High density flip chip interconnections produced with in-situ underfills and compatible solder coatings

Applicability of three no-flow fluxing encapsulants and one nonconductive adhesive (NCA) to underfilling of high-density flip chip assemblies was studied by bonding Sn37Pb-Bi or Sn-Bi bumped test structures on FR4 substrates. The Sn37Pb or Sn contact pads on the substrates were coated with a thin layer of bismuth. The test chip bump pitches were 250 and 100 μm. The results showed that reliable, intermetallic-free joints can be produced with one of the fluxing no-flow underfills and with NCA together with metallurgically compatible solder coatings. Detailed C-SAM, fine-line focus X-ray, scanning electron microscopy and microanalysis confirmed that metallurgical bonding can be achieved with the fluxing underfill and the NCA, when a thin layer of bismuth top coating is utilized, but only the nonconductive adhesive enables good void-free encapsulation for high density transfusion flip chip attachment. The other no-flow underfills produced numerous voids, especially when the bonding was carried out at such low temperatures as those used for the transfusion bonding