Okay, I'll buy that.
Tell me this; when you say using less copper, why is that a selling point? If you use a solid pour, there is less copper etched away. I kinda had the idea that less etching of the copper was good. Maybe not? Do they recover the copper?
John S
No no.
Yellow is top, red is bottom.
The ground is the entire bottom layer, minus a few "island" areas you see on the right. The yellow is the top layer, the big yellow square is a +Vcc "powerplane" supplying the 3.3V pins. The four traces coming from the corners go to decoupling capacitors. All the vias which don't have red tracks going to them are ground vias. You can't actually see the groundplane under the QFP in the screenshot but it is continuous there.
Sure but this was a relatively large, sparsely populated board with otherwise simple circuitry (it's large for backwards-compatibility reasons). I thought of using a "plug in" 4-layer module too, like the SIMstick(?) idea. But this was easier in the end.
-- John Devereux
I have no idea, and I have to admit, when I said, "Less copper," it was kind of a conceptual thing - I really have no idea about how they plate them, or the cost of milligrams of copper vs. the other factors - guess I was just atlking out of the top of my hat.
Thanks, Rich
'scuse me - I may have been atlking, but I intended to be talking. %-}
I got that (vias give that part away). What I didn't get that top was Vcc.
Yeah, connectors are expensive, too. I rarely have designs that are that sparse. I've been going to smaller and smaller parts to fit stuff in. One board I did recently fits in the back shell of a DE-0 connector. It has a couple of LDOs on it, a TCXO, and a CPLD (.5mm BGA). Even with four layers, the raw board was about $.50. ;-)
Oh,
My main PCB supplier seems to scale prices *entirely* based on sq mm. So boards get cheap real fast as the linear dimension is reduced. A couple of time I had a few hundred ~1/2 sq inch "boards" made basically for use just as mechanical components. One was a spacer (no layers!) and one was a shield (one layer all "groundplane"). No bare board test needed for those ha ha.
When I moved to SMT designs ~10 years ago I standardized on 0805 as the "default" size, at the time they were probably the easiest to procure and to work with. I'm going to 0603 now for new projects, perhaps I should be more ambitious.
-- John Devereux
Oh,
All things being the same, sure, cost scales roughly with area since more get stuffed on a panel. Some of our board sizes are chosen so two or four can get placed on a panel (1 1/2 doesn't help ;-). Obviously other things matter, too (number of layers, minimum feature size, drill density, number of drills,...).
0603 was pretty much standard when I started with the company ('08). They were scared of anything smaller. I've recently been moving to 0402s for dense stuff, particularly for decoupling caps and pullups where there are large numbers of the same value. We're limited to 10 0402 feeders, right now, but I'm sure they'll buy more if I can show justification. I've also gone to SOT-523s and SC70s, rather than SOT-23, where possible. Outside some grumbling from the inspection and repair folks, production has had zero problems with the smaller parts. Amazingly they had zero problems with .5mm pitch BGAs, once they got them put into reels (they have problems picking parts up out of trays).They want more function in a smaller footprint, that's what it takes. Oh, well.
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