3 ms·

> To do a decent PCB layout you'll need to use very small passives in order to get the placement right. 0402 (inches) is small but doable. They really start to

> To do a decent PCB layout you'll need to use very small passives in order to get the placement right. 0402 (inches) is small but doable. They really start to feel like grains of sand at that point. But I personally stick with 0805 and 0603 when doing my own board layouts. I can't say I've ever felt space constrained. The placement of decoupling capacitors can be a few mil off, and in fact the "extra space" for placement helps your tweezers anyway, so you don't really want to push everything so close together. Especially for hobby projects, I don't think anyone is really in the business of counting up the savings of 0.01" in the hobby world. Like, how small are you actually aiming for, and is it really so bad that you can't add another 0.5" to your board? -------- Professionals use 0201 (inches) for maximum flexibility and minimal sizes. So even 0402 is larger than professional projects. If we're all accepting "larger hobbyist sizes" anyway, might as well go all the way up to the much easier 0603 or 0805 parts. While a 0402 is slightly larger than a grain of sand, I'd say 0805 is approximately the size of a grain of rice. So yeah, doable with tweezers and solder paste. Just imagine lining up dry rice and its well within the capabilities of most hobbyists. Its just problematic when you get smaller than that.

dave78 2y ago
> Especially for hobby projects, I don't think anyone is really in the business of counting up the savings of 0.01" in the hobby world. Like, how small are you actually aiming for, and is it really so bad that you can't add another 0.5" to your board? It's not really about saving board space. Decoupling capacitors need to be placed "close" to the chip, and at higher clock frequencies this can be an issue. There's enough decoupling caps needed on a RP2040 that doing them in 0805 would require moving them quite a bit further away from the chip just to have room to place them all. An ATmega (Arduino) at something like 8MHz is really forgiving and you can take a lot of liberties with the layout. The RP2040 runs much faster at 133MHz, so presumably the tolerances are much tighter. Admittedly, I didn't try a doing a design with 0805s for the RP2040 but I read enough from people more experienced than me that gave me the impression that compromising the layout with larger passives had a greater chance of things not working right. Since assembly is so cheap at places like JLC these days, even in small quantities, it really wasn't worth the hassle. I've done many other boards by hand with 0805s and and agree they're pretty easy to deal with.
- dragontamer 2y ago
  > Decoupling capacitors need to be placed "close" to the chip All of these rules are just rules of thumb. The "rule" is that your Power Delivery Network (PDN) needs to have low-enough impedance to function properly. High parallel capacitance and low series inductance/resistance. Longer leads increases inductance and resistance so closer is preferred. But for even 100MHz designs, you're well under the size where 100mil or 0805 would cause any serious problem. ------------- So the "secret" is that all faster designs (100MHz to 300MHz) have substantial on-package capacitance. Take a look at this design, which I admit is Microchip/Atmel MCU, but its running at 300MHz and not just the relatively low 133MHz of the RP2040. https://lcamtuf.coredump.cx/bob-the-cat/ https://lcamtuf.coredump.cx/bob-the-cat/ Those are LARGE 1206 1uF capacitors. Which is actually scary to me because we're not looking at tight 100nF decoupling caps anymore but instead substantially relying upon "on-package" capacitance. Still, it shows that lcamtuf was confident in this 300MHz processor handling far-away 1206 capacitors, showing how much wiggle room we have in practice in these designs. You shouldn't worry about 100mil of movement of 0805 caps on a 133MHz design. After all, there are real designs that are closer to 500 mil that use 1206 caps on a 300MHz MCU. -------- I'm honestly scared for lcamtuf here and would never design a board like this. But I'm really not worried about 0805 caps on relatively low speed 100MHz (or even 133MHz) MCUs. Especially if you're properly "teaming" them up so that their resistances are paralleled and inductances are paralleled. (Notice that lcamtuf's 300MHz design doesn't even have the 10x recommended parallel 100nF capacitors close to any of the pins!! He's really stretching the specs) ----------- But yeah, my personal preference is majority of 0805s and 0603s for the "close" decoupling capacitors. I know there's plenty of wiggle room here (even if I'm not as aggressive as lcamtuf's designs). If you're using PCBA from another shop, I guess its all "free" to you to use 0402s or whatever they got loaded in their chip-shooters. So might as well take your free pre-loaded resistors. But if I'm assembling a board myself, I definitely prefer the larger size.
  - dave78 2y ago
    Fair enough. I've taken digital logic classes etc. in college but when it comes to practical circuit design I'm mostly self-taught, having worked my way up from all through-hole components with ATmega and similar devices to dipping my toe in the SMD waters with larger TQFP parts to now doing a few designs with the RP2040. I've tried to educate myself on best practices and follow recommended layouts and things like that where possible, to try to learn to do things the "right way" - but I definitely don't have enough experience to know when the rules of thumb can be broken or not. It's good to know that the "closeness" requirement of decoupling caps is perhaps not as important as I had believed.

