Molding Techniques Support Thin Gold Wires, Low-k Materials

Kenji Tsuda, Asia Contributing Editor -- Semiconductor International, 7/21/2008

New low-pressure molding technologies developed by two Japan-based molding machine manufacturers support thinner IC packages created with package-on-package (PoP), wafer-level packaging (WLP), and transparent resin packaging. The thin packages for new cell phone designs are a market driver.

Towa Corp. (Kyoto, Japan) said it has developed a new compression molding machine, while competitor Apic Yamada Corp. (Nagano, Japan) said it has come up with a liquid-based resin molding machine. Both are expected to lower molding pressures to a fraction of conventional transfer-molding systems, leading to a thinner and more complex system-in-package (SiP) and WLP.

In conventional transfer-molding technology, plastic resin tablets are pushed into a gate and flowed through runner pipes to reach wire-bonded ICs. The gold wires are covered with the molten resin at high pressure, leading to the possibility of deformation, electrical shorts and lay-down defects. Today's high-density logic chips with low-k materials with lower mechanical strength are easily deformed by high pressure.

Towa Corp. developed the Flow Free Thin molding technique for thin packages.

Also, the current 25 μm diameter typical for gold bonding wires is becoming expensive, and many companies want to thin to 16 μm or less to reduce material costs. However, the high pressure of the molten resin used in conventional transfer molding tends to deform the thinner gold wires.

Apic Yamada Corp. uses liquid resin in its injection molding technique.

Plastic resins include a variety of materials such as basic epoxy material, filler (silica) and mold-release material. As the filler volume is reduced, the compound resin flows more easily onto the chip, but a higher difference in the coefficient of thermal expansion leads to cracking and breaking. Finding the optimum resin content has been a challenge, particularly for the thinner IC packages, which require easy-to-flow resins. Resin manufacturers and molding machine manufacturers have cooperated to develop an optimum resin.

For mobile devices such as cellular phones, games and music players, molded packages with a thickness of 0.4 mm are available commercially. Recent semiconductor packaging technologies require thinning wafers to &100 μm, using gold wires with a lower profile, and thinner moldings. Conventional transfer-molding techniques are difficult to employ for chips using the thinner resin.

Towa’s molding machine makes use of compression molding. The FFT (Flow Free Thin) molding system forms an IC package by putting the chips on a board and immersing them into a molten resin mold. The approach mitigates pressure on the chips, and the gold wires are not destroyed or cut. The molding technique is designed for map molding or wafer-level packaging, molding dozens of chips at once.

A board mounted with chips is immersed in the molten resin in the lower mold and solidified on the board. The molding does not require a gate or runner, which reduces waste, particularly important for encapsulating high-brightness LEDs with expensive transparent resins.

Apic Yamada also developed a new molding machine using liquid resin, using Cavity Direct Injection Molding (CDIM) technology. The molding technique ejects the liquid resin on a board and the top mold presses on the cavity of the chips to solidify the resin. Unlike conventional transfer molding, the CDIM method also uses little horizontal pressure, which is effective for logic chips using mechanically fragile low-k materials and multiple gold wires. The company said data shows wire deformation in &1% of the cases for 25 μm diameter wires.

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Making the gold wire thinner whilst reducing costs seems like it would be problematic due to...

David Cutler - 2008-21-7 23:25:00

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