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With solder in the title this might be a good time to remember how much less of a conductor solder is compared to copper. This also may not apply very much to

With solder in the title this might be a good time to remember how much less of a conductor solder is compared to copper. This also may not apply very much to gaming consoles in particular. If the wire carrying high current has copper that is too thin gage, it will heat to some extent since its resistance is not quite zero, and at any one current the more resistance then the more heat will need to be dissipated. If there is a point where it is not pure copper from end-to-end but has a solder joint in between, the solder will be the point of highest resistance so it will autoheat to a higher temperature than the copper does. Plus when a solder joint cools properly it solidifies as a eutectic alloy for the most part rather than a crystalline solid. A "cold" solder joint can have more of a crystalline nature, plus can have lots of metal oxides included if it was allowed to remain molten for too long or at much higher temperature than necessary considering its melting point. IOW a cold solder joint can be even less of a conductor than normal, so much less it can even sometimes act as a bit of a ~semi~conductor in exactly the spot you would least want that.

mystified5016 2y ago
Resistance is a function of cross-sectional area of a conductor. Current capacity is a function of resistance, length, and heat dissipation (largely surface area). The cross-sectional area of a copper trace is on the order of ten micrometers square. The cross-sectional area of a solder joint is usually on the order of one or two square millimeters, if not more. For a BGA ball, you can approximate it as a cylinder with radius equal to the ball radius. Again, this is on the scale of millimeters. No matter what, your solder joint will have an order of magnitude larger area than the trace feeding it. Solder joints generally have far lower resistance than a copper trace due to this fact. Besides that, many (most?) lead-free solder alloys use silver, which is famously quite a remarkable conductor. Cold joints and the other problems you mention are basically irrelevant electrically. They're mechanical problems that are likely to cause a break in the circuit. Any added resistance from inclusions or crystal structure are completely irrelevant unless you're doing something extremely specialized like high sensitivity metrology or crazy RF stuff.

mystified5016 2y ago

Resistance is a function of cross-sectional area of a conductor. Current capacity is a function of resistance, length, and heat dissipation (largely surface area). The cross-sectional area of a copper trace is on the order of ten micrometers square. The cross-sectional area of a solder joint is usually on the order of one or two square millimeters, if not more. For a BGA ball, you can approximate it as a cylinder with radius equal to the ball radius. Again, this is on the scale of millimeters. No matter what, your solder joint will have an order of magnitude larger area than the trace feeding it. Solder joints generally have far lower resistance than a copper trace due to this fact. Besides that, many (most?) lead-free solder alloys use silver, which is famously quite a remarkable conductor. Cold joints and the other problems you mention are basically irrelevant electrically. They're mechanical problems that are likely to cause a break in the circuit. Any added resistance from inclusions or crystal structure are completely irrelevant unless you're doing something extremely specialized like high sensitivity metrology or crazy RF stuff.