Bigger Chip Packages Run Into a Piece of Japanese Glass Cloth

Bigger Chip Packages Run Into a Piece of Japanese Glass Cloth

At an October circuit-board show, Japan's Nittobo published specifications for its next-generation glass cloth, Vlex: a coefficient of thermal expansion of 2.0, below the 2.8 of the T-glass used in high-end packaging today and far below ordinary E-glass at 5.6.

A package substrate is the board under the chip, pressed together from resin and glass cloth. Silicon expands at roughly 3, and the assembly heats and cools repeatedly. If the board moves at a different rate, a larger substrate warps more and the joints crack. Lowering the cloth's expansion lifts a constraint on substrate size, and size is exactly what this generation of accelerator packaging is fighting for.

It is not a straight upgrade. Vlex has a tensile modulus of 71 against T-glass at 86, so it is less stiff. Nittobo says on the same slide that which one wins depends on the resin: Vlex is better with low-expansion resins, T-glass with the rest. Vlex currently exists only as thick cloth and remains under development.

On the list of bottlenecks for AI servers, materials like this appear earlier than capacity numbers, and they are harder to replace.

Source: Nittobo technical presentation at a circuit-board show (Image: @SemiAnalysis_)