Definitive guide to SMT resistor dissipation?

Sod it, I'll rephrase that. What are the best 1% tolerance resistors in SMT to replace 0.6W and 2W metal film? (0.5W is also acceptable).

Lostgallifreyan wrote in news:Xns99EBBFE4E1F8Czoodlewurdle@140.99.99.130:

Better rephrase that too... I want metal film. Just not axial lead types.

Usually 250mW at 70°C "Ta" derated to zero at or around 155°C. The exact definition of Ta is another question.

All the reputable makers have data sheets.

Best regards, Spehro Pefhany

Spehro Pefhany wrote in news: snipped-for-privacy@4ax.com:

Thanks. 250 mW is what I've seen enough times to think it's reasonable to assume most will match that. Same with 1210 doing 500 mW, I think, but I haven't seen enough examples.

I guess Ta means the actual temperature of the resistor, as opposed to ambient, which really could mean anything depending how proximate it was, or whether the air moved...

I don't even know who they are, with SMT resistors, and specific maker's guides should never be relied on for general standards, which is why I asked the question about general guides. Asking in a place where people have direct experience of various standards is the best thing, rather than blundering on Google for days...

Most resistors found on eBay might have a size declared but no hint of who made them.

Lostgallifreyan wrote in news:Xns99EBBE5647666zoodlewurdle@140.99.99.130:

Ok, found one answer to this:

The numbers are dimensions in 10-mil (thou) units:

Power dissipation depends to a large extent on how they are mounted, size of pad/trace, proximity, the type of resistor construction and material.

Consult the manufacturers' data for recommended dissipation limits, it's freely available (Vishay, Yageo, etc).

Fred Abse wrote in news: snipped-for-privacy@cerebrumconfus.it:

Thankyou. Vishay I discovered in the last half hour. :) Looked like a good reference capable of being extended to other firms parts, in many cases I saw.

I noticed that the dissipation specs are conservative, so I guess it's possible to increase it in the case of one or two with a good anchorage on broad pads. I might just try a test to see what it takes to overheat them in an actual-size test mounting. Can't do that till I get them though, hence the questions...

Is there any SMT resistor that is ideal to substitute for an 0.6W metal film type? Something standardised enough that it won't matter exactly who I buy it from?