Polymer chip, bonding adhesive

One of the more common difficulties in bonding pine veneers and chips is adhesive dryout. Dry-out is associated with the high liquid absorbancy of pine sapwood and it appears especially during long assembly times. This problem can be overcome by using resins modified through reaction with alkylated phenols, especially 3,4-xylenol [52]. Another technique used to achieve similar results is the manipulation of synthesis procedures used in preparing a standard PF resin [52]. The dry-out resistance imparted by alkylated phenols is due to an initial semithermoplastic character in the resin. This is derived from their monomer bifunctionality and the linear polymer that is consequently formed. 

The above concept of forming adhesive bonds in the solid state has been used to demonstrate the possibility of the parallel processing of a multi-chip module substrate, consisting of a multilayered polymer substrate with circuitry embedded on each polymer layer via lithographic processing [43], In this case, it is essential that the polymeric layer retains its dimensional stability so that registration and interconnections between the layers can be achieved using a Pb-Sn solder (see Fig. 28). A copolyester which appears to be ideally suited for this purpose is the 4/1 PHBA/BPT which melts at 320 °C in the randomized form... 

In a similar process, known as polymer-film interconnect (PFI), an insulative thermoplastic film is laminated over the devices at the wafer stage, and vias are opened over the bonding pads using a laser. At that point, either the normal solder bumps can be formed or a silver-filled conductive adhesive can be stencil printed into the vias to form polymer bumps. After printing, the epoxy is B-staged and the flip-chip devices are diced. In assembly, the devices are heated to a temperature that completes the cure of the B-staged bumps and simultaneously reflows the thermoplastic underfill material. 

An additional consideration for the selection of an adhesive system is that robust bonds must be formed to all surfaces involved in the interconnection. Materials commonly found include metallizations on the substrate and components (e.g., gold, solder, copper, aluminum, and indium tin oxide), polymer substrates and coatings (e.g., polyimide, polyester, epoxy, and acrylic adhesives), and chip passivation layers (e.g., Si02 and SI3N4). Adhesion promoters may be required. 

Structuring of Microfluidic Channels into the Tape Bonding with adhesive tapes is an attractive low-cost and low-temperature method for polymer and glass-polymer bonding. If the glass chip carries a sensor or electrodes, it is necessary to cut microfluidic channels into the tape in order not to block the sensor or electrode surface. There are two methods to accomplish that. 

Figure 32 Process for the fabrication of polymer bumps (a) bumping sites delineated by screen printing silica-filled polyimide paste (b) silver-filled epoxy resin composition is printed over the metal pads (c) polymer bumps are formed by screen printing a second layer of conductive epoxy. Steps (d) and (e) sketch the subsequent flip chip process using a layer of conductive adhesive coated on the substrate bonding pads.

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