Polymer bumps

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. 

Protective Chip Pad Layer. As with virtually all flip chip processes, the A1 bond pads must be protected to eliminate the formation of nonconductive aluminum oxide. This ensures a low and stable resistance at bond-bond pad interface. The PFC process utilizes an electroless plating technique, using Ni/Au or Pd, to cover the A1 bond pads prior to polymer bumping. The typical metal thickness is 0.5-1.0 pm for Pd and 3.0-5.0 pm for Ni/Au. 

Curing. The polymer bumps are then either fully cured or partially cured to the so-called B-stage for thermosetting polymer bumps. For thermoplastic polymer bumps, after stencil printing the solvent is removed to form solid bumps. Bump heights are typically 50-75 pm and process can accommodate pitches down to 5 mil. Bump densities of up to 80,000 bumps/wafer have been formed with excellent coplanarity. 

Once the bumped wafers are diced, chips are picked from the wafers, flipped over, and then placed on and bonded to chip carriers. Different process procedures are utilized to bond thermosetting polymer bumps to similar thermoplastic bumps, as noted in Figure 13. Final processing involves a heat cure for thermosetting bumps, while thermoplastic bump connections only require in a few seconds under heat and pressure to melt the thermoplastic. 

Fig. 14. Schematic depicting a flip chip technology utilizing chips with micromachined poljrmer bumps, (a) Process flow for creating micromachined polymer bumps in the wafer state, (b) Die attachment to a chip carrier.

After the polyimide layer is cured, the conductive polymer paste is printed to the substrate in order to fill the holes over the metal pads and then cured (Fig. 32(b)). Using the same stencil and conducting paste, the polymer bumps are then screen printed on top of the conductive polymer pads and cured (Fig. 32(c)). Current printing techniques allow production of 50-100 p,m diameter bumps, 30-50 p.m high, with a centre-to-centre pitch of 125-150 p,m, and flat, conic, or hemispherical shapes. Once the bumps are cured and the dice sawed,... 

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.

Vied, B., Rosner, B. (1994). Flip chip with polymer bumps on various substrates. Adhesives in Electronics 94, International Conference on Adhesives Joining Technology Electronics Manufacture, VDl/VDE-lT. 

The polymer bumps are then either fully cured or partially cured to the so-called B-stage for thermosetting polymer bumps. For thermoplastic polymer bumps, after stencil printing, the... 

Lin, J. Drye, J. Lytle, W. Scharr, T. Subrahmanyan, R. Sharma, R. Conductive polymer bump interconnects. Proceedings of the 46th Electronic Components and Technology Conference, Orlando, FL, May 1996 1059-1068. 

Oh, K.E. Flip chip packaging with micromachined conductive polymer bumps. IEEE J. Sel. Top. Quantum Electron. January-February 1999, 5 (1), 119-126. 

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