I need to attach a heat sink to a PCB. The PCB is a 560 mm x 9 mm strip of LEDs. The LEDs vary from mid-power to high power. I have a pure copper solution for the high power. I am investigating the lower power end where a 25 mm wide strip would be sufficient for thermal management. The lower end, mid power, is budget sensitive and am experimenting with aluminum extruded heat sinks.
My concern about an aluminum heat sink is galvanic corrosion where aluminum touches copper. In the first prototype for the pure copper the thermal pad on the PCB were unplated bare copper.
This image shows the pure copper thermal path from the LED's thermal pad to 1/2" copper water pipe.
This is the footprint for another prototype recently fabed. It shows the copper thermal path from a Cree XP or OSRAM high power LED. The center pad of the footprint is the LED's thermal pad. It goes to a copper strip along the edge which gets screwed to either an aluminum bar or copper bar.
With a thinner copper bar I can get the copper bar closer to the LED reducing thermal resistance.
I have new strips with ENIG plating.
HASL plating is not an option
This is using a footprint for an Oslon SSL similar to the KiCad image above.
The next experiment is to try three different aluminum extrusions.
I used the ENIG assuming I can screw either aluminum or copper to ENIG with very little chance of galvanic corrosion but have no idea why I believe that may be true or not. If not, it's still a nice planar surface.
I am going to assume for this issue that the aluminum will do as well as the copper.
The bars screw to the PCB and the heat sinks screw to the bar.
I assume there will be a problem with a copper bar mating with the aluminum heat sink. I could use a thermal tape TIM like 3M's 5 mil Thermally Conductive Adhesive Transfer Tapes. Or screw the parts together with a thermal compound.
I could use an aluminum bar screwed to the PCB and there will be no problem with the mating of the bar and heat sink.
Will there be a galvanic corrosion issue with the ENIG and copper or aluminum?
Will there be an issue with the copper bar and aluminum heat sink?