The present application generally relates to a transmission cable. More particularly, the present application relates to a transmission cable which involves interlayer connections using bumps, and further relates to a manufacturing method thereof.
Data and the like is transferred over various types of networks via transmission cables (network cables), and flat transmission cables which are easily laid have been developed. Flat transmission cables are made by patterning signal lines using a method similar to that used with so-called flexible wiring substrates, and have the advantage that differences in level are not formed in a location such as underneath flooring materials where wiring is installed.
Incidentally, the coming of the broadband era has created a need for high speed and large capacity transfer, and improvements to the transmission cables mentioned above are desired. For example, a serious problem is caused by malfunctioning due to noise and mutual cross-interference of signals with transmission cables formed simply of signal lines.
Due to these circumstances, transmission cables have been proposed with a design in which ground lines are disposed between signal lines which are electrically connected to a shield layer formed via an insulating layer. A flat cable is disclosed in which shield patterns and shield layers are electrically connected by conductive bumps formed from a conductive resin paste. See, for example, Japanese Patent laid-open publication No. 11-162267.
However, conductive paste has the problems of having high electrical resistance and high connection resistance with the copper foil which makes up the ground lines. Such high electrical resistance and connection resistance prevent the ground lines from fully functioning and make it impossible to fully eliminate noise and interference between signal lines. This is a serious obstacle to the increased speeds mentioned above.
Furthermore, employing a manufacturing process of forming conductive bumps using the conductive paste and force-inserting these bumps into and through an insulating material layer leads to a problem of increased manufacturing costs. For example, with the technology described in Japanese Patent laid-open publication No. 11-162267, conductive bumps are formed on predetermined locations on a stainless steel plate using, for example, a stamping method designed to pressurize a conductive composition (a paste) contained in a casing and extrude it through holes in a mask. In this case, special equipment such as a stamping machine is needed, and there is also a need to repeat a printing and drying process at the same position, and also to perform a heat hardening process, rendering the process complex.