Curing substrate-attach adhesives

Visual-inspection criteria for cured adhesives used for element or substrate attachment are specified in paragraph 3.1.2 of Method 2017, Internal Visual (Hybrid) of M1L-STD-883F. Visual-inspection criteria for adhesives are classified according to element and substrate attachment and according to coverage and defects. 

Adhesion promoters can be applied either by incorporating them directly into the adhesive formulation or by applying them directly to a substrate, similar to a primer. When applied in situ, through the adhesive formulation, the adhesion promoter migrates to the interface region and attaches itself between the adhesive molecule and the substrate before the adhesive cures. 

First, in terms of their physical form, adhesives may be either pastes or preforms (also known as films or tapes). Pastes are semisolid materials, easily dispensed through a needle or applied by screen or stencil printing. Film adhesives are solid sheets of thermoplastic or partially cured (B-staged) thermosetting polymers that can be cut to size and generally used to attach large-area components, substrates, and lids. 

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. 

Hot-meltS/ radiation-curable adhesives, and epoxies can all be applied to a substrate in film or sheet form. The substrate can be metal, plastic, or paper. In addition, the substrate can be woven, nonwoven, or knot. The porous substrates can be used to manufacture filter aids or medical adhesives, e.g., to attach prostheses to the skin. An interesting form of substrate is a foam that joins surfaces of irregular gaps. The foam can also be formed instantaneously during curing to fill gaps. 

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