A wiring circuit substrate, comprising: a single layer base member in which are formed: a first metal layer in which first conductor circuit traces are formed, and etched metal protrusions that are electrically connected to said first metal layer; a second metal layer (54,76) that is formed on top surfaces of said protrusions, wherein said second metal layer is formed of a solder-plated layer or a solder layer; and an interlayer-insulating layer formed on said first metal layer; and characterized by: a third metal layer overlaid on said second metal layer and said interlayer-insulating layer and etched to form second conductor circuit traces; wherein said first and second conductor circuit traces are each exposed and suitable for directly contacting conductive elements of a further wiring circuit substrate upon the further wiring circuit substrate being overlaid directly onto said wiring circuit substrate or said wiring circuit substrate being overlaid directly onto the further wiring circuit substrate. a single layer base member in which are formed: a first metal layer in which first conductor circuit traces are formed, and etched metal protrusions that are electrically connected to said first metal layer; a second metal layer (54,76) that is formed on top surfaces of said protrusions, wherein said second metal layer is formed of a solder-plated layer or a solder layer; and an interlayer-insulating layer formed on said first metal layer; and characterized by: a third metal layer overlaid on said second metal layer and said interlayer-insulating layer and etched to form second conductor circuit traces; wherein said first and second conductor circuit traces are each exposed and suitable for directly contacting conductive elements of a further wiring circuit substrate upon the further wiring circuit substrate being overlaid directly onto said wiring circuit substrate or said wiring circuit substrate being overlaid directly onto the further wiring circuit substrate. a first metal layer in which first conductor circuit traces are formed, and etched metal protrusions that are electrically connected to said first metal layer; a second metal layer (54,76) that is formed on top surfaces of said protrusions, wherein said second metal layer is formed of a solder-plated layer or a solder layer; and an interlayer-insulating layer formed on said first metal layer; and characterized by: a third metal layer overlaid on said second metal layer and said interlayer-insulating layer and etched to form second conductor circuit traces; wherein said first and second conductor circuit traces are each exposed and suitable for directly contacting conductive elements of a further wiring circuit substrate upon the further wiring circuit substrate being overlaid directly onto said wiring circuit substrate or said wiring circuit substrate being overlaid directly onto the further wiring circuit substrate. a third metal layer overlaid on said second metal layer and said interlayer-insulating layer and etched to form second conductor circuit traces; wherein said first and second conductor circuit traces are each exposed and suitable for directly contacting conductive elements of a further wiring circuit substrate upon the further wiring circuit substrate being overlaid directly onto said wiring circuit substrate or said wiring circuit substrate being overlaid directly onto the further wiring circuit substrate.