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Can you address or touch on thermal management for spot welding of flat power leads to pads on the PCB? Some techniques include flat nickel-plated contact points soldered to the PCB and then flat nickel leads spot welded to those contact points, and I’ve also heard of the flat leads being spot welded directly to the PCB pad.
Are there some good rules of thumb for figuring thermal conductivity between a top layer pad and:
- isolated adjacent plane layer;
- isolated adjacent plane layer sandwiched between a top layer and inner layer pad connected by vias;
- top layer pad and thru-hole vias to the bottom side?
I’ve never heard of spot welding power leads to a PCB. I have experience with battery pack spot welding, so I can understand that, but I’m not exactly sure what heat you want to manage?
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I don’t have any rules of thumb unfortunately, but thermal resistance is often pretty easy to calculate. Every material has a thermal resistance, which is proportional to the area and thickness of the material.
I go through some of this on my heat management/signal integrity full day class. I talk about calculating thermal resistances, but not specifically about PCB thermal resistance calculations.
Obviously vias help between layers, but it’s a mostly localized effect. Normal 1 oz or thinner copper doesn’t conduct heat so well, so the vias will conduct well where they are, but it doesn’t spread out much past a few cm unless you have thicker copper like 2 oz or thicker.
Pads are very good at spreading heat, and so are through hole vias. You can calculate this using the thickness and length of the via along with the conductivity of the copper. I’d consider the pad (and the lead and solder on the pad) to be all one temperature (not much thermal resistance there). Obviously 2 vias will conduct 2x better than 1 via, and so on.
I hope this helps. I can also say you can calculate real world thermal resistance after the fact (once it’s built) much easier with some steady state measurement of the heat spreading. Then you know what’s actually happening and can model your heat spread across a larger range of ambient temps, etc.
I can talk about calculating thermal resistances of a built PCB from measured temperatures tomorrow in the webinar if you ask the question.
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Hi Kevin,
The heat of spot welding the flat nickel power lead to the battery management PCB assembly is what I’m thinking of. I’ve seen some assemblies where the ‘flash heating’ appeared to be so high that solder balls were created and the copper expanded & partially lifted from the PWB substrate.
I was curious about recommendation for designing to handle that and direct spot welding to the PWB.
Interesting.
I’m not an expert on this kind of construction, but I have been involved in commercial battery pack and BMS design many times. I’ve never seen a tab spot welded directly to a PCB. I’ve only seen them soldered on, bolted on, or go through a connector.
Any heat of spot welding heat is generated almost instantly, and so cannot be spread out by any thermal management method normally available, and if it could, it would likely result in poor welds. The weld, by definition, has to be hot enough to melt metal, and this is always going to be higher than the glass transition temperature of the PCB and so the pad will likely lift.
As an alternate idea, you could try ultrasonic welding. This uses vibrations to join metal and could be much less destructive. Otherwise I’d recommend the normal soldering of the battery tabs to the BMS circuitry since I don’t see any way to spot weld to the PCB without weakening the pad’s connection to the PCB from excessive heat.
You could try really large pads to see if the damage could be contained to just one part of the pad and the connection over the rest of the pad may remain cool enough to hold? That’s all I can think of for this problem. I hope that helps.