My project uses a 4S 21700 Li-ion battery pack. I plan to attach the batteries to the back of the board with this battery holder. It has 8 TH pins that will attach to large copper planes (82mm x 20mm) making the 4S circuit. I will be hand soldering this part to the finished board.
I will be drawing significant current (6-8 A) from the batteries for short periods of time (300 - 500 msec), with long pauses (2-10+ min) between pulses. My question is what type of pad should I use for the 8 TH pins on the battery holder - thermal relief or solid?
I have learned from various posts that thermal relief pads are easier to hand solder, but I have not seen any discussion if there are different current capabilities of thermal relief versus solid pads.
In general, should high current (1A or more) connections use solid pads, or is there little difference between thermal relief and solid pads when it come to current carrying capabilities? My battery fuel gauge has a few SMD parts in the high current path; same with the charging circuit. Should those SMD pads be thermal relief or solid? Does it matter?
8A is not much current. You can still use thermal reliefs if you make the spokes a bit wider. Note that with KiCad’s calculator tools, 0.8mm wide track can already handle 2A (4 spokes on a thermal relief). The thermal reliefs are also very short. Only at longer tracks you get a significant voltage drop and heat dissipation.
When you’re going to 20A or more it becomes a bit different, Around there somewhere it’s better to use a solid connection instead of the thermal reliefs.
But also, if you have decent soldering tools, then maybe there is no need to use thermal reliefs at all. Thermal reliefs are always a compromise. If you are hand soldering without thermal reliefs, then using bigger pads may help, because more contact with the soldering iron improves heat transfer to the copper plane.
@paulvdh Thanks for the reference to the Calculator Tools. Very helpful.
Can you give me a little more explanation on how you arrived at 4 spokes @ 0.8 mm results in 2 A carrying capacity?
Looking at the Track Width calculator,
are you assuming internal traces or external traces?
how long is the trace?
how does the number of spokes figure into the calculation?
Thanks!
P.S Is it a coincidence or planned that your calculation of 2A is approximately the RMS values of a train of pulses 500 msec wide and 10 min between pulses (actual value is 2.2A)?
The calculator is (in it’s default settings) based on a 10degree centigrade temperature rise of the PCB.
If you type in 2A in the wanted current, it suggest 0.79mm track width. And 4 spokes with each 2A is your 8A total.
But do keep in mind numbers like these are not very accurate, but it gives you some idea of what is needed in practice.
As a power supply designer, I seldom use thermal reliefs. The main disadvantage of thermal reliefs is that they defeat (or greatly reduce) the heatsinking capability of the copper zone. This is particularly an issue when connecting semiconductor power tabs (Such as a D-pak) to a copper zone when the semiconductor needs some cooling in operation.
Soldering without the thermal reliefs is more difficult but I can generally get that done without too much of a problem.
Except SMD soldering iron I’m using 100W trafo-iron (gun-iron). I make soldering tips from copper wire with a diameter of 1.5 mm. I would not expect any problem with soldering this with solid zone connection.
That seems rather thick for the inner layers, but anyway, even if you just use one 2oz layer for 8A and 10C increase in temperature you only need 2.7mm width. Across 4 spokes needs 4 x 0.7mm wide spokes, seems very doable . . . as long as your pads are big enough. Small pads can only have small spokes.
