What I Learned this Month

I wrote a column in 2001 on the first commercial use of high temperature superconductors (HTS) and have followed the technology since its creation in 1987. It was with great interest I read about a superconducting power transmission cable installed in Long Island, NY. This is a joint project between the Long Island Power Authority (LIPA) and the American Superconductor Corp (ASC). It was commissioned on April 22, 2008.

The cable is basically a pipe for liquid nitrogen with layers of high temperature superconductor (HTS) and insulation wrapped around the outside. As a power transmission system, the voltage rating is 138 kV, carrying 574 MegaWatts. This comes out to about 4kA. It’s only 2,000 feet long, but is a start.

While transmission losses from IR drop are typically 7-10% of the power transmitted, superconducting cables are lossless in principle. However, there is still the energy used in refrigerating the cable. There is a lot of room for improvement in insulation and heat pipe design, so in the long term, superconductor cabling for power transmission will be an important element to a large sacle, efficient power distribution network.

I visited Neoconix last month. They have a novel interposer technology which turns any PCB surface into a compliant, probing surface. On a separate sheet, an array of bent leads is manufactured. This sheet is laminate to the surface of a circuit board with landing pads to match the footprint of the beams.

The composite board is plated up so that the plating provides the electrical connection between the attached beam and the substrate. With this approach, the compliant beams can be placed on one or both sides of the board. The board can be a simple array of through hole vias connecting one side of the board to the other, or have functionality, like power and ground planes, decoupling capacitors, or a geometry transforming footprint.

For a while, I’ve been promoting the idea of integrating geometry transformers for test sockets to relieve the interconnect burden from load boards. In testing devices with pads or leads on 0.5 mm centers or finer, you pay a premium for the load board to get the fine pitch required by the socket. For 0.4 mm, the yield of the load board is so low, you have to build 3 or 4 boards to get one that is not shorted.

Why not have the fine pitch for the device on one side of a small interposer board and a coarse pitch for the board surface on the other side, with a multilayer circuit board providing the fan out? Even a 4 layer board could have controlled impedance fan out, with low impedance power and ground distribution. I think Neoconix technology is the perfect solution to implement this cost effective approach.