Flexible printed circuit board and method of manufacturing flexible printed circuit board

A flexible printed circuit board comprising:

• an electrically insulating layer having a first surface and a second surface opposite to the first surface;
• a first wiring disposed on the first surface;
• a second wiring disposed on the second surface;
• and
• an electrically conductive layer,
• wherein the first wiring includes a first land,
• wherein the second wiring includes a second land,
• wherein the first wiring includes a first layer disposed on the first surface and a second layer disposed on the first layer,
• wherein the second wiring includes a third layer disposed on the second surface and a fourth layer disposed on the third layer,
• wherein the first land and the second land overlap each other in a plan view,
• wherein the electrically insulating layer is provided with a through hole extending through the electrically insulating layer in a thickness direction and overlapping the first land and the second land at least partially in the plan view,
• wherein palladium particles are disposed directly on the inner wall surface of the through hole,
• wherein the electrically conductive layer is disposed directly on the palladium particles and is connected to the first land and the second land,
• wherein the second layer, the fourth layer, and the electrically conductive layer are electrolytic plating layers made of the same material,
• wherein a thickness of each of the first land and the second land is from 5 μm to 100 μm, wherein a thickness of the electrically insulating layer is from 5 μm to 100 μm,
• wherein the thickness of each of the first land and the second land is 0.75 times or more as large as the thickness of the electrically insulating layer, and 6.0 times or less as large as the thickness of the electrically insulating layer,
• wherein, when the flexible printed circuit board is subjected to a heat shock test in which the temperature is repeatedly cycled between −40° C. and 130° C. for a predetermined time at each temperature for 1000 cycles, electrical connection between the first land and the second land is maintained, and
• wherein an inner diameter of the through hole is more than or equal to the thickness of the electrically insulating layer.

A method of manufacturing a flexible printed circuit board, the method comprising:

• preparing an electrically insulating layer;
• and
• forming a first wiring including a first land, and a second wiring including a second land,
• wherein the electrically insulating layer has a first surface and a second surface opposite to the first surface,
• wherein the electrically insulating layer is provided with a through hole extending through the electrically insulating layer in a thickness direction and overlapping the first land and the second land at least partially in a plan view,
• wherein the forming of the first wiring and the second wiring includes forming a first layer on the first surface, forming a first resist layer on the first layer, forming a second layer on the first layer exposed from the first resist layer, forming a third layer on the second surface, forming a second resist layer on the third layer, and forming a fourth layer on the third layer exposed from the second resist layer,
• wherein the forming of the second layer and the forming of the fourth layer are performed by electrolytic plating,
• wherein, in the forming of the second layer and the forming of the fourth layer, the second layer and the fourth layer grow along an inner wall surface of the through hole to form an electrically conductive layer directly on the inner wall surface of the through hole,
• wherein the first resist layer and the second resist layer are removed after the second layer and the fourth layer are formed,
• wherein the first layer below the first resist layer that has been removed and the third layer below the second resist layer that has been removed are removed by etching after the second layer and the fourth layer are formed, and
• wherein a thickness of each of the first land and the second land is 0.5 time or more as large as a thickness of the electrically insulating layer.