Control of Bond Line Thickness

If the adhesive has a propensity to flow easily before and during cure, then one risks the possibility of a final joint that is starved of adhesive material. If the adhesive flows only with the application of a great amount of external pressure, then one risks the possibility of entrapping air at the interface and too thick of a bond line. These factors could result in localized high-stress areas within the joint and reduction of the ultimate joint strength. 

Flow characteristics can be regulated by the incorporation of fillers, by the use of scrims or woven tapes as internal shims within the adhesive itself, or by the careful regulation of the cure cycle. All these options along with a few more are available to the adhesive for-mulator and end user. Generally, fillers are incorporated to control the viscosity of the adhesives as well as other properties such as thixotropy and sag resistance. 

The type and amount of fillers are chosen so that a practical bond line thickness will result after application of the necessary pressure (usually only contact pressure, approximately 5 psi) during cure. Ordinarily, the objective is a bond line thickness of 2 to 10 mils. Consideration, of course, must be given to the curing temperature. Viscosity of the formulation could drastically be reduced at elevated temperatures, and unless there is a furrow designed into the joint to contain the adhesive, much of the adhesive could flow out of the joint area before the adhesive is completely cured. 

polyester, and cotton fabric or mat are often used as a carrier in tape or film adhesive systems. In addition to being a carrier and a reinforcement, the strands of the fabric offer an internal shim so that the bond line cannot be thinner than the thickness of these strands. Sufficient pressure need only be applied to cause the adhesive to flow so that the shims meet the substrate surfaces to provide a positive stop. Paper, mat, and other carrier materials may also be used for this purpose. 

Glass or polymeric microballoons, incorporated directly into the adhesive formulation, can also provide the shimming function. Here the diameters of the microballoons are the positive stops that will prevent too thin of a bond line. 

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Many film adhesives have a supporting carrier or reinforcement fabric incorporated into the adhesive to improve handling of the film and provide control of bond line thickness. The carriers are usually glass, polyester, or nylon fabrics of knitted, woven, or nonwoven construction. The difficulty with such carriers is that they can provide an effective way of moisture entering the bulk of the adhesive. Moisture can wick along the fiber-adhesive interface. Nylon carriers should especially be reviewed since they have a strong tendency to absorb moisture. 

The control of bond line thickness is essential for high-power devices. A thin bond line is required since thickness is directly proportional to the junction-to-case thermal resistance, jc (see Chapter 2). Specially formulated solvent-based hybrid adhesives... 

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