Quote from: someone42 on May 02, 2013, 05:36:17 PM

Quote from: BkkCoins on May 02, 2013, 04:47:46 PM

Smaller is harder but I'm not sure it matters. One of those magnifying circular light dealies is recommended. Parts count is the killer. I just finished the prelim parts list and pushed it to github. There's 320 parts on that 10x10cm board. Ouch!

(Someone please send me a Pick n Place machine)

70% of those parts are decoupling caps for the ASICs. Is it really necessary to have 14 per ASIC? As a back-of-the-envelope calculation, I get about 5 nC of charge per clock cycle (based on 1.5 A @ 282 MHz). With 0.8 uF of lumped capacitance, neglecting ESR/ESL, that's about a 7 mV drop, which is small. Maybe you can get away with less? Maybe you can use larger capacitance values but fewer caps overall?

I suppose the only way to know is to do some in-circuit testing on actual ASICs.

When you add an SMT capacitor to a circuit, what you're really adding is a series RLC circuit. R for the ESR of the capacitor and the traces, L for the parasitic inductance of the capacitor and the traces, and the C you actually want

If you look at it on a network analyzer you'll see indeed the impedance has a minimum at a given frequency based on the ESL and Capacitance.

You can add more capacitance but you will shift this frequency lower, and the minimum impedance will increase somewhat.

Generally start with one say, 0.1uF capacitance, and if it doesn't work, try changing the value lower or higher (depending on the frequency you need decoupling at) - once that is ideal, the best thing to do is add more of the exact same value capacitor in parallel. this increases the capacitance but also DECREASES the paraistic inductance, so it gets BETTER and has more decoupling capacitance.

note : People used to add say, 0.1uF, 10nF, 100pF caps in parallel because in theory this would give a wide range of decoupling

the problem is it produces anti-resonance where the coupling gets worse. it's a very, very tricky thing to try and fine tune, and should be avoided in 95% of cases.

my suggestion - put pads for lots of 0603 0.1uF capacitances, but only populate the reference PCB amount. if you need more or have to tweak, the pads are right there for it. It's standard practice to have pads for parts you don't actually populate going into production.