disparate materials and to accommodate complex joint geometries. Production benefits such as the absoiption of manufacturing tolerances through the use of variable thickness bondlines also have obvious economic benefits. As a result, as adhesive technology has become more established then its application within other more commonplace industries, such as the automotive sector, has increased. 

The adhesives are used in the construction industry that includes residential and commercial buildings, roads, bridges, and other elements of the infrastructure. They are utilized for manufacturing a number of structural products, such as softwood plywood, laminated timbers, laminated paper-based panels, etc. More recently, adhesives have been used to replace mechanical fastenings in the assembly operation of building construction. These are called construction adhesives. The compounds must be capable of producing adequate bonds in poorly fitting joints with thick and variable thickness bondlines.t " ... 

StiTJCtural adhesives are also available in paste form. Bondline thickness control and void elimination are more difficult with paste materials but they can be very useful for unusual designs and innovative manufacturing methods. 

For a typical assembly, first part qualification begins with a rigorous dimensional check and painstaking prefit of all details on the bond tool. The assembly details are placed on the tool without adhesive, close contact between bond surfaces is verified and any detail or tool interference is corrected prior to proceeding. This is followed by fabrication of a verification film , or a simulated bond cure cycle of the assembly to allow measurement of the adhesive bondline thickness. 

After a satisfactory verification film is produced, an assembly may be fabricated specifically for destructive inspection to validate that the verification film was accurate. This correlation allows the use of verification film rather than more expensive destructive inspection for future changes such as duplicate tool fabrication and tool or detail modification. Simple assemblies are usually not destructively inspected because of high confidence that the verification film is entirely representative of the expected bondlines. Complex or large parts may not be destructively inspected because of the cost of the details and assembly time. In these cases other means of validating the verification film are used. Meticulous pre-bond detail and post-bond assembly thickness measurements may be sufficient to prove bondline thickness control. Ultrasonic inspection and X-ray photography (discussed previously) may be sufficient to prove that details are in the correct places and bonds are good. 

If an assembly is destructively inspected, the verification film is used to identify potential problem areas for particular attention. These areas and others randomly selected are cross-sectioned to determine bondline thickness (Fig. 22), bond details are peeled apart to inspect for voids and honeycomb core bonds are... 

The results above suggest that it may be possible to apply fracture mechanics data to determine failure loads of more complex structures, provided that (i) the adhesives used are not too ductile, (ii) bondline thickness is known and controlled, (iii) non-linear behaviour due to adherend and interface damage is limited, and (iv) the specimens employed to determine... 

To determine adhesive failure, it was necessary to apply appropriate algorithms to the data For quantitative analysis data were imported to a spreadsheet, smoothed to remove noise from the LVDTs, and then sorted to remove edge effects. Because there was considerable warp in all specimens due to the durability test, a parabolic function was fit to this distortion and subtracted from the raw data to produce a flat bondline. The data were again sorted (in ascending order) to produce a cumulative frequency distribution of surface irregularities (wood failure). Conceptually, a thickness tolerance could then be specified to define the bondline region as well as a depth tolerance for shallow wood failure. The relative population of data within these regions represented the percent e of adhesive, shallow, and deep wood failure. 

At this point in the analysis, it was possible to define a tolerance for bondline thickness, whereby all points within a prescribed tolerance would be considered adhesive failure, and all points outside this tolerance would be considered wood failure. Furthemtore, a second tolerance could be specified to distinguish shallow" wood failure fi-om deep" wood failure (Fig. 10). For the specimens evaluated in this study, two tolerances ( 40 pm and 60 pm) were selected for both the bondline thickness and the depth of shallow wood failure. A typical bondline thickness for block-shear specimens is about 80 pm, and the thickness of a small fiber bundle is 40-60 pm (or 2-5 fiber diameters). Table 1 summarizes the results from this analysis as well as the visual grading values obtained from the trained observers. 

Specimen 5 (Fig. 1) was selected for analysis because like specimen 1, it too is an obvious adhesive failure. However, unlike specimen 1, it contained several unique surface features that made it difficult to analyze. Among these were thick adhesive fragments, bondline voids (probably from air bubbles), and shallow (a few fibers) wood failure. Four of the observers declared near complete adhesive failure. However, one observer viewed it veiy differently (85 % wood failure). In this case, the profilometer analysis did not agree well with visual observations. This was due to considerable warp (cup and bow), which severely compromised the analysis. 

Ideally, this method would be fairly simple to implement on flat specimens. However, all the specimens tested contained significant degrees of warp due to the effects of a vacuum-pressure exposure test. In many specimens, the magnitude of this distortion was several times that of the surface irregularities. Therefore, it was necessaiy to flatten the bondline by fitting a parabolic function to the cup and then subtract the fit from the raw data. Next, the data were sorted in ascending order to produce a cumulative distribution for all measured surface displacements. A thickness tolerance could then be specified to define the bondline region as well as a depth... 

All specimens were constructed in a honded-beam configuration. Aluminum adherends were used in both the static and dynamic DCB configurations, and the composite adherends were used in the static and dynamic DCB and dynamic ELS and SLB configurations. Aluminum (6061) adherends were P2 etched to provide an adequate bonding surface and a bondline of either 0.8 mm or 0.5 mm was used. Because they were shipped in panel form, the composite adherends were bonded in 300 x 300 mm sheets. Each surface was abraded and then cleaned with acetone prior to bonding. The bondline was agmn set using either 0.8 mm or 0.5 mm wire at the center and 20 mm from both ends of the composite panels. Initially, this wire was only placed at the ends of the panels, but this resulted in inconsistent bondlines due to deformation at the center of the composite panels. Once the bondline thickness was set, a thermocouple was added to the center of the specimen to monitor the bondline temperature profile. 

As Fig. 15 shows, the 1 mm bondline thickness specimen has a very non-uniform fracture surface, whereas the fracture surface of the 0.5 mm bondline thickness specimen is fairly smooth and uniform. The precrack region, noted by the circle in the im e. is also very non-uniform in the case of the 1 mm bondline thickness specimen. Because the only data point collected from these tests is an initiation from this precrack, the data may be suspect due to the non-uniformit of the precrack. 

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