Optical module

1. An optical module comprising:

an optical sub-assembly; and

a flexible substrate including an insulating film, an interconnection pattern, and a spacer layer, the flexible substrate being connected to the optical sub-assembly,

the insulating film having some projections, the projections protruding from a basic area in a first direction, the projections being arranged in a second direction perpendicular to the first direction, the insulating film having a flat shape with a recess between an adjacent pair of the projections,

the interconnection pattern including some pads in the basic area on a first surface of the insulating film, the pads being arranged in the second direction,

the pads including some first pads adjacent to the respective projections, the pads including at least one second pad adjacent to the recess,

the spacer layer being on the first surface and at each of the protrusions.

2. The optical module according to claim 1 , wherein the at least one second pad is one of the pads.

3. The optical module according to claim 1 , wherein the at least one second pad is two of the pads.

4. The optical module according to claim 1 , wherein the projections are three or more projections.

5. The optical module according to claim 1 , wherein

the flexible substrate includes a cover layer on the first surface and in the basic area, the cover layer covering the interconnection pattern except for the pads, and

the spacer layer is equal in material and thickness to the cover layer.

6. The optical module according to claim 5 , wherein

the interconnection pattern includes a ground plane on the first surface, and

the ground plane is between the insulating film and the cover layer.

7. The optical module according to claim 6 , wherein the ground plane is continuous from the first pads.

8. The optical module according to claim 1 , wherein the interconnection pattern includes some dummy terminals, the dummy terminals being between the insulating film and the spacer layer, the dummy terminals being at the projections, the dummy terminals being continuous from the first pads.

9. The optical module according to claim 1 , wherein

the interconnection pattern includes at least one signal line on a second surface opposite to the first surface, and

the at least one signal line penetrates the insulating film and is connected to the at least one second pad.

10. The optical module according to claim 1 , further comprising:

a printed circuit board including a conductor pattern, the conductor pattern including some electrodes opposed to the respective pads, the printed circuit board being opposed to the projections and in contact with the spacer layer; and

a filler metal between the pads and the electrodes.

11. The optical module according to claim 10 , wherein the conductor pattern includes a conductive plane continuous from at least one of the electrodes.

12. The optical module according to claim 11 , wherein

the conductor pattern includes at least one transmission line continuous from another at least one of the electrodes, and

the conductive plane is on both sides of the at least one transmission line.

13. The optical module according to claim 12 , wherein the electrodes include some first electrodes opposed to the respective first pads and at least one second electrode opposed to the at least one second pad.

14. The optical module according to claim 11 , wherein the spacer layer is in contact with the conductive plane.

15. The optical module according to claim 10 , wherein

the printed circuit board further includes a protective layer covering the conductor pattern except for the electrodes, and

the spacer layer is in contact with the protective layer.

16. The optical module according to claim 1 , wherein the optical sub-assembly is brazed or soldered to the interconnection pattern except at the pads.