[Big Ideas]

It's from a PC so sputtering process is out, too expensive. Not bonded, too expensive.

The base part is an aluminum extrusion to the outside profile. The gaps between fins are formed in one of 2 ways most likely.

1) there is a gang saw with many saw blades, with spacers in between, on a horizontal shaft that is mounted on a horizontal mill type machine. The roughness would be a function of shoving the parts thru at a high rate. Since aluminum recycles readily, it's better to have a fast process with lots of scrap then a slow process with little scrap. Remember, must be cheep but effective.

2) (this is the fun option) I have seen heatsinks that are formed by a knife that cuts into a solid block of of aluminum in progressive cuts. Just like whittling a fire starter. But this gives slightly curved fins with the root with a sharp angle (not flat).