Лако-красочные материалы — производство

A. Dispensing

Conductive adhesives are applied to the substrates using a wide range of standard tech­niques. For die attach applications, the isotropically conductive adhesive is applied using either a stamping tool or a syringe dispenser. To ensure complete void-free coverage, sophisticated multineedle syringe dispensers have been developed. Stencil and screen print­ing are used for the placement of isotropically and anisotropically conductive adhesive pastes on PWBs for the assembly of surface-mounted components. The development of ink-jet technology may prove to be another economical means of dispensing precise quan­tities of conductive adhesive paste [54]. Placement of anisotropically conductive films is accomplished by hand or by pick-and-place equipment.

B. Assembly

When using isotropically conductive adhesives, placement of components is performed by the same equipment as used for nonadhesive attachment techniques. Die bonders are similar to those used for eutectic bonding except for the type of adhesive dispenser. Surface-mounted placement machines developed for solder paste assembly can also be

used with conductive adhesive pastes. After placement, the adhesives are cured in either batch or conveyorized ovens.

Assembly with anisotropically conductive adhesives is more complex, as the electrical interconnections require the simultaneous application of heat and pressure. For assembly of flex to rigid substrate interconnections, thermal compression bonders are used with either metal or ceramic hot bars. Pressures are typically in the range 25 to 100kg/cm2 [9]. Surface mounting of components with anisotropically conductive adhesives is usually accomplished by serial processes. Equipment similar to solder rework stations is used, where a device is aligned, placed into the adhesive, and cured under pressure with the application of heat by hot gas jets. This process is slow and difficult to control. Equipment costs are high due to low throughput resulting from curing each device in the placement machine. The pressure required to achieve intercon­nection will depend on the number and type of leads. A reliable batch processing technique, where all components on a PWB are cured simultaneously under pressure, is required before anisotropically conductive adhesives can be used for large-volume manufacturing of surface-mounted components.

Assembly of silicon chips onto substrates with anisotropically conductive adhesives uses specialized equipment, initially developed for flip-chip solder and TAB inner lead bonding. Heat and pressure are transmitted to the adhesive through a thermode attached to a robotic arm or a high-precision linear translator. Equipment requirements are more demanding than for solder assembly, as no self-alignment can occur. A minimum placement accuracy of ± 0.0005 in. is required. Coplanarity between the substrate and die is critical; one study reports maintaining coplanarity to within 0.00004 in. [19]. The pressure required to achieve interconnection depends on the size of the die, the type of conductive particle used, and the viscosity of the adhesive at the bonding temperature.