SMD "power" resistors : what constraints on PCB layout

Question

I'm looking for current sens resistors. As those are rather big (and beeing for now the limiting element for my PCB size), I would like to stop using over-rated one and take some with "just" the power rating needed.

So I was wondering what exactly the powerrating means?

If for exemple I choose a 0.5W resistor. Does that mean that it is meant to be used at 0.5W, or that it might get destroyed at 0.500001W?

And how about power dissipation (the datasheets I looked at have no information about it) : can I do my layout as "bad" as I want and have the 0.5W rating, or do I need to have a layout optimized for heat dissipation if I want to get the ratings from the datasheet?

If you want a more specific use case, there is the one I'm currently working on :

• using the resistors for measuring motor current (usualy for now 0.1A nominal, worst case 1A for about 1 minute (motor stalling), driver has internal *5 voltage amplifier for sens resistor voltage)
• resistors in the range 0.2-0.3 ohms
• I started my design with 0.2 ohms, 3W, 2512 package (wanting some margin to switch to bigger motors if needed, the driver can handle 2.5A, so 0.2*2.5^2=1.25W, so 3W seemed safe). Now I realized that the resistors are too big to fit, so I'd rather loose the possibility to get bigger motor than increase the size of the PCB. -I would like to go down to 2010 package or smaller (nb : I mainly care about length, so for exemple 1812 is also perfectly fine)
• one of the resistors I was think to use now is this one : 0.5W, 0.24 ohms, 1812 package
• air temperature will be 30°C max (while testing at home), 14°C max in real use (it's for a caving robot)

Answer 1

The manufacturer generally specifies a footprint (or multiple) for their parts. In the case of your 0.5W resistor, it's the standard reflow soldering footprint for the 1812 package size. The resistor will be able to withstand the rated power indefinitely with the manufacturer recommended PCB footprint and an ambient temperature of 70°C (as specified in the datasheet you linked).

It won't immediately break if you apply slightly higher power than it's rated for, especially at lower temperatures (below 70°C). Make sure that the components around the resistor don't heat it up excessively (I wouldn't place a linear regulator right next to it, for example). Most resistors can also withstand higher pulse loads as long as the maximum power rating is observed on average. That means you can pulse the resistor with 1W for a second, then turn it off for another second, and it'll be fine.

You can also get resistors with a higher power rating in the same small package. These are then simply designed to operate at higher temperatures under load. High-power resistors operating at something like 200°C under load isn't unheard of.

TL;DR: As long as you don't put more than 0.5W through that resistor on average and don't exceed an ambient temperature of 70°C because of other components heating it, you can lay out your PCB however you want and it'll be fine.

2.5A is way too much for that 0.5W resistor, though - that'd be 1.5W of power dissipation. 1.5A is the most it can take continuously. It might be able to handle 2A for limited amounts of time (i.e. for a few seconds during a fault condition) but you will risk failure if you do that.

An example of a 1812-size resistor ("wide" style) that can take 1W at 70°C is the RCWL1218R220JNEA from Vishay Dale. It's just designed to run hotter (as can be seen by its 155°C maximum temperature rating). You can comfortably push 2A continuous through it and 2.5A during (time-limited) faults won't kill it either.