A solder layer and an electronic device bonding substrate using the layer are provided which avoid deteriorating qualities of the electronic device to be bonded. In a solder layer 14 free from lead and formed on a substrate 11 or an electronic device bonding substrate 10 having such a solder layer, the solder layer 14 has a specific resistance of not more than 0.4 Ω·μm. The electronic device bonding substrate 10 can have a thermal resistance of not more than 0.5 K/W and a thickness of not more than 10 μm. Then, voids contained in the solder layer 14 have a maximum diameter of not more than 0.5 μm and the substrate can be a submount substrate.