Signal transmission line comprising stacked insulating layers having a signal line and a ground conductor respectively spaced apart by a spacer with a hollow portion

An electronic device comprising:

• a circuit board;
• and
• a transmission line connected to the circuit board;
• wherein
• the transmission line includes:
• a first structure including a first insulating substrate and a first ground conductor on or in the first insulating substrate;
• a second structure including a second insulating substrate and a signal line, a second ground conductor, and an interlayer connection conductor on or in the second insulating substrate;
• a spacer including a third insulating substrate including an opening;
• and
• a metal bonding material that bonds the first structure and the second structure to each other with the spacer interposed therebetween;
• the first insulating substrate and the second insulating substrate are stacked with the third insulating substrate interposed therebetween to provide at least one hollow portion;
• the signal line and the first ground conductor partially face each other across the at least one hollow portion in a bonding direction;
• and
• the first ground conductor includes at least one opening in a region that overlaps the signal line but does not overlap the at least one hollow portion when looking in plan view in the bonding direction.