Why would solder joints look like this from a hired contractor?

The lumpy solder and that brown gunk are not indicative of acceptable quality for anything that needs to last past the next repair cycle. You're seeing several common failure points, most notably what's called cold joint syndrome and oxidation buildup. When the solder just sits in blobs instead of flowing smoothly and bonding cleanly, it means either the joint didn't get hot enough, or the soldering iron wasn't applied correctly to transfer enough heat. If the wire feels loose, it's a definite cold joint—a failure to melt the joint components fully. Furthermore, that brownish smear is likely leftover flux residue combined with oxidation, which acts as an insulator and dramatically reduces the current capacity and overall reliability of the connection over time. This whole situation is compounded by the fact that you're soldering onto an aluminum heatsink. Aluminum conducts heat extremely fast, which makes getting enough heat into a small copper pad without melting the whole bar a massive challenge. While that environment is difficult, a skilled technician should at least be using appropriate flux and reflow techniques to acheeve a reliable electrical connection, even if the job is rushed. The result you have here is neither pretty nor functional. When you talk to the contractor, don't start by just complaining about the looks. Frame it as a functional concern. Point out that the appearance suggests inadequate heat application and residue, which guarantees resistance will spike and the joint will fail under thermal stress or vibration. You need a repair that meets basic industrial standards for permanence, not just something that looks like it was finished in ten minutes. The current work is highly likely to fail prematurely.

I think focusing so much on how fast aluminum conducts heat misses the real primary problem. Even if the thermal transfer was perfect, if the copper pad itself was oxidized—which that brown residue totally suggests—the solder simply won't adhere properly. You can't overcome surface contamination just by applying maximum heat to the bar. The issue is more about surface prep than heat distribution across the aluminum.

I reckon the vibration thing is as bad as the heat issue. When you're soldering stuff like that onto aluminum, that constant thermal expansion and contraction is gonna put serious stress on the joints. It's not just about the current flow; you're gonna see stress cracks or loose connections popping up pretty fast under normal shop vibration. They definatly didn't account for the long term physical stability.

You gotta understand that even if the wire seems to hold right now, that kind of shoddy work is just borrowing time. What they've done is created a high point of electrical resistance right where the current flows, and that's a waiting game. My grandpappy used to say if you can't see a clean solder fillet, you've allready lost. They likely didn't use enugh thermal mass to truly flow the solder, just to force a connection. This will turn hot under load and eventually cause a short.

The point about surface contamination being the real killer is spot on. Even if the thermal transfer was perfect, you can't fix a bad bond if the copper pad is oxidized. I've run into that exact problem when working on older wiring harnesses. It’s not enough to just heat the joint; you first have to strip off that gunk. I ussually use a fine wire brush or even a gentle sanding technique before applying any flux to ensure the metal is clean. That initial prep step is what decides if the solder will actually grab the pad.