Ive always used only 15%, but 15% too high now. Went to dynaflow then 5% I find dynaflow doesn't flow as well as 5%. Cant speak for joint strength but never found a leaking brazed joint unless it was obviously misbrazed ( Bugars and/or gaps) Ive heard the brazing material is stronger than the copper and at the looks of todays copper I believe it!
As previously stated, it depends on what your sticking together. Normal everyday copper to copper and copper to brass I prefer 15%. But when you start talking about carbon steel to copper, then you need 45% or higher and probably flux coated. If you have something like stainless steel to copper you need to look at 56% and higher. There are charts out there from Harris and others detailing what each alloy is for.
Been using Rothenberger S2 rods for copper to copper. (seems to be industry standard here in the UK)
5% for braze in sight glasses and driers.
15% for braze in txv's.
38% (flux coated) for copper to steel or brass.
56% for copper to stainless with special flux.
The S2 rods are about 50p and the 15% ones work out around 14 times that...
I prefer the 5% and 15% rods but have to cut my cloth according to my means
I like 15% because it has better gap filling ability than the lower silver content alloys.
If all of the joints are good and snug, the difference between 0% and 15% is barely noticeable, but if there is some slop in the joints, as is often the case when working with soft copper, or the stub outs on residential equipment, using 0% can be frustrating. 15% is king for copper to copper.
For copper to carbon steel, I like Harris Safty Silv 45. It has a blue flux coating on it, so with carefully prepared fittings/tubing, no additional flux is needed, reducing the chance for getting flux in the system.
For copper to stainless steel, or when I'm unsure of what kind of steel it is, I like Harris Safty Silv 56 It has a pink flux coating.
I'm still working on a "favorite" for copper to aluminum, and aluminum to aluminum. I picked up a sample of something at the AHR expo that I'm going to try out though.
If more government is the answer, then it's a really stupid question.
I haven't ever heard anything bad about the 5% rods for plain copper to copper.Although I have found old connections that have cracked at the joint and leaking refrigerant.I know that 15% produces a higher tensile strength on connections,and has a thicker liquid state.Also as stated above it fills in gaps better than 5%.May be overkill but I have always used 15%.