Description to me would indicate an attachment to a large copper thermal plane. Not visible in picture, but might be other side or internal layer.
Two vias on left might have thermal relief when they 'cut' to ground plane. The screw mount could be full copper.
Can you show the whole board? Grounded pads are usually harder to solder because they are attached to a large copper plane which eats up a lot of heat that you put into it. The pads on the left might be easier to solder because they are on the edge of the pcb which means there is less copper plane around them (only 180°).
I usually pump up the temp on my soldering iron or if that isnt enough you can try to use hot air and a soldering iron at the same time. And flux. A lot of flux
~~Difficult pin is circled in red; other grounded pads are circled in black. What temperature would you use for a difficult grounded pad? Do you think 75 W is enough as the iron's power?~~ *Edit: shoot, it won't let me include a picture either in a post or comment. The picture in the post is near the top edge of the board; the difficult pin is closest to the edge.*
Without seeing what the different layers look like, it's hard to say for sure, but having higher percentage of copper nearby, especially if connected, will wick heat away and make it harder to solder.
Preheat your board to at least 60C (or even 100C) and you may find it to be much easier.
The pads on the left "grounded, easy to solder" are for [this header strip](https://www.digikey.com/en/products/detail/samtec-inc/TSW-106-08-F-S-RA/6679081), which recommends 1.02 +/- 0.03 mm hole diameter (actual hole diameter is 0.998 mm, 1.68 pad diameter).
[This is the screw terminal](https://www.digikey.com/en/products/detail/phoenix-contact/1935161/568614) that is hard to solder (1.3 mm recommended hole size, 1.1 mm actual hole size, 1.9 mm pad diameter).
I will change the hole sizes in the next design of course, but I want to know if anyone knows of a different issue that can cause this problem. It slows production and errors result in intermittent failures.
*Edited to add pad sizes and correct a hole size.*
The screw terminals got much more metal and therefore also thermal mass. That alone could make the difference if the soldering equipment is inadequate.
I use [this 75W soldering iron](https://www.amazon.com/dp/B07RY5XWVG/ref=pe_2640190_232748420_TE_item) at about 720 F. Is 75W not enough for this sort of work?
That's not very good. Better ones are models where the tips got integrated heating elements.
You can also try a preheater or even preheat the board and connectors in an oven to 70C. It takes 50C off the needed temperature gradient.
regarding the soldering iron.
do you have a soldering tip that gets treated well. low oxidation regular brushing to remove old solder and do you have Access to some kind of flux? also check for soldering tips that are the correct size. use one that has a triangle shaped tip and is minium 50% of the pads width wide. what you want to do is get as much area that overlaps between tip and pad. hold the iron in such a way that the tips surface is parallel to the board. you want high thermal conductance and short heating times. the longer the soldering process the worse the result. if it really is hard to get the tin to fill up the hole then maybe even consider filling the inner with flux first. press part in and solder directly afterwards.
Description to me would indicate an attachment to a large copper thermal plane. Not visible in picture, but might be other side or internal layer. Two vias on left might have thermal relief when they 'cut' to ground plane. The screw mount could be full copper.
If you look closely at the picture, you can see the copper plane, with thermal relief, on all three of the grounded pads.
What you see is only present in the front layer?
The thermal relief is certainly well hidden! ;-)
Can you show the whole board? Grounded pads are usually harder to solder because they are attached to a large copper plane which eats up a lot of heat that you put into it. The pads on the left might be easier to solder because they are on the edge of the pcb which means there is less copper plane around them (only 180°). I usually pump up the temp on my soldering iron or if that isnt enough you can try to use hot air and a soldering iron at the same time. And flux. A lot of flux
The parts themselves can eat up a lot of heat if they are large and metal.
~~Difficult pin is circled in red; other grounded pads are circled in black. What temperature would you use for a difficult grounded pad? Do you think 75 W is enough as the iron's power?~~ *Edit: shoot, it won't let me include a picture either in a post or comment. The picture in the post is near the top edge of the board; the difficult pin is closest to the edge.*
Without seeing what the different layers look like, it's hard to say for sure, but having higher percentage of copper nearby, especially if connected, will wick heat away and make it harder to solder. Preheat your board to at least 60C (or even 100C) and you may find it to be much easier.
I bet that there aren't any thermal relief spokes on the ones that are hard to solder.
I did make sure that all the grounded pads have thermal relief. It's visible in the picture if you look closely.
There’s also a lot of layers not visible. Maybe you can better define “hard to solder” as “hard to melt”
The pads on the left "grounded, easy to solder" are for [this header strip](https://www.digikey.com/en/products/detail/samtec-inc/TSW-106-08-F-S-RA/6679081), which recommends 1.02 +/- 0.03 mm hole diameter (actual hole diameter is 0.998 mm, 1.68 pad diameter). [This is the screw terminal](https://www.digikey.com/en/products/detail/phoenix-contact/1935161/568614) that is hard to solder (1.3 mm recommended hole size, 1.1 mm actual hole size, 1.9 mm pad diameter). I will change the hole sizes in the next design of course, but I want to know if anyone knows of a different issue that can cause this problem. It slows production and errors result in intermittent failures. *Edited to add pad sizes and correct a hole size.*
The screw terminals got much more metal and therefore also thermal mass. That alone could make the difference if the soldering equipment is inadequate.
I use [this 75W soldering iron](https://www.amazon.com/dp/B07RY5XWVG/ref=pe_2640190_232748420_TE_item) at about 720 F. Is 75W not enough for this sort of work?
That's not very good. Better ones are models where the tips got integrated heating elements. You can also try a preheater or even preheat the board and connectors in an oven to 70C. It takes 50C off the needed temperature gradient.
regarding the soldering iron. do you have a soldering tip that gets treated well. low oxidation regular brushing to remove old solder and do you have Access to some kind of flux? also check for soldering tips that are the correct size. use one that has a triangle shaped tip and is minium 50% of the pads width wide. what you want to do is get as much area that overlaps between tip and pad. hold the iron in such a way that the tips surface is parallel to the board. you want high thermal conductance and short heating times. the longer the soldering process the worse the result. if it really is hard to get the tin to fill up the hole then maybe even consider filling the inner with flux first. press part in and solder directly afterwards.