Method of preparing a high density interconnect printed circuit board including microvias filled with copper

A method of preparing a high density interconnect printed circuit board (HDI PCB) including microvias filled with copper, the method comprising the steps of:

• a2) providing a multi-layer substrate (10) comprising
• (i) a stack assembly of an electrically conductive interlayer (14) embedded between two insulating layers (12) having a peripheral surface,
• (ii) a microvia (20) extending from the peripheral surface of the insulating layers (12) of the multi-layer substrate (10) and ending on the conductive interlayer (14);
• b2) depositing a conductive layer (30) on the peripheral surface of the insulating layers (12) of the multi-layer substrate (10) and on an inner surface of the microvia (20);
• c) electrodepositing a copper filling (42) in the microvia (20) and a first copper layer (40) on the conductive layer (30) wherein a thickness of the first copper layer (40) is from 0.1 to 3 μm and wherein the copper filling (42) and the first copper layer (40) form together a planar surface (32);
• and, subsequent to step c):
• d) forming a patterned masking film (50) on the first copper layer (40);
• e) electrodepositing a second copper layer (60) in the area not covered by the patterned masking film (50);
• and
• f) removing the patterned masking film (50).