鑞 combines 金 (metal) with 巤, which likely contributes the sound ロウ. The character was created to name a specific metal alloy used for joining.
Meaning mnemonic
The left side is metal 金, and the right side 巤 looks like a complex tool with a container and flowing lines. Picture molten metal flowing from a tool to join two pieces: that is solder.
Reading mnemonic
For ロウ, imagine a row of metal pieces being joined with solder: row -> ロウ, and the metal 金 on the left holds them together.