dave78 2y ago

> Especially for hobby projects, I don't think anyone is really in the business of counting up the savings of 0.01" in the hobby world. Like, how small are you actually aiming for, and is it really so bad that you can't add another 0.5" to your board? It's not really about saving board space. Decoupling capacitors need to be placed "close" to the chip, and at higher clock frequencies this can be an issue. There's enough decoupling caps needed on a RP2040 that doing them in 0805 would require moving them quite a bit further away from the chip just to have room to place them all. An ATmega (Arduino) at something like 8MHz is really forgiving and you can take a lot of liberties with the layout. The RP2040 runs much faster at 133MHz, so presumably the tolerances are much tighter. Admittedly, I didn't try a doing a design with 0805s for the RP2040 but I read enough from people more experienced than me that gave me the impression that compromising the layout with larger passives had a greater chance of things not working right. Since assembly is so cheap at places like JLC these days, even in small quantities, it really wasn't worth the hassle. I've done many other boards by hand with 0805s and and agree they're pretty easy to deal with.

dragontamer 2y ago
> Decoupling capacitors need to be placed "close" to the chip All of these rules are just rules of thumb. The "rule" is that your Power Delivery Network (PDN) needs to have low-enough impedance to function properly. High parallel capacitance and low series inductance/resistance. Longer leads increases inductance and resistance so closer is preferred. But for even 100MHz designs, you're well under the size where 100mil or 0805 would cause any serious problem. ------------- So the "secret" is that all faster designs (100MHz to 300MHz) have substantial on-package capacitance. Take a look at this design, which I admit is Microchip/Atmel MCU, but its running at 300MHz and not just the relatively low 133MHz of the RP2040. https://lcamtuf.coredump.cx/bob-the-cat/ https://lcamtuf.coredump.cx/bob-the-cat/ Those are LARGE 1206 1uF capacitors. Which is actually scary to me because we're not looking at tight 100nF decoupling caps anymore but instead substantially relying upon "on-package" capacitance. Still, it shows that lcamtuf was confident in this 300MHz processor handling far-away 1206 capacitors, showing how much wiggle room we have in practice in these designs. You shouldn't worry about 100mil of movement of 0805 caps on a 133MHz design. After all, there are real designs that are closer to 500 mil that use 1206 caps on a 300MHz MCU. -------- I'm honestly scared for lcamtuf here and would never design a board like this. But I'm really not worried about 0805 caps on relatively low speed 100MHz (or even 133MHz) MCUs. Especially if you're properly "teaming" them up so that their resistances are paralleled and inductances are paralleled. (Notice that lcamtuf's 300MHz design doesn't even have the 10x recommended parallel 100nF capacitors close to any of the pins!! He's really stretching the specs) ----------- But yeah, my personal preference is majority of 0805s and 0603s for the "close" decoupling capacitors. I know there's plenty of wiggle room here (even if I'm not as aggressive as lcamtuf's designs). If you're using PCBA from another shop, I guess its all "free" to you to use 0402s or whatever they got loaded in their chip-shooters. So might as well take your free pre-loaded resistors. But if I'm assembling a board myself, I definitely prefer the larger size.
- dave78 2y ago
  Fair enough. I've taken digital logic classes etc. in college but when it comes to practical circuit design I'm mostly self-taught, having worked my way up from all through-hole components with ATmega and similar devices to dipping my toe in the SMD waters with larger TQFP parts to now doing a few designs with the RP2040. I've tried to educate myself on best practices and follow recommended layouts and things like that where possible, to try to learn to do things the "right way" - but I definitely don't have enough experience to know when the rules of thumb can be broken or not. It's good to know that the "closeness" requirement of decoupling caps is perhaps not as important as I had believed.