Heat pumps often have pressures inside reaching 300 psig. The method used for joining is to silver solder (which is really a kind of brazing) the connections. This requires cleaning the metal, applying silver brazing flud, heating the metal to dull red, then applying the rod to the joint. It takes some skill.
If the tubing is not purged with nitrogen during the heating, flaking will occur inside the tubing to pass throughout the system potentially clogging the small metering devices.
Ordinary solder will hold for an undetermined time, then there will be a sudden release of all environmentally damaging refrigerant and oil on the surrounding area. This is the reason that refrigerant work requires a license.