Peer review request on PTH GND PCB design idea

Thread Starter

mgookin

Joined Mar 21, 2017
11
Problem: When hand soldering a through hole connection for a pin connected to a ground plane, the time & temperature requisite to get good solder flow and good connection are considerably greater than that required for a pin connected to something other than a ground plane. This is caused by the thermals in the board design dissipating the heat from the soldering iron into the ground plane so fast that it's hard to get the pin & plated through hole up to temperature. At times this leads to failure even when it looks like the connection is good.

When I measure the thermals in Eagle Cad I see 20 mils four times each side of a 2 layer 2 oz. copper board. That's cumulatively 160 mils of 2 oz. copper dissipating the heat. The current on this pin is just a few mA. There is certainly no need for this much trace.

My idea is to modify the package in Eagle Cad to place an arc around the pin, some distance away from the plated through hole. I clone the arc so I have one on the top layer and one on the bottom layer. The arc, a copper trace connected to nothing, obeys the rules of spacing set in the design rules. I limit the arc to slightly more than 180 degrees to prevent 3 of the thermals from being generated on each layer.

This provides me with one 20 mil thermal on the top ground plane and one 20 mil thermal on the bottom ground plane, cumulatively 40 mils, still much more than I need for just a few mA passing through this connection.

As to pros & cons:

Pros:
This should help significantly with the problem of failed connections due to insufficient heat when soldering resulting in faster production and diminished failure rate.

Cons:
I can't think of any.
Can anyone else think of any reason I shouldn't spend the money to have some boards made up with this design and give it a try?

Has anyone ever tried this before?

Thanks!
 

ebp

Joined Feb 8, 2018
2,332
As long as the through-plating is good, you really don't need solder to make it to the top of the board, though I always liked to. I used to do boards with four ounce copper on 3/32" laminate, which is an "enhanced" challenge.

As long as your arcs don't generate rule checking problems or routing problems, it seems like it might work. My first approach would be to simply change the relief pad definition for 10 mil connectors, which shouldn't cause any fab problems with 2 ounce copper. Are you sure there is no capability to set them for just two spokes?
 

Thread Starter

mgookin

Joined Mar 21, 2017
11
ebp:

I'm not trying to get solder all the way to the top of the board. I just want a good cone/ good connection. What happens presently is the solder wants to ball up above the pad sometimes not even connected to the pad.

There is no means in Eagle Cad (that I can find) which allow me to control the size of the connectors. The only control I get is to turn thermals on or off. If I turn thermals off I get continuous connection from pad to ground plane (worse) and if I turn thermals on I get 4 spokes. There is no means (again, which I can find) that allows me to control the number of spokes. By placing the arc I am effectively controlling the number of spokes.

It looks a little funny on the board. There are no errors. The board house may (or may not) question it.

Thanks very much for the reply. Sounds like you are unaware of any problems this may cause and that's a good thing!
 

Thread Starter

mgookin

Joined Mar 21, 2017
11
I think you need a better, cleaner, soldering iron.
Our biggest pin on that board carries 15A with 100 mil trace each side of the board and it solders like a dream. Great looking work with 100% success. We have the best equipment. It's these intermediate pins which are a challenge. We've tinkered with different tips & temps and even solders with limited success which is why I'm now looking at the design to address the heating and solder flow issue. But thanks.
 

dl324

Joined Mar 30, 2015
11,317
Use an iron at the correct temperature and with sufficient thermal mass. I use my 60W Weller WTCP with a 700F conical tip to solder pieces of copper clad together to make boxes or supports.
 

ebp

Joined Feb 8, 2018
2,332
A work-around, never a great thing to have to do but sometimes worth the bother, would be to do the normal routing avoiding using 20 mil tracks. You might use 19 or 21, depending on which would least interfere with your design rules checking. After the Gerbers are generated, you manually (or using a script - I highly recommend AutoHotkey if you use Windows) edit the aperture table to change the 20 mil aperture to 10 mils, then the 19/21 to 20 (if you even care about the 1 mil difference). The only thing I can think of that you would need to watch for is where then ends of the spokes fall, though I doubt it would be a problem. The CAD packages I've used draw the tracks ending right on the centres of the pads (i.e. the centre of the track aperture would align with the centre of the pad aperture) so there shouldn't be issues with the spokes being too long or too short - but I'd certainly want to review the Gerbers carefully to be confident.

When I used things like that I liked to put some text in a notes layer in the design to remind myself what I needed to do. I also typically made notes elsewhere on how to do things because I'd just forget if I was away from it for awhile.

Incidentally, if you need to desolder parts from ground plane boards, a good preheat of a reasonable area using a hot air tool can be a big help.
 

ebp

Joined Feb 8, 2018
2,332
' ... I use my 60W Weller WTCP with a 700F conical tip ... '

I've used that series of Weller stations for decades. Because of those tips, I still think of soldering in Fahrenheit, but everything else in Celsius.
 

dl324

Joined Mar 30, 2015
11,317
I've used that series of Weller stations for decades. Because of those tips, I still think of soldering in Fahrenheit, but everything else in Celsius.
I've used my iron for virtually all of my soldering since 1977. Still using the original conical tip.

My Wife used the military version for her stained glass work.
 

Thread Starter

mgookin

Joined Mar 21, 2017
11
Use an iron at the correct temperature and with sufficient thermal mass. I use my 60W Weller WTCP with a 700F conical tip to solder pieces of copper clad together to make boxes or supports.
We have an arsenal of equipment and we've tried using the same setup we use to solder .060" thick brass plug blades and even turned up the heat hotter and all we're doing is pumping heat into the board (and components) without getting the result we want.

By placing the arc around the pin it eliminates 3 of the 4 spokes on each side for a 75% reduction in trace going into the ground planes. It will be interesting to see how (if) this works out.

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