2

This question is further to Why thermal reliefs on vias?

I have recently been advised (by someone I consider an expert) that I should add thermal reliefs to vias which are connected to planes, even if they will not be soldered.

This is to alleviate thermal stress between the different types of copper in the plane and the via. The plane has rolled copper, whereas the via has deposited copper, and these behave differently across temperature.

Has anyone seen any reliability issues due vias being directly connected to planes?

Added: My personal project will be restricted to the commercial temperature range. However, I would still be interested to know if this is a problem in any other temperature range.

Community
  • 1
Rocketmagnet
  • 26,933
  • 17
  • 92
  • 177
  • What kind of operating temperature range are we talking about here? – Anindo Ghosh May 17 '13 at 13:17
  • @AnindoGhosh - Added. – Rocketmagnet May 17 '13 at 13:20
  • 1
    I just checked through boards from several electronic devices around me that I have "liberated" over the years, including printers, scanners, a computer monitor, and so on. None of them seems to have any thermal reliefs on the vias, at least in those cases where a copper pour is visible. Admittedly that isn't conclusive, hence not writing an answer. :-) – Anindo Ghosh May 17 '13 at 13:26
  • The only reason I've done it in the past is in the off chance that I'd need a ground connection for the scope probe by putting a lead in the hole and soldering it in. – placeholder May 17 '13 at 14:10
  • What are reliefs going to do to help? Thermal reliefs promote the preservation of temperature differences, and temperature differences are a culprit (except when we are soldering) right? Ideally, we would want the entire board: copper, glass, resin, coatings: to be at the same temperature. The much bigger issue is that both coppers are bonded to something completely dis-similar. I'm calling "onion in the varnish" on this practice. – Kaz May 17 '13 at 14:34
  • @Kaz - I think his point was that even at the same temperature, the two types of copper behave differently. – Rocketmagnet May 17 '13 at 14:36
  • Is the rationale that expansion space is being created around the via? – Kaz May 17 '13 at 15:03
  • @Kaz - Now that I run over it in my head, I'm not exactly sure what the rational was. It was something like: the thermal is mechanically a little more forgiving, which reduces the mechanical stress as the two types of copper expand differently at the same temperature. – Rocketmagnet May 17 '13 at 15:28
  • @Rocketmagnet: Aha, I thought so. It's about the mechanics of expansion, not about heat relief. The thin traces of the thermal relief are more elastic than a solid plane. – Kaz May 17 '13 at 15:50
  • @Kaz - So does it make a difference or not? Should I add thermals to all vias, even if they aren't going to be soldered? – Rocketmagnet May 17 '13 at 20:57

3 Answers3

3

Reliability is a somewhat slippery slope, once you start chasing after after it, sort of like the 'in the interests of national security' in contemporary media plotlines. It helps to know what you're protecting against - are you going to vibrate it? Will it experience mechanical shock? Thermal shock? Thermal cycling? Environmental effects like the weather? Direct exposure to sunlight?

Different kinds of risk exposure call for different methods of risk mitigation. All the while, you also have to keep in mind the requirements for performance. For instance, tell an RF designer he has to do that and he'll laugh you out the room - they use vias to build walls to copper. They won't let you break the plane for it, because it defeats the purpose.

However, in a general setting, thermal vias are definitely mechanically forgiving. When in doubt, take the safer route as long as it isn't more expensive. Cost could be as money, performance, or design time.

With low end fabrication vendors, it also gives you a cleaner way to target the drills if it's done by hand and eye(not a problem in the first world as much).

And as for not soldering the vias, I had to use very high reliability fabrication protocols in the past that involve filling every via with solder. Someone literally sits down with a microscope and pours copper into every single via, atleast those big enough for solder to go through, with special dispensation for the smaller ones. Those are used in a very high vibration, extended thermal cycling sort of environment with many other headaches. I don't know how widepread the practice is in regular electronics. We certainly dont do it for lab instrumentation.

Chintalagiri Shashank
  • 2,241
  • 17
  • 26
0

There isn't a reliability issue, per se. Its a making the connection in the first place. Since a regular power via connects to a plane, you have to heat up the whole plane to solder. The thermal relief makes that connection less, so you don't have to heat up as much.

markrages
  • 19,905
  • 7
  • 59
  • 96
Brian Carlton
  • 13,252
  • 5
  • 43
  • 64
0

I agree w/ Brian. Thermal pads/vias are mainly used so that you can solder a pad easily w/o having to heat up the whole surrounding area, which you'd otherwise have to do with a std pad/via.

gman
  • 190
  • 3