Super-Heavy Copper planes:   what solder and flux??? - Page 1
Forum section: electronics / projects

author: frogblender
author_rank: Regular Contributor
timestamp: June 24, 2025, 09:22:43 pm
content: I have an 8layer PCB with 2oz copper per layer: 16 oz total. And it is all plane, no signals.So soldering on connectors is proving difficult.I tried an InSn (melting point 118°C, digikey# SMDIN52SN48), but wettability is poor, even using the flux included with the solder.Can anyone provide advice on what flux and solder to use?

author: thm_w
author_rank: Super Contributor
timestamp: June 24, 2025, 09:26:47 pm
content: What soldering iron? Tip? Temperature?Any decent brand flux core solder should work fine, though you may need to preheat the board.I don't think indium solder is needed for this.

author: IDEngineer
author_rank: Super Contributor
timestamp: June 24, 2025, 10:02:23 pm
content: Agreed on the preheating. And on the tip (and iron) selection. You're basically soldering to a large copper heat sink, which is going to wick heat away from the iron in real time. You'll need an iron with plenty of heat capacity, a tip with lots of mass, and you may need to kick up the temperature to compensate for the heat sinking.Don't try to make this happen with a too-wimpy iron and tip. You'll just bake the laminate and get cold solder joints, if you get any "soldering" at all. Proper tools and preparation is the key to success.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 24, 2025, 10:16:50 pm
content: Yeah the choice of flux/solder is not really important. It's how you're transferring heat to the work, mainly your soldering iron and the choice of tip.If your soldering iron is underpowered, preheating with hot air might be necessary.And you will need to be patient.

author: Psi
author_rank: Super Contributor
timestamp: June 24, 2025, 10:21:30 pm
content: Sounds like your issue is entirely related to getting the PCB up to soldering temp.+1 to buying/using a preheater. I had to do that for my 6 layer 3oz boards.Or if you don't have one, use a hot air rework gun + soldering iron at the same time. But be expected for it to take a while.Boards with that much copper pull heat away like crazy.Get the board up to temp before you bring the connector in for soldering. Otherwise the connector will probably melt from all that hot air for so long.

author: Smokey
author_rank: Super Contributor
timestamp: June 24, 2025, 10:45:50 pm
content: It's (probably) not a solder/flux problem. It's a PCB design problem.If you didn't include thermal relief on the PCB (spokes) then you are already pretty hosed if you are trying to hand solder through hole pins directly to many planes (2oz or not). It will be extremely difficult to get the solder to actually flow all the way through the hole, even with pre-heating. You almost have to get the whole thing to reflow temps to have a good chance.Go back and redo the layout and add thermal spokes to all the through hole pads on every layer. If you need to conduct so much current through one pin that you can't manage thermal relief then you may want to go back and redesign your connectors as well.

author: Psi
author_rank: Super Contributor
timestamp: June 25, 2025, 12:58:01 am
content: Yeah, use thermal relief where possible.It's not always possible, sometimes you need full width copper into high current mosfets for example, and they can be annoying to solder. You really do need a preheater in those cases.

author: Smokey
author_rank: Super Contributor
timestamp: June 25, 2025, 01:06:49 am
content: Quote from: Psi on June 25, 2025, 12:58:01 amFor something like a through hole MOSFET, bring the current down to inner layers with a bunch of vias next to the thermally relieved through hole pad. You may think you are getting "more current" by using a direct pour connection, but if the solder doesn't actually flow all the way through the hole then you are only actually conducting through the hole plating, which is really really thin compared to the surface plating.One more thing... (for example) 4 x 20mil spokes = 80mil connecting the pin to a plane. While yes, that is 80mil of copper, it's not the same current capacity as a long 80mil trace because the spokes are super short and the plane is acting as a big heatsink keeping the spokes from melting.

author: Psi
author_rank: Super Contributor
timestamp: June 25, 2025, 02:14:03 am
content: I was thinking SMT mosfets, the 100A+ ones in SOIC8 style packages.But yeah valid point about TH ones.

author: IanB
author_rank: Super Contributor
timestamp: June 25, 2025, 03:00:58 am
content: Has no one considered an oxyacetylene torch?But seriously, I have sometimes used a butane micro-torch where a lot of heat is required on non-PCB applications. I imagine the danger with PCBs is you could easily delaminate the layers if you overdo it. Probably not enough finesse for a circuit board.