Ball Grid Array
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Plastic top BGA
One of the first versions of BGA capsules were the fiber based with a wire bonded chip on top covered in black resin. These are very easy to process and most of the gold recides in the black plastic on top.
Flip chip BGA
On these chips the connection between the substrate and the die is made with tiny solder pillars. Not a lot of gold in them. The die in it self is turned upside down with the circuit against the fiber base. It is the silicon die you see at the top.
Copper base BGA
In some high end equipment there is a type of BGA with a copper base and a flexible mylar substrate that connects the solder balls with the die. The die is very thin and sits on the solder side of the chip under a thin plastic cover. These are probably bonded too but with very short wires.
A good approximation seems to be about 5 g/kg of BGA chips and 10g/kg for the plastic top without the fiber bottom. The fiber bottom contains circa 0.7 g/kg of gold but a lot of tin solder complicates the refining process and is better handled by large scale refineries that makes copper bars out of circuit boards.