Yukihiro Yamamoto

Micro-ball wafer bumping for flip chip interconnection

A new wafer bumping method using micro-balls was developed that can be used for high-density LSI assembly, specifically for Flip Chip interconnection. Micro solder balls with the diameter ranging from 60 /spl mu/m to 200 /spl mu/m were first formed with a high level of accuracy and sphericity. These balls were transferred and bonded to the whole electrode-pads of an 8-inch wafer in one stroke using a fully automated micro ball mounter, which was newly developed. The balls were held on fluxed pads and melted in a reflow furnace. The fluxing was performed using unique stamp system. The productivity and the yield were evaluated under the following conditions. The number of chips on an 8 inch wafer was 616, Pad pitch was 250 /spl mu/m, Pad number of a chip was 625 (25/spl times/25 area array), and the total number of balls on a wafer was 385,000. The yield of forming bumps was confirmed to be higher than 99.995% without repairing and the cycle time of micro ball bumping was ca. 5 min. For an 8 inch wafer. The bump height variation, the bump shear strength and the bond reliability were evaluated in comparison with other methods.

Significant reduction of wire sweep using Ni plating to realise ultra fine pitch wire bonding

Significant reduction of the wire sweep in molding is proposed because the wire sweep is considered to be the major problem to realise wire bonding with ultra fine pitches of under 30 micrometers. In the present proposal, Ni was plated for several micrometers before molding on bonded Au wires. Ni plating was carried out by means of electroless plating for several minutes in the aqueous solution kept at 358 K containing Ni and P. The wire sweep ratio for Ni plated wire (total diameter was 21 micrometers) was almost half of that for Au wire with the diameter of 15 micrometers except Ni plate, and was slightly smaller than that for Au wire with the diameter of 25 micrometers except Ni plate even the total diameter was smaller. It is considered that wire sweep suppression by this technique was due to the enhancement of both elastic and plastic properties.

An Applicatin of Micro-Ball Wafer Bumping to Double Ball Bump for Flip Chip Interconnection

Microball wafer bumping method was applied to forming double ball bumps, which increased the bump height to improve the reliability of the flip chip interconnection. The micro solder balls of 100mum in diameter were transferred and connected to the whole electrode-pads covered with UBMs (under bump metals) of an 8 inch wafer in one stroke using a fully automated micro ball mounter, which was originally developed. The balls were held on fluxed pads and melted in a reflow furnace. After cleaning the flux residue, the wafer bumped with microballs was then encapsulated with epoxy resin containing silica fillers by using Apic Yamadas wafer level molding system. The molding resin was spread to the whole wafer by compressing with a heated flat plate, where the top of the solder bumps were covered with an elastic parting film. The supplied resin volume was previously adjusted to the desired molding thickness. The exposed top of the ball bumps was cleaned and then second microball bumping was processed on the top of the first bumps. The first and second balls were connected by reflowing to form the double ball bumps. The height variation and shear strength of double ball bumps were evaluated. To compare the reliability for the different type of bumps the TCTs and FEM analysis were performed for the chips connected with PCBs