I've just received my first batch of professionally manufactured boards. When designing the boards I tried to do things to best practice recommendations as picked up on this forum and elsewhere.
One of these was to use thermal pads onto the ground plane as per the attachment below (it's a standard 0.8mm hole). However I'm having real problems soldering pad in question. It has four way links (I'm sure there's a proper term) to the ground pour on both sides of the board. I couldn't get the pad in question to take solder at all, ended up with a classic "keep lashing solder on until you get some sort of connection". Standard lead based solder, tip at 330 degrees.
The rest of the pins on that device (an isolating DC-DC convertor) soldered fine so it would seem fairly cut and dry that it's the links to the ground pour causing the problem but I'd have thought that was a fairly standard design so wanted to make sure I wasn't missing something?
It's amazing how much more heat those pads need. I usually run the iron 340-370°C. Use the side of the iron, rather than the very tip, put a little more pressure on both the pad and the wire, and take a little more time. Still, it's rare for solder to flow through to the other side of the board, which I usually see on most non-thermal pads. Oddly, when soldering power jacks into 4.8mm holes, I seem to have more than enough heat.
330 is not hot enough. 680F (360C) is a good place to start for ground planes, up to 720F (380C) as needed but dial it back down when you can. Use a big tip since that will have a larger thermal mass and transfers heat much more efficiently.
Even better, get an underboard preheater (Hakko makes a relatively inexpensive one....can't think of the part number just now) and pre-heat the board to 150C (300F) to minimize the temperature differential when soldering up top. It really makes a difference. I used to have the same problems as you (many moons ago!) but with a) right temperature, b) right tip, c) underboard preheat I can make good connections to ground planes in <3 seconds.
If you just kept lashing solder on....you may very well have a cold solder joint, you just can't see it. Solder will quite happily stay firmly seated on a pin, riding on a nice wave of insulating flux thin enough that you can't see it, but thick enough to insulate nicely from the pad!
BTW, those "four-way links" are called a "thermal relief" or "thermal pad". They're actually supposed to help in soldering to the ground plane by minimizing heat loss away from the pad. I don't think they help that much
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at this temperature the contained flux vapors quickly.
And the tip can burn out if you leave it for many hours.
It's correct turn back the temperature.
Lead solder only seems to be reasonable because lower melting point.
GilchristT
"keep lashing solder on until you get some sort of connection"
The solder must fuse properly between the metal wires and the copper plating.
If you can break it off easily it is no good connection.
Terminals soldered into PCBs like you showed, break off after a year or two sometimes.
So it's all reflow solder engineered these days.
The latest trick I saw in televisions is to crimp high power diodes actually, not solder them.
Then big massive rings are pressed into the PCB without soldering.
Modern lead-free solder needs higher temperature - 360 C should do. You must have solder and flux on the iron's tip, this means clean the tip immediate prior to soldering, tin the soldering iron and immediately apply it to the joint to be made and then immediately apply the solder to the tip and pad - fresh flux is always required on soldering.
Heat only conducts well when the iron has enough solder to bulge out a bit and press against the pad - and there must be flux to reduce the oxide coating and let the molten solder actually touch the pad. Then there is rapid heat transfer (at which point you are adding more solder to the joint and immediately removing the iron so it can cool.
Hot iron, use fresh flux, work fast, don't breath the fumes...
You didn't say what sort of soldering iron you are using. To solder pads to ground planes:
Use a temperature controlled soldering iron with a heat output of around 50W or more, not a constant-power soldering iron (typical heat output of those is 18 to 25W).
The bit on the soldering iron needs to have a large enough flat area to conduct sufficient heat to the pad. The fine bit that you probably use for soldering the other pins of the chip won't conduct enough heat when soldering pads to ground planes.
Tin the flat area of the bit first to aid heat conduction.
