PCB design question - thermals?

[Family Hoof]

How do you decide when to use thermals? In case anyone is not familiar, it is the concept of connecting solder pads to a common plane indrectly, using small traces instead of just having the solder pad be part of the plane. I've always thought this was to keep the plane from sucking all of your heat while soldering to pads on it, but someone should correct me if I'm wrong.

As for me, I have been using and enjoying them but sometimes I worry that it'd be better for signal strength to have all pads fully connected to the ground plane.

 

[PRR]

That gimmick is for mass production wave-soldering. All joints on the board get the same heat input. If some are the end of a thin trace, and others are the middle of a big copper area, there is no time/temperature setting that will bond the big joints without cooking the little joints or the chips.

No, I'd never do that. Not for signal reasons, but because I have a big frikkin iron and heat until hot, one joint at a time. I'm not real good with the tiny joints, but I can solder a DIP or a piano (key-rail) if I want to.

 

[Crusty2]

As PRR said, it's for wave soldering purposes. No reason to use them for hand soldering. I would certainly avoid them in high current areas such as the rectifier section of a psu, though I've seen it done.

 

[mnats]

How do you decide when to use thermals?...I've always thought this was to keep the plane from sucking all of your heat while soldering to pads on it, but someone should correct me if I'm wrong.

An observation - whenever I'm designing a pcb and I have a small, possibly heat-sensitive part that has one leg going to the ground plane I use a thermal pad. Even though I could heat it with my iron if the "pad" was just a hole in the ground plane I just feel better if it is thermally isolated a bit so I don't have to cook the hell out of everything to get a good joint.

It also looks better as the solder tends to stay on the pad instead of migrating out onto the ground plane in a random fashion.

As for me, I have been using and enjoying them but sometimes I worry that it'd be better for signal strength to have all pads fully connected to the ground plane.

The prof gave a nice dissertation on a related topic here.

 

[mcs]

[quote author="mnats"]It also looks better as the solder tends to stay on the pad instead of migrating out onto the ground plane in a random fashion.[/quote]
The solder mask takes care of that...

Best regards,

Mikkel C. Simonsen

 

[mnats]

The solder mask takes care of that...

I'm not quite up to putting solder masks on my home-etched boards...

 

[bcarso]

Sometimes I am happy to have regis pads when dealing with boards where the values may have to be changed---it makes things much easier to desolder.

However, PRR is quite correct that their primary function is to permit proper wavesoldering in manufacturing.

 

[CJ]

We changed from a 1 trace to ground plane to 4 trace deal. I can't stand it. If you have to remove a component while troubleshooting, you need a blow torch, like Brad said.
Another problem is thru hole diameter. The new guy shrunk all the thru holes down a size. Now the parts don't come out easy like they used to. And the solder dosen't wick thru as good.

A trick I borrowed from the welders, they can adjust the heat while welding by varying the feed rate of the welding rod. Adding the rod faster cools off the joint tempeture a bit, since you are adding cold steel to the hot puddle. Slowing the feed rate increases tempeture. You can use the same thing while soldering. If you have a hot iron and a sensitive part, feed the solder in quicker to control the heat. And vice versa.

 

[rlaury]

I all ways use them. It's just a good engineering practice.
The only time I don't use them is if im trying to bond 2 planes together
thermally.

 

[Bauman]

I just use ´em for commercial boards, the ones will be fabbed and assembled by machine. For my own I don´t.

:guinness:
Fabio