Mitsubishi, Renesas Develop Solder Tool for Fine-Pitch BGAs

Kenji Tsuda, Asia Contributing Editor -- Semiconductor International, 5/14/2008 8:56:00 AM

Aimed at small-pitch ball grid array (BGA) packages, Mitsubishi Electric Corp. (Tokyo) and Renesas Technology Corp. (Tokyo) have jointly developed a solder ball machine that can deposit 13-µm-diameter balls using a lead-free solder alloy of silver and tin (Ag-Sn). The tool thus far has a relatively slow throughput of ~300 droplets/sec, making it suitable for prototyping but not for mass production.

The head of the tool ejects molten solder droplets onto gold bonding pads, similar to an inkjet printer. The smaller ball enables the tighter BGA pitch; the companies have demonstrated a 30 µm pitch on a silicon substrate with 1 pL solder droplets.

The solder ejection head was redesigned, with the gas flow and solder ejection direction in parallel.

The I/O density of BGA packaging has been limited by the relatively wide pitch of the solder balls, which have been difficult to shrink. The Mitsubishi engineers developed a nozzle that drives the solder droplets by a piezoelectric element with a diaphragm. By driving a pulsed voltage to the diaphragm, the molten solder balls are ejected from the nozzle. Near the nozzle, nitrogen gas is also blown down onto the chip to prevent the balls from oxidizing. The nozzle has a convex form, rather than the flat shape used in conventional nozzles, which leads to a regulated flow of nitrogen gas blowing parallel to the droplets. In conventional nozzles, the solder droplets emerge perpendicular to the direction of the flow of the nitrogen gas.

In addition to the nozzle improvement, the engineers investigated ejection times of 3.3 msec, 5-10 msec, corresponding to 300, 200 and 100 droplets/sec. Position accuracy varies from 1.60 to 0.81 µm at a single-sigma rate of deviation. Changing the nozzle sizes leads to changing the size of the solder balls, ranging from 13 µm in diameter (1 pL) to 450 µm (48 nL). The metallic gold pads create a self-alignment effect in the molten solder.

The tool can deliver solder bumps at a 30 µm pitch with a flow of 100 droplets/sec.

The Mitsubishi engineers produced a prototype chip-on-chip structure with a 50 µm pitch ball array on two 3 × 3 mm chips. They have also applied the approach to the formation of through-silicon via (TSV) technology for 3-D integration of system-in-a-package (SiP) and MEMS stacked chips. The solder droplets are injected into the through-hole via to fill it with solder. For example, 70 solder droplets with a 40 µm diameter are injected into a through hole with a diameter of 100 µm and a depth of 300 µm. The engineers said the approach was able to fill the hole with a solder that was free of voids.