Power Mac G5: Processor Mounting Hardware - Precise Models

[Nullcaller]

Why?​

Lately I've been dwelling on the idea of using a CPU from the set of Late 2005 PMG5 components I bought (CPU, logic board and some other bits and bobs) to turn a Late 2005 G5 that I have into a weird 2.0 GHz Quad configuration. As far as I can tell, all of the required components for the second CPU seem to be there on the 2.0 GHz board that I have, and the few empty pads that seemingly don't have the components populated are actually also not populated on the real Quads.

To this end, I have successfully desoldered a CPU socket from a board of an earlier model that I just had lying around. This was just a test using a board that I can screw up and not be too sad about. The pads did not get damaged, and the socket got desoldered cleanly, so, you know, no harm done, in the end. I'm still waiting on 0.65 mm solder balls (my uneducated guess as to what size I need for this socket) to arrive so that I can test my solder-the-thing-back-on skills and then go for the real thing.

Spoiler: Socket Desoldering Instructions (if you're interested)

If anyone's gonna attempt something similar, I needed to raise the temperature to 350 degrees Celcius on my hot air station. I heated it at 250 degrees for 3-5 minutes, then at 300 deegrees for 3-5 minutes and at 350 for 3-5 minutes, and only then was I finally able to desolder it. Be careful when pulling it up and away from the board, as one end of the socket might not be hot enough to be desoldered while the other one is, so pull away really slowly and stop if you feel or see any resistance. There should not be any resistance whatsoever. If there is, that means you're pulling pads from the board.

I'll upload a video of the procedure when I have time for it.

But while I'm waiting on the solder balls, I thought I'd try and address another issue with this mod, which is that the heatsinks from the 2.0 and 2.3 GHz models cover the area designated for the second processor.

What?​

To start somewhere, I took some measurements of the processor mounting system of the Late 2005 PMG5, and made OpenSCAD models of:

1. the giant 7 mm thick plate to which everything mounts (without the heatpipe support piece in the area where the hotplate sits, because it can't be reached without disassembling the actual heatsink) - heatsink_plate_wo_hp_holder.scad,

2. the VRM heatsink (without the indentation where the heatpipe should sit) - vrm_heatsink_wo_hp_cutout.scad,

3. a WIP modded version of the 7 mm plate, which would allow me to mount two processors (I will need to acquire appropriate PC coolers and modify the plate further to add mountpoints for them) - heatsink_plate_mod.scad.

Couple of things about the models. They should be quite precise. All the measurements were done with callipers that should allow 0.01 mm precision measurements, and pretty much all of the variables that have their values set numerically in .scad files contain actual real measurements with no math done to them. The math is done automagically using other derived variables. All that said, however, (1) I'm a big goof and could've made a few mistakes here or there and (2) the 7 mm plate is cast and it is therefore very difficult to measure it correctly, so no guarantees. I did, however, print out a few 0.2 mm thick versions of the models on my 3d printer to compare with the real hardware, and I can attest that all the holes and openings align with the real ones quite well.

I have also made measurements of the processor board itself with actual 0.01 mm precision and will be updating the archives with board models soon™. Same for all the fasteners (except the spring, for which I don't have the equipment to measure the Young's modulus). There's a dimension 'drawings' (more like frantic notes) file attached to the post if you want the dimensions now.

Just to be clear, all model files (.scad and .stl) published here are made by me and are licensed to anybody who wants them under CC-BY 4.0, unless the forum rules or EULA state otherwise, in which case they take precedence.

What's Next?​

Currently, the preliminary plan for making the 'permanent solution' is 3d-printing a plate, then using it to cast a part out of metal. Or perhaps paying somebody to print the part out of high-performance plastic, like PEEK. The idea is that the result should be able to fit all Quads regardless of whether or not I succeed with my weird custom 2.0 GHz thing that I'm doing. The plan for the VRM heatsink is to add the heatpipe cutouts, convert the model to a CNC-mill-friendly format and have it made in a professional workshop.

If anyone reading happens to have a pair of callipers and a Quad G5 CPU daughtercard just, you know, lying around detached from the LCS, I would be very grateful if you took some measurements of the heatpipe cutouts and the little extra copper bits right next to them, along with some measurements of the two screw holes on the side of the VRM heatsink which are different on the Quad version of the VRM heatsink.

Model Version History​

1.0 - Initial release with 2.0 GHz heatsink plate (without heatpipe backing section), 2.0 GHz VRM heatsink (without heatpipe cutouts), WIP modded 2.0 GHz heatsink plate.

 

[Nullcaller]

By the way, here's some information on how the G5 heatsinks are put together, while I struggle with the socket.

They seem to be using what I'm assuming is the standard Tin-Bismuth[-Silver] low temperature solder to affix the heatpipes to the cold plate. Here's a photo of the Late 2005 G5 heatsink with that shiny metal stuff:

The older model heatsinks I have also seem to be using the same stuff:

The cooling fins (on all heatsinks, including the VRM heatsinks), however, seem to be using thermal epoxy of some sorts. It's that dark-grey stuff on the heat pipes:

I've tried to remove some of it with a sandpaper, and sure enough, I see none of that metallic sheen:

The fact that the thermal epoxy isn't applied along the entire circumference of the heatpipes, but instead in relatively thin lines going along the length of the heatpipe seems to suggest that the heatsinks mostly rely on press fit for thermal conductance, rather than on the thermal epoxy. The thermal epoxy is probably there mostly to hold the heatsinks together, and not to act as a thermal interface material. The full disassembly of the processor mounting hardware seems to involve removing the cooling fins both on the later and the older models, which would involve somehow undoing both the epoxy and the press fit.

 

[Bug-Creator]

As far as I can tell, all of the required components for the second CPU seem to be there on the 2.0 GHz board that I have, and the few empty pads that seemingly don't have the components populated are actually also not populated on the real Quads.

Even if this is true, and if there is no lock against it in FirmWare, it still seems like an a## backwards approach.

If you have access to a real Quad, use that board, slap in 2 2.0GHz CPUs (this works the other way round) and maybe downgrade the PSU as a next step.

You should now have a 2.0GHz Quad with a little bit less power used.

 

[Nullcaller]

Even if this is true, and if there is no lock against it in FirmWare, it still seems like an a## backwards approach.

Depends on what it is you think I'm trying to achieve. I highly doubt I'll be able to come across a Quad where I'm at, and my goal with this project is not to get a cool weird Quad anyway. It is cool, and it is weird, but what's more important is that I want to understand how these machines work and improve my skills to a point where I'm comfortable working on them. If there's a firmware restrisction, so be it, I'll try and learn how to bypass it. If the hardware just plain doesn't support it, hey, I learnt something. I don't want to have a Quad. I want to make it.